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Deciphering mantle source components in basalts from hotspot tracks and oceanic islands Nobre Silva, Inês G. 2011

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DECIPHERING M ANTLE SOURCE COMPONENTS IN BASALTS FROM HOTSPOT TRACKS AND OCEANIC ISLANDS by Inês Garcia Nobre Silva  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in The Faculty of Graduate Studies (Geological Sciences)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) April 2011  © Inês Garcia Nobre Silva, 2011  A BSTRACT The isotopic compositions of oceanic island basalts erupted at the Earth’s surface can be used to directly constrain the isotopic signatures of their deep mantle sources. Basaltic rocks are highly susceptible to seawater alteration, which can significantly modify elemental contents and potentially hinder the use of radiogenic isotopes as geochemical tracers. In this study, multi-isotopic (e.g., Pb-Hf-Sr-Nd) analyses on the same acid-leached sample aliquot are shown to produce reliable results for use in the discrimination of mantle source components of oceanic basalts. Application to basalts from the Ninetyeast Ridge in the Indian Ocean and from Mauna Kea volcano on Hawaii in the Pacific Ocean allows for an enhanced resolution of their source components and distribution in the deep mantle. Isotopic geochemistry of the Ninetyeast Ridge sampled during Ocean Drilling Project Leg 121 reveals a Kerguelen and Amsterdam-St. Paul mantle plume origin with the presence of at least three source components and no contribution from an Indian mid-ocean ridge basalt source. The isotopic characteristics of the Ninetyeast Ridge basalts are typical of the Dupal isotopic domain and consistent with recycling of altered oceanic crust and a mixture of pelagic sediments and lower continental crust into their mantle source. This supports a deep origin for the enriched mantle “EM-1”-like Dupal signatures encountered in Indian Ocean island basalts. In the Pacific Ocean, isotopic heterogeneity in basalts from the deepest parts of the Hawai’i Scientific Drilling Project (HSDP2) core on Mauna Kea, combined with prior results from overlying flows, indicate that shield basalts can be explained by mixing of variable proportions of four isotopically distinct components intrinsic to the Hawaiian mantle plume. The “Kea” component is the prevailing composition in Mauna Kea basalts and  ii  ABSTRACT  throughout volcanic activity of the Hawaiian hotspot. The relatively depleted isotopic compositions of this “Kea” component are shared by other Pacific oceanic island basalt groups and are very similar to those of the common mantle component “C”. This suggests that “Kea” may be a common and widespread composition within the deep mantle beneath the Pacific Ocean basin.  iii  PREFACE 
 This dissertation encompasses four chapters (2 to 5) that were prepared in manuscript format appropriate for submission to international scientific journals. I am the leading author of the four chapters and all are co-authored by my supervisors Dr. Dominique Weis and Dr. James S. Scoates. Dr. Jane Barling, member of my supervisory committee, is also a coauthor of Chapters 2 and 4. All samples used for this dissertation were part of the working sample collection of Dr. Dominique Weis, as a participant of major international scientific projects (IODP and HSDP). All research, analytical work (except as indicated below), and manuscript preparation was carried out by me. For each chapter, my supervisors and co-authors contributions included exchange of ideas, advice, insightful comments, careful reviewing, as well as financial support.  CHAPTER 2 Leaching Systematics and Matrix Elimination for the Determination of High-Precision Pb Isotope Compositions of Ocean Island Basalts — a version of this chapter has been published in the electronic journal Geochemistry Geophysics Geosystems from the American Geophysical Union. Nobre Silva, I. G., D. Weis, J. Barling, and J. S. Scoates (2009), Leaching systematics and matrix elimination for the determination of high-precision Pb isotope compositions of ocean island basalts, Geochem. Geophys. Geosyst., 10. Q08012, doi:10.1029/2009GC002537.  
 iv  PREFACE  CHAPTER 3 The Effects of Acid Leaching on the Sr-Nd-Hf Isotopic Compositions of Ocean Island Basalts — a version of this chapter has been published in the electronic journal Geochemistry Geophysics Geosystems from the American Geophysical Union. Nobre Silva, I. G., D. Weis, and J. S. Scoates (2010), Effects of acid leaching on the Sr-NdHf isotopic compositions of ocean island basalts, Geochem. Geophys. Geosyst., 11, Q09011, doi:10.1029/2010GC003176. The Sr isotopic compositions presented in this chapter were analyzed by Dr. Bruno Kieffer. CHAPTER 4 The Ninetyeast Ridge and its Relation to the Kerguelen, Amsterdam and St. Paul Hotspots in the Indian Ocean — is intended for publication. Authors: Inês G. Nobre Silva, Dominique Weis, James S. Scoates and Jane Barling Dr. Bruno Kieffer and Claude Maerschalk analyzed the Sr isotopic compositions and some of the Nd isotopic compositions presented in this chapter. CHAPTER 5 Pb-Sr-Nd Systematics of the Early Mauna Kea Shield Phase and Insight Into the Deep Mantle Beneath the Pacific Ocean — is intended for publication. Authors: Inês G. Nobre Silva, Dominique Weis and James S. Scoates APPENDIX B Ninetyeast Ridge Supplementary Samples Pb-Sr-Nd-Hf Isotope Systematics of the Ninetyeast Ridge from Dredged Samples – Implications for its Mantle Sources This appendix includes a preliminary study on 32 samples that were collected during a scientific expedition to the Ninetyeast Ridge, in the summer of 2007, on which I participated. Dr. Frederick A. Frey was one of the Principal Investigators and provided the major and trace element XRF and ICP-MS analyses on the selected Ninetyeast Ridge samples. I was responsible for acquiring Pb-Sr-Nd-Hf isotopic compositions on all 32 samples. After further analyses of additional samples, this work is intended for publication.  
 v  T ABLE OF C ONTENTS Abstract ................................................................................................................................ii Preface.................................................................................................................................iv Table of Contents.................................................................................................................vi List of Tables........................................................................................................................x List of Figures....................................................................................................................xiii Acknowledgements............................................................................................................xix  CHA PTE R 1 Introduction ........................................................................................................................1 1.1. Motivation and Rationale of the Research Project ...............................................2 1.2. Mantle Heterogeneity, Components and End-member Compositions ..................2 1.3. Large Igneous Provinces and Mantle Plumes ......................................................3 1.4. Hawaiian Mantle Plume – The Hawaiian-Emperor Seamount Chain ...................7 1.4.1. Hawai’i Scientific Drilling Project (HSDP) ............................................. 10 1.5. Kerguelen Mantle Plume System ...................................................................... 14 1.5.1. Ninetyeast Ridge: Longest Linear Feature on Earth................................. 16 1.6. Analytical Challenges ....................................................................................... 19 1.7. Overview of this Dissertation............................................................................ 21 1.8. Contributions to this Research Project............................................................... 26  CHA PTE R 2 Leaching Systematics and Matrix Elimination for the Determination of High-Precision Pb Isotope Compositions of Ocean Island Basalts........................................................... 31 2.1. Synopsis ........................................................................................................... 32 2.2. Introduction ...................................................................................................... 32 2.3. Samples ............................................................................................................ 34  vi  TABLE OF CONTENTS  
 2.4. Analytical Techniques ...................................................................................... 36 2.4.1. Leaching Procedure................................................................................. 36 2.4.2. Sample Digestion and Pb Separation ....................................................... 37 2.4.3. Mass Spectrometry.................................................................................. 38 2.4.3.1. Pb Concentrations............................................................................ 38 2.4.3.2. Pb Isotopic Compositions ................................................................ 38 2.5. Results.............................................................................................................. 39 2.5.1. Sequential Leaching ................................................................................ 40 2.5.2. Pb Purification ........................................................................................ 51 2.6. Discussion ........................................................................................................ 53 2.6.1. Implications of Sequential Acid-Leaching............................................... 53 2.6.2. Effect of Matrix Elimination ................................................................... 57 2.7. Conclusions ...................................................................................................... 60  CHA PTE R 3 The Effects of Acid Leaching on the Sr-Nd-Hf Isotopic Compositions of Ocean Island Basalts................................................................................................................................ 62 3.1 Synopsis ............................................................................................................ 63 3.2. Introduction ...................................................................................................... 63 3.3. Samples ............................................................................................................ 65 3.4. Analytical Techniques ...................................................................................... 67 3.4.1. Sample Preparation ................................................................................. 67 3.4.2. Mass Spectrometry.................................................................................. 68 3.4.2.1. Sr, Nd, and Hf Abundances.............................................................. 68 3.4.2.2. Sr, Nd, and Hf Isotopic Compositions.............................................. 68 3.5. Results.............................................................................................................. 69 3.6. Discussion ........................................................................................................ 77 3.6.1. Effects of Acid Leaching of OIB in Sr-Pb Isotopic Space........................ 77 3.6.2. Effects of Acid Leaching on Trace Element Abundances and Nd-Hf Isotopic Compositions of OIB ................................................................. 84 3.7. Conclusions ...................................................................................................... 88  CHA PTE R 4 The Ninetyeast Ridge and its Relation to the Kerguelen, Amsterdam and St. Paul Hotspots in the Indian Ocean ........................................................................................... 90 4.1. Synopsis ........................................................................................................... 91 4.2. Introduction ...................................................................................................... 91 
 vii  TABLE OF CONTENTS  
 4.3. Geographic and Geological Setting ................................................................... 96 4.4. Previous Petrologic and Geochemical Results................................................... 99 4.5. Samples and Methods ..................................................................................... 101 4.5.1. Samples................................................................................................. 101 4.5.1.1. Summary of Alteration Features and Secondary Mineralogy of Samples ......................................................................................... 105 4.5.2. Analytical Techniques........................................................................... 106 4.5.3. Age-Correction of the Isotopic Ratios of the Ninetyeast Samples .......... 108 4.6. Results............................................................................................................ 112 4.6.1. Site 758 (~5.38˚N; ~82 Ma) .................................................................. 112 4.6.2. Site 757 (~17˚S; ~58 Ma)...................................................................... 115 4.6.3. Site 756 (~27.35˚S; ~43 Ma) ................................................................. 118 4.6.4. Comparison Between Sites.................................................................... 120 4.7. Discussion ...................................................................................................... 122 4.7.1. Defining the Mantle Source Components of the Ninetyeast Ridge Basalts .................................................................................................. 122 4.7.1.1. Is the Relatively Depleted Ninetyeast Ridge Component Indian MORB? ......................................................................................... 125 4.7.1.2. Are the Ninetyeast Ridge Enriched Components Related to the Kerguelen and Amsterdam-St. Paul Mantle Plumes?...................... 129 4.7.2. Implications for the Source of Indian Ocean Basin Volcanic Rocks and the Dupal Anomaly ........................................................................ 133 4.8. Conclusions .................................................................................................... 137  CHA PTE R 5 Pb-Sr-Nd Systematics of the Early Mauna Kea Shield Phase and Insight Into the Deep Mantle Beneath the Pacific Ocean.................................................................................. 139 5.1. Synopsis ......................................................................................................... 140 5.2. Introduction .................................................................................................... 141 5.3. Hawaii Scientific Drilling Project: Geological Setting and Core Stratigraphy.. 142 5.4. Previous Geochemical Results from the HSDP2 ............................................. 146 5.5. Samples .......................................................................................................... 150 5.6. Analytical Techniques .................................................................................... 150 5.7. Results............................................................................................................ 153 5.7.1. Stratigraphic Variations in Pb-Sr-Nd Isotope Compositions .................. 153 5.7.2. Isotope Correlations .............................................................................. 158 5.8. Discussion ...................................................................................................... 160 
 viii  TABLE OF CONTENTS  
 5.8.1. HSDP2: A Record of the Evolution of a Single Volcano or the Output of Different Volcanoes?............................................................. 160 5.8.2. HSDP2 Isotope Variability and the Hawaiian Source Components........ 166 5.8.3. Chemical Structure of the Hawaiian Plume During the Growth of Mauna Kea............................................................................................ 167 5.8.4. The Nature of the “Kea” Component and the Deep Pacific Mantle ........ 169 5.9. Conclusions .................................................................................................... 171  CHA PTE R 6 Conclusions ..................................................................................................................... 173 6.1. Summary and Conclusions.............................................................................. 174 6.1.1. Leaching Systematics of Ocean Island Basalts....................................... 175 6.1.2. Identifying Mantle Components in Basalts from Major Hotspot Tracks on Earth..................................................................................... 175 6.1.3. Fingerprinting Deep Mantle Heterogeneities Beneath the Indian and the Pacific Ocean Basins ....................................................................... 176 6.2. Directions for Future Research........................................................................ 177 6.2.1. Broader Leaching Studies...................................................................... 177 6.2.2. Additional Sampling on the Ninetyeast Ridge ....................................... 178 6.2.3. Drilling on a Loa Trend Volcano........................................................... 179  B I B L I O G R A P H Y .................................................................................................. 181 A P PE N D I X A HSDP2-B and -C Analytical Precision and Accuracy........................................................ 206  A P PE N D I X B NER Supplementary Dredge Samples ............................................................................... 211  A P PE N D I X C Scientific Communications During the Ph.D. .................................................................... 225  
 ix  LIST OF TABLES 
  CHAPTER 2 TABLE 2.1. Summary of Sample Geochemical Characteristics: Hawaii and Kerguelen ........ 35 TABLE 2.2. Pb Content and Isotopic Composition of Each Leaching-Step Solution for Two Hawaiian Basalts................................................................... 41 TABLE 2.3. Pb Content and Isotopic Composition of Each Leaching Step Solution for Two Kerguelen Basalts.................................................................. 43 TABLE 2.4. Pb Isotopic compositions of Tholeiitic Basalts from Mauna Loa and Mauna Kea ....................................................................................................... 47 TABLE 2.5. Pb Isotopic Compositions of Subaerial Alkalic Basalts From Mont Crozier on the Kerguelen Archipelago........................................................................... 48 TABLE 2.6. Pb Isotopic Compositions of Subaerial Tholeiitic Transitional Basalts From Mont des Ruches, Mont Fontaine, and Mont Bureau on the Kerguelen Archipelago and of a Submarine Tholeiitic Basalt from ODP Leg 183, Site 1140, on the Northern Kerguelen Plateau............................................................................................. 50  CHAPTER 3 TABLE 3.1. Summary of Sample Geochemical Characteristics, Hawaii and Kerguelen ........ 66 TABLE 3.2. Sr, Nd, and Hf Elemental Contents and Isotopic Compositions of Unleached Whole-rock Powders, Leachate Solutions, and Leached Residues for Hawaiian Basalts......................................................................................... 70  x  LIST OF TABLES  TABLE 3.3. Sr, Nd, and Hf Elemental Contents and Isotopic Compositions of Unleached Whole-rock Powders, Leachate Solutions, and Leached Residues for Kerguelen Basalts........................................................................................ 71 TABLE 3.4. Summary of the Weight and Relevant Element % Losses During the Multi-step Acid Leaching Procedure................................................................. 78 TABLE 3.5. ICP-MS Trace Element Abundances (ppm) of Unleached and Leached Splits of the Mont Crozier (Kerguelen Archipelago), Basalt ....................................... 85  CHAPTER 4 TABLE 4.1. Summary of the Geochemical Characteristics of the Ninetyeast Ridge Basalts Sampled During ODP Leg 121........................................................................ 102 TABLE 4.2. Parent-Daughter Concentrations and Ratios of the Ninetyeast Ridge Basalts Sampled During ODP Leg 121........................................................................ 103 TABLE 4.3. Pb-Sr-Nd-Hf Isotopic Compositions by MC-ICP-MS and TIMS of Hawaiian Basalt Reference Materials Analyzed in this Study ......................................... 109 TABLE 4.4. Pb Isotopic Compositions by MC-ICP-MS from the Ninetyeast Ridge Basalts ODP Leg 121.................................................................................................. 113 TABLE 4.5. Sr-Nd-Hf Isotopic Compositions by TIMS from the Ninetyeast Ridge Basalts ODP Leg 121.................................................................................................. 116  CHAPTER 5 TABLE 5.1. Geochemical Characteristics of Basalts Sampled from the Last Drilling Phases (-B and -C) of the HSDP2 Core ...................................................................... 147 TABLE 5.2. Pb Isotopic Compositions by MC-ICP-MS of Mauna Kea Samples From the Last Drilling Phases (-B and -C) of the HSDP2 Core.............................................. 154 TABLE 5.3. Sr and Nd Isotopic Compositions by TIMS and MC-ICP-MS of Mauna Kea Samples From the Last Drilling Phases (-B and -C) of the HSDP2 Core ......... 156  xi  LIST OF TABLES  APPENDICES TABLE B1. Ninetyeast Ridge KNOX06 Dredge Summary ................................................. 213 TABLE B2. Major Element and Trace Element Abundances of NER Whole Rock Basalts Collected During the KNOX06 Cruise............................................................ 217 TABLE B3. Pb Isotopic Compositions by MC-ICP-MS from a Subset of Nineyeast Ridge Basalts Collected During the KNOX06 Cruise................................................ 220 TABLE B4. Sr, Nd, and Hf Isotopic Compositions by TIMS and MC-ICP-MS from a Subset of Nineyeast Ridge Basalts Collected During the KNOX06 Cruise ................. 221  xii  LIST OF FIGURES 
  CHAPTER 1 FIGURE 1.1. Diagrams of 87Sr/86Sr versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb for ocean island basalts...........................................................................................4 FIGURE 1.2. World map showing the global distribution of Phanerozoic Large Igneous Provinces ..........................................................................................................6 FIGURE 1.3. Bathymetric map of the Northern Pacific Ocean showing the Hawaiian-Emperor Seamount chain and its distinctive 60° bend .....................................................9 FIGURE 1.4. Photographs from the main drilling phase of the Hawaii Scientific Drilling Project 2 at Hilo.............................................................................................. 11 FIGURE 1.5. Photographs of representative core recovered during the HSDP2–B and HSDP2–C ....................................................................................................... 12 FIGURE 1.6. Bathymetric map of the Indian Ocean showing its major topographic features including those related to the Kerguelen hotspot ............................................. 15 FIGURE 1.7. Photomicrographs in plane-polarized light of typical ocean island basalts analyzed in this study from Hawaii and Kerguelen.......................................... 20 FIGURE 1.8. Photographs showing the analytical approach used in this study for the isotopic analysis of ocean island basalts, from sample collection to laboratory and chemical treatment, to data acquirement ................................................... 22  xiii  LIST OF FIGURES  FIGURE 1.9. Photographs showing some of the geological highlights and typical rocks recovered during the two-month KNOX06RR expedition to the Nineyeast Ridge in the summer of 2007 .......................................................................... 27 FIGURE 1.10. Photographs showing some of the natural and geological highlights during the 4-week expedition to the Northern Hawaiian Islands....................................... 29  CHAPTER 2 FIGURE 2.1. Diagrams of 208Pb/204Pb versus 206Pb/204Pb showing the results of the leaching experiments for two tholeiitic Hawaiian basalts............................................... 42 FIGURE 2.2. Diagrams of 208Pb/204Pb versus 206Pb/204Pb showing the results of the leaching experiments for two Kerguelen basalts............................................................ 44 FIGURE 2.3. Diagrams of 207Pb/204Pb versus 206Pb/204Pb showing the results of the leaching experiments..................................................................................................... 45 FIGURE 2.4. Diagrams of 208Pb/204Pb versus 206Pb/204Pb showing the reproducibility of the Pb isotopic compositions of powder splits from the same alkalic basalt................ 49 FIGURE 2.5. Diagrams of 208Pb/204Pb versus 206Pb/204Pb diagrams for powder aliquots of (a) a tholeiitic Hawaiian basalt (sample SR0954-8.00 (Mauna Kea)) and (b) an alkalic Kerguelen basalt (samples OB93-165 (Mont Crozier)) showing the effect of purifying samples by anion exchange chromatography................................ 52 FIGURE 2.6. Diagrams of 208Pb/204Pb versus 206Pb/204Pb showing the isotopic compositions obtained from the acid-leaching and matrix elimination experiments for tholeiitic Hawaiian basalts............................................................................... 54 FIGURE 2.7. Diagrams of 208Pb/204Pb versus 206Pb/204Pb showing the isotopic compositions obtained with the acid-leaching and matrix elimination experiments for Kerguelen basalts............................................................................................ 56 FIGURE 2.8. Diagrams of 207Pb/204Pb versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb for leached and unleached samples from this study compared to fields for reported compositions of Hawaiian and Kerguelen basalts............................................ 58  xiv  LIST OF FIGURES  CHAPTER 3 FIGURE 3.1. Variation of Pb, Sr, Nd, and Hf concentrations throughout the leaching procedure for basalts from (a) Mauna Loa, (b) Mauna Kea, (c) Mont Crozier, and (d) the Northern Kerguelen Plateau........................................................... 72 FIGURE 3.2. 87Sr/86Sr variations throughout the acid leaching procedure for basalts from (a) Mauna Loa, (b) Mauna Kea, (c) Mont Crozier, and (d) the Northern Kerguelen Plateau ........................................................................................... 74 FIGURE 3.3. 143Nd/144Nd variations throughout the acid leaching procedure for basalts from (a) Mauna Loa, (b) Mauna Kea, (c) Mont Crozier, and (d) the Northern Kerguelen Plateau ........................................................................................... 75 FIGURE 3.4. 176Hf/177Hf variations throughout the acid leaching procedure for basalts from (a) Mauna Loa, (b) Mauna Kea, (c) Mont Crozier, and (d) the Northern Kerguelen Plateau ........................................................................................... 76 FIGURE 3.5. 87Sr/86Sr versus 206Pb/204Pb diagrams showing the relationships between the unleached sample powders, leached residues, and leachate solutions for the two Hawaiian basalts compared to the isotopic compositions of possible external contaminants................................................................................................... 80 87  FIGURE 3.6. Sr/86Sr versus 206Pb/204Pb diagrams showing the isotopic compositions of each sample fraction obtained throughout the leaching procedure for the Kerguelen basalts compared to the isotopic compositions of possible external contaminants................................................................................................... 82 FIGURE 3.7. Extended primitive mantle-normalized trace element diagram.......................... 87  CHAPTER 4 FIGURE 4.1. Bathymetry map (500 m interval contours) of the eastern Indian Ocean showing the major topographic features ........................................................................ 93 FIGURE 4.2. (a) Topographic profile along the Ninetyeast Ridge (north–south). Vertical exaggeration = 171x. Profile drawn using GeoMapApp: http://www.geomapapp.org. (b) Schematic summary of the core stratigraphy with depth.................................................................................... 97  xv  LIST OF FIGURES  FIGURE 4.3. Total alkalis versus silica diagram (after LeBas et al. [1986]) showing the compositional range of the Ninetyeast Ridge basalts recovered during ODP Leg 121................................................................................................ 100 FIGURE 4.4. Extended primitive mantle-normalized incompatible trace element abundances of the Ninetyeast Ridge basalts collected during ODP Leg 121 ..................... 104 FIGURE 4.5. Incompatible trace element concentration variations for the ODP Leg 121 basalts........................................................................................................... 111 FIGURE 4.6. Diagrams of 208Pb/204Pb and 207Pb/204Pb versus 206Pb/204Pb showing the comparison between the MC-ICP-MS results from this study (symbols) and published TIMS results (shaded grey fields) from Weis and Frey [1991] for the Ninetyeast Ridge, ODP Leg 121 basalts .................................................. 114 FIGURE 4.7. Diagrams of 143Nd/144Nd versus 87Sr/86Sr and 176Hf/177Hf versus 143Nd/144Nd for the Ninetyeast Ridge, ODP Leg 121 basalts .................................................. 117 FIGURE 4.8. Comparison of the measured (a) 206Pb/204Pbm; (b) 87Sr/86Srm; εHfm; and (d) 143  Nd/144Ndm variations as a function of drilling depth .................................. 119  FIGURE 4.9. Variations of the measured 206Pb/204Pb, εHf, and 87Sr/86Sr as a function of drilling depth ................................................................................................ 121 FIGURE 4.10. Variations of the initial isotopic compositions of the Ninetyeast Ridge drilled basalts as a function of latitude...................................................................... 123 FIGURE 4.11. Diagrams of (a) initial 208Pb/204Pb versus 206Pb/204Pb, and (b) measured 208  Pb/206Pb versus 207Pb/206Pb for the Ninetyeast Ridge basalts compared to  other Indian Ocean basalts ........................................................................... 127 FIGURE 4.12. Variations of (a) 87Sr/86Sri and (b) 176Hf/177Hfi with 206Pb/204Pbi for the Ninetyeast Ridge basalts compared to other Indian Ocean basalts ................ 131 FIGURE 4.13. (a) 87Sr/86Sri versus 143Nd/144Ndi and (b) 143Nd/144Ndi versus 176Hf/177Hfi of the Nineyeast Ridge basalts compared to other Indian Oceans basalts and to the global isotopic end-members......................................................................... 134  xvi  LIST OF FIGURES  CHAPTER 5 FIGURE 5.1. Location maps of the Hawaiian Islands and HSDP2 drill site ......................... 143 FIGURE 5.2. Simplified lithologic column of the HSDP2 drill core .................................... 145 FIGURE 5.3. Pb, Sr, and Nd isotopic variations with depth (meters below sea level, mbsl) in the HSDP2 drill core................................................................................. 157 FIGURE 5.4. Diagram of 143Nd/144Nd versus 87Sr/86Sr for the HSDP2 Mauna Kea basalts compared to shield basalts from volcanoes on the Big Island of Hawaii (Kohala, Hualalai, Mauna Loa, Kilauea, Loihi) and to the post-shield lavas of Mauna Kea and Kohala................................................................................. 159 FIGURE 5.5. Diagram of 87Sr/86Sr versus 206Pb/204Pb for the HSDP2 Mauna Kea basalts compared to shield basalts from volcanoes on the Big Island of Hawaii (Kohala, Hualalai, Mauna Loa, Kilauea, Loihi) and to the post-shield lavas of Mauna Kea and Kohala................................................................................. 161 FIGURE 5.6. Diagram of 143Nd/144Nd versus 206Pb/204Pb for the HSDP2 Mauna Kea basalts compared to shield basalts from the other volcanoes on the Big Island of Hawaii (Kohala, Hualalai, Mauna Loa, Kilauea, Loihi) and to the post-shield lavas of Mauna Kea and Kohala .................................................. 163 FIGURE 5.7. (a) 206Pb/204Pb versus 206Pb/204Pb and (b) 208Pb/206Pb versus 207Pb/206Pb diagrams for the HDP2 Mauna Kea basalts compared to shield basalts from the other volcanoes on the Big Island of Hawaii (Kohala, Hualalai, Mauna Loa, Kilauea, Loihi) and to the post-shield lavas of Mauna Kea and Kohala ....................... 165 FIGURE 5.8. Comparison of the isotopic compositions of Hawaiian basalts and of the “Kea” component to those of other groups of Pacific Ocean island basalts............... 170  APPENDICES APPENDIX A FIGURE A1. Down-hole comparison of the Pb and Nd isotopic compositions determined in this study and by Blichert-Toft and Albarède [2009] for the same set of samples recovered during the HSDP2-B and -C ......................................................... 208  xvii  LIST OF FIGURES  FIGURE A2. Binary diagrams comparing Pb and Nd isotopic compositions determined in this study and by Blichert-Toft and Albarède [2009] for samples recovered during the HSDP2-B and -C .................................................................................... 209 FIGURE A3. Pb-Pb and Nd-Pb isotope diagrams comparing the isotopic compositions determined in this study and those by Blichert-Toft and Albarède [2009] ...... 210 APPENDIX B FIGURE B1. Diagram of measured 208Pb/204Pb versus 206Pb/204Pb for the Ninetyeast Ridge KNOX06RR dredged samples compared to other Ninetyeast Ridge basalts recovered by drilling and to other Indian Ocean basalts ..................... 222 FIGURE B2. Diagram of measured (a) 143Nd/144Nd and (b) 176Hf/177Hf versus 206Pb/204Pb for the Ninetyeast Ridge KNOX06RR dredged samples compared to other Ninetyeast Ridge basalts recovered by drilling and to other Indian Ocean basalts........................................................................................................... 223 FIGURE B3. Diagram of measured (a) 143Nd/144Nd versus 87Sr/86Sr and (b) εHf versus εNd for the Ninetyeast Ridge KNOX06RR dredged samples compared to other Ninetyeast Ridge basalts recovered by drilling and to other Indian Ocean basalts........................................................................................................... 224  xviii  A CKNOWLEDGMENTS  My PhD research could be metaphorically described as a “Journey to the interior of the Earth”. It was a great personal journey and, as such, there have been many people who, in their own way, contributed throughout the years to the completion of this long adventure. Whether by providing guidance and help at the academic and technical level, or by simply being present at certain stages of my life here in Vancouver, they made this a fuller, character-building, and worthwhile experience. For all that, I thank: Firstly, my supervisors Dominique Weis and James S. Scoates, for their mentorship, patience, and support throughout the years. For their constant availability to their students, for encouraging me to keep going in the right direction, and for allowing me to learn and grow with my research. To Dominique (“Zee Boss”) I am especially grateful for her patience and all the chocolate comfort made in Belgium that she provided to us all when our brains were in need of a sugar high. Jane Barling (“Aunty Jane”), for the long hours of training on the MC-ICP-MS, for her patience and friendly ear when I needed one, either regarding my analytical work and my writing, and for the enumerable trips to IKEA. Bruno Kieffer and Claude Maerschalk, for introducing me to the clean lab work at the PCIGR, for their help with the chemical separation and for running Sr and some Nd on the TRITON TIMS. I also thank Claude for mailing extra chocolate whenever our supply in Vancouver was running low. To Bruno, special thanks for always being available to help and for all the funny translation moments between Portuguese, French and English. Bert Mueller and Vivian Lai, for their invaluable assistance during HR-ICP-MS work; and to Richard Friedman, for always providing whatever was needed in the lab and for his friendly smile at all times. Mike O. Garcia and Fred. A. Frey for giving me the opportunity to be part of the scientific party of the “Underwater Volcanoes of the Northern Hawaiian Islands” and  xix  ACKNOWLEDGEMENTS  “KNOX06RR” scientific expeditions, respectively, as well as for the scientific interactions and advice throughout this project. Catherine Chauvel is also thanked for her advice. All the residents of the “Bowling Alley”, past and present, with whom I shared the highlights of being a graduate student and have become my good friends. I am especially grateful to those who have been with me since the start. Alyssa Shiel, my “twin sista” and long term office mate, for being there all the way until the end. For allowing me to distract her especially during the times we both felt frustrated with our thesis. Elspeth Barnes, for always having time for a friendly conversation and a friendly hug. A special thanks to “Madame” Caroline-Emmanuelle Morisset and Diane Hanano (“Puffin DeeDee”), whose friendship I deeply deeply treasure. For always cheering me up and being there for me, and with who I shared so many unforgettable emotions and experiences. My good Portuguese friends, Nuno Costa, Dina Branco, Gémeas, Nuno Eládio, Hugo Machado, Rita Ferreira, João Geraldes, Sara Ferreira, Rita Folha, who even from afar supported and cheered me. Filipa Xavier, who I met here in Vancouver and who contributed to smooth as saudades da minha alégre casinha. Frederico Henriques, for having started this journey with me. Que pelo menos um de nós triunfe. A very special Thank You to Luke P. Beranek (my dear “Mr. L.B.”), for being a great surprise in my life. For his love, support, and understanding. The most special note of thanks goes to my mum and dad, and my family. Without their unconditional love and support I could have not endured being away from home all these years. Muito obrigada por estarem sempre presentes!  xx  C HAPTER 1  Introduction  1  CHAPTER 1: INTRODUCTION  1.1 Motivation and Rationale of the Research Project Geochemical studies on ocean island basalts related to mantle plumes provide essential information about the composition and evolution of the Earth’s deep mantle. This research focuses on the acquisition of high-precision radiogenic isotopic compositions on stratigraphically controlled samples of basalt related to two major mantle plume systems, Kerguelen in the Indian Ocean and Hawaii in the Pacific Ocean. Specifically, this study focuses on two of the longest hotspot tracks on Earth, the ~5000 km-long Ninetyeast Ridge and the ~6000 km-long Hawaii-Emperor Seamount chain. They were chosen because they formed in different settings and overlie different mantle domains as expressed, for example, by the lead (Pb) isotopic differences between basalts from the Pacific and Indian ocean basins [Dupré and Allègre, 1983]. Hawaii corresponds to the simplest geodynamic case, being the archetype example of intra-plate volcanism, formed by interaction of a hotspot with an over-riding plate [Morgan, 1971]. The Ninetyeast Ridge formed in a more complex geodynamic case, where the associated Kerguelen mantle plume has also interacted with a spreading centre [Luyendyk, 1977; Royer et al., 1991]. The primary goal of this study is to identify the composition and relative proportions of different mantle components sampled during melting of these long-lived mantle plumes and to use this information to assess the nature of their deep mantle domains. In the following sections, a brief overview of the relevant background information related to the major topics covered in this dissertation are provided.  1.2. Mantle Heterogeneity, Components and End-member Compositions Over the past four decades, studies of mantle-derived rocks have established that the mantle is chemically and isotopically heterogeneous at different scales, from centimeters to thousands of kilometers [e.g., Gast, 1969; Tatsumoto, 1978; Zindler and Hart, 1986; Sun and McDonough, 1989; Hofmann, 2003]. Such heterogeneities are the result of differentiation and recycling processes throughout Earth history that continuously generate  2  CHAPTER 1: INTRODUCTION  continental crust, and trace element enriched and depleted mantle domains [e.g., Zindler and Hart 1986; Hart, 1988; Weaver, 1991]. These mantle domains are commonly referred to as reservoirs [e.g., Sun and McDonough, 1989; Willbold and Stracke, 2006]. The distinctive isotopic compositions of a specific reservoir in the mantle are referred to as components, which have no physical implications, and upon mixing are better described by their endmember compositions [e.g., Zindler and Hart, 1986; Stracke et al., 2005; Albarède, 1995; Hofmann, 2003]. The isotopic variability observed among oceanic basalts and the recognition of specific isotopic characteristics within certain ocean island groups are interpreted as resulting from mixing between these isotopically distinct mantle components (Figure 1.1). These components are considered to derive from separate mantle sources characterized by different histories of parent and daughter elemental fractionation [e.g., Zindler et al., 1982; White, 1985; Zindler and Hart, 1986]. Zindler and Hart [1986] identified four components, which they defined as: DMM (depleted mid-ocean ridge basalt (MORB mantle), HIMU (high U/Pb mantle), EM-1 and EM-2 (enriched mantle 1 and 2). In addition, the observation that most oceanic basalts, both OIB and MORB, converge towards a common area in multi-isotope space, led to the recognition of an additional common mantle component, which Zindler and Hart [1986] termed PREMA for prevalent mantle component. In subsequent isotopic studies, other common components, differing from each other only in minor details, were defined and termed FOZO (focal zone) [Hart et al., 1992; Stracke et al., 2005], “C” [Hanan and Graham, 1996], or PHEM (primitive helium mantle) [Farley et al., 1992]. The presence of specific and traceable components in the mantle source of oceanic basalts provides a framework for discussing the origins of these basalts as well as the dynamics of mantle cycling.  1.3. Large Igneous Provinces and Mantle Plumes Large igneous provinces (LIP) are the largest magmatic events known to occur on Earth and encompass a wide range of geological structures in both continental and oceanic  3  CHAPTER 1: INTRODUCTION  87  Sr/86Sr  0.710  MORB Hawaii Galapagos Easter Samoa Society Is. Marquesas Austral-Cook Pitcairn-Gambier Juan Fernandez  0.709 EM II  0.708 0.707 0.706  EM I  Crozet Comores Kerguelen & Heard Amsterdam St. Paul Mascarene Is. Marion-Prince Edward Jan Mayen Iceland Azores Cape Verde Canary Is. Fernando de Noronha Ascension & St. Helena Tristan & Gough Bouvet  0.705 0.704 HIMU  0.703 DMM 0.702 17.5 18 208  a 18.5  19  19.5 206  Pb/204Pb  20  Pb/204Pb  20.5  21  21.5  22  22.5  41.0 40.5  HIMU  40.0  EM II  39.5 39.0  EM I  38.5 38.0 37.5  DMM  37.0 36.5 17.5  b 18.0  18.5  19.0  19.5 206  20.0  20.5  21.0  21.5  22.0  22.5  Pb/204Pb  Figure 1.1. Diagrams of 87Sr/86Sr versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb for ocean island basalts worldwide. End-member compositions (DMM = depleted MORB mantle; EM-1 = enriched mantle 1; EM-2 = Enriched mantle 2; HIMU = high μ) are from Zindler and Hart [1986]. Data from the compilation of White [2010]; GEOROC database (http://www.georoc.mpch-mainz.gwdg.de).  4  CHAPTER 1: INTRODUCTION  environments (Figure 1.2). These include continental flood basalt provinces, volcanic passive margins and aseismic ridges, oceanic plateaus, submarine ridges, and seamounts and oceanic islands, which are the focus of this study. Large igneous provinces represent massive and rapid emplacements of predominantly mafic extrusive and intrusive rock at the Earth’s surface. The processes responsible for their formation involve large amounts of thermal energy and are independent from “normal” seafloor spreading [Coffin and Eldholm, 1994]. LIP volumes can be up to 106 km3 in a continental setting (e.g., Siberian traps) and up to 40x106 km3 in an ocean setting (e.g., Ontong Java Plateau). They are typically emplaced within a geologically short period of time (<10 Myr), with the bulk of the magmatism commonly occurring within <1 Myr. In some cases, the magmatic activity may persist at a much-reduced rate for significantly longer time intervals (up to 100 Myr) and generate hotspot tracks or seamount chains [Ernst et al., 2005; Coffin et al., 2006] (e.g., Ninetyeast Ridge, Emperor-Hawaii Louisville, Chagos-Laccadive). The voluminous volcanism associated with LIP is assumed to account for ~10% of the heat released from the mantle [Davies, 1988; Sleep, 1992] and to have significant environmental impacts [e.g., Larson, 1991; Kerr, 2005; Saunders, 2005]. The origin of LIP was first explained by Morgan [1971] to be related to mantle plumes. These are buoyant upwellings of hot mantle that are chemically distinct from the upper mantle MORB source. Subsequently, numerous models of mantle plumes [e.g., Richards et al., 1989; White and McKenzie, 1989; Campbell and Griffiths, 1990; Gonnerman et al., 2004; Jellinek and Manga, 2004] and alternative theories have been developed and proposed (back arc processes, mantle “edge” convection, delamination, and bolide impact [Saunders, 2005; references therein]), to explain the origin of large igneous provinces. Despite the differences between the models, most agree that large amounts of thermal energy are required to produce the large volumes of magma over a geologically short period of time [Saunders, 2005]. Numerical and laboratory models for mantle plumes typically predict high melt production rates and high temperatures. The large volumes of magma erupted within short  5  CHAPTER 1: INTRODUCTION  120  80  40  0  40  80  120  160  High-Arctic NAVP  60 40 20  Wrangellia  Siberian Traps  Columbia River  Emperor CAMP  Deccan Traps  Caribbean  Afar  E Mariana CLR  0  Galapagos  40 60  Nauru  Reúnion  NER  Paraná  20  Shatsky  Hawaii  Manihiki  Karoo Etendeka  OJP  Madagascar Broken Ridge Kerguelen  Hikurangi  Ferrar  Figure 1.2. World map showing the global distribution of Phanerozoic Large Igneous Provinces (LIP). Short-lived or transient (“plume head”) LIP are identified in orange and long-lived or persistent (“plume tail”) LIP are identified in green, including the Ninetyeast Ridge and Hawaii hotspot tracks. Large Igneous Provinces are better preserved in the oceans where they are not subjected to terrestrial erosional processes and thus offer a prime target for scientific ocean drilling. Abbreviations: NAVP = North Atlantic Volcanic Province; CAMP = Central Atlantic Magmatic Province; CLR = Chagos-Laccadive Ridge; NER = Nintetyeast Ridge; OJP = Ontong Java Plateau. Map modified from Jules Verne Voyager maps (http://jules.unavco.org) and adapted from Coffin et al. [2006].  6  CHAPTER 1: INTRODUCTION  time intervals indicate that melt production rates for LIP must be at least 2 km3/yr, when including the lower crustal intrusives, and require the involvement of deep mantle plumes [White and McKenzie, 1995]. Normal-temperature mantle is incapable of producing such large volumes of melt, even where it decompresses close to the Earth’s surface, as it does beneath mid-ocean ridge spreading centers. This argument is supported by the petrology and geochemistry of the igneous rocks preserved in LIP, which suggests that plume temperatures are typically 150-300ºC higher than normal asthenospheric mantle [e.g., Putirka, 2005; Herzberg et al., 2007]. These temperatures are also consistent with theoretical models of mantle plumes [e.g., Campbell and Griffith, 1991; White and McKenzie, 1995; Davies 1999; Campbell, 2005; Herzberg et al., 2007]. The mantle plume model is the most widely accepted hypothesis for the origin of LIP, which are considered to result from decompression melting within the head of deep mantle plumes rising through the mantle from a hot boundary layer, most probably the coremantle boundary (CMB) [e.g., Campbell and Griffith, 1990; Coffin and Eldholm, 1994; Montelli et al., 2004; Jellinek et al., 2003; Jellinek and Manga, 2004; Campbell, 2005; Coffin et al., 2006]. Several theoretical studies [e.g., Lenardic and Kaula, 1994; Farnetani, 1997; Kellogg et al., 1999] have focused on the composition, structure and geophysical properties of the D’’ region, to gain a better understanding of the source of mantle plumes. Assuming that the D’’ (the lowermost 200 km of the mantle) is the source region of most mantle plumes, the study of plume-related volcanic rocks is the best way geoscientists have to assess the geochemical variability of the deepest mantle [DePaolo and Weis, 2007]. Hence, study of the trace element and isotopic geochemistry of volcanic rocks associated with LIP provides a means to investigate the compositions and processes that occur in regions of the mantle other than the asthenospheric upper-mantle, which is the source of mid-ocean ridge basalts.  1.4 Hawaiian Mantle Plume – The Hawaiian-Emperor Seamount Chain Located in the northern Pacific Ocean, the ~6000 km long Hawaiian-Emperor Seamount Chain (HESC) is one of the most remarkable features in the Pacific Ocean basin  7  CHAPTER 1: INTRODUCTION  (Figure 1.3). It comprises at least 129 volcanoes [Regelous et al., 2003; Sharp and Clague, 2006] and is thought to result from a relatively simple geodynamic setting of the northward motion of the Pacific Plate over a long-lived, relatively fixed hotspot – the Hawaiian mantle plume [e.g., Morgan, 1971; Clague and Dalrymple, 1987]. The distinctive 60º bend at 32ºN separates the northward-trending Emperor seamounts and the westward-trending Hawaiian islands, and is interpreted to have resulted from a change of the Pacific Plate motion around 50 Ma [Keller et al., 2000; Tarduno et al., 2003; Sharp and Clague, 2006]. These two segments constitute an age-progressive chain of seamounts and islands, the oldest being Meiji seamount (85 Ma) located at 53ºN 165ºE and the youngest being Loihi seamount at 19ºN 155ºE (35 km southeast of the Big Island) and which is still in its pre-shield stage of volcanic activity [Keller et al., 2000; Regelous et al., 2003; Dixon and Clague, 2001]. Combined, the total volume of volcanic material erupted for the entire chain (7.08 x 106 km3) [Vidal and Bonneville, 2004] is comparable to that of other large igneous provinces. Of all the seamounts and islands, the Big Island of Hawai’i is the best studied. It is formed by the coalescence of five different shield volcanoes: Kohala, Mauna Kea, Hualalai, Mauna Loa and Kilauea, of which the last three are still historically active [e.g., Clague and Dalrymple, 1987]. As the different volcanoes grow, their lavas overlap and inter-layer, causing older volcanoes to become partially covered by lavas from younger volcanoes. From studies of lavas accessible at the surface of Hawaiian volcanoes, an ideal volcano growth model has been developed [e.g., Clague, 1987; Moore and Clague, 1992; Chen and Frey, 1983; Wolfe and Morris, 1996] to explain the change in different lava types, eruption rates and magma volumes encountered at each volcano as the Pacific Plate moves over the Hawaiian hotspot. In a thermally and compositionally concentric model for the Hawaiian plume [Griffith and Campbell, 1990; Frey and Rhodes, 1993; Lassiter et al., 1996, DePaolo et al., 2001], melt production is low at the cool periphery of the melt zone, initiating volcanism on the ocean floor with low eruption rates of alkalic and transitional tholeiitic basalts during the pre-shield phase. As the volcano migrates over the hotter central part of the plume, melt production increases, and high eruption rates of tholeiites and picrites are observed during the shield-building phase. During this phase, which accounts for ~95% of  8  CHAPTER 1: INTRODUCTION  150°E  170°E  170°W  150°W  130°W  h  50°N  A  ian leut  c Tren  50°N  Emperor Seamounts Pacific Plate v = 9-10 cm/yr  30°N  Ha w aiian  30°N  Chai  n KEA  LOA  10°N  150°E  170°E  170°W  150°W  10°N  130°W  Figure 1.3. Bathymetric map of the Northern Pacific Ocean showing the Hawaiian-Emperor Seamount chain and its distinctive 60º bend. Inset shows the Hawaiian Islands and the distinctitve double chain of volcanoes including the southwestern Loa chain and the northeastern Kea chain. Adapted from the NOAA Surface of the Earth poster (http://www.ngdc.noaa.gov/mgg/fliers/00mgg05.html; http://www.ngdc.noaa.gov/mgg/global/global.html).  9  CHAPTER 1: INTRODUCTION  the volcano’s volume, the volcano reaches subaerial heights (up to 4000-5000 m above sea level). As the volcano moves away from the melting zone, the melt production and eruption rates decline, giving way to the post-shield phase, characterized by inter-bedded eruptions of tholeiites, transitional tholeiites and alkalic basalts. As the volcano continues to move away, a period of quiescence (<0.4 to 2.5 Myr) follows, after which the volcano can undergo infrequent small-volume eruptions of strongly alkalic basalts from vents not associated with pre-existing structures [e.g., Clague and Dalrymple, 1987].  1.4.1. Hawai’i Scientifc Drilling Project (HSDP) Drilling of ocean island volcanoes provides unique access to a continuous sequence of samples that can be studied and interpreted in the context of shallow-level magmatic processes and deeper mantle evolution with time. With the recovery of ~3500 m long continuous stratigraphic sequence of predominantly Mauna Kea lavas during the Hawaii Scientific Drilling Project (Figures 1.4 and 1.5), major advances have been made in terms of understanding the geochemical and petrological structure of an Hawaiian volcano [e.g., DePaolo et al., 1996; Stolper et al., 2009]. The HSDP provided a remarkable opportunity to quantify and characterize the geochemical evolution of a single volcano (in this case, Mauna Kea) as it moves with the Pacific Plate across a mantle plume and samples different zones of the melting region. During the second phase of HSDP, a large amount of high-precision geochemical data was acquired [e.g., Blichert-Toft et al., 2003; Eisele et al., 2003; Kurz et al., 2004; Bryce et al., 2005], leading to a better understanding of the chemical structure of the Hawaiian mantle plume, involving different scales of heterogeneity within the plume. “Large scale” heterogeneities relates to the overall geochemical structure of the Hawaiian mantle plume and is best discriminated when comparing the several volcanoes that make up the Hawaiian Islands. The correspondence of the two geographically parallel chains (Loa and Kea, [Dana, 1849; Jackson et al., 1972]) to two geochemically distinct trends is a reflection of this large-scale structure [Tatsumoto,1978]. “Small scale” heterogeneities relates to  10  Top  CHAPTER 1: INTRODUCTION  Unit 41  Mauna Loa Bottom  Mauna Kea Unit 42  Top  b  Unit 168  Unit 169  Submarine  Bottom  Subaerial  a  c  Figure 1.4. Photographs from the main drilling phase 2 of the Hawaii Scientifc Drilling Pproject at Hilo. (a) The HSDP2 drill rig (owned by the Drilling, Observation and Sampling of the Earth’s Continental Crust Corporation, DOSECC). (b) Working half of core where the stratigraphic contact between subaerial basalts from Mauna Loa and Mauna Kea can be observed; (c) Working half of core where the stratigraphic contact between Mauna Kea subaerial and submarine lavas was encountered. Photographs from the HSDP public data at http://www.icdp-online.org.  11  CHAPTER 1: INTRODUCTION  a  b  c  d  Figure 1.5. Photographs of representative core recovered during the HSDP2–B and HSDP2–C. (a) Intrusion with abundant olivine clasts. (b) Working box of core containing moderately plagioclase-clinopyroxene-olivine-phyric basalt, recovered at ~3098 mbsl. (c) Working box of core containing massive aphyric basalt, recovered at ~3348 metres below sea level (mbsl). (d) Pillow breccia – team scientist points to the clay of a pillow breccia (normally green) that was baked to a white color by an overlying intrusion. Photographs taken from HSDP public data at http://www.icdp-online.org.  12  CHAPTER 1: INTRODUCTION  smaller and more common geochemical variations within the mantle plume and is best distinguished when looking at the isotopic variations within a single volcano. Elemental and isotopic distinctions observed between basalts from the Loa and Kea trend volcanoes have been interpreted as reflecting sampling of two different areas of the Hawaiian mantle plume. Some authors [e.g., Lassiter et al., 1996; DePaolo et al., 2001; Kurz et al., 2004; Bryce et al., 2005] have interpreted these two areas to be more central and peripheral zones, respectively, within a concentrically zoned mantle plume. At a broader scale, the elemental and isotopic differences between basalts from the Loa and Kea trends might reflect concentric zonation of the plume. However, this zonation model does not account for the variations observed in Pb isotopic space, nor of the other isotopic systems when combined with Pb for both the Loa and Kea trend volcanoes. Based on the Pb isotopic compositions of several Hawaiian volcanoes, Abouchami et al. [2005] interpreted the systematic differences between the two trends as reflecting a bilateral compositional difference between the southwest and northeast sides of the plume. Pb isotopes are extremely useful in identifying different mantle components [e.g., Tastumoto, 1978; Sun, 1980]. Pb isotope systematics of Mauna Kea samples collected during HSDP-2 allowed for the identification of three distinct Pb linear arrays, which were inferred to represent at least four mantle components involved in the origin of Mauna Kea lavas [Eisele et al., 2003]. These Pb trends were interpreted as reflecting small-scale vertical heterogeneities within the plume stem. Whereas Sr, Nd, Hf and He isotopic compositions analyzed to date appear to mimic the concentric temperature distribution within the hotspot [Blichert-Toft et al., 2003; Kurz et al., 2004; Bryce et al., 2005], Pb isotopes are not as affected and are more sensitive to the superposition of vertical and lateral heterogeneities within the plume source.  13  CHAPTER 1: INTRODUCTION  1.5. Kerguelen Mantle Plume System The Indian Ocean basin is dominated by volcanic features related to activity of the Kerguelen mantle plume, including the Kerguelen Plateau, Kerguelen Archipelago, Broken Ridge and the Ninetyeast Ridge (Figure 1.6). Together with their continental counterparts (Bunbury basalts, southwestern Australia; Rajmahal Traps, northeast India; lamprophyres in conjugate margins of India and Antarctica), they represent ~130 Myr of magmatic activity and ~2.5 x 107 km3 of magma output by the Kerguelen mantle plume [Coffin et al., 2002; Frey et al., 2002]. The formation of these features involves a considerably more complex geodynamic setting than that of Hawaii and is intimately related to the break-up of Gondwana and evolution of the Indian Ocean during the Cretaceous and Cenozoic by the interaction of the deep-seated Kerguelen mantle plume with the newly forming spreading centre. Based on new radiometric  40  Ar/39Ar ages [Coffin et al., 2002; Duncan, 2002], the  Bunbury basalts represent the first clear manifestation of the Kerguelen plume, from ~132 to ~123 Ma. This period was characterized by low output rates (<0.1 km3/yr), and coincides with the continental breakup between Australia/Greater India and Australia/Antarctica [Frey et al., 2003]. Subsequently, the Naturaliste Plateau formed at ~124 Ma [Davies et al., 1989]. Between ~123 and ~110 Ma, as spreading between India and Antarctica proceeded, magma output rates increased substantially (to ~0.9 km3/yr) and the emplacement of the Southern Kerguelen Plateau, Rajmahal Traps and lamprophyres on the Indian and Antarctic continental margins took place. From ~110 to ~105 Ma, low rates of volcanism gave rise to volcanic rocks found at Elan Bank [Coffin et al., 2002], a western salient with different orientation than that of the Kerguelen Plateau, that has been interpreted as a fragment of continental crust isolated within oceanic lithosphere as pieces of East India broke off [e.g. Coffin et al., 2002; Frey et al., 2002a]. Another peak of volcanism resulted in the formation of the Central Kerguelen Plateau, between ~105 and ~100 Ma, and Broken Ridge between ~100 and ~95 Ma. For the period between ~95 and ~82 Ma, no record of volcanic activity by the Kerguelen plume has  14  CHAPTER 1: INTRODUCTION  45°E  75°E  105°E 20°N  Chagos-Laccadive Ridge  20°N  0°  Ninetyeast Ridge  0°  Mascarene Plateau  CIR  Réunion  Broken Ridge  IR  40°S  SW  AmsterdamSt.Paul  40°S  SE  IR  Crozet  Kerguelen Archipelago  Kerguelen Plateau 60°S  60°S  45°E  75°E  105°E  Figure 1.6. Bathymetric map of the Indian Ocean showing its major topographic features, including those related to the Kerguelen hotspot. Abbreviations: CIR = Central Indian Ridge; SWIR = Southweast Indian Ridge; SEIR = Southeast Indian Ridge. Adapted from the NOAA Surface of the Earth poster. (http://www.ngdc.noaa.gov/mgg/fliers/00mgg05.html).  15  CHAPTER 1: INTRODUCTION  yet been found, but may correspond to the oldest part of the Ninetyeast Ridge now buried beneath the Bengal Fan [Frey et al., 2003]. A subsequent magmatic phase characterized by lower output rates (~0.1 km3/yr) [Coffin et al., 2002] gave rise to Skiff Bank (~68 Ma) and to the Ninetyeast Ridge from ~82 to ~38 Ma [Duncan, 1991], as the Indian Plate moved rapidly northward over the hotspot [Frey et al., 2002]. Around 40 Ma, the recently formed Southeast Indian Ridge (SEIR) jumped northeastward intersecting the hotspot and leading to the breakup between the Central Kerguelen Plateau and Broken Ridge [Coffin et al., 2002; Frey et al., 2003]. Consequently, plume-related magmatism became confined to the Antarctic plate and the Northern Kerguelen Plateau formed on relatively old oceanic lithosphere. From ~30 Ma to ~24 Ma [Nicolaysen et al., 2000], the Kerguelen Archipelago flood basalts formed on top of the Northern Kerguelen Plateau, at variable output rates, and significantly less voluminous alkalic volcanism continued until <1 Ma [Weis and Giret, 1994; Weis et al., 1998]. Since ~21 Ma, magmatism related to the Kerguelen plume has created Heard and McDonald Islands [Barling et al., 1990; 1994], as well as a northwest-southeast trending chain of submarine volcanoes (Kerimis seamounts) between these islands [Weis et al., 2002], on the central sector of the Kerguelen Plateau. As Heard and McDonald Islands are currently volcanically active, they represent the most likely present-day position of the Kerguelen hotspot and underlying mantle plume [Weis et al., 2002].  1.5.1. Ninetyeast Ridge: Longest Linear Feature on Earth With a length of ~5000 km, the Ninetyeast Ridge (NER), an aseismic volcanic ridge, is the longest linear feature on earth. Located in the eastern Indian Ocean along the 90ºE meridian (Figure 1.4), it extends from 34ºS at its intersection with Broken Ridge to ~17ºN where it is buried beneath sediments of the Bengal Fan [Royer et al., 1991; Mukhopadhyay and Krishna, 1995; Krishna et al., 2001]. The ridge varies in width from 100 to 200 km [Royer et al., 1991] and it is elevated to ~3.5 km above the Central Indian Basin to the west and the Warton Basin to the east [Krishna et al., 2001].  16  CHAPTER 1: INTRODUCTION  The Ninetyeast Ridge was the scientific destination of several oceanographic cruises, among which two were from the Deep Sea Drilling Project (DSDP) and one from the Ocean Drilling Program (ODP). The basement of the NER was sampled in 1972 by DSDP drilling at four sites (Leg 22, Sites 214 and 216; and Leg 26, Sites 253 and 254) and in 1988 by ODP drilling at three sites (Leg 121, Sites 756, 757 and 758), where ~180 m of tholeiitic basalt, affected by variable degrees of post-magmatic low temperature alteration, were recovered from a total penetration of ~310 m [Shipboard Scientific Party, 1989; Weis et al., 1991]. Paleontological, paleomagnetic and radiometric studies on samples recovered during these cruises indicate that basement rocks were erupted subaerially or in shallow water depths [Shipboard Scientific Party, 1989; Saunders et al., 1991] and that ages become systematically older in a northward direction, from ~38 Ma at DSDP Site 254 to ~82 Ma at ODP Site 758 [Duncan, 1978, 1991; Royer et al. 1991]. The origin of the Ninetyeast Ridge has been debated for decades [Whitford and Duncan, 1979; Royer et al., 1991] with many tectonic models (e.g., horst structure, overthrusted plate boundary, interaction between a spreading center and transform fault, hotspot track) being proposed for its formation. After drilling results from the Ninetyeast produced during DSDP and ODP, the general consensus for its formation was that the ridge formed from enhanced volcanism generated by a fixed hotspot at the edge of the Indian Plate as it moved rapidly northward [Whitford and Duncan, 1979; Royer et al., 1991; Duncan, 1991; Frey et al., 1991; Weis & Frey, 1991]. However, the identity and number of mantle plumes (Kerguelen, Amsterdam-St. Paul) involved in the generation of the ridge [Saunders et al., 1991; Royer et al., 1991; Weis and Frey, 1991] remained open questions. Basement paleolatitudes measured at drill sites 757 to 217 are mostly consistent with a volcanic source that remained at a constant paleolatitude near 50ºS. This was interpreted as the present-day latitude of the Kerguelen plume [Royer et al., 1991], under the westernmost extremity of the Kerguelen Plateau. Given that no recent activity or seismicity has been reported from this area [Royer et al., 1991] and that recent radiometric (40Ar/39Ar) dating from ODP Leg 183 at Site 1139 (Skiff Bank) give ages of ~68 Ma [Duncan, 2002], this  17  CHAPTER 1: INTRODUCTION  cannot be the present location of the Kerguelen mantle plume. According to Steinberg (2000), a present-day positioning of the plume under the Kerguelen Archipelago improves the fit of the Ninetyeast Ridge and Rajmahal Traps in a fixed hot-spot reference frame. However, the most recent volcanic activity is encountered at Heard and McDonald Islands ~400 km to the southeast of the archipelago [Barling et al., 1994; Weis et al., 2002]. As suggested by Royer et al. (1991) and others, the mismatch of the model tracks for the Ninetyeast Ridge assuming the Kerguelen hotspot is underneath Heard and McDonald Islands might imply a southern migration of the plume. This is coherent with the general trend of recent volcanism occurring in a southeasterly direction from the Northern Kerguelen Plateau to the Kerguelen Archipelago to Heard and McDonald Islands [Weis et al., 2002]. However, some paleolatitudes (~43ºS) measured at Site 757 at Site 756 [Klootwijk et al., 1991] do not agree with the latitude of Kerguelen, and may be related to the interaction of Amsterdam-St. Paul hotspot (which at present is at latitudes of ~40ºS) in the formation of the Ninetyeast Ridge [Saunders et al., 1991; Royer et al., 1991]. The uncertainty of the mantle source, or sources, for basalts of the Ninetyeast Ridge is also reflected in the different interpretations based on isotopic compositions. Ninetyeast Ridge basement lavas show incompatible element abundances and radiogenic isotope signatures (Sr-Nd-Pb) intermediate between those of Indian MORB and very enriched ocean island basalt compositions, with more similarities to the latter [Frey et al., 1991; Saunders et al., 1991; Weis and Frey, 1991; Weis et al., 1991]. Weis and Frey [1991] and Saunders et al. [1991] interpreted the Ninetyeast Ridge isotopic compositions as indicating mixing between at least three mantle components: a depleted MORB-like component, an enriched Kerguelen hotspot-type component, and a component comparable to the one sampled by basalts from Amsterdam-St. Paul Islands along the Southeast Indian Ridge. Differences in the isotopic compositions between sites thus reflect differing relative proportions of each component due to the change in the tectonic setting, from hotspot centred volcanism at Site 758 to hotspotridge interaction volcanism at Site 756. Class et al. [1993] interpreted increasing Pb isotope trends with decreasing eruption age from Site 758 (~82 Ma) to Site 757 (~58 Ma) to Site 756 (~43 Ma) as reflecting the compositional evolution of a single young homogeneous plume by  18  CHAPTER 1: INTRODUCTION  secular changes in isotopic ratios resulting from radioactive decay in the source. This compositional trend with age, however, is not observed in the Sr and Nd isotopic systems [Weis and Frey, 1991; Saunders et al., 1991]. Finally, numerous recent studies (especially those resulting from ODP Leg 183) on other parts of the Kerguelen LIP have significantly increased the trace element and isotopic dataset for the overall Kerguelen plume system. With this new dataset, new interpretations of the Kerguelen plume end-member compositions have been developed [e.g., Mattielli et al., 2002; Ingle et al., 2003; Doucet et al., 2005]. Ingle et al. [2003] proposed the existence of two distinct parts of the Kerguelen plume system, the “plume head” and the “plume tail”, to explain the isotopic differences between the Kerguelen Plateau-Broken Ridge and Kerguelen Archipelago lavas. The significantly expanded isotopic dataset for components of the Kerguelen plume system, combined with these new interpretations, offers the opportunity to re-examine the origin of the Ninetyeast Ridge and deep mantle composition beneath the Indian Ocean basin.  1.6. Analytical Challenges The combination of analyses of different isotopic systems (e.g., Sr-Nd-Hf-Pb) on samples of oceanic basalts constitutes a powerful tool for the determination and characterization of their mantle sources and components [e.g., Gast et al., 1964; Tatsumoto, 1966, 1978; White and Hofmann, 1982; Dupré and Allègre, 1983; Hart, 1984; White, 1985; Hamelin et al., 1986; Zindler and Hart, 1986; Weaver, 1991; Hofmann, 1997; 2003; Stracke et al., 2005]. Inherent to their emplacement in an oceanic environment, oceanic basalts are susceptible to seawater alteration (Figure 1.7), and the effects of this alteration on Rb-Sr and U-Pb isotope systematics have long been recognized [e.g., Hart et al., 1974; Hawkesworth and Morrison, 1978; Cohen and O’Nions, 1982; Verma, 1992; Staudigel et al., 1995; Krolikowska-Ciaglo et al., 2005]. Another well-known cause of disturbance of the isotopic compositions of basalts, especially for Pb isotopes, is contamination during crushing and  19  CHAPTER 1: INTRODUCTION  c  a Plag.  Opx.  Plag.  Ol.  Ol. Ol.  1 mm  Plag.  J2-020-23 (ML, Hawaii)  0.5 mm  OB93-165 (Mt. Crozier, KA)  Submarine Tholeiitic basalt ~450 ka MgO (wt%) = 14.3 A.I. =-0.87 LOI (wt%) = 0.99  Subaerial Alkalic basalt ~24 Ma MgO (wt%) =3.5% A.I. = 0.67  b  d Ol. Ol.  Ol.  Ol.  Idd  ing  .  Plag.  1 mm  Plag. Plag.  SR0954-8.00 (MK, Hawaii) Submarine Tholeiitic basalt ~550 ka MgO (wt%) = 18.2% A.I. = -1.31 LOI (wt%) = 0.86  1 mm  ODP Leg183, 1140A-31R-1, 57-61 (NKP) Submarine Tholeiitic basalt ~34 Ma MgO (wt%) = 5.7 A.I. = -0.81 LOI (wt%) = 1.65  Figure 1.7. Photomicrographs in plane-polarized light of typical ocean island basalts analyzed in this study from Hawaii and Kerguelen. Abbreviations: ol. = olivine; plag. = plagioclase; Idding. = iddingsite. (a) Hawaiian basalt from Mauna Loa (sample J2-020-23), characterized by large unaltered olivine phenocrysts in a fine-grained groundmass with trace alteration. (b) Hawaiian basalt from Mauna Kea (sample SR0954-8.00), showing minor alteration of the olivine phenocrysts and moderate alteration of the groundmass (yellow-green patches). (c) Basalt from Mont Crozier on the Kerguelen Archipelago (sample OB93-165), characterized by unaltered, zoned euhedral plagioclase phenocrysts and minor olivine phenocryst partially to completely iddingsitized, set in a medium-grained groundmass, moderately altered. (d) Basalt from the northern Kerguelen Plateau (sample ODP Leg183, 1140A-31R-1, 57-61), showing plagioclase and olivine phenocrysts set in a fine-grained groundmass. Olivine phenocrysts are completely altered. Photomicrographs taken by M.O. Garcia and D. Hanano.  20  CHAPTER 1: INTRODUCTION  grinding [e.g., Woodhead and Hergt, 2000; McDonough and Chauvel, 1991; Weis et al., 2005]. Over the last decade, advances in analytical techniques, particularly the development of the multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS), have allowed the isotope ratios of numerous elements to be determined with significantly improved precision [e.g., Albarède et al., 2004] compared to that obtained by conventional TIMS. With the increase in precision afforded by the triple spike thermal ionization mass spectrometry (TS-TIMS) and MC-ICP-MS, the differences between unleached and leached samples is now resolved for Pb isotope compositions, with the unleached showing generally a more radiogenic character than the leached samples [McDonough and Chauvel, 1991; Abouchami et al., 2000; Eisele et al., 2003; Baker et al., 2004; Weis et al., 2005]. To overcome the chemical disturbance caused by alteration, plus possible contamination during sample handling, careful acid leaching of samples prior to dissolution and analysis (Figure 1.8) has proven to be an effective and essential step in sample processing for mantle geochemistry studies [e.g., Manhès et al., 1978; Dupré and Allègre, 1980; McDonough and Chauvel, 1991; Eisele et al., 2003; Stracke et al., 2003; Weis et al., 2005; Thompson et al., 2008; Hanano et al., 2009; Nobre Silva et al., 2009].  1.7 Overview of this Dissertation The contents of this dissertation focus on identification and interpretation of mantle source components in oceanic basalts whose variations in isotopic compositions reflect the presence of geochemical heterogeneities within the Earth’s mantle [e.g., White et al., 1985; Zindler and Hart, 1986; Hanan and Graham, 1996]. Before one can confidently attribute a certain correlation of isotopic compositions observed in a suite of oceanic basalts to a specific mantle source component, one must be confident that the measured isotopic compositions are indeed primary or magmatic. Inherent to their emplacement in the oceanic environment, oceanic basalts (both MORB and OIB) are highly susceptible to the effects of seawater alteration that can vary from moderate to extensive depending on how long the  21  CHAPTER 1: INTRODUCTION  a) Sample collection  b) Sequential Acid Leaching 1st step  Last acid step (>15x)  Intermediate steps  d) High-Precision Isotopic Analyses  MC-ICP-MS  c) Chemical Separation  TIMS  Figure 1.8. Photographs showing the analytical approach used in this study for the isotopic analysis of ocean island basalts, from sample collection to laboratory and chemical treatment to data acquirement. (a) Submarine basalts recovered by dredging. (b) Some of the acid leaching steps. Note the change in colour of the supernatant from green to pale yello and translucid. (c) Chromatographic columns used in the separation of the elements of interest for isotopic analysis. (d) Mass spectrometers used for the acquisision of the isotopic compositions.  22  CHAPTER 1: INTRODUCTION  basalts have interacted with the seawater and at what temperature [e.g., Hart et al., 1974; Staudigel et al., 1995; Thompson et al., 2008]. Interaction with seawater can change the original elemental contents of the fresh basalts, which in turn can affect their isotopic systematics, especially with respect to the Rb-Sr and U-Pb isotopic systems. Additional disturbances to the isotopic compositions of basalts, especially for Pb isotopes, can be introduced by contamination during sample collection, crushing and grinding [e.g., Weis et al., 2005]. To obtain the most accurate and precise isotopic compositions measured on basalts required to decipher mantle source components, one must then overcome the chemical disturbances produced by alteration and/or possible contamination during sample handling. An assessment of the effect of alteration on the Pb-Sr-Nd-Hf isotopic compositions of oceanic island basalts and its efficient removal by acid-leaching was a major part and contribution of this study, and is the subject of Chapters 2 and 3 of this dissertation. Chapter 2 focuses specifically on the effects of acid-leaching and matrix elimination on the Pb isotopic compositions of ocean island basalts. The preliminary results were presented at the 2005 AGU Fall meeting and at the 2006 GAC-MAC Joint Annual Meeting (Appendix C: Nobre Silva et al., 2005; 2006a) and a version of this chapter was published in Geochemistry Geophysics Geosystems [Nobre Silva et al., 2009]. This study documents the results of a systematic investigation of how the Pb elemental and isotopic compositions of two Hawaiian and two Kerguelen basalts are affected during multi-step (up to 14 steps) acidleaching treatment prior to isotopic analysis (Figure1.8). Following these results, the Pb isotopic compositions of unleached and leached sets, of three sample aliquots each, from seven and eight different basalts from Hawaii and Kerguelen, respectively, are compared after being subjected to one and two passes through the purification process, to assess the importance of the removal of residual elemental matrix in the Pb fraction after anionexchange chromatography. With a total of 174 isotopic analyses, the major conclusion of this study is that accurate and reproducible high-precision Pb isotopic compositions of oceanic basalts can be achieved by careful sample treatment, comprising both thorough acid leaching and two passes through the purification process.  23  CHAPTER 1: INTRODUCTION  Chapter 3 constitutes a complementary study to the previous chapter. The preliminary results were presented at the 2009 AGU Fall Meeting (Appendix C: Nobre Silva et al., 2009) and a version of this chapter was published in Geochemistry Geophysics Geosystems [Nobre Silva et al., 2010]. In this study, the effects of multi-step acid leaching on the Sr, Nd, and Hf elemental contents and isotopic compositions of two Hawaiian and two Kerguelen basalts are assessed through a comprehensive study of elemental and isotopic analyses of unleached and leached aliquots of the same samples. The effects of acid leaching on the trace element abundances are also addressed. Integration of the results of this study with those from Chapter 2 leads to the conclusion that the Sr and Pb isotope compositions of oceanic basalts are highly susceptible to low temperature seawater alteration, as well as to interaction with external contaminants such as Fe-Mn oxides, deep-sea sediments, or loess. In contrast, Nd and Hf isotope compositions are not significantly affected by alteration, nor by the acid leaching treatment, despite the significant elemental losses during acid leaching. Although accurate and precise Pb, Sr, Nd, and Hf isotopic compositions representative of the magma source can be obtained on the same aliquot of acid-leached sample, the same cannot be concluded for elemental abundances. Therefore, in studies that require calculation of initial isotopic ratios at the time of crystallization, parent/daughter ratios should be determined from elemental concentrations measured on unleached sample powders. The results achieved in Chapters 2 and 3 provide assurance that by following a careful sample treatment prior to isotopic analysis one can confidently assign the isotopic compositions measured on acid-leached oceanic basalts to their mantle source components. In the following two chapters of this dissertation (Chapters 4 and 5), this same sample treatment was applied to basalts from two of the major hotspot tracks on Earth – the Ninetyeast Ridge and Hawaii – in two major ocean basins (Indian and Pacific oceans, respectively) to discriminate their source components and relative proportions, and to provide insight into the origin of their distinct compositional mantle domains. Chapter 4 details a high-precision Pb-Sr-Hf-Nd isotopic study of 38 Ninetyeast Ridge basalts recovered from Sites 758 (82 Ma), 757 (58 Ma), and 756 (43 Ma) during ODP  24  CHAPTER 1: INTRODUCTION  Leg 121 campaign in the late 1980s. The isotopic variations reported here, with significantly better precision and reproducibility than 20 years ago, are inconsistent with the hypothesis of an aging mantle plume for the Ninetyeast Ridge [Class et al., 1993]. They support instead the involvement of three, possibly four, distinct components intrinsic to the mantle source of the ridge. Indian asthenospheric depleted mantle is not the “depleted” component within the Ninetyeast Ridge lavas. A Kerguelen and Amsterdam-St. Paul mantle plume origin for the ridge implies that the plume sources are compositionally more heterogeneous in time and space than previously assumed. The Ninetyeast Ridge, together with other Indian Ocean island basalts, has isotopic characteristics typical of the Dupal isotopic domain [e.g., Weis et al., 1992]. Whereas the Dupal–like characteristic of Indian MORB and OIB that formed at the onset of continental rifting (e.g., Cretaceous Kerguelen Plateau) can be attributed to shallow level contamination by lower crust and/or subcontinental lithospheric mantle, the Ninetyeast Ridge basalts and other OIB formed in intraplate setting show no evidence for such shallow contamination. The EM-1-like compositions of the Ninetyeast Ridge basalts are best explained by a contribution of a mixture of pelagic sediments and lower continental crust recycled together with altered oceanic crust in their deep mantle source. The preliminary results from this study were presented at the 16th and 17th Annual Goldschmidt Conference (Appendix C: Nobre Silva et al., 2006b; 2007). Chapter 5 presents a high-precision Pb-Sr-Nd isotopic study of 40 basalts from the last 408 metres of core recovered during the final drilling phase of the Hawai’i Scientific Drilling Project (HSDP2-B and -C) on Mauna Kea. The isotopic compositions of these deeper and older basalts show compositional continuity with the overlying younger basalts, making the ~3500 metre-long HSDP2 core the longest record of the magmatic output of a single volcano. These older basalts extend the isotopic compositions of Mauna Kea to both significantly more and less radiogenic values, similar to those of “ancestral” Kilauea lavas, and those of late shield and post-shield lavas (<400 kyr) from Mauna Kea and Kohala, respectively. The Pb isotopic compositions of the HSDP2 basalts are consistent with the presence of four source components during the growth of Mauna Kea. The “Kea” component is the prevailing component throughout the evolution of Mauna Kea, whereas the remaining  25  CHAPTER 1: INTRODUCTION  three components (“Loihi”, Enriched Makapuu [EMK] and Depleted Makapuu [DMK]) were involved in different stages of volcano development and contributed to the short-term isotopic variability of Mauna Kea basalts. At a broader, regional scale, the isotopic compositions of the “Kea” component are shared by other Pacific OIB. In Pb-Sr-Nd binary isotope diagrams, “Kea” occupies an intermediate position towards which the main trends formed by other Pacific Ocean island groups (from EM-I, EM-II and HIMU) converge. This indicates that the “Kea” component is not only the common composition within the Hawaiian mantle plume, but also that it is a common composition within the deep Pacific mantle. The preliminary results from this study were presented at the 2006 AGU Fall Meeting and at the 18th Annual Goldschmidt Conference (Appendix C: Nobre Silva et al., 2006c; 2008a). A summary of the significant findings during the course of this dissertation is presented in the concluding Chapter 6, as well as ideas of additional future research that could be undertaken related to the topics focused on this dissertation. Additional material to this study is presented in appendices A and B. Appendix A reports the analytical precision and accuracy of the isotopic analyses on basalts from the HSDP2-B and –C. Appendix B reports a summary of the preliminary geochemical results obtained on the Ninetyeast Ridge basalts recovered by dredging during the IODP site survey cruise in the summer of 2007.  1.8. Contributions to this Research Project Within the framework of this research project I had the remarkable opportunity to participate in two scientific research expeditions over the summer of 2007 and interact with scientists and graduate and undergraduate students in the field of marine geology, geochemistry and geophysics. The first expedition was a 48.6-day (departed Phuket on June 18th, 2007 and arrived at Singapore on August 6th, 2007) NSF – IODP funded site survey cruise to the Ninetyeast Ridge in the Indian Ocean – KNOX06RR, on board the R/V Roger Revelle (Figure 1.9). The expedition was led by Dr. Wiliam Sager (Texas A&M University; geophysics) and Dr. Frederick A. Frey (Massachusetts Institute of Technology;  26  CHAPTER 1: INTRODUCTION  a c  d  e  b  f  g  h  Figure 1.9. Photographs showing some of the geological highlights and typical rocks recovered during the two-month KNOX06RR expedition to the Ninetyeast Ridge in the summer of 2007. (a) The research vessel Roger Revelle. (b) Bathymetric map produced during the cruise, showing the course of the expedition. (c) Dredge basket with samples on deck. (d) Samples on board and scientists washing and cutting rocks. (e) Aphyric massive basalt. (f) Palagonite breccia. (g) Altered vesicular olivine-phyric basalt. (h) Altered aphyric massive basalt. Scale in centimetres.  27  CHAPTER 1: INTRODUCTION  geochemistry) and included the participation of included 29 members in the scientific party. The aim of the expedition was to collect geophysical data (bathymetry, magnetic, and seismic) and sample the igneous basement by dredging at several sites along the ~5000 km long ridge, for a proposed drilling program and to test the hotspot hypothesis for the Ninetyeast Ridge. As a PhD graduate student within the “petrology team”, I worked under the supervision of Dr. Fred Frey and Dr. Malcolm Pringle, helping throughout the process of sample collection and preparation (washing and cutting samples as soon as they were on deck), as well as being in charge of the description and grouping of the numerous samples collected during each of the 33 dredges. As part of my research and contribution to this international project, I was responsible for the acquisition of Pb-Sr-Nd-Hf isotopic compositions on 55 selected samples. The preliminary results (on 31 samples) were presented at the 2008 AGU Fall Meeting (Appendix C: Nobre Silva et al., 2008b) and are reported in the Appendix B of this dissertation. The full set of isotopic results on the 55 dredged samples is intended for future submission for publication. The second expedition "Underwater Volcanoes of the Northern Hawaiian Islands" was a 28-day (from September 8th to October 8th, 2007) NSF funded cruise to the Northern Hawaiian Islands, on board the R/V Kilo Moana (Figure 1.10). The expedition was led by Dr. M.O. Garcia (University of Hawaii at Manoa; petrology and geochemistry) and Dr. G. Ito (University of Hawaii at Manoa; geophysics) and included the participation of 3 UBC scientists among 15 altogether plus the JASON crew (Woods Hole Oceanographic Institution). This expedition aimed at mapping the Kaua'i and Ni'ihau submarine volcanic fields (by multibeam bathymetry and acoustic imagery) and sampling many of its volcanoes formed by secondary volcanism using the JASON2 ROV to characterize the petrology, geochemistry and ages of these lavas. My participation in this international expedition included helping in the collection of geophysical (gravity, magnetics, multibeam and sidescan sonar) data, navigation and data logging, still imagery, and rock sampling during JASON2 dives, and subsequent sample preparation and description. Preliminary results from this expedition were published in the Eos Transactions of the American Geophysical Union [Garcia et al., 2008; Appendix C].  28  CHAPTER 1: INTRODUCTION  b a  c  d  e  g  f  h  Figure 1.10. Photographs showing some of the natural and geological highlights during the 4-week “Underwater volcanoes of the Northern Hawaiian Islands” expedition to the Northern Hawaiian Islands. (a) Bathymetric map of the Northern Hawaiian Islands produced during the cruise, showing the cruise track and the location of the Jason2 dives [Garcia et al., 2008; http://www.soest.hawaii.edu/expeditions/Kauai/]. (b) The research vessel Kilo Moana. (c) and (d) Submarine pillow lavas. (e) The remotely operated vehicle Jason 2. (f) Sampling with the robotic arm of Jason 2. (g) and (h) Marine wildlife. Submarine photographs were taken with the camera coupled to Jason2 (Jason Midnight-4 AM team, D. Weis (UBC) team leader and photographer, Kilo-Moana September 2007, University of Hawaii).  29  CHAPTER 1: INTRODUCTION  As part of my early training in the clean laboratory, I was involved in a study carried out at the Pacific Centre for Isotopic and Geochemical Research regarding the Hf isotopic characterization of United States Geologic Survey reference materials. I was responsible for determining the trace element concentrations and Sr, Nd and Hf isotopic compositions of different labware materials including borosilicate glass, glass frit material and quartz from the columns. The preliminary results of this study were presented at the 2005 AGU Fall Meeting (Appendix C: Weis et al., 2005), and the completed study was published in the electronic journal Geochemistry Geophysics Geosystems [Weis et al., 2007; Appendix C]. During the course of this project, funding was provided by a four-year scholarship from Fundação para a Ciência e Tecnologia (FCT), a Thomas and Marguerite MacKay Memorial Scholarship, and NSERC Discovery Grants to Dominique Weis and James Scoates.  30  C HAPTER 2  Leaching Systematics and Matrix Elimination for the Determination of High-Precision Pb Isotope Compositions of Ocean Island Basalts1  1  A version of this chapter has been published. Nobre Silva, I. G., D. Weis, J. Barling, and J. S. Scoates (2009), Leaching systematics and matrix elimination for the determination of high-precision Pb isotope compositions of ocean island basalts, Geochem. Geophys. Geosyst., 10. Q08012, doi:10.1029/2009GC002537. 31  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  2.1. Synopsis Ocean island basalts from Hawaii and Kerguelen were analyzed for their Pb isotopic compositions to assess the effect of acid-leaching and matrix elimination by Pb anionexchange columns on reproducibility and accuracy. Unleached samples consistently yield Pb isotopic ratios that reflect the incorporation of foreign material. Leaching removes up to 7080% of the total Pb content of the samples with corresponding weight losses between 3560%. The older and more altered Kerguelen basalts show better external reproducibility than the Hawaiian basalts, which appears to be due to the presence in the Hawaiian samples of more radiogenic contaminants (e.g., seawater Pb, drilling mud and related alteration phases). All leached samples purified twice on anion-exchange columns show more radiogenic Pb isotopic ratios than those processed once. The difference is larger for tholeiitic basalts (Hawaiian and Kerguelen Plateau) than for transitional to alkalic basalts (Kerguelen Archipelago). The small differences in measured ratios of total procedural triplicates reflect differential elimination of residual alteration via leaching and matrix effects. The effectiveness of matrix elimination depends on the specific basalt composition and tholeiitic basalts (i.e., low-Pb concentrations) require two passes on anion-exchange columns. This study shows that all steps in sample processing are critical for achieving accurate highprecision Pb isotopic compositions of ocean island basalts.  2.2. Introduction The Pb isotopic compositions of oceanic basalts are extremely useful for evaluating and characterizing the mantle sources and components of both ocean island basalts (OIB) and mid-ocean ridge basalts (MORB) [e.g., Gast et al., 1964; Tatsumoto, 1966; 1978; Abouchami et al., 2000; 2005; Eisele et al., 2003; Blichert-Toft et al., 2003; Marske et al., 2007]. In order for the measured Pb isotopic compositions of basalts to be representative of their mantle source, any foreign elemental contribution introduced by post-magmatic processes, including secondary phases produced during seawater alteration and weathering or any surficial contamination from sample crushing, must be eliminated. Acid-leaching of  32  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  samples prior to dissolution and isotopic analysis has long been recognized as a means of removing such secondary material [e.g., Manhès et al., 1978; Dupré and Allègre, 1980; Hamelin et al., 1986; McDonough and Chauvel, 1991; Weis et al., 2005] and a wide variety of leaching protocols are used by different laboratories [e.g., Mahoney, 1987; McDonough and Chauvel, 1991; Weis and Frey, 1991; Stracke and Hegner, 1998; Abouchami et al., 2000; Thirlwall, 2000; Eisele et al., 2003; Stracke et al., 2003; Baker et al., 2004; Weis et al., 2005]. In the past, the reproducibility of leached samples was within the analytical precision of Pb isotope ratio measurements (per mil range). However, with the increase in precision (e.g., <200 ppm range on  208  Pb/204Pb) achieved by recent analytical developments, such as  triple spike thermal ionization mass spectrometry (TS-TIMS) [e.g., Galer, 1999; Woodhead and Hergt, 2000] and double spike or Tl-corrected multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) [e.g., Longerich et al., 1987; Walder and Furuta, 1993; Belshaw et al., 1998; Rehkämper and Halliday, 1998; Thirlwall, 2000; White et al., 2000; Weis et al., 2005], it has become clear that not all leaching techniques provide reproducible results. Reproducibility problems have been reported for Pb isotopic compositions in several studies of oceanic basalts, such as those from the Hawaii Scientific Drilling Project (HSDP) [Abouchami et al., 2000; Eisele et al., 2003] and some Icelandic basalts [Stracke et al., 2003; Baker et al., 2004; 2005; Albarède et al., 2005]. The poor reproducibility in some of these studies was attributed to variable degrees of sample contamination, sample heterogeneity, and/or the inability of the leaching procedures to consistently eliminate various contaminants. Recently, it has also been documented that the accuracy of radiogenic isotope ratio measurements determined by MC-ICP-MS can be affected by non-spectral interferences (matrix effects that affect the ionization and transmission of the analyte, as well as instrumental mass bias) due to residual sample matrix [e.g., Thirlwall, 2002; Woodhead, 2002; Barling and Weis, 2008]. In this contribution, we report Pb isotopic analyses of basalts from two major hotspot systems, Hawaii (Pacific Ocean) and Kerguelen (Indian Ocean), by MC-ICP-MS to assess  33  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  the efficiency of multi-step acid-leaching in obtaining reproducible Pb isotopic compositions of OIB. This is a companion paper to that of Hanano et al. [2009], which documented different alteration assemblages in Hawaiian and Kerguelen basalts and their behavior during acid-leaching based on scanning electron microscopy (SEM) of thin sections and X-ray diffraction (XRD) characterization of sample powders (unleached and leached). To investigate how Pb isotopic ratios are affected by acid-leaching process, two Hawaiian and two Kerguelen samples were chosen from the sample set, where both the Pb contents and isotopic compositions were measured in the acid solutions (leachates) of each leaching step and in the bulk leachates (all leaching step solutions combined), as well as for unleached and leached powders (residues). We then compared the isotopic compositions of unleached and leached powders for other samples from the same hotspot systems. To assess the importance of the residual elemental matrix in the Pb fraction after anion-exchange chromatography, we also compared the isotopic compositions of two sets of full procedural triplicates of leached powder splits of samples that were passed once (1x) and twice (2x) through the purification process.  2.3. Samples Fifteen basalts from the Hawaiian and Kerguelen oceanic islands were selected for this leaching investigation (see Table 2.1 for brief sample characterization). These samples are representative of basalts typically analyzed for radiogenic isotopic compositions from these two islands and span a wide range of MgO (3.5–18.0 wt%) with weak to moderate alteration (e.g. 0.40–2.8 wt% LOI) (Table 2.1). Four samples from this study were examined in detail for alteration mineralogy by Hanano et al. [2009]. For the Hawaiian hotspot system, we chose seven tholeiitic samples from the Mauna Loa and Mauna Kea volcanoes; the Mauna Kea samples were all collected from the HSDP-2 drill core. All of the Hawaiian basalts have ages less than 500 ka [Sharp and Renne, 2005] and, with the exception of one sample from Mauna Loa (SW-70), all are submarine in origin. For the Kerguelen hotspot system, seven samples from the Kerguelen Archipelago were chosen, varying from transitional tholeiites to alkalic basalts with ages ranging from 29 to 24 Ma [Nicolaysen et  34  Table 2.1. Summary of Sample Geochemical Characteristics: Hawaii and Kerguelen Sample  Eruption Environment  Depth a (mbsl/masl)  Rock Type  SiO2 (wt%)  MgO (wt%)  Na2O (wt%)  K2O (wt%)  Pb (ppm)  A.I.b  LOI c (wt%)  Age (Ma) d  References  Hawaiian Volcanoes Mauna Loa SW-70 J2-020-23 J2-019-04  subaerial submarine submarine  – 489 1986  tholeiitic basalt tholeiitic basalt tholeiitic basalt  51.52 50.14 49.86  9.33 6.50 14.19  2.35 2.72 1.62  0.41 0.52 0.27  2.50 2.00 1.00  -1.87 -0.87 -2.28  – 0.99 0.42  0.140 – 0.450  Rhodes, J. M. (unpublished data, 2003) Rhodes, J. M. (unpublished data, 2003) Rhodes, J. M. (unpublished data, 2003)  Mauna Kea (HSDP-2 e) SR0705-0.15 SR0768-11.20 SR0954-8.00 SR0956-18.35  submarine submarine submarine submarine  1823.2 2157.4 3009.2 3019.0  hyaloclastite hyaloclastite tholeiitic basalt tholeiitic basalt  49.16 50.39 47.67 49.70  11.69 6.76 18.03 7.01  1.81 2.26 1.65 2.25  0.17 0.37 0.23 0.35  0.73 1.09 0.66 1.08  -1.76 -0.95 -1.31 -1.34  2.32 0.40 0.85 0.61  0.488 0.506 0.550 0.550  Rhodes and Vollinger Rhodes and Vollinger Rhodes and Vollinger Rhodes and Vollinger  [2004] [2004] [2004] [2004]  subaerial subaerial subaerial  515 380 78  alkalic basalt alkalic basalt alkalic basalt  50.91 48.75 48.93  3.54 3.89 4.05  3.52 3.54 3.19  1.45 1.86 1.65  6.34 5.58  0.67 1.90 1.28  – – –  24.5 24.5 24.5  Weis, D. (unpublished data, 2005) Weis, D. (unpublished data, 2005) Weis, D. (unpublished data, 2005)  Mont des Ruches BY96-27 BY96-31  subaerial subaerial  455 408  transitional basalt transitional basalt  46.90 49.91  10.65 4.31  2.16 2.84  0.72 1.21  0.96 1.69  -0.04 0.01  2.01 1.82  28.0 28.0  Doucet et al. [2002] Doucet et al. [2002]  Mont Fontaine BY96-86  subaerial  300  transitional basalt  45.87  11.50  1.94  0.77  1.71  0.17  2.80  28.0  Doucet et al. [2002]  Mont Bureau GM92-48  subaerial  250  transitional basalt  47.09  8.71  2.50  0.43  0.27  0.05  –  29.5  Yang et al. [1998]  submarine  270.07  tholeiitic basalt  49.64  5.54  2.64  0.49  1.52  -0.81  1.65  34.3  Weis and Frey [2002]  Northern Kerguelen Plateau ODP Leg 183, 1140A-31R-1, 57-61  For submarine basalts, mbsl means meters below sea level; for subaerial basalts, masl means meters above sea level. A.I., alkalinity index (AI = total alkalis – (SiO2 × 0.37 – 14.43)) [Rhodes, 1996]. c LOI, weight loss on ignition after 30 min at 1020ºC. d Ages from Mauna Loa, B. Singer (unpublished data, 2007); Mauna Kea samples, Sharp and Renne [2005]; Mont Crozier and Mont Bureau, Nicolaysen et al. [2000]; Mont des Ruches and Mont Fontaine, Doucet et al. [2002]; Site 1140, Duncan [2002]. e HSDP-2, Hawaii Scientific Drilling Project, phase 2. a  b  35  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  Kerguelen Basalts Mont Crozier OB93-165 OB93-177 OB93-202  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  al., 2000], and all were erupted in a subaerial environment. In addition, to evaluate if compositional differences between samples would influence Pb isotopic compositions during leaching, a submarine tholeiitic basalt from the 34 Ma Northern Kerguelen Plateau (NKP) recovered during ODP Leg 183 [Weis and Frey, 2002] was also analyzed. All Hawaiian sample powders were prepared from chips that were repeatedly rinsed in deionized water and pulverized following the crushing and washing procedures described in Rhodes [1996] and Rhodes and Vollinger [2004]. For the Kerguelen samples, the powders were prepared following the method described in Doucet et al. [2002].  2.4. Analytical Techniques All leaching and chemical separation were carried out in Class 1000 clean labs and the mass spectrometric analyses were performed in Class 10,000 labs at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia. Sample handling in all labs was carried out in Class 100 laminar flow hoods. All reagents used for leaching, dissolution and separation were sub-boiled, all dilutions were made using ≥18.2 MΩ.cm deionized water, and all labware was acid-washed prior to use. Whenever sample size permitted, the complete analytical procedure (separate leaching, dissolution and chemistry) was carried out in triplicate (i.e., three separate aliquots or splits of the same starting sample powder).  2.4.1. Leaching Procedure The sequential acid-leaching procedure used in this study follows that of Weis et al. [2005], which was slightly modified after Mahoney [1987]. Approximately 0.2–0.4 g of sample powder (grain size <100 µm) were acid-leached with 10 mL of 6M HCl in a 15 mL screw-top Savillex® beaker in an ultrasonic bath (frequency of 40 kHz) for 20 minutes. This process brought the temperature up to ~50ºC. The supernatant (leachate solution) was immediately decanted before the fines had time to settle. This procedure was repeated until a transparent (i.e., free of fine-size particles), pale-yellow to colorless solution was obtained.  36  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  The same procedure was repeated two more times using ≥18.2 MΩ.cm water to eliminate any trace of acid. The leached rock powders were then dried to completion on a hot plate at ~120°C and weighed after cooling. A minimum of six acid-leaching steps was required for the least altered samples and up to 14 steps for the most altered ones. In addition, for four samples (Hawaii: J2-020-23 [Mauna Loa], SR0954-8.00 [Mauna Kea]; Kerguelen: OB93165 [Mont Crozier, Kerguelen Archipelago], ODP leg 183, 1140A-31-R1, 57-61 [Northern Kerguelen Plateau]), the leachate solutions were collected at each step, as well as accumulated for bulk leachate analysis, and were measured for both their Pb contents and Pb isotopic compositions.  2.4.2. Sample Digestion and Pb Separation Unleached and leached rock powders were digested in a closed vessel using a 1:10 mixture of concentrated HNO3 and HF acids on a hotplate at ~120°C over ~48 hours. During this period, the sample solutions were ultrasonicated for ~30 minutes to ensure complete digestion, after which they were dried, re-digested in 10 mL of 6M HCl for ~24 hours and dried again. In preparation for Pb chemistry, the samples were re-dissolved in 2 mL of 0.5M HBr. As a final precaution, sample solutions were ultrasonicated for ~20 minutes and centrifuged at 14,500 rpm (14,100 × g) for 6 minutes before being loaded onto a pre-cleaned and conditioned 200 µL column of fresh AG1-X8 100-200 mesh resin (Bio-Rad Laboratories, USA). The matrix was washed out with 0.5M HBr after which Pb was eluted in 6M HCl [Weis et al., 2006]; this column chemistry results in close to 100% recovery of Pb [D. Weis, unpublished data]. To assess the efficiency of the Pb purification and potential matrix effects, a second set of leached triplicates of 11 samples was subjected to two passes through the same anion-exchange resin. After chemical purification, the eluted Pb fraction was dried, a small quantity of concentrated HNO3 was added to destroy any organic material eluted from the resin along with the Pb, and it was dried again. In preparation for isotopic analyses, the dried Pb fractions were re-dissolved in 1 mL of 0.05N sub-boiled HNO3 in an ultrasonic bath. Seven total procedural blanks, including leaching, were measured by IDTIMS using a  205  Pb spike; their Pb concentrations were between 50 to 160 pg (average 100  37  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  pg), which is negligible in comparison to the Pb content of the sample powders analyzed (52 to 428 ng).  2.4.3. Mass Spectrometry 2.4.3.1. Pb Concentrations The Pb concentrations of leached and unleached sample powders, as well as of each acid-leaching step solution and bulk leachates were measured to determine the amount of Pb present at each step of the acid-leaching procedure. The analyses were performed on an ELEMENT2 high-resolution (HR)-ICP-MS (Thermo Finnigan, Germany) and were quantified using external calibration curves and indium (In) as an internal standard. Standard solutions were prepared from a 1000 ppm Specpure® (Alfa Aesar®, Johnson Matthey Company, USA) Pb standard solution. Samples and standards were diluted and run in 0.15M HNO3. All analyses were normalized to the internal standard and blank subtracted. 2.4.3.2. Pb Isotopic Compositions The Pb isotopic analyses were performed on a Nu Plasma MC-ICP-MS (Nu Instruments Ltd, UK) under dry plasma conditions using a membrane desolvator (Nu DSN100) for sample introduction. Analyses were made by static multi-collection with masses 202 to 208 measured in collectors L2 to H4, respectively. Instrumental mass fractionation was monitored and corrected on-line using a Specpure® Tl standard solution with a  205  Tl/203Tl = 2.3885. This in-house value provides SRM-981 Pb standard (NIST,  USA) ratios within error of the triple spike isotopic values [Galer and Abouchami, 1998] and has remained constant since instrument installation (fall 2002). The potential interference on the  204  204  Hg isobaric  Pb ion beam was monitored at mass 202 and, when necessary, was  corrected for assuming natural abundances (202Hg/204Hg = 4.35) adjusted for instrumental mass fractionation. As is standard practice for all MC-ICP-MS analyses at the PCIGR, all samples and standards were prepared fresh for each analytical session, which can be particularly important for Pb-Tl solutions [Kamenov et al., 2004]. Standard solutions were prepared by combining the SRM-981 Pb standard and the Tl standard solutions to give a  38  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  [Pb]/[Tl] of ~4 and diluted with 0.05M HNO3 to obtain an optimal 208Pb ion beam of 8V (and no less than 2V). Samples were also run in 0.05M HNO3 with Tl added to match the standard [Pb]/[Tl] of ~4. Matching of [Pb]/[Tl] is done to ensure that sample and standard solutions are matrix-matched as much as possible. To accomplish this, the Pb content of each solution was determined by HR-ICP-MS analysis of a small aliquot prior to isotopic analysis of the sample by MC-ICP-MS. Sample analysis followed a modified sample-standard bracketing protocol in which the SRM-981 Pb standard was run after every second sample. The results were then normalized off-line to the triple spike values (206Pb/204Pb = 16.9405, and  208  207  Pb/204Pb = 15.4963,  Pb/204Pb = 36.7219) [Galer and Abouchami, 1998], using the ln-ln method as  described in Albarède et al. [2004]. For sessions where a systematic drift in the SRM-981 isotopic ratio reference values was observed (i.e., drift >2SD on the average of the analyses for the day), the sample-standard bracketing normalization was used instead. In all cases, the agreement between the two normalization methods was excellent (better than 2x10-3, or <50 ppm) for all Pb isotopic ratios. During the course of this study, 176 analyses of the SRM-981 Pb standard yielded mean values of 208  206  Pb/204Pb = 16.9421 ± 0.0030,  207  Pb/204Pb = 15.4985 ± 0.0025, and  Pb/204Pb = 36.7190 ± 0.0078, which are within error of the triple spike values [Galer and  Abouchami, 1998]. Reproducibility of the SRM-981 Pb standard on a per session basis was significantly better (±30, ±42 and ±60 ppm/amu for  208  Pb/204Pb,  207  Pb/204Pb and 206Pb/204Pb,  respectively).  2.5. Results All the analyses acquired during the course of this study are presented in Tables 2.2 to 2.6 and the results are illustrated in Figures 2.1 to 2.8. For the four samples indicated in section 3.1 that were studied in detail for characterization of the acid-leaching procedure, we report the Pb isotopic compositions for selected individual leachate solutions, which were  39  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  chosen to cover, as best as possible, the entire leaching profile. For the last leaching steps, the Pb isotopic compositions are not reported as their Pb contents were insufficient (<<7 ng) for analysis. In addition, we subjected six splits each of powder from five Hawaiian and six Kerguelen samples to acid-leaching following the procedure described in section 2.3.1. To assess the influence of residual sample matrix (e.g., Ca, Al, Fe, Mg) still present in the Pb fraction after anion-exchange chromatography on the accuracy and reproducibility of the Pb isotopic compositions, one-half of these same powder splits were passed 1x on columns and the other half was passed 2x, as described in section 2.3.2.  2.5.1. Sequential Leaching Throughout the multi-step acid-leaching procedure, the Hawaiian basalts lost ~35% of their initial weight and the Kerguelen basalts lost ~60%, which is consistent with the higher degree of alteration presented by the older Kerguelen basalts [Hanano et al., 2009]. For both Hawaiian and Kerguelen samples, the amount of Pb (as well as Sr, Nd and Hf) that is leached out decreases significantly in the first 1–2 steps and then slowly decreases with each progressive step (Tables 2.2 and 2.3; Figures 2.1, 2.2 and 2.3). Leaching removes up to 70–80% of the original Pb content of each sample. Unleached sample powders are isotopically more radiogenic than their respective leached sample residues, with the exception of the submarine Kerguelen basalt (ODP leg 183, sample 1140A-31R-1, 57-61), and fall along a mixing line formed by the leached sample residues and the respective bulk leachate solutions. Most of the foreign Pb (~40 to 50% for the Hawaiian and submarine Kerguelen tholeiites and ~25% for the subaerial Kerguelen Archipelago basalts) is removed during the first leaching step (Figures 2.1, 2.2 and 2.3), as demonstrated by the similarity between the Pb isotopic compositions of the first leachate solutions and the bulk leachate solutions. As the number of acid-leaching steps increases, the relative amount of Pb that is removed becomes increasingly smaller (<1.5%), after 3–4 steps for the Mauna Loa and Mont Crozier basalts and after 6–7 steps for the Mauna Kea and Northern Kerguelen Plateau basalts, respectively (Tables 2.2 and 2.3).  40  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  Table 2.2. Pb content and Pb Isotopic Composition of Each Leaching Step Solution for Two Hawaiian Basalts Fraction  ng Pb in Each % Pb in Each Fraction Fraction  Column Passes  Pb/204Pb  206  2 SE  Pb/204Pb  2 SE  207  Pb/204Pb  2 SE  208  Sample J2-020-23 (Mauna Loa) Unl.a  572.6  100  1  18.3052  0.0067  15.5121  0.0021  38.1726  0.0016  B.L.Homog.b B.L.Homog. B.L.Acid c B.L.Acid  313.7 313.7 269.1 269.1  54.8 54.8 47.0 47.0  1 2 1 2  18.3463 18.3473 18.4045 18.4026  0.0006 0.0007 0.0008 0.0005  15.5306 15.5319 15.5575 15.5553  0.0006 0.0006 0.0007 0.0005  38.2486 38.2538 38.3594 38.3528  0.0016 0.0017 0.0019 0.0014  1st 2nd 3rd 4th 5th 6th 1st H 2O 2nd H 2O  340.5 18.6 12.1 9.2 6.7 3.8 2.4  59.5 3.2 2.1 1.6 1.2 0.7 0.4  1 1 1 1  18.3978 18.2983 18.2192 18.2400 –d – – –  0.0013 0.0014 0.0021 0.0025 – – – –  15.5552 15.5096 15.4716 15.4816 – – – –  0.0012 0.0012 0.0019 0.0020 – – – –  38.3484 38.1544 37.9844 38.0073 – – – –  0.0031 0.0036 0.0046 0.0056 – – – –  Total e  393.2  68.7  Leach. Res.f  179.4  31.33  1  18.1799  0.0007  15.4577  0.0006  37.9441  0.0015  Unl.  258.8  100  B.L.Homog. B.L.Homog. rep.g B.L.Acid  138.6 138.6 139.7  1st 2nd 3rd 4th 5th 6th 7th 8th 1st H 2O 2nd H 2O  105.5 14.5 16.3 11.3 11.1 9.2 4.7 5.5 2.6 2.6  40.8 5.6 6.3 4.3 4.3 3.5 1.8 2.1 1.0 1.0  Total  183.2  70.8  Leach. Res.  75.6  29.2  Sample SR0954-8.00 (Mauna Kea) 1  18.6083  0.0025  15.4969  0.0029  38.1771  0.0028  1 1 1  18.6633 18.6735 18.6610  0.0008 0.0036 0.0008  15.4987 15.5084 15.5037  0.0008 0.0030 0.0007  38.2166 38.2398 38.2191  0.0020 0.0060 0.0017  1 1 1  18.6687 18.6288 18.6219 – – 18.6317 18.6477 – – –  0.0013 0.0019 0.0012 – – 0.0024 0.0036 – – –  15.5020 15.4962 15.4942 – – 15.4984 15.5139 – – –  0.0013 0.0017 0.0011 – – 0.0020 0.0031 – – –  38.2270 38.1966 38.1924 – – 38.1959 38.2144 – – –  0.0041 0.0040 0.0033 – – 0.0058 0.0082 – – –  18.6091  0.0011  15.4874  0.0010  38.1790  0.0029  1 1  1  Unl., unleached rock powders. B.L.Homog., bulk leachate solution homogenized (acid + fine powder particles). B.L.Acid, bulk leachate solution without the fine powder particles. d Dashes indicate no isotopic analysis performed. e The Pb content of the bulk leachate solutions does not equal the sum of the individual leaching steps because they were measured on different aliquots of the same powders. f Leach. Res., leached powder residues. g Replicate analyses of the same sample solution. a  b  c  41  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  208  Pb/204Pb  38.35 38.25  60  60  50  50  Pb (ppm)  38.45  Hawaii 40  Pb (%)  38.55  30  30  10  10  0  #1  #1 #1  #2  #2  #3  #3  #4  #4  #5  #5  Leaching steps  #6  #2 H2O  Unleached Sample  37.95  Leached Sample  a  37.85 18.15 38.265  45  2 SE  10  0  5 #1  #2  38.205  18.39  18.43  SR0954-800 (Mauna Kea)  Bulk leachate (acid solution) #1  #3  #2  #3  #4  #5  #4  #5  #6  #6  #7  #7  Leaching Steps  #2 Leached Sample  18.61  18.35  Pb Nd Hf Sr/100  Pb (ppm)  Pb (%)  15  5  b  18.31  Pb/204Pb  10  20  38.165 18.60  206  15  25  38.185  18.27  20  30  0  18.23  25  35  38.225  #4  #3  18.19  40  Pb/204Pb  Bulk leachate (solution+particles)  #2  38.05  208  Bulk leachate (acid solution)  #6 #2 H2O  38.15  38.245  (Mauna Loa)  20  20  0  J2-020-23  Pb Nd Hf Sr/100  40  #1  #2 #8 #1 H2O H2O  #8  #1 #2 H2O H2O  #7 Bulk leachate (solution+particles)  #3 #6 Unleached Sample  2 SE  18.62  18.63 206  18.64  18.65  Pb/ Pb  18.66  18.67  18.68  204  Figure 2.1. Diagrams of 208Pb/204Pb versus 206Pb/204Pb showing the results of the leaching experiments for two tholeiitic Hawaiian basalts: (a) sample J2-020-23 (Mauna Loa) and (b) sample SR0954-8.00 (Mauna Kea). Each circle represents the Pb isotopic composition for each individual leaching fraction and the size of the circle reflects the relative amount of Pb present in the fraction. The open circles indicate the analyses of the unleached sample powders and correspond to 100% of the Pb content in the sample. The filled circles without outlines (labeled as “Leached Sample”) indicate analyses of the leached sample residues, and the symbol size corresponds to the amount of Pb that remained after the entire leaching procedure. The smaller filled circles indicate the analyses of leachates and the number labeled adjacent to each of these circles refers to the leaching step (see inset diagrams). The dark and light grey outlined circles represent the bulk leachate solution recovered throughout the leaching procedure; the size of these two circles is the same, as the Pb content is equivalent at the scale of the figure. The lighter grey circle refers to the isotopic composition of the acid solution plus silt size particles that were removed during leaching, whereas the darker grey circle refers to the isotopic composition of the acid solution alone, after separation of the particles by centrifugation. In the right lower corner of each panel, the average 2SE of the Pb isotopic analysis for each experiment is indicated. The inset diagrams show the percentage of Pb that was eliminated at each leaching step (histogram) and the amount of Pb (ppm) present in each leachate (line graph), as well as the amounts of Sr, Nd and Hf (also in ppm) for comparison (see chapter 3).  42  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  Table 2.3. Pb Content and Isotopic Composition of Each Leaching Step Solution for Two Kerguelen Basalts Fraction  ng Pb in Each % Pb in Each Fraction Fraction  Column Passes  Pb/204Pb  206  2 SE  Pb/204Pb  207  2 SE  Pb/204Pb  208  2 SE  Sample OB93-165 (Mont Crozier) Unl.a  1827.8  B.L.Homog.b B.L.Homog. B.L.Acid c B.L.Acid B.L.Acid rep.d  443.3 443.3 419.8 419.8 419.8  1st 2nd 3rd 4th 5th 6th 7th 8th 1st H2O 2nd H2O  445.8 96.8 28.5 45.3 21.9 22.2 8.6 8.3 3.2 1.8  100  24.4 5.3 1.6 2.5 1.2 1.2 0.5 0.5 0.2 0.1  Total f  682.3  37.3  Leach. Res.g  1145.6  62.7  1  18.6045  0.0017  15.5817  0.0018  39.3219  0.0036  1 2 1 2 2  18.6066 18.6118 18.6045 18.6025 18.5987  0.0009 0.0006 0.0016 0.0006 0.0007  15.5790 15.5846 15.5882 15.5872 15.5830  0.0008 0.0005 0.0018 0.0007 0.0006  39.3221 39.3402 39.3534 39.3482 39.3352  0.0024 0.0015 0.0048 0.0019 0.0018  1 1 1  18.6128 18.5939 18.6016 –e – 18.6014 18.6286 – – –  0.0011 0.0007 0.0015 – – 0.0017 0.0042 – – –  15.5872 15.5787 15.5811 – – 15.5786 15.5839 – – –  0.0009 0.0006 0.0012 – – 0.0017 0.0030 – – –  39.3470 39.2950 39.2880 – – 39.2741 39.2643 – – –  0.0026 0.0017 0.0034 – – 0.0049 0.0086 – – –  18.5959  0.0010  15.5769  0.0008  39.2961  0.0020  1 1  1  Sample ODP Leg 183, 1140A-31R-1, 57-61 (Northern Kerguelen Plateau) Unl. B.L.Homog. B.L.Homog. B.L.Acid B.L.Acid  572.5 332.7 332.7 314.2 314.2  100.00  1 1 2 1 2  18.5451 18.5409 18.5420 18.5428 18.5431  0.0008 0.0011 0.0025 0.0010 0.0026  15.5619 15.5624 15.5622 15.5625 15.5623  0.0009 0.0013 0.0031 0.0010 0.0030  38.9056 38.9003 38.9019 38.9025 38.9039  0.0029 0.0042 0.0105 0.0027 0.0099  1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th 13th 14th 1st H2O 2nd H2O  262.6 59.9 27.3 19.9 15.6 14.9 11.9 7.5 8.8 5.1 5.6 4.8 5.1 3.1 2.6 2.0  45.9 10.5 4.8 3.5 2.7 2.6 2.1 1.3 1.5 0.9 1.0 0.8 0.9 0.5 0.4 0.3  1 1 1 1  18.5487 18.5484 18.5461 18.5436 – 18.5521 18.5635 18.5741 18.5856 18.6111 – – – – – –  0.0011 0.0008 0.0012 0.0022 – 0.0012 0.0018 0.0027 0.0018 0.0032 – – – – – –  15.5685 15.5646 15.5635 15.5627 – 15.5648 15.5659 15.5648 15.5723 15.5922 – – – – – –  0.0011 0.0007 0.0012 0.0023 – 0.0011 0.0017 0.0023 0.0016 0.0029 – – – – – –  38.9182 38.9090 38.8882 38.8781 – 38.8814 38.8893 38.8627 38.9003 38.8502 – – – – – –  0.0030 0.0019 0.0028 0.0064 – 0.0031 0.0056 0.0064 0.0040 0.0067 – – – – – –  Total  456.8  79.8  Leach. Res.  115.7  20.2  18.5575  0.0006  15.5648  0.0007  38.9405  0.0020  1 1 1 1 1  1  Unl., unleached rock powders. B.L.Homog., bulk leachate solution homogenized (acid + fine powder particles). B.L.Acid, bulk leachate solution without the fine powder particles. d Replicate analyses of the same sample solution. e Dashes indicate no isotopic analysis performed. f The Pb content of the bulk leachate solutions does not equal the sum of the individual leaching steps because they were measured on different aliquots of the same powders. g Leach. Res., leached powder residues. a  b c  43  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  Kerguelen  39.365  Bulk leachate (solution+particles)  #1  25  70  Pb/204Pb  10  Unleached Sample  Leached #2Sample  39.285  Pb (%)  15  39.305  50 40 30 20 10  5  0  0  #3  #1  #2  #1  #3  18.598  206  Leached Sample  Pb/204Pb  Bulk leachate (solution)  38.920  Pb (%)  ODP Leg 183, 1140A-31-R1, 57-61 (NKP)  38.940  208  18.605  #1  18.613  Pb/ Pb 204  50  #4  #5  #5  #6  #6  #7  #7  Leaching steps  #1 #2 H 2 O H2 O  #8  #8  #1 #2 H2O H2O  18.620  2 SE  18.628  40  30 20  18.635  Pb Nd Hf Sr/100  35  40  Pb (ppm)  39.245 18.590  30 25 20 15 10 5  10  0  #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14 #1 #2  H 2O H 2O  0  #2 Unleached Sample #3  38.900 38.880 38.860  #4  #3  #7  c  38.960  #2  #6  39.265  38.980  Pb Nd Hf Sr/100  60  20  39.325  208  (Mont Crozier, KA)  Pb (ppm)  39.345  OB93-165  Bulk leachate (solution)  #4  #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14 #1 #2 H 2O H2O Leaching Steps  #8 #7  #6  Bulk leachate (solution+particles)  #9  d  38.840 18.527  #10  18.547  18.567 206  18.587  Pb/ Pb 204  18.607  2 SE  18.627  Figure 2.2. Diagrams of 208Pb/204Pb versus 206Pb/204Pb showing the results of the leaching experiments for two Kerguelen basalts: (a) sample OB93-165 (Mont Crozier, Kerguelen Archipelago) and (b) sample 1140A-31-R1, 57-61 (ODP Leg 183, Northern Kerguelen Archipelago). Color coding, symbol sizing, labelling, and insets as described in the legend to Figure 2.1. KA = Kerguelen Archipelago, NPK = Northern Kerguelen Plateau.  44  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  J2-020-23 (Mauna Loa)  15.586  15.540 Bulk leachate (acid solution)  Leached Sample  15.450  #3  Unleached Sample  #4  15.520 15.514  18.230  18.270  206  18.310  Pb/204Pb  18.350  18.390  18.430  #7  15.484  Leached Sample  #3 Unleached Sample  Bulk leachate (solution+particles)  18.620  18.630  206  18.640  18.650  Pb/204Pb  18.660  #10  Bulk leachate (solution+particles)  18.670  Bulk leachate (acid solution)  #1 #2 #3 #6 #4  15.565  2 SE  18.610  (NKP)  15.570  15.560  b  15.478 18.600  Pb/204Pb  ODP Leg 183, 1140A-31R1, 57-61  15.575  207  #2  15.490  Pb/204Pb  Pb/204Pb 207  15.496  15.580  #1  #6  2 SE  15.585  15.508 15.502  c  206  15.590  Bulk leachate (acid solution)  Bulk leachate (solution+particles)  18.590 18.595 18.600 18.605 18.610 18.615 18.620 18.625 18.630 18.635  15.595  SR0954-8.00 (Mauna Kea)  15.574  #7  Unleached Sample  #6  Leached Sample  15.577 2 SE  18.190  #3  15.580 #2  a  15.420 18.150  (Mont Crozier, KA)  Bulk leachate (acid solution)  15.583  207  207  #2  15.480  OB93-165  #1 #1  15.510  Kerguelen  15.589  Bulk leachate (solution+particles)  Pb/204Pb  15.570  Hawaii  Pb/204Pb  15.600  18.680  d  Unleached Sample  #8 #7 Leached Sample  #9 2 SE  15.555 18.527 18.537 18.547 18.557 18.567 18.577 18.587 18.597 18.607 18.617 18.627  206  Pb/204Pb  Figure 2.3. Diagrams of 207Pb/204Pb versus 206Pb/204Pb showing the results of the leaching experiments: (a) Hawaiian basalt sample J2-020-23, (b) Hawaiian basalt sample SR0954-8.00, (c) Kerguelen basalt sample OB93-165 and (d) Northern Kerguelen Plateau basalt sample 1140A-31-R1, 57-61. Colour coding, symbol sizing and labeling as described in the legend to Figure 2.1; abbreviations as indicated in the legend to Figure 2.2.  45  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  For the Hawaiian basalts (Figures 2.1, 2.3a and 2.3b), the Pb isotopic compositions of the individual leachate solutions generally plot along a mixing trend between a more radiogenic end-member and the leached sample residue. In contrast, for the Kerguelen basalts (Figures 2.2, 2.3c and 2.3d), the Pb isotopic compositions of the individual leachate solutions after leaching step #5 deviate from the mixing line formed by the residual leached sample powder and bulk leachate solutions. The leachate solutions corresponding to higher leaching steps are insignificant in terms of their Pb content (Figures 2.1 to 2.3). Nevertheless, these leachate solutions yield more radiogenic Pb isotope ratios, which indicates the presence of an additional minor component. With the exception of three Hawaiian samples, the unleached sample powders show better reproducibility (e.g., 122 and 93 ppm on  206  Pb/204Pb for samples SR954-8.00 and  OB93-165, respectively) than their respective leached sample residues (e.g., 254 and 230 ppm on 206  206  Pb/204Pb for the same two samples) (Tables 2.4 to 2.6). In a  208  Pb/204Pb vs.  Pb/204Pb diagram (Figure 8a), the individual leaching trends have steeper slopes than the  “Kea-mid8” and “Kea-hi8” groups of HSDP-2 basalts defined by Eisele et al. [2003]. The isotopic compositions of the unleached powders for three Hawaiian basalts (J2-019-04, J2020-23, SR0705-0.15) yield much higher dispersion and plot distinctly outside the fields of their respective volcano (Figure 8), which reflects the presence of a much higher proportion of contamination and secondary components (Figure 2.6; and see section 2.5.1 below). The Hawaiian basalts, despite significantly younger ages and less alteration than the Kerguelen basalts, show much larger differences between the Pb isotopic compositions of unleached powders, bulk leachate solutions and their residue. The number of acid-leaching steps affects the final Pb isotopic reproducibility of a sample. In Figure 2.4, we compare results for an alkalic basalt (OB93-165) from the Kerguelen Archipelago, for which powder splits were subjected either to six-acid leaching steps (3 aliquots) or eight acid-leaching steps (7 aliquots). The Pb isotopic compositions of the powder splits that were subjected to six acid-leaching steps have a reproducibility of 230 ppm on 206Pb/204Pb, whereas those subjected to eight steps have a reproducibility of 136 ppm  46  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  Table 2.4. Pb Isotopic Compositions of Tholeiitic Basalts From Mauna Loa and Mauna Kea  Sample  Acid Leaching % Weight Steps Loss  Column Passes  Pb/204Pb  2 SE  206  Pb/204Pb  2 SE  207  Pb/204Pb  2 SE  208  Mauna Loa SW-70 -1 a SW-70 -2 SW-70 -3 Ext. Reprod.b (ppm)  – – –  – – –  1 1 1  18.1092 18.1059 18.0993 558  0.0014 0.0020 0.0014  15.4842 15.4824 15.4759 566  0.0016 0.0023 0.0016  37.9295 37.9235 37.9069 619  0.0045 0.0079 0.0051  SW-70 -Ic SW-70 -II SW-70 -III Ext. Reprod. (ppm)  6 6 6  39.15 34.50 35.50  1 1 1  18.0721 18.0562 18.0551 1053  0.0007 0.0009 0.0008  15.4841 15.4590 15.4568 1960  0.0008 0.0011 0.0009  37.9454 37.8660 37.8588 2536  0.0023 0.0033 0.0029  J2-020-23 -1  –  –  1  18.3052  0.0016  15.5121  0.0021  38.1726  0.0067  J2-020-23 -I J2-020-23 -II J2-020-23 -III J2 020 23 -III rep.d Ext. Reprod. (ppm)  7 7 7 7  37.28 26.76 38.45 38.45  1 1 1 1  18.1801 18.1772 18.1797 18.1800 153  0.0007 0.0005 0.0012 0.0009  15.4571 15.4539 15.4568 15.4571 198  0.0006 0.0006 0.0013 0.0008  37.9415 37.9323 37.9401 37.9414 230  0.0018 0.0014 0.0041 0.0022  J2-020-23 -a e J2-020-23 -b J2-020-23 -c J2-020-23 -c rep Ext. Reprod. (ppm)  7 7 7 7  37.39 36.21 30.74 30.74  2 2 2 2  18.1799 18.1805 18.1809 18.1779 146  0.0008 0.0008 0.0013 0.0013  15.4568 15.4573 15.4576 15.4544 187  0.0007 0.0009 0.0013 0.0013  37.9401 37.9418 37.9437 37.9355 185  0.0020 0.0026 0.0042 0.0036  J2-019-04 -1  –  –  1  18.4951  0.0012  15.5703  0.0014  38.4676  0.0042  J2-019-04 -I J2-019-04 -II Ext. Reprod. (ppm)  7 7  34.78 34.83  1 1  18.1796 18.1774 170  0.0008 0.0007  15.4583 15.4558 227  0.0008 0.0006  37.9751 37.9677 278  0.0022 0.0018  7  28.89  2  18.1806  0.0009  15.4601  0.0009  37.9795  0.0024  SR0705-0.15 -1 SR0705-0.15 -2 SR0705-0.15 -3 Ext. Reprod. (ppm)  – – –  – – –  1 1 1  18.5130 18.6619 18.7945 15096  0.0009 0.0013 0.0020  15.5042 15.5289 15.5518 3065  0.0009 0.0011 0.0023  38.1661 38.2650 38.3567 4983  0.0028 0.0037 0.0073  SR0705-0.15 -I SR0705-0.15 -II SR0705-0.15 -III Ext. Reprod. (ppm)  6 6 6  40.05 38.63 37.02  1 1 1  18.3372 18.3201 18.3475 1514  0.0017 0.0010 0.0017  15.4662 15.4768 15.4686 719  0.0020 0.0011 0.0020  38.0123 38.0054 38.0264 564  0.0067 0.0029 0.0063  SR0768-11.20 -1 SR0768-11.20 -I SR0768-11.20 -II SR0768-11.20 -III Ext. Reprod. (ppm)  – 11 11 10  – 30.63 27.31 33.72  1 1 1 1  18.5598 18.5380 18.5338 18.5373 243  0.0015 0.0011 0.0006 0.0010  15.4854 15.4819 15.4776 15.4817 309  0.0019 0.0011 0.0006 0.0009  38.1574 38.1435 38.1313 38.1422 351  0.0064 0.0031 0.0019 0.0027  SR0768-11.20 -a SR0768-11.20 -b SR0768-11.20 -c Ext. Reprod. (ppm)  9 9 9  31.42 32.67 29.72  2 2 2  18.5374 18.5357 18.5376 115  0.0014 0.0017 0.0011  15.4801 15.4817 15.4813 106  0.0016 0.0020 0.0013  38.1426 38.1379 38.1422 137  0.0049 0.0062 0.0039  SR0954-8.00 -1 SR0954-8.00 -2 SR0954-8.00 -3 Ext. Reprod. (ppm)  – – –  – – –  1 1 1  18.6336 18.6346 18.6323 122  0.0014 0.0012 0.0010  15.4921 15.4921 15.4908 98  0.0012 0.0011 0.0011  38.1946 38.1943 38.1898 142  0.0034 0.0033 0.0034  SR0954-8.00 -I SR0954-8.00 -II SR0954-8.00 -III Ext. Reprod. (ppm)  6 6 6  27.60 19.09 49.92  1 1 1  18.5986 18.6031 18.6020 254  0.0012 0.0013 0.0014  15.4811 15.4832 15.4804 186  0.0016 0.0014 0.0013  38.1574 38.1641 38.1561 225  0.0043 0.0050 0.0041  SR0954-8.00 -a SR0954-8.00 -b SR0954-8.00 -c Ext. Reprod. (ppm)  6 6 6  28.64 28.75 31.82  2 2 2  18.6100 18.6089 18.6075 135  0.0006 0.0008 0.0010  15.4874 15.4864 15.4869 62  0.0006 0.0008 0.0009  38.1787 38.1761 38.1764 74  0.0018 0.0019 0.0027  SR0956-18.35 -1 SR0956-18.35 -I SR0956-18.35 -II Ext. Reprod. (ppm)  – 9 8  – 34.93 34.62  1 1 1  18.5075 18.4892 18.4864 210  0.0009 0.0006 0.0009  15.4786 15.4748 15.4717 283  0.0009 0.0005 0.0009  38.1409 38.1281 38.1182 370  0.0025 0.0014 0.0026  SR0956-18.35 -a  8  35.45  2  18.4857  0.0010  15.4742  0.0011  38.1250  0.0033  J2-019-04 -a Mauna Kea (HSDP-2)  Numbers 1, 2, and 3 refer to complete procedural duplicates of unleached samples. b Ext. Reprod., external reproducibility of the set of the same type of analyses (i.e., 1, 2, and 3; I, II, and III; and a, b, and c), expressed in ppm (2 SD on the mean of individual analyses of each set of triplicatesof the same sample: 2 SD/mean × 106). c Roman numerals I, II, and III refer to complete procedural duplicates of leached samples that were passed once on anion exchange columns. d Replicate analyses of the same sample solutions by MC-ICP-MS are indicated by rep. e Letters a, b, and c refer to complete procedural duplicates of leached samples that were passed twice on anion exchange columns. a  47  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  Table 2.5. Pb Isotopic Compositions of Subaerial Alkalic Basalts From Mont Crozier on the Kerguelen Archipelago Sample OB93-165 -1 a OB93-165 -2 OB93-165 -3 OB93-165 -3 rep.b  Acid Leaching Steps  % Weight Loss  Column Passes  – – – –  – – – –  1 1 1 1  OB93-165 -I OB93-165 -II OB93-165 -III  6 6 6  36.20 35.49 34.67  1 1 1  Ext. Reprod. (ppm) OB93-165 -A2 OB93-165 -A3 OB93-165 -B2 OB93-165 -B3 OB93-165 -C2 OB93-165 -C3 OB93-165 -C3 rep e  0.0015 0.0015 0.0011 0.0009  18.5969 18.5930 18.5936  8 8 8 8 8 8 8  50.44 50.44 45.31 45.31 41.31 41.31 41.31  1 1 1 1 1 1 1  18.5892 18.5895 18.5895 18.5868 18.5868 18.5871 18.5881  7 7 7 7  62.00 52.49 52.49 62.65  2 2 2 2  18.5890 18.5924 18.5920 18.5920  8 8 8  50.44 45.31 41.31  2 2 2  18.5898 18.5868 18.5892  0.0010 0.0010 0.0009  2 SE  15.5830 15.5812 15.5811 15.5815  0.0012 0.0017 0.0010 0.0013  15.5788 15.5769 15.5751  0.0007 0.0009 0.0014 0.0021 0.0015 0.0011 0.0018  15.5765 15.5763 15.5778 15.5750 15.5749 15.5746 15.5744  0.0008 0.0008 0.0009 0.0009  15.5758 15.5756 15.5775 15.5770  0.0015 0.0014 0.0028  15.5773 15.5749 15.5765  0.0010 0.0012 0.0010  2 SE  39.3245 39.3212 39.3207 39.3212  0.0037 0.0034 0.0028 0.0043  39.3010 39.2908 39.2894  0.0030 0.0036 0.0033  324 0.0007 0.0010 0.0018 0.0026 0.0017 0.0010 0.0019  39.2840 39.2853 39.2870 39.2786 39.2803 39.2778 39.2773  0.0009 0.0011 0.0011 0.0010  39.2823 39.2878 39.2890 39.2898  0.0019 0.0015 0.0035  39.2851 39.2808 39.2833  158  0.0023 0.0032 0.0055 0.0091 0.0059 0.0028 0.0064  199  120  175  Pb/204Pb  208  90  239  172  Ext. Reprod. (ppm)  Pb/204Pb  207  114  136  Ext. Reprod. (ppm) OB93-165 -A1 OB93-165 -B1 OB93-165 -C1  18.6054 18.6038 18.6037 18.6050  230  Ext. Reprod. (ppm) OB93-165 -a f OB93-165 -b OB93-165 -b rep OB93-165- c  2 SE  93  Ext. Reprod. (ppm) c d  Pb/204Pb  206  0.0025 0.0031 0.0026 0.0031  172  161  0.0065 0.0049 0.0120  110  OB93-177 -1  –  –  1  18.4802  0.0013  15.5682  0.0015  39.1292  0.0044  OB93-177 -I OB93-177 -II OB93-177 -III  7 6 4  55.99 36.86 80.64  1 1 1  18.4459 18.4493 18.4534  0.0009 0.0010 0.0010  15.5606 15.5609 15.5615  0.0009 0.0010 0.0009  39.0594 39.0603 39.0610  0.0022 0.0026 0.0025  Ext. Reprod. (ppm)  410  57  43  OB93-177 -a OB93-177 -b OB93-177 -c Ext. Reprod. (ppm)  6 5 5  61.99 54.36 45.91  2 2 2  18.4528 18.4534 18.4480 322  0.0014 0.0008 0.0008  15.5624 15.5630 15.5598 220  0.0014 0.0008 0.0011  39.0639 39.0654 39.0583 191  0.0035 0.0020 0.0021  OB93-202 -1  –  –  1  18.4917  0.0009  15.5734  0.0010  39.1247  0.0029  OB93-202 -I OB93-202 -II  7 5  62.62 52.21  1 1  18.4957 18.4934  0.0008 0.0007  15.5724 15.5728  0.0007 0.0007  39.1123 39.1090  0.0015 0.0017  Ext. Reprod. (ppm) OB93-202 -a  176 5  58.25  2  18.4976  35 0.0009  15.5740  118 0.0009  39.1166  0.0025  Numbers 1, 2, and 3 refer to complete procedural duplicates of unleached samples. Replicate analyses of the same sample solutions by MC-ICP-MS are indicated by rep. Ext. Reprod., external reproducibility of the set of the same type of analyses (i.e., 1, 2, and 3; I, II, and III; and a, b, and c), expressed in ppm (2 SD on the mean of individual analyses of each set of triplicates of the same sample: 2 SD/mean × 106). d Roman numerals I, II, and III refer to complete procedural duplicates of leached samples that were passed once on anion exchange columns. e Capital letters A2, A3, B2, B3, C2, and C3 refer to portions of three leached duplicates that were passed once on anion exchange columns, and capital letters A1, B1, and C1 refer to fractions of three leached duplicates that were passed twice on Pb anion exchange. f Letters a, b, and c refer to complete procedural duplicates of leached samples that were passed twice on anion exchange columns. a  b  c  48  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  39.310 39.305  OB93-165  (Mont Crozier, KA)  39.300  Pb/204Pb  39.295  208  39.290  6 leaching steps  39.285  2SDm: 230; 324 ppm  39.280 39.275  8 leaching steps  2SDm: 136; 199 ppm  39.270 39.265 18.584  18.586  18.588  18.590  18.592 206  18.594  18.596  18.598  18.600  Pb/204Pb  Figure 2.4. Diagram of 208Pb/204Pb versus 206Pb/204Pb showing the reproducibility of the Pb isotopic compositions of powder splits from the same alkalic basalt (OB93-165) from the Kerguelen Archipelago that were subjected to six acid-leaching steps (three aliquots) and eight acid-leaching steps (seven aliquots). All these samples were purified once through Pb anion exchange columns prior to analysis by MC-ICP-MS. The error bars on each individual symbol represent the 2 SE of the individual run. The average (mean) Pb isotopic composition and 2 SD for each set of aliquots subjected to different numbers of leaching steps are represented by the larger grey symbols with thicker error bars. The number of acid-leaching steps is indicated as well as the external reproducibility (2 SDm in parts per million (ppm)) of 206Pb/204Pb and 208Pb/204Pb.  49  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  Table 2.6. Pb Isotopic Compositions of Subaerial Tholeiitic Transitional Basalts From Mont des Ruches, Mont Fontaine, and Mont Bureau on the Kerguelen Archipelago and of a Submarine Tholeiitic Basalt from ODP Leg 183, Site 1140, on the Northern Kerguelen Plateau Sample Mont Des Ruches BY96-27 -1 a BY96-27 -2 BY96-27 -3 Ext. Reprod.b (ppm) BY96-27 -I c BY96-27 -II BY96-27 -III Ext. Reprod. (ppm)  Acid Leaching Steps  % Weight Loss  Column Passes  – – –  – – –  6 6 6  Pb/204Pb  2 SE  Pb/204Pb  2 SE  1 1 1  18.2551 18.2543 18.2534 95  0.0006 0.0011 0.0011  15.5272 15.5273 15.5258 110  29.46 30.03 31.34  1 1 1  18.2539 18.2542 18.2495 288  0.0014 0.0010 0.0006  206  Pb/204Pb  2 SE  0.0005 0.0010 0.0011  38.8273 38.8260 38.8210 171  0.0014 0.0029 0.0035  15.5201 15.5211 15.5167 301  0.0016 0.0010 0.0006  38.7853 38.7885 38.7737 402  0.0055 0.0031 0.0021  207  208  BY96-31 -1  –  –  1  18.3490  0.0008  15.5516  0.0010  39.0494  0.0042  BY96-31 -I BY96-31 -II Ext. Reprod. (ppm)  11 10  44.32 41.67  1 1  18.3222 18.3225 28  0.0014 0.0019  15.5514 15.5497 155  0.0017 0.0022  39.0013 38.9980 118  0.0055 0.0075  BY96-31 -a d BY96-31 -b BY96-31 -c Ext. Reprod. (ppm)  12 12 11  58.54 54.62 41.74  2 2 2  18.3258 18.3200 18.3228 318  0.0015 0.0012 0.0018  15.5528 15.5479 15.5510 320  0.0014 0.0010 0.0007  39.0097 38.9925 39.0034 445  0.0037 0.0029 0.0007  BY96-86 -1 BY96-86 -1rep.e Ext. Reprod. (ppm)  – –  – –  1 1  18.3610 18.3624 106  0.0008 0.0014  15.5436 15.5448 110  0.0007 0.0017  39.0256 39.0217 138  0.0022 0.0037  BY96-86 -I BY96-86 -II Ext. Reprod. (ppm)  13 10  58.93 59.20  1 1  18.3803 18.3776 212  0.0008 0.0012  15.5397 15.5387 85  0.0007 0.0009  39.0083 39.0034 178  0.0019 0.0027  BY96-86 -a  9  55.45  2  18.3788  0.0009  15.5406  0.0009  39.0084  0.0021  – –  – –  1 1  18.4760 18.4748 90  0.0013 0.0008  15.5660 15.5639 196  0.0012 0.0008  38.5495 38.5457 142  0.0036 0.0024  10 9 11  59.85 60.08 61.24  1 1 1  18.4475 18.4472 18.4454 123  0.0014 0.0015 0.0014  15.5199 15.5179 15.5170 194  0.0014 0.0014 0.0013  38.6163 38.6113 38.6106 160  0.0043 0.0040 0.0036  Mont Fontaine  Mont Bureau GM92-48 -1 GM92-48 -1rep Ext. Reprod. (ppm) GM92-48 -I GM92-48 -II GM92-48 -III Ext. Reprod. (ppm)  Northern Kerguelen Plateau: ODP Leg 183, Site 1140 1140A-31R-1, 57-61 -1 – –  1  18.5451  0.0008  15.5619  0.0009  38.9056  0.0029  1140A-31R-1, 57-61 -I 1140A-31R-1, 57-61 -II 1140A-31R-1, 57-61 -III Ext. Reprod. (ppm)  13 13 10  64.01 54.72 50.99  1 1 1  18.5583 18.5519 18.5567 359  0.0012 0.0015 0.0009  15.5637 15.5570 15.5631 475  0.0010 0.0015 0.0008  38.9384 38.9214 38.9349 461  0.0027 0.0046 0.0023  1140A-31R-1, 57-61 -a 1140A-31R-1, 57-61 -b 1140A-31R-1, 57-61 -c Ext. Reprod. (ppm)  10 11 14  49.73 54.89 59.99  2 2 2  18.5571 18.5622 18.5594 274  0.0008 0.0009 0.0013  15.5636 15.5642 15.5646 68  0.0007 0.0008 0.0012  38.9383 38.9369 38.9420 134  0.0020 0.0021 0.0032  Numbers 1, 2, and 3 refer to complete procedural duplicates of unleached samples. Ext. Reprod., external reproducibility of the set of the same type of analyses (i.e., 1, 2, and 3; I, II, and III; and a, b, and c), expressed in ppm (2 SD on the mean of individual analyses of each set of triplicates of the same sample: 2 SD/mean × 106). c Roman numerals I, II, and III refer to complete procedural duplicates of leached samples that were passed once on anion exchange columns. d Letters a, b, and c refer to complete procedural duplicates of leached samples that were passed twice on anion exchange columns. e Replicate analyses of the same sample solutions by MC-ICP-MS are indicated by rep. a  b  50  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  (Table 2.5). Comparable improvements in reproducibility were observed for for  207  Pb/204Pb and  208  Pb/204Pb. Although just within error, the average isotopic composition of the powder  splits that were subjected to eight acid-leaching steps is slightly less radiogenic (i.e., further from the composition of the unleached powder and leachate solutions) than the average of those that were leached six times.  2.5.2 Pb Purification The matrix elimination experiments show differences between full procedural sample triplicates that were processed once and twice by anion-exchange chromatography (Tables 2.4 to 2.6, Figure 2.5). Sample triplicates subjected to the same number of acid-leaching steps (six for the Hawaiian sample and eight for the Kerguelen sample) that were purified twice yield Pb isotopic compositions that are in general more reproducible (e.g., 135 and 74 ppm difference in  206  Pb/204Pb and  208  Pb/204Pb for Hawaii and 175 and 110 ppm for  Kerguelen, respectively) than the triplicates that were purified only once (e.g., 254 and 225 ppm difference in  206  Pb/204Pb and  208  Pb/204Pb for Hawaii and and 136 and 199 ppm for  Kerguelen, respectively). The differences in reproducibility are more significant for the Hawaiian sample and may reflect greater matrix effects (see section 2.5.2 below). The number of acid-leaching steps that the powder splits are subjected to affects the relative differences in the measured isotopic ratios of the samples processed once vs. twice on columns. For the Mont Crozier sample OB93-165 (Figure 2.7a), the difference in the Pb isotopic ratios of powder splits that were purified once or twice decreases with an increasing number of acid-leaching steps. For the Mauna Loa samples J2-019-04 and J2-020-23 (Figure 2.6a), the powder splits were subjected to seven acid-leaching steps and show negligible difference in their isotopic compositions after one and two passes on the Pb anion-exchange columns, whereas for the Mauna Kea sample SR0954-8.00 (Figures 2.6a and 2.6b), where the powder splits were subjected to six leaching steps, the Pb isotopic signatures are clearly distinct.  51  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  38.183 38.178  Hawaii (SR0954-8.00) 6 Leaching steps  2x  208  Pb/204Pb  38.173  2x on columns 2SDm: 135; 74 ppm  38.168 38.163 1x  38.158 38.153  1x on columns 2SDm: 254; 225 ppm  a  38.148 18.596  18.598  18.600  18.602  18.604  206  39.300  18.606  18.610  18.612  Pb/204Pb  Kerguelen (OB93-165)  39.295  18.608  2x on columns 2SDm: 175; 110 ppm  8 Leaching steps  208  Pb/204Pb  39.290 39.285 1x  39.280  2x  39.275 39.270  1x on columns 2SDm: 136; 199 ppm  b  39.265 18.584 18.585 18.586 18.587 18.588 18.589 18.590 18.591 18.592 18.593 206  Pb/204Pb  Figure 2.5. Diagrams of 208Pb/204Pb versus 206Pb/204Pb for powder aliquots of (a) a tholeiitic Hawaiian basalt (sample SR0954-8.00 (Mauna Kea)) and (b) an alkalic Kerguelen basalt (sample OB93-165 (Mont Crozier)) showing the effect of purifying samples by anion exchange chromatography. For each sample, the powder aliquots were subjected to the same number of acid-leaching steps (six steps for the Hawaiian sample and eight steps for the Kerguelen sample) and processed once or twice through the Pb anion-exchange columns prior to analysis by MC-ICP-MS. The error bars on the individual symbols represent the 2 SE of individual run. The average Pb isotopic compositions and 2 SD for the different groups of analyses are represented by the larger grey symbols and thicker error bars. Also identified are the number of acid-leaching steps that each group of powders was subjected to, as well as their reproducibility (2 SDm expressed in ppm) for 206Pb/204Pb and 208Pb/204Pb.  52  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  Finally, all triplicates, from Hawaiian and Kerguelen samples, that were processed twice on columns show more radiogenic Pb isotopic compositions (Tables 2.4 to 2.6). The results for the triplicates of Mauna Kea sample SR0954-8.00 that were subjected to one pass vs. two passes on columns are not within error of each other (Figures 2.5a and 2.6b). For the Kerguelen Archipelago basalts, the differences in Pb isotopic compositions for samples with one- vs. two-passes on columns are within error (Figures 2.5b and 2.7a). For the submarine basalt on the Northern Kerguelen Plateau (Figure 2.7b), even though the powder splits were subjected to higher numbers of acid-leaching steps (10–14), the relationship between one vs. two passes on columns is preserved.  2.6. Discussion 2.6.1. Implications of Sequential Acid-Leaching Acid-leaching of whole rock powders of oceanic basalts leaves a residue of plagioclase and clinopyroxene ± olivine and oxides that provides the closest estimate of the magmatic isotopic compositions [e.g., Mahoney, 1987; Regelous et al., 2003; Hanano et al., 2009]. For both the Hawaiian and Kerguelen basalts examined in this study, the relationship between full procedural triplicates of unleached and leached samples defines trends that are oblique to calculated mass fractionation lines (Figures 2.6 and 2.7). These trends result from the removal by acid-leaching of foreign Pb hosted in low-temperature alteration minerals or in any potential contaminant (i.e., drilling mud, secondary minerals related to seawater alteration) [Hanano et al., 2009]. For the tholeiitic Hawaiian basalts, the difference in the Pb isotopic compositions between full procedural triplicates of unleached and leached sample splits reflects the incorporation of a local contaminant, such as seawater Pb as represented by Pacific Fe-Mn nodules [Abouchami and Galer, 1998; Eisele et al., 2003] (Figure 2.6a). For the two submarine tholeiites from Mauna Loa (J2-019-04 and J2-020-23), the compositions of the unleached and leached sample splits trend towards the field of the Pacific Fe-Mn nodules  53  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  39.60  Hawaii a  HSDP-2  Bore Hole Mud  39.00  HSDP-2  s  Mud Slag Pond  Fe -M nN od  ule  38.70  n  Mauna Loa  tio  na mi  nta  co  cif ic  38.40  Pa  208  Pb/204Pb  39.30  J2-019-04 J2-020-23  Mauna Kea  38.10  SR0954-8.00  Fractionation lines 37.80 18.10  18.30  18.50  18.70  18.90 206  38.20  208  Pb/204Pb  38.19  19.10  19.30  19.50  19.70  Pb/204Pb  b SR0954-8.00 (Mauna Kea)  unleached  leached 2x col.  38.18  n  co  n  tio  na  i tam  38.17 leached 1x col.  38.16 38.15 18.595  18.601  18.607  Fractionation line Fract. line 2sd  18.613 206  18.619  18.625  18.631  18.637  Pb/204Pb  Figure 2.6. Diagrams of 208Pb/204Pb versus 206Pb/204Pb showing the isotopic compositions obtained from the acid-leaching and matrix elimination experiments for tholeiitic Hawaiian basalts (samples J2-019-04, J2-020-23, SR0954-8.00). (a) Larger scale. Potential contaminants are also reported, including seawater Pb, represented by the field for Pacific Fe-Mn nodules [Abouchami and Galer, 1998], and HSDP-2 borehole mud and mud from the right slag pond [Abouchami et al., 2000; Eisele et al., 2003], represented as triangles. Also shown are the mass fractionation lines (red solid lines) calculated for these samples. (b) Enlarged portion of Figure 6a, focusing on the isotopic results for leaching and matrix elimination experiments of sample SR0954-8.00 from Mauna Kea. The calculated mass fractionation line is shown as the red solid line and its associated 2 SD as subparallel red dashed lines. The grey arrows on both diagrams show the direction of possible contamination that could explain the relationship between the isotopic results for the leached (colored symbols) and unleached (open symbols) samples.  54  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  (Figure 2.6a), which is consistent with the observation by Hanano et al. [2009] of finely banded Mn-oxides filling void spaces in sample J2-019-04. For the Mauna Kea samples collected from the HSDP-2 drill core (Figures 2.6 and 2.8), the trends of the leaching results do not intercept the field for Pacific Fe-Mn nodules indicating that seawater Pb is not a major contaminant. The trends point towards a much more radiogenic contaminant, with potentially variable Pb isotopic compositions, such as the drilling mud used during HSDP-2 (bore hole mud and mud from the right slag pond [Abouchami et al., 2000; Eisele et al., 2003]). Distinct patches of barite/celestite have been identified within sample SR0954-8.00 [Hanano et al., 2009], thus despite the careful sample washing procedures employed for the HSDP sample suite prior to crushing [Rhodes and Vollinger, 2004], unleached HSDP-2 samples may be contaminated by interaction with drilling mud. The distinctly larger differences in Pb isotopic compositions between unleached and leached powder splits observed for the Hawaiian basalts compared to the Kerguelen basalts (Figures 2.8a and 2.8b) is likely accounted for by contamination of the HSDP-2 samples with drilling mud and by interaction with seawater for the Mauna Loa samples. These larger differences were surprising as the Hawaiian basalts are much younger (≤500 ka vs. 24-34 Ma) and significantly less altered than the Kerguelen basalts. For the Kerguelen Archipelago basalts, the unleached powder splits plot within 2σ error of each other and typically have more radiogenic Pb isotopic compositions than the leached splits (Figures 2.7a and 2.8b). The results for the leached powder splits for sample OB93-165 (Figure 2.7a) trend towards less radiogenic isotopic compositions. This trend is systematic and correlates with an increasing number of leaching steps, which is consistent with progressive removal of secondary alteration minerals by sequential acid leaching. For this subaerial alkalic basalt, the trend corresponds to a mixing line with an unidentified component associated with alteration. All the other Kerguelen samples show comparable alteration systematics (Figure 2.8b), except the Mount Bureau sample GM92-48. Identification of the contaminant in the subaerial basalts from the Kerguelen Archipelago is more difficult to trace than that for the submarine basalts and may be related to the wide  55  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  39.33 39.32  Kerguelen OB93-165  (Mont Crozier, KA)  39.31  Pb/204Pb  al  208  39.30  7 leaching steps, 2x col.  39.27  Fractionation line Fract. line 2sd  8 leaching steps, 1x and 2x col.  39.26 18.584  18.588  18.592  18.596  206  38.95  Pb/204Pb  18.600  leached 2x col.  Pb/204Pb  39.25  a 18.608  b  39.15  208  Pb/204Pb  18.604  39.45 39.35  208  ing  6 leaching steps, 1x col.  39.28  38.93  unleached  ch  lea  39.29  38.94  ion  at ter  39.05  KP + Crozet B. + Aust-Ant. B. Fe-Mn deposits  38.95 38.85 38.75 18.5  18.6  18.7  18.8 206  38.90 18.5435  19.0  19.1  19.2  ing  ch  lea  38.92  38.91  18.9  Pb/204Pb  leached 1x col.  co  n  tio  ina  m nta  ODP Leg 183, 1140A-31R-1, 57-61 (NKP)  unleached  18.5475  18.5515 206  18.5555  Pb/204Pb  18.5595  18.5635  Figure 2.7. Diagrams of 208Pb/204Pb versus 206Pb/204Pb showing the isotopic compositions obtained with the acid-leaching and matrix elimination experiments for Kerguelen basalts. (a) The results for the subaerial alkalic sample OB93-165 from Mont Crozier on the Kerguelen Archipelago and (b) the results for the submarine tholeiitic sample 1140A-31R-1, 57-61, recovered during ODP Leg 183 (Site 1140) from the Northern Kerguelen Plateau (NKP). Also reported are the mass fractionation lines (red solid lines) calculated for these samples and their associated 2 SD (red dashed lines); the pale grey areas represent the range of uncertainty of the calculated mass fractionation. The inset diagram in the top left of Figure 7b shows the results for sample 1140A-31R-1, 57-61, and the Pb isotopic composition of Indian Ocean seawater, which is represented by the field for Fe-Mn deposits from the Kerguelen Plateau, Crozet Basin and Australian-Antarctic Basin [Vlastélic et al., 2001]. The relationship between the results for the unleached and leached duplicates in these experiments cannot be explained by interaction with seawater.  56  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  variety of secondary minerals (carbonates, oxides, sulfides, quartz, clays, epidotes, zeolites) produced during subsequent hydrothermal alteration of the subaerial basalts [Nougier et al., 1982; Verdier, 1989; Hanano et al., 2009]. In contrast to all the other studied Hawaiian and Kerguelen basalts, the submarine tholeiite (ODP leg 183, 1140A-31R-1, 57-61) from the Northern Kerguelen Plateau (Figures 2.2, 2.3 and 2.7b) shows more radiogenic isotopic compositions for the leached sample splits than for the unleached sample. The reason for this reverse leaching behaviour is enigmatic, especially as the field for Indian seawater Pb is distinctly more radiogenic (Figure 2.7b). Thus, interaction with seawater Pb, as represented by the Fe-Mn deposits from the Kerguelen Plateau, Crozet Basin and Australian-Antarctic Basin [Vlastélic et al., 2001], cannot account for this observation.  2.6.2. Effect of Matrix Elimination The accuracy of radiogenic isotope ratio measurements by MC-ICP-MS can be affected by non-spectral interferences (matrix effects that affect the ionization and transmission of the analyte and instrumental mass bias) due to residual sample matrix [e.g., Thirlwall, 2002; Woodhead, 2002; Barling and Weis, 2008]. Samples for isotope ratio analysis by MC-ICP-MS thus need to be as pure as possible to obtain accurate high-precision results. Another possible source of inaccuracy could be mass fractionation during Pb anionexchange chromatography [Blichert-Toft et al., 2003], as under certain circumstances, incomplete elution of the Pb fraction leads to lighter (less radiogenic) Pb isotopic compositions. In this case, if a significant difference was to be observed in the measured Pb isotopic ratios for a sample that was purified once and twice on columns (with close to 100% recovery in the Pb chemistry), then the twice-purified Pb fraction would be isotopically lighter than the once-purified Pb fraction. In this study, all samples processed twice on columns have heavier isotopic ratios (i.e., more radiogenic) than those processed once, thus we observe the opposite relationship  57  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  Hawaii  Pb/204Pb  15.54 15.52  SW-70 J2-020-23 J2-019-04  Mauna Loa  39.40  208  39.20  8  -mid  Kea  8  lo a-  37.90  Ke  Mauna Kea  SR0954-8.00 bulk leachates individual leachates  Mauna Loa  37.70 18.00 18.10 18.20 18.30 18.40 206  OB93-165 OB93-177 OB93-202  Mont Bureau  Mt. Bureau GM92-48  Mont des Ruches & Mont Fontaine  Mont Crozier  39.00  Kea-hi8  38.00  Mt. Crozier  Mont des Ruches & Mont Fontaine  Pb/204Pb  leached basalts, 2x on columns  38.10  NKP  15.52  15.48  leached basalts, 1x on columns  38.20  37.80  b  Mont Crozier  BY96-27 BY96-31 BY96-86  15.50  SR0954-8.00 SR0768-11.20 SR0956-18.35 SR0705-0.15  unleached basalts  Pb/204Pb  38.30  Mauna Kea  208  207  Mauna Kea  15.46  38.40  Mt. des Ruches & Fontaine  15.54  15.48  38.50  15.58 15.56  15.50  15.44  15.60  a  Mauna Loa  Pb/204Pb  15.56  207  15.58  Kerguelen  18.50 18.60 18.70 18.80  Pb/204Pb  38.80 38.60  NKP  Mont Bureau  38.40 38.20 18.10  NKP  ODP Leg 183, 1140A-31R-1, 57-61  18.20  18.30  206  18.40  18.50  Pb/204Pb  18.60  18.70  Figure 2.8. Diagrams of 207Pb/204Pb versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb for leached and unleached samples from this study compared to fields for reported compositions of Hawaiian and Kerguelen basalts. (a) Hawaii. Fields for high-precision Pb isotopic compositions by TS-TIMS [Abouchami et al., 2000; Eisele et al., 2003] and MC-ICP-MS [Blichert-Toft et al., 2003; Wanless et al., 2006; Weis, D. unpublished data, 2005] are reported for basalts from Mauna Loa and Mauna Kea volcanoes. Lines for Kea-lo8, Kea-mid8 and Kea-hi8 are those defined by Eisele et al. [2003] on HSDP-2 basalts. Results for Hawaiian basalts analyzed in this study are shown as individual symbols (the analytical uncertainty for individual samples is smaller than the symbol sizes). Open symbols correspond to unleached samples, light-coloured symbols (with dark outlines) correspond to leached samples purified once on Pb anion-exchange columns, and dark-coloured symbols indicate leached samples passed twice on Pb anion exchange columns. For reference, dashed lines connect results for unleached and leached samples (colour-coded to match the corresponding symbols); they have been extrapolated to more radiogenic values for the HSDP-2 samples to clearly show the difference in slope compared to the Pb-Pb arrays of Eisele et al. [2003]. (b) Kerguelen. Fields for Pb isotopic compositions by TIMS [Yang et al., 1998; Doucet et al., 2002] and MC-ICP-MS [Doucet et al., 2005; Weis, D. unpublished data, 2005] are reported for basalts from the Kerguelen Archipelago and Northern Kerguelen Plateau [Weis and Frey, 2002]. Results for the Kerguelen basalts analyzed in this study are shown as individual symbols. Analytical uncertainty, colour coding and legend are as noted above. In both diagrams, note the significantly reduced dispersion of high-precision Pb isotopic ratios for leached samples compared to results from unleached samples and TIMS analyses.  58  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  of what would be expected from column fractionation experiments [Blichert-Toft et al., 2003] (Figures 2.6b and 2.7b). These observed differences between the two types of triplicates (purified 1x and 2x on columns), which are more pronounced in the tholeiitic basalts (Hawaiian and submarine Northern Kerguelen Plateau basalts), appear to fall along the calculated mass fractionation lines (within their 2σ deviations). However, these differences are best explained by a non-spectral matrix effect as Barling and Weis [2008] show that the presence of magnesium and calcium in the matrix leads to lighter Pb isotopic compositions. The lack of a mass fractionation effect produced during chromatography is also supported by the Pb isotopic results for the transitional and alkalic basalts from the Kerguelen Archipelago, which have comparable CaO, but lower MgO and higher NaO and SiO2 contents than the Hawaiian basalts. For these Kerguelen basalts, the Pb isotopic compositions for full procedural triplicates of leached powder splits that were purified once and twice on Pb anion-exchange columns are within error of each other (Tables 2.5 and 2.6). In addition, their 2σ mean deviations (≤160 ppm for  206  Pb/204Pb and  208  Pb/204Pb for the  alkalic basalt OB93-165, Figure 7a) are within instrumental external reproducibility (≤200 ppm). One possible explanation for the occurrence of a non-spectral matrix effect due to residual sample matrix during processing of the tholeiitic basalts may lie in their relatively low Pb concentrations (sub-ppm to ppm range) compared to the alkalic basalts (Table 2.1) and thus the amount of sample powder (mg) that is purified. For the tholeiitic samples, more sample powder is needed to ensure enough Pb for MC-ICP-MS analysis after leaching and chromatography. All of the sample splits for which the leached residue exceeded ~170 mg (e.g., 250-300 mg for Mauna Kea [HSDP-2] samples and 130-190 mg for the NKP sample) show differences in Pb isotopic compositions when passed once and twice on columns. Full procedural sample triplicates where less sample was loaded (e.g., 150-170 mg for the Mauna Loa samples, and 76-130 mg for the Kerguelen Archipelago samples) show smaller to negligible differences (i.e., within errors) in Pb isotopic compositions for one and two passes  59  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  on columns. Larger sample sizes, still well below column saturation, mean that more sample matrix is loaded into the columns and this may not be effectively removed with just a single pass through columns [Barling and Weis, 2008]. Thus, for oceanic tholeiitic basalts, a second pass of chemical separation may be needed to reduce the sample matrix to negligible levels.  2.7. Conclusions Acid-leaching and matrix elimination experiments on the Pb isotopic compositions of ocean island basalts from Hawaii and Kerguelen yield the following conclusions: •  Acid-leaching removes the effects of contamination and alteration that disturb the magmatic Pb isotopic composition of OIB, which is essential for evaluating the isotopic composition of the mantle source of these basalts.  •  In most cases, acid-leaching leads to distinctly less radiogenic (>500 ppm) Pb isotopic ratios of the samples compared to unleached samples and is associated with a slight decrease in reproducibility.  •  Independent of basalt composition (i.e., tholeiitic vs. alkalic), leaching results in 3560% weight loss throughout the entire process and 70-80% of the Pb is lost in the first three acid-leaching steps.  •  The reproducibility of Pb isotopic compositions of OIB improves with the number of acid-leaching steps that the samples are subjected to, even if most of the Pb is leached out in the first steps.  •  Another factor controlling the reproducibility of Pb isotopic compositions of leached oceanic basalts is matrix elimination through column chromatography. Samples purified twice on columns show more radiogenic Pb isotopic compositions. The effect is stronger for tholeiitic (Hawaiian and Kerguelen Plateau basalts) than for alkalic (Kerguelen Archipelago) compositions.  •  The difference in the Pb isotopic compositions of leached samples that are passed once vs. twice through chromatography is not due to fractionation on the anionexchange columns, but instead depends on the specific basalt composition (i.e., the Pb concentration of the sample). Whenever a relatively large amount of material  60  CHAPTER 2: Pb ISOTOPIC COMPOSITIONS OF ACID-LEACHED OIB  (>170 mg) needs to be dissolved for analysis due to low Pb concentrations (sub-ppm to ppm), a second chemical purification step is recommended to avoid matrix effects. •  All steps are crucial during sample processing for obtaining accurate, high-precision Pb isotopic compositions, to discern the mantle sources and components of ocean island basalts.  61  C HAPTER 3  The Effects of Acid Leaching on the Sr-Nd-Hf Isotopic Compositions of Ocean Island Basalts 1  1  A version of this chapter has been published. Nobre Silva, I. G., D. Weis, and J. S. Scoates (2010), Effects of acid leaching on the Sr-Nd-Hf isotopic compositions of ocean island basalts, Geochem. Geophys. Geosyst., 11, Q09011, doi:10.1029/2010GC003176. 62  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  3.1. Synopsis The ability to conduct multi-isotopic analyses (e.g., Sr-Nd-Hf-Pb) on the same sample is critical for studies that evaluate the mantle source components of oceanic basalts. The isotopic compositions of relatively immobile elements, such as Nd (and other REE) and Hf, are considered to be relatively resistant to alteration, however, accurate Sr and Pb isotopic analyses of oceanic basalts require thorough acid leaching prior to dissolution. A detailed study of the Sr, Nd and Hf isotopic systematics of acid-leached oceanic basalts from Hawaii and Kerguelen was undertaken to assess how acid leaching affects their isotopic compositions. Most of the Sr, Nd and Hf was removed in the first acid leaching steps. Hawaiian basalts lose up to 35% and 40% of their total Sr and Hf contents, respectively, whereas for Kerguelen basalts the corresponding losses are 63% and ~70%. Acid leaching leads to significant loss of the original Nd content (up to 90%), which cannot be solely explained by the elimination of alteration phases and is likely related to preferential removal of the REE in the constituent silicate minerals (e.g., plagioclase, clinopyroxene). The leached residues yield Sr isotopic ratios significantly less radiogenic than their respective unleached powders and Nd-Hf isotopic compositions that are within analytical uncertainty of the respective unleached powders. This study shows that multi-isotopic analyses on the same acid-leached sample aliquot can produce reliable results for use in the discrimination of mantle source components of oceanic basalts.  3.2. Introduction The combination of analyses of different isotopic systems (e.g., Sr-Nd-Hf-Pb) on samples of oceanic basalts constitutes a powerful tool for the determination and characterization of their mantle sources and components [e.g., Gast et al., 1964; Tatsumoto, 1966, 1978; White and Hofmann, 1982; Dupré and Allègre, 1983; Hart, 1984; White, 1985; Hamelin et al., 1986; Zindler and Hart, 1986; Weaver, 1991; Hofmann, 1997; 2003; Stracke et al., 2005]. Inherent to their emplacement in an oceanic environment, oceanic basalts are susceptible to seawater alteration, and the effects of this alteration on Rb-Sr and U-Pb isotope systematics have long been recognized [e.g., Hart et al., 1974; Hawkesworth and  63  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  Morrison, 1978; Cohen and O’Nions, 1982; Verma, 1992; Staudigel et al., 1995; Krolikowska-Ciaglo et al., 2005]. Another well known cause of disturbance of the isotopic compositions of basalts, especially for Pb isotopes, is contamination during crushing and grinding [e.g., Woodhead and Hergt, 2000; McDonough and Chauvel, 1991; Weis et al., 2005]. To overcome the chemical disturbance caused by alteration, plus possible contamination during sample handling, careful acid leaching of samples prior to dissolution and analysis has proven to be an effective and essential step in sample processing for mantle geochemistry studies [e.g., Manhès et al., 1978; Dupré and Allègre, 1980; McDonough and Chauvel, 1991; Eisele et al., 2003; Stracke et al., 2003; Weis et al., 2005; Thompson et al., 2008; Hanano et al., 2009; Nobre Silva et al., 2009]. Given the geochemical nature of Nd and Hf (rare earth element (REE) and high field strength element (HFSE), respectively), they are considered to be relatively immobile elements in aqueous solutions and their isotopic compositions to be relatively resistant to alteration [e.g., Cohen and O’Nions, 1982; Verma, 1992; Staudigel et al., 1995; Lassiter et al., 1996; Chauvel and Blichert-Toft, 2001; Mattielli et al., 2002; Krolikowska-Ciaglo et al., 2005]. Also, in contrast to Sr, which is relatively abundant in seawater, the concentrations of Nd and Hf in seawater are very low [e.g., Faure, 1986]. Hence, an acid-leaching treatment prior to chemical separation for Nd and Hf isotopic analysis is not considered to be a requirement. Several studies, however, do show evidence for some mobility of Nd (and other REE) during hydrothermal alteration [e.g., Ludden and Thompson, 1979; Cotten et al., 1995; Staudigel et al., 1995; Bau et al., 1996; Smith et al., 2000; Kempton et al., 2002]. Furthermore, the 143Nd/144Nd and 176Hf/177Hf ratios of basalts may be affected to some extent by seawater alteration under some specific conditions [e.g., Kempton et al., 2002; Thompson et al., 2008]. Depending on the sample collection method in studies of oceanic basalts (i.e., by hammering an outcrop, dredging along the slope of a seamount, ridge, or ocean floor, or by drilling), the sample sizes available for geochemical analysis can range from kilograms to less than a few grams. To properly characterize the sample and avoid any sample  64  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  heterogeneity effect, it is important that multi-isotopic analyses are performed on the same sample aliquot (same sample powders, chips, and/or glasses). This means that sequenced chemical separations should be performed on a single dissolution of each leached sample. A wide variety of leaching protocols are used by different laboratories [e.g., Mahoney, 1987; McDonough and Chauvel, 1991; Weis and Frey, 1991; Stracke and Hegner, 1998; Abouchami et al., 2000; Thirlwall, 2000; Eisele et al., 2003; Stracke et al., 2003; Baker et al., 2004; Weis et al., 2005], but not all leaching techniques seem to provide results with the desired reproducibility for high-precision isotopic studies and interlaboratory comparison [Abouchami et al., 2000; Eisele et al., 2003; Stracke et al., 2003; Baker et al., 2004; 2005; Albarède et al., 2005]. The recent improvements in precision and reproducibility offered by the current generations of multiple collector mass spectrometers (TIMS and MC-ICP-MS) have enabled researchers to discriminate different isotopic trends within data sets that previously were within analytical uncertainty, especially for Pb and Hf. The question then arises, to what extent, if any, are the Nd and Hf isotopic compositions of OIB affected by acid leaching? To complement an earlier study on the effect of acid leaching and matrix elimination on Pb isotopes of samples from ocean island basalts [Nobre Silva et al., 2009], we carried out a comprehensive study of the Sr, Nd and Hf isotopic systematics of two Hawaiian and two Kerguelen oceanic basalts (0-1.7 wt% LOI) that were subjected to multi-step acid leaching (up to 14 steps). Here we report the Sr, Nd, and Hf elemental contents and isotope compositions for the unleached and respective leached powders (residues), as well as for the acid solutions (leachates) of each leaching step and the bulk leachates (all solutions combined). A full suite of trace element concentrations on both unleached and leached powders was also measured for one of the basalts. These data are used to assess the effects of our leaching procedure on basaltic rock compositions.  3.3. Samples For this study, we selected two Hawaiian and two Kerguelen basalts amongst the 14 analyzed by Nobre Silva et al. [2009]. We focused on these samples as they are  65  Table 3.1. Summary of Sample Geochemical Characteristics, Hawaii and Kerguelen Sample  Eruption Deptha Environment (mbsl/masl)  Rock Type  References  Hawaiian Basalts  Mauna Loa (Southwest Rift Zone) J2-020-23  Age Pb Sr Nd Hf SiO2 MgO Na2O K2O A.I.b LOIc (wt%) (wt%) (wt%) (wt%) (ppm) (ppm) (ppm) (ppm) (Ma)d  submarine  489  tholeiitic basalt  50.14  6.50  2.72  0.52  2.00  306  32.08  6.60  -0.87 0.99  -  J. M. Rhodes (unpublished data, 2003)  submarine  3009.2  tholeiitic basalt  47.67  18.03  1.65  0.23  0.66  239  15.43  2.95  -1.31 0.85  0.55  Rhodes and Vollinger [2004]  24.5  Mauna Kea (HSDP-2 e) SR0954-8.00  OB93-165 Northern Kerguelen Plateau ODP Leg 183, 1140A 31R-1 57-61  subaerial  515  alkalic basalt  50.91  3.54  3.52  1.45  6.34  417  36.30  5.99  0.67  -  submarine  270.07  tholeiitic basalt  49.64  5.54  2.64  0.49  1.52  262  20.4  3.75  -0.81 1.65  34.3  D. Weis (unpublished data, 2005) Weis and Frey [2002]  mbsl = meters below sea level, for submarine basalts; masl = meters above sea level, for subaerial basalts. A.I. = alkalinity index (AI = total alkalis - (SiO2 x 0.37 - 14.43)) [Rhodes, 1996]. c LOI = weight loss-on-ignition after 30 min at 1020ºC. d Ages from Mauna Kea samples: Sharp and Rennes [2005]; Mont Crozier and Mont Bureau: Nicolaysen et al. [2000]; Mont des Ruches and Mont Fontaine: Doucet et al. [2002]; Site 1140: Duncan [2002]. e HSDP2 = Hawaii Scientific Drilling Project, phase 2. a  b  66  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  Kerguelen Basalts  Mont Crozier  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  representative of basalts typically analyzed for radiogenic isotopic compositions from these two islands and span a wide range of MgO (3.5–18.0 wt%) with relatively weak alteration (e.g., 0.85–1.65 wt% LOI). The samples are: J2-020-23 from the Mile High Section of Mauna Loa volcano, SR0954-8.00 from the Mauna Kea volcano collected from the Hawaii Scientific Drilling Project (HSDP), OB93-165 from Mont Crozier on the Courbet Peninsula of the Kerguelen Archipelago, and 1140A-31R-1, 57-61 cm recovered during ODP Leg 183 on the Northern Kerguelen Plateau (see Table 3.1 for additional sample characterization). To assess the effects of our multistep acid leaching procedure on the elemental concentrations and Sr, Nd, and Hf isotopic compositions of these four samples, all column separations and analyses were performed on the same chemical digestions of all sample fractions (i.e., unleached and leached sample powders, leachate solutions collected at each leaching step, and respective accumulate bulk leachates) as the Pb isotopic analyses published by Nobre Silva et al. [2009].  3.4. Analytical Techniques 3.4.1. Sample preparation All chemical digestion and separation were carried out in Class 1000 clean laboratories and the mass spectrometric analyses were performed in Class 10,000 laboratories at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia. Sample handling in all labs was carried out in Class 100 laminar flow hoods. All reagents used for leaching, dissolution and separation were subboiled, all dilutions were made using ≥18.2 MΩ.cm de-ionized water, and all labware was acid-washed prior to use. The acid leaching procedure used follows that of Weis et al. [2005] and is detailed by Nobre Silva et al. [2009]. Brieftly, the sample powders were acid-leached using 10 mL of 6 M HCl in screw-top Savillex® beakers, in a warm (~50ºC) ultrasonic bath (40 kHz) for 20 min. Immediately after, the supernatant (leachate solution) was decanted without centrifugation, to ensure the removal of the silt to fine-size particle fraction, which is mostly dominated by secondary alteration phases. The whole process was repeated (6 to 14 times for the samples in this study) until the leachate was clear and free of fine-size particles.  67  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  For all sample specimens, Sr, Nd and Hf elemental concentrations and isotopic compositions were determined on a single dissolution, using methods similar to those described in Weis et al. [2005, 2006, and 2007]; the total Sr, Nd and Hf procedural blanks were of the order of 200, 60, and 30 pg, respectively. The results are reported in Tables 3.2 and 3.3. In Table 3.4, a summary of the percentages of powder and elemental losses is presented.  3.4.2. Mass Spectrometry 3.4.2.1 Sr, Nd, and Hf Abundances To determine the amount of Sr, Nd, and Hf present at each step of the acid-leaching procedure, and consequently how much is lost, the respective concentrations were measured on a 10% aliquot of the unleached sample powders, individual leachates and bulk leachate solutions. The analyses were performed on an ELEMENT2 high-resolution (HR)-ICP-MS (Thermo Finnigan, Germany) and were quantified using external calibration curves and indium (In) as an internal standard. Multi-element (Sr, Nd, and Hf) standard solutions were prepared from 1000 ppm Specpure® (Alfa Aesar®, Johnson Matthey Company, USA) single element standard solutions. Samples and standards were diluted accordingly ([Sr] and [Hf ] = 1–500 ppb; [Nd] 0.1–25 ppb) and run in 0.15M HNO3. All analyses were normalized to the internal standard and blank subtracted. 3.4.2.2 Sr, Nd, and Hf Isotopic Compositions All Sr and Nd isotopic ratios were measured on a TRITON (Thermo Finnigan) thermal ionization mass spectrometer (TIMS) in static mode with relay matrix rotation on single Ta (Sr) or double Re-Ta (Nd) filaments. Sr and Nd isotopic compositions were corrected for mass fractionation with the exponential law using 146  86  Sr/88Sr = 0.1194 and  Nd/144Nd = 0.7219, respectively. The data were then normalized using the average of the  corresponding standard (NIST SRM 987 for Sr and La Jolla for Nd) ran in the barrel, relative to the values of NIST SRM 987  87  Sr/86Sr = 0.710248 and La Jolla  143  Nd/144Nd = 0.511858  [Weis et al., 2006]. During the course of the analyses presented in this study, the average  68  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  value of the SRM 987 Sr standard was 0.710244 ± 0.000027 (n = 11) and the La Jolla Nd standard was 0.511854 ± 0.000014 (n = 9). Hf isotopic compositions were determined on a Nu Plasma (Nu021; Nu Instruments Ltd, UK) multiple collector inductively coupled plasma mass spectrometer (MC-ICP-MS), under dry plasma conditions using a membrane desolvator (Nu DSN100) for sample introduction, following the analytical procedures detailed by Weis et al. [2007]. All analyses were obtained in static multi-collection mode, with interference corrections for 176  Yb on  176  Hf, and  174  Yb on  174  176  Lu and  Hf, respectively. All sample and standard solutions were  prepared fresh for each analytical session and sample analysis followed a modified samplestandard bracketing protocol in which the JMC 475 standard solution was run after every two samples to monitor for any systematic daily drift of the standard value. Hf isotopic ratios were corrected for instrumental mass fractionation by exponentially normalizing to 179  Hf/177Hf = 0.7325. The sample results were then normalized to the  176  Hf/177Hf value of  0.282160 [Vervoort and Blichert-Toft, 1999] using the daily average of the JMC 475 Hf standard. The JMC 475 Hf standard analyzed over the period of the analyses gave an average value of 176Hf/177Hf = 0.282181 ± 0.000008 (n = 32).  3.5. Results Throughout the multistep acid-leaching procedure, the Hawaiian basalts lose ~35% of their initial weight (up to 8 acid steps; Tables 3.2 and 3.4) and Kerguelen basalts lose up to 60% (up to 14 acid steps; Tables 3.3 and 3.4). For all samples, the largest amount of Sr, Nd and Hf is leached out in the first 1-2 acid leaching steps (Figure 1), as is the Pb. Overall, leaching removes up to 35% of the total Sr content and 40% of the Hf content for Hawaiian basalts, and up to 63% and ~70% of the Sr and Hf contents, respectively, for the Kerguelen basalts (Tables 3.2, 3.3, and 3.4). With the exception of one Hawaiian basalt, all samples lose ~90% of their initial Nd content during leaching. Similar proportions of Sr and Nd losses after acid leaching have been reported for basalts from the first drilling phase of the HSDP [Hauri et al., 1996].  69  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  Table 3.2. Sr, Nd, and Hf Elemental Contents and Isotopic Compositions of Unleached Whole-rock Powders, Leachate Solutions, and Leached Residues for Hawaiian Basalts Sr Total ng in % Sr in Each Each Fraction Fraction  Sample  Nd 87  Sr/86Sr  2 SEa  Total ng in % Nd in Each Each Fraction Fraction  Hf Nd/144Nd  143  2 SEa  Total ng in % Hf in Each Each Fraction Fraction  176  Hf/177Hf  2 SEa  Sample J2-020-23 (Mauna Loa) Unleached b  8960  100  0.512943  7  1843  100  9  4754 4395  53.1 49.0  0.512934 0.512931  647 503  35.1 27.3  9 8 9 – 8 8 – –  4490 259 176 121 101 52.8 – 18.8  50.1 2.89 1.97 1.35 1.13 0.589 – 0.209  0.512943 0.512938 0.512948 – 0.512956 0.512939 – –  9 8 8 7 12 – 18 14 – –  540 61.9 44.5 21.8 20.4 7.35 – 0.450  29.3 3.36 2.41 1.18 1.11 0.40 – 0.024  5218  58.2  696  37.8  3742  41.8  0.512948  7  1147  85466  100  0.703818  B.L.Homog. B.L.Acid d  29768 18415  34.8 21.5  0.703930 0.704203  1st 2nd 3rd 4th 5th 6th 1st H2O 2nd H2O  18266 3033 3233 2012 1738 669 –e 343  21.4 3.55 3.78 2.35 2.03 0.783 – 0.401  0.704310 0.703782 0.703671 – 0.703687 0.703704 – –  Total f  29294  34.3  Leach. Res.g  56172  65.7  0.703683  10  Unleached  94286  100  0.703728  8  6085  100  0.513009  8  B.L.Homog. B.L.Acid  19718 19579  20.9 20.8  0.704556 0.704634  8 9  6944 7020  114 115  0.512997 0.512614  1st 2nd 3rd 4th 5th 6th 7th 8th 1st H2O 2nd H2O  15597 1951 1548 1175 1978 1130 635 601 245 222  16.5 2.07 1.64 1.25 2.10 1.20 0.673 0.637 0.260 0.236  0.704511 0.703831 0.703661 – 0.703570 – 0.703592 – – –  9 8 17 – 7 – 9 – – –  4902 229 76.3 46.2 48.0 32.6 16.2 15.7 6.80 7.80  80.6 3.77 1.25 0.759 0.789 0.536 0.266 0.258 0.112 0.128  0.512995 – 0.513027 – 0.512872 – – – – –  9 10 7 – 42 – 112 – – – – –  261.7 55.00 39.91 25.80 28.10 18.90 11.35 10.75 0.800 -  22.5 4.73 3.43 2.22 2.41 1.62 0.975 0.924 0.069 -  Total  25083  26.6  5381  88.4  452  38.9  Leach. Res.  69203  73  0.703535  9  704.4  11.6  0.513000  3  711  61  c  9 10  0.283189 –  33 –  0.283109 – – – – – – – –  10 – – – – – – – –  62.2  0.283084  6  1164  100  407.7 406.5  35.0 34.9  0.283158 – – –  12 – – –  – – – – – – – – –  – – – – – – – – –  0.283120  5  Sample SR0954-8.00 (Mauna Kea)  2 SE, indicates the uncertainty in the last digits of the measured isotopic ratios b Unleached, unleached rock powders. c B.L.Homog., bulk leachate solution homogenized (acid + fine powder particles). d B.L.Acid, bulk leachate solution without the fine powder particles. e Dashes indicate no isotopic analysis performed. f Total = sum of the elemental contents of all individual leaching steps. The Sr, Nd, and Hf contents of the bulk leachate solutions does not equal the sum of the individual leaching steps because they were measured on different aliquots of the same sample powders. Note: the elemental contents (ng) of the unleached powders are calculated from the initial amounts of powder and the respective sample concentrations (see Table 3.1). The contents of the bulk and individual leachate solutions are those measured by HR-ICP-MS on a 10% aliquot of each solution. The elemental contents (ng) of the leached residues are calculated by mass balance between the total ng in the starting unleached powders and the sum of the elemental contents in all the individual leachate solutions. g Leach. Res., leached powder residues. a  70  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  Table 3.3. Sr, Nd, and Hf Elemental Contents and Isotopic Compositions of Unleached Whole-rock Powders, Leachate Solutions, and Leached Residues for Kerguelen Basalts Sr Sample  Total ng in % Sr in Each Each Fraction Fraction  Nd 87  Sr/ Sr 86  2 SE  a  Total ng in % Nd in Each Each Fraction Fraction  Hf Nd/ Nd  143  144  2 SE  a  Total ng in % Hf in Each Each Fraction Fraction  176  Hf/177Hf  2 SEa  Sample OB93-165 (Mont Crozier) Unleached b  120221  100  0.705340  8  10465  100  0.512621  6  1727  100  0.282850  B.L.Homog.c B.L.Acid d  37016 34489  30.8 28.7  0.705430 0.705440  10  86.8 87.9  0.512617 –  7  28.3 19.4  0.282849 –  39774 13813 3808 7740 3289 3607 1297 1661 313 194  33.1 11.5 3.17 6.44 2.74 3.00 1.08 1.38 0.261 0.161  0.705414 0.705303 0.705297 –e 0.705291 – 0.705284 – – –  8774 357 83.5 153 75.7 79.7 27.8 35.3 9.60 4.10  83.8 3.41 0.798 1.46 0.723 0.762 0.265 0.337 0.092 0.039  0.512626 0.512632 0.512633 – 0.512062 – 0.512632 – – –  – 8 10 10 – 11 – 40 – – –  488 334  1st 2nd 3rd 4th 5th 6th 7th 8th 1st H2O 2nd H2O  9 7 8 9 – 8 – 9 – – –  9088 9196  677 168 49.3 88.5 36.7 36.1 13.8 18.2 1.35 –  39.2 9.72 2.85 5.12 2.13 2.09 0.796 1.05 0.078 –  0.282761 0.282835 – – – – – – – –  – 15 8 – – – – – – – –  10  0.512625  6  Total f  75495  62.8  Leach. Res.g  44726  37.2  0.705298  Unleached  103438  100  0.704422  8  8037  100  0.512808  7  B.L.Homog. B.L.Acid  54762 46404  52.9 44.9  0.704545 0.704668  6289 6143  78.3 76.4  0.512809 0.512806  1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th 13th 14th 1st H2O 2nd H2O  32782 5508 2567 1718 1981 2652 3483 2263 2773 1014 1399 1455 1688 850 373 154  31.7 5.32 2.48 1.66 1.92 2.56 3.37 2.19 2.68 0.98 1.35 1.41 1.63 0.821 0.360 0.149  0.704720 0.704615 0.704629 0.704548 0.704300 0.704226 0.704227 0.704228 – – – –  8 8 6 8 9 9 8 8 8 10 – – – –  5745 307 87.7 49.5 58.6 73.4 113 76.6 109 35.0 53.2 54.4 64.8 25.8 16.7 6.85  71.5 3.8 1.1 0.6 0.7 0.9 1.4 1.0 1.4 0.4 0.7 0.7 0.8 0.3 0.2 0.1  0.512818 0.512213 0.512806 0.512833 0.512712 – – – –  7 7 8 19 20 22 34 – – – –  6876  85.6  0.704222  23  1161  14.4  0.512839  6  9599  91.7  866  8.28  6 9  1089  63.1  638  36.9  0.282849  6  1481  100  0.283020  5  885 779  59.8 52.6  0.283023 0.282962  672 127 37.9 21.6 20.7 23.7 32.5 16.4 24.2 2.20 7.19 13.3 16.0 4.65 0.15 –  45.4 8.55 2.56 1.46 1.40 1.60 2.19 1.11 1.63 0.148 0.486 0.898 1.08 0.314 0.010 –  0.282908 0.283023 – – – – – – – – – – – – – –  5 10 18  1019  68.8  462  31.2  ODP Leg 183, 1140A-31R-1, 57-61 (Northern Kerguelen Plateau)  Total  62660  60.6  Leach. Res.  40778  39.4  0.283028  6 – – – – – – – – – – – – – – 5  2 SE, indicates the uncertainty in the last digits of the measured isotopic ratios Unleached, unleached rock powders. B.L.Homog., bulk leachate solution homogenized (acid + fine powder particles). d B.L.Acid, bulk leachate solution without the fine powder particles. e Dashes indicate no isotopic analysis performed. f Total = sum of the elemental contents of all individual leaching steps. The Sr, Nd, and Hf contents of the bulk leachate solutions does not equal the sum of the individual leaching steps because they were measured on different aliquots of the same sample powders. Note: the elemental contents (ng) of the unleached powders are calculated from the initial amounts of powder and the respective sample concentrations (see Table 3.1). The contents of the bulk and individual leachate solutions are those measured by HR-ICP-MS on a 10% aliquot of each solution. The elemental contents (ng) of the leached residues are calculated by mass balance between the total ng in the starting unleached powders and the sum of the elemental contents in all the individual leachate solutions. g Leach. Res., leached powder residues. a  b  c  71  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  60  Element (ppb)  30 20  0  Pb Sr/ 100  #2  #3  #4  #5  #6  Leaching Step  25  Hf  20  SR0954-8.00 (Mauna Kea, Hawaii)  Nd  10  0  40  Pb  b  #1  20  #3  #4  #5  #6  #7  Leaching Step  #8  #1 #2 H 2O  Pb  100  c  #1  40 35 Hf 30  #2  #3  #4  #5  #6  #7  Leaching Step  #8  #1  #2 H 2O  ODP Leg 183, 1140A-31R-1, 57-61 (Northern Kerguelen Plateau)  25 20 15 Pb 10 5  #2  OB93-165 (Mt. Crozier, Kerguelen Archipelago)  Nd  30 Sr/  0  #2H2O  Sr/ 15 100  5  50  10  a #1  Hf  60  Element (ppb)  Element (ppb)  40  10  Element (ppb)  Nd  Hf  50  70  J2-020-23 (Mauna Loa, Hawaii)  0  d  Nd Sr/100  #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11#12 #13 #14 #1 #2 H 2O  Leaching Step  Figure 3.1. Variation of Pb, Sr, Nd, and Hf concentrations throughout the leaching procedure for basalts from (a) Mauna Loa, (b) Mauna Kea, (c) Mont Crozier, and (d) the Northern Kerguelen Plateau. Elemental concentrations (ppb) refer to the abundances in each leachate solution measured by HR-ICP-MS. The Sr abundances are divided by 100 to allow for plotting on the same graph as the other elements.  72  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  The Hawaiian and Kerguelen basalts present a similar pattern of Sr isotopic variation throughout the leaching process (Figure 3.2). The magnitudes of the isotopic variations at each leaching step are different between the four samples as they are dependent on the amount of fine particles (mostly alteration phases and contaminants) that are removed when decanting the individual leachate solutions. The leached powder residues of all four samples yield less radiogenic Sr isotopic compositions than the respective unleached powders. The Sr isotopic compositions of the individual leachate solutions and unleached powders, of both Hawaiian and Kerguelen basalts, generally plot along a mixing trend between a more radiogenic end-member (represented by the bulk leachate solutions and 1st leachate solution) and the leached sample residue (Figure 3.2). For both Hawaiian and Kerguelen basalts, no significant difference (i.e., outside individual analytical error) is observed between the Nd isotopic compositions of the unleached rock powders, the bulk leachate solutions, the individual leachate solutions, and the leached residues (Figure 3.3). For each individual sample, the results for nearly all sample fractions plot within error of each other. The few exceptions with lower 143Nd/144Nd represent a very minor relative proportion of the whole sample Nd content (e.g., leachate #2 for sample 1140A-31R-1, 57-61; leachate #5 for sample OB93-165; and a bulk leachate solution for sample SR0954-8.00). After acid leaching, the leached powder residues of both Hawaiian and Kerguelen basalts yield similar Nd isotopic compositions to those of their respective unleached sample powders. Statistically acceptable Hf runs (i.e., >1.5 V on the  177  Hf beam) could only be  obtained for the Kerguelen samples. This was because during the course of analysis of the leachate solutions, we encountered tungsten (W) interference problems on some of the Hawaiian samples processed in a tungsten carbide (WC) crusher and mill. In addition, the Hf contents of some fractions were not sufficient to provide good runs, hence no data is reported. For the two Kerguelen samples, the Hf isotope compositions of the unleached sample powders, bulk leachate solutions, and leached sample residues are within error of each other (Figure 3.4). In both cases, the first leachate solution yields lower  176  Hf/177Hf  73  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  87  Hawaii  Sr/86Sr  0.7045  #1  0.7041  87  Sr/86Sr  0.7048  0.7053 #2  Unleached  0  #5 #6 #3 leaching steps  #1 #2 #3  Sample Fraction  Leached residue  0.7052  #5 #6  87  SR0954-8.00 (MK, Hawaii)  2SE  Bulk leachate Bulk leachate (solution) (solution+particles)  0.7044  #1  Bulk leachate (solution)  2SE  c  #2  Unleached  b  #3  #5 #7  leaching steps  0  #1 #2 #3 #5 #7  Sample Fraction  Leached residue  0.7043  0.7041  #3  #5  #7  leaching steps  0  #1  #2  #3  #5  Sample Fraction  Sr/86Sr  0.7049  #2  Unleached  #7  ODP Leg 183, 1140A-31R-1, 57-61 (NKP)  2SE  #1 Bulk leachate (solution)  0.7045  0.7036  OB93-165 (Mt. Crozier, KA) #1  Bulk leachate (solution+particles)  0.7047  0.7040  0.7032  0.7055  Leached residue  Bulk leachate (solution+particles)  a  Kerguelen  Sr/86Sr  0.7054  Bulk leachate (solution)  0.7039  0.7035  J2-020-23 (ML, Hawaii)  2SE  0.7043  0.7037  87  #2  #3 #5  Bulk leachate (solution+particles)  Unleached  d  #7 leaching steps  0  #11  #1  #2 #3  #9  Leached residue  #12  #5#7#9#11#12  Sample Fraction  Figure 3.2. 87Sr/86Sr variations throughout the acid leaching procedure for basalts from (a) Mauna Loa, (b) Mauna Kea, (c) Mont Crozier, and (d) the Northern Kerguelen Plateau. The numbers refer to the respective acid leaching step (see Tables 3.1 and 3.3). The size of the spheres is proportional to the amount of Sr in each sample fraction. The analytical uncertainty of each analysis is smaller than the symbol sizes. In the top left corner of the diagrams, the average 2SE of the Sr isotopic analysis of each experiment is shown.  74  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  143  Hawaii  Nd/144Nd  0.51298  143  J2-020-23 (ML, Hawaii)  2SE  0.51297  #1  0.51294 Unleached  #2  0.51291  0  2SE  a  0.5121  Leached Residue  #7  Bulk leachate (solution+particles)  0.5120  OB93-165 (Mt. Crozier, KA) 0  Leached Residue  0.5126  c  #2 #3 #5 #7  Sample Fraction  Nd/ Nd  0.5127  #5 leaching steps  #1  144  Bulk leachates (solution+ particles)  0.5128 #3  #2 #3  0.5123  0.5129  (solution)  #11 #1  Leached residue  #9  Unleached  2SE  #12  0.5125  Bulk leachate (solution)  0.5124  #5  0.5123  SR0954-8.00 (MK, Hawaii) 0  0.5124  143  Unleached  0.5127  0.5125  #5 #6  Unleached  0.5130  #1  0.5128  0.5126  #1  0.5122  Bulk leachate (solution+particles)  0.5130 0.5129  #2 #3  Sample Fraction  Nd/ Nd  0.5131  #5  #3  leaching steps  #1  144  Leached residue  #6  Bulk leachate (solution+ Bulk leachate (solution) particles)  0.51292  0.5132  2SE  0.5127  0.5125  0.51295  143  0.5128  0.5126  0.51296  0.51293  Kerguelen  Nd/144Nd  leaching steps  #1  #3 #5  Sample Fraction  b  0.5122 0.5121  ODP Leg 183, 1140A-31R-1, 57-61 (NKP) 0  #2  leaching steps  #1  #2  Sample Fraction  #9#11#12  d  Figure 3.3. 143Nd/144Nd variations throughout the acid leaching procedure for basalts from (a) Mauna Loa, (b) Mauna Kea, (c) Mont Crozier, and (d) Northern Kerguelen Plateau. Symbol colours and sizes are as described in the caption of Figure 3.2. The analytical uncertainty is smaller than the symbol sizes, except where represented. The average 2 SE of the Nd isotopic analysis of each experiment is shown in the upper corners of the diagrams.  75  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  176  Hf/177Hf  176  seawater  0.28288  2SE  0.28286  Unleached  #2  Bulk leachate (solution)  Leached residue  0.28300 0.28298  Unleached  0.28294  0.28278  0.28274  #2 Leached residue  Bulk leachate (solution+particles)  0.28296  0.28280  0.28276  2SE  0.28304 0.28302  0.28284 0.28282  Hf/177Hf  0.28306  OB93-165 (Mt. Crozier, KA) 0  #1 leaching steps  #1  Bulk leachate (solution)  0.28292  #2  Sample Fraction  a  0.28290 0.28288  ODP Leg 183, 1140A-31R-1, 57-61 (NKP) 0  #1 leaching steps  #1  Sample Fraction  b  #2  Figure 3.4. 176Hf/177Hf variations throughout the acid leaching procedure for basalts from (a) Mont Crozier and (b) the Northern Kerguelen Plateau basalts. Symbol colours and sizes are as described in the caption of Figure 3.2. The average 2 SE of the Hf isotopic analysis of each experiment is shown in the upper right corner of the diagrams. Note the non-overlapping range of Hf isotopic ratios between the two diagrams.  76  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  values. After acid leaching, the leached powder residues of both Hawaiian and Kerguelen basalts yield identical Hf isotopic compositions to those of their respective unleached sample powders (Tables 3.2 and 3.3).  3.6. Discussion 3.6.1. Effects of Acid Leaching of OIB in Sr-Pb Isotopic Space The bulk leachate solutions consist of acid, a combination of secondary mineral phases, and any contaminant removed from the sample by the acid leaching process. The isotopic compositions of the bulk leachate solutions are thus a proxy for the total alteration and contaminant removed with the acid leaching procedure, whereas those of the leached residues can be considered to represent the original isotopic composition of the samples. In multi-isotopic space, the bulk leachate solutions and leached residues are hence expected to lie along mixing trends that encompass the isotopic compositions of the initial unleached rock powders and that provide insight about the nature of the contaminant. In Figures 3.5 and 3.6, we combine the Sr isotopic results of our leaching experiments with their respective Pb isotopic compositions from Nobre Silva et al. [2009], and discuss the plausibility of several natural contaminants to explain the mixing trends defined by the bulk leachates and the leached sample residues. The differences between the Sr and Pb isotopic compositions of the unleached and leached rock powders of the submarine tholeiites from Mauna Loa (sample J2-020-23) and Mauna Kea (sample SR0954-8.00) reflect exchange with a more radiogenic contaminant (Figures 3.2a, 3.2b, and 3.5). Given their high Sr contents and radiogenic Sr isotopic compositions, possible natural contaminants to consider for these two Hawaiian basalts are seawater, local deep-sea sediments and/or Fe-Mn crusts, or a mixture of each. A distal contaminant that must also be considered, and that has been identified in soil horizons of Hawaiian lavas, is wind-blown Chinese loess into the central north Pacific [e.g. Kennedy et al., 1998; Abouchami et al., 2000; Jones et al., 2000].  77  Table 3.4. Summary of the Weight and Relevant Element % Losses During the Multi-step Acid Leaching Procedure Each leaching step Sample  Initial Weight (g)  # Acid Leaching Steps  Final Weight (g)  Weight Loss (%)  Pb Loss (%)  Sr Loss (%)  Nd Loss (%)  Hf Loss (%)  0.2793  6  0.1828  34.6  68.7  34.3  58.2  37.8  0.3945  8  0.259  34.3  70.8  26.6  88.4  38.9  0.2883  8  0.106  63.2  37.3  62.8  91.7  63.1  0.3948  14  0.1598  59.5  76.1  60.6  85.6  68.8  Mauna Loa J2-020-20 Mauna Kea  OB93-165 Northern Kerguelen Plateau ODP Leg 183, 1140A-31R-1, 57-61  78  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  SR0954-8.00 Mont Crozier  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  In Pb-Sr isotopic space, the leaching experiments for the Mauna Loa basalt (sample J2-020-23) produce a shallow trend defined by the leached sample residue, unleached sample powder, bulk leachates, and individual leachate solutions (Figure 3.5a). This trend reflects a larger difference in the Pb isotopic values of the leached fractions relative to their Sr isotopic compositions. Mixing between the fresh basaltic rock sample and a contaminant with a high Pb content is thus needed to explain this “leaching” trend. Given their very low Pb contents, Pacific Ocean seawater ([Sr] = ~ 8 ppm [after Faure, 1986]; 87Sr/86Sr = 0.70918 [Thomas et al., 1996]; [Pb]1000–2000 m = ~5 x 10-5 ppm [after Chester, 2003]) and authigenic carbonate deposits can be ruled out as viable contaminants (see mixing curves in Figure 3.5a). Fe-Mn deposits, which are efficient scavengers of trace metals, have very elevated Pb contents (e.g., Central Pacific Fe-Mn deposits: 997 – 1054 ppm [after Ling et al., 1997]). Even a contribution of 1% of Fe-Mn oxides would be too high to explain the observed trend between the sample and leachate fractions (Figure 3.5a). Chinese eolian loess has elemental and isotopic compositions ([Sr] = ~200 ppm [after Nakai et al., 1993];  87  Sr/86Sr = 0.717 [after  Kennedy et al., 1998]; [Pb] = ~20 ppm [after Gallet et al., 1996]; 206Pb/204Pb = 18.775 [after Jones et al, 2000]) suitable to explain the trend produced by leaching. However, incorporation of as much as 15% of loess into the fresh lava would be needed (Figure 3.5a). This is a very large amount of sediment to be incorporated into a basalt erupted in a submarine environment and thus unlikely to be the sole explanation for the observed trend. A preferred explanation is incorporation of a composite mixture of deep-sea interbedded claybearing carbonate sediments, similar in composition to those now subducting at the IzuBonin trench [Hauff et al., 2003; Plank et al., 2007], and some minor Fe-Mn oxides, deposited on the surface of the basalt (Figure 3.5a). For the HSDP-2 Mauna Kea submarine tholeiite (sample SR0954-8.00), the isotopic trend produced by the acid leaching reflects interaction of the basalt with a highly radiogenic contaminant, different than that discussed above for the submarine Mauna Loa basalt (Figures 3.2b and 3.5b). Several previous studies have suggested possible contamination of the HSDP samples by drilling mud [e.g., Hauri et al., 1996; Abouchami et al., 2000; Eisele et al., 2003; Nobre Silva et al., 2009]. Hanano et al. [2009] also identified distinct patches of  79  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  87  (solution)  0.90 mixing with clay bearing carbonates  0.70175 18.55  ulk lo es s  f lo ct .o fra  at e  xid  es  no  0.3  0.3 0.1  0.5  in mix  it h gw  0.1  Pb content Sr content  Bulk Leachate (solution + particles)  SR0954-8.00 Mauna Kea (Hawaii)  2SE  18.65  of lo ess  authig. carbonate  rac  0.1  18.90  HSDP-2 drilling mud  0.01 #7  18.80  Fe-Mn oxides  Leached Unleached Residue  18.60  with b  ate frac t.  mixin g with  -M  18.70  Pb/204Pb  mix of 30% drilling mud + 70% clay bearing carb.  #1  #3  18.60  J2-020-23 Mauna Loa (Hawaii)  Seawater  0.5  #2  18.50  Pb content Sr content  te f  0.95 Bulk Leachate  mixing with carbonate +Fe-Mn Oxides  si l ica  0.70575  0.5  lic si  loess  Sr/86Sr  0.96  206  Fe  th bul k  0.97  0.70675  18.40  ith  ing wi  0.98  th wi  tion  18.30  0.99  0.70775  carbon  Bulk Leachate (solution + particles)  18.20  0.70875  Bulk Leachate (solution)  Unleached  2SE  b  0.70275  0.01  mixing with carbonate frac. of loess  0.70220 18.10  0.1 mixing w  #3  mi  g xin  mix  Leached Residue  0.70300  0.2  #2  0.80  0.3  0.05  #1  m  0.85  0.70380  on ate s  rin g  c la yb ea  0.90 0.2  0.1  wi th  0.70460  0.5  ix in g  0.95  0.70375  ge n  gw  0.70540  0.4  0.5  mi xin  87  0.4  ca  s  au th i  Sr/86Sr  0.5  ic  ate on rb  ca rb  0.6  0.98  0.70620  0.70475  clay bearing carbonates  1  0.99  0.70700  Fe-Mn oxides  of loess  mixing  0.995  0.70780  0.70975  Seawater  es s  0.999 with seawater 0.998  0.70860  mixin g  a  ith  0.70940  18.70  18.75 206  18.80  Pb/204Pb  18.85  18.90  18.95  19.00  Figure 3.5. 87Sr/86Sr versus 206Pb/204Pb diagrams showing the relationships between the unleached sample powders, leached residues, and leachate solutions for the two Hawaiian basalts compared to the isotopic compositions of possible external contaminants. Pb isotopic compositions and concentrations are from Nobre Silva et al. [2009]. In both diagrams, the size of the symbols is proportional to the Sr (lighter colour) and Pb (darker colour) contents in each sample fraction. The analytical uncertainty of each analysis is represented by the 2 SE, which is much smaller than the symbol sizes. (a) The results for the Mauna Loa sample (J2-020-23) and several possible contaminants: seawater (elemental abundances (EA): Faure [1986]; isotopic compositions (IC): Abouchami and Galer [1998] and Eisele et al., [2003]); authigenic carbonates (EA: Faure [1986] and Chester [2003]; IC: same as seawater); Fe-Mn oxides (EA and IC: Ling et al. [1997]); deep-sea interbedded clay-bearing carbonate sediments such as those now subducting at the Izu-Bonin trench (EA and IC: Hauff et al. [2003] and Plank et al. [2007]); Chinese loess (EA: Nakai et al. [1993] and Gallet et al. [1996]; IC: Kennedy et al. [1998] and Jones et al. [2000]). The thick red line represents the mixing trend between the leached sample residue and the bulk leachate solutions (circles indicate 10% increments). (b) The results for the Mauna Kea sample (SR0954-8.00) and three external contaminants: seawater, carbonates, HSDP-2 bulk mud [Abouchami et al., 2000], and a mixture of clay-bearing carbonates plus drilling mud. The mixing trend between each of these contaminants and the leached sample residue are shown. All unlabelled tick marks on the mixing curves indicate 10% increments.  80  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  barite/celestite within a thin section of this sample. This is unexpected given the careful sample washing procedures employed for the HSDP sample suite prior to crushing [Rhodes, 1996; Rhodes and Vollinger, 2004]. The bulk mud used during HSDP-2 has distinctly more radiogenic Sr and Pb isotopic compositions [Abouchami et al., 2000; Eisele et al., 2003], however its Pb isotopic composition is too radiogenic to explain the trend defined by the leached sample residue, unleached sample powder, bulk leachates and individual leachate solutions of the Mauna Kea basalt (SR0954-8.00) (Figure 3.5b). Interaction of loess particles with this sample could potentially account for the observed trend produced by leaching. Based on its Pb isotopic composition, the carbonate fraction of Asian loess has been considered a possible source of the radiogenic Pb contamination of Hawaiian basalts recovered during the HSDP [Abouchami et al., 2000; Eisele et al., 2003]. However, as much as 25% of the carbonate fraction of the loess alone would be needed (Figure 3.5b). This represents an excessively large amount of sediment to be incorporated into the sample, and requires a process that would facilitate interaction with the carbonate fraction of the loess, but not with the silicate fraction. Another alternative is the interaction of the fresh basalt with 20% of drilling mud and deep sea carbonates mixed in a proportion of 30:70 (black thick mixing curve in Figure 3.5b). However, 20% is also a large amount of a contaminant to be incorporated into the basalt. A more plausible explanation for the observed trend is interaction of the basaltic rock sample with a combination of distinct end-members, including the drilling muds used during the HSDP-2. Acid leaching of the submarine tholeiite from the Northern Kerguelen Plateau (sample 1140A-31R-1, 57-61) lowers its Sr isotopic composition (Figures 3.2 and 3.6b). In contrast, its Pb isotopic composition increases after being acid leached [Nobre Silva et al., 2009]. This implies that the contaminant being leached out has more radiogenic  87  Sr/86Sr  values, but less radiogenic Pb isotopic compositions, than those of the basalt. Given the low Pb content and radiogenic Sr and Pb isotopic compositions of seawater, alteration of this basalt by Indian Ocean seawater or contamination with carbonated sediments (Figure 3.6a) cannot account for this opposite behaviour during leaching. Assuming that the isotopic compositions of the drilling muds used during the ODP Leg 183 are comparable to those  81  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  0.70550 0.70530  ODP Leg 183, 1140A-31-R1, 57-61 (Northern Kerguelen Plateau)  mixing with Indian Ocean Seawater  0.9  0.70510 0.70490  Pb content Sr content  Sr/86Sr  Bulk Leachate (solution)  0.70470  #1  87  #3  0.70450  #2  Bulk Leachate (solution + particles)  0.7  0.6 0.3  mixing with older volcanic components from the Kerguelen Plateau  0.70390 18.520  18.530  18.540  0.7  0.5  Unleached  0.70430 0.70410  0.8  Leached Residue  18.550  0.70543  interaction Bulk Leachate with (solution) Indian Ocean Seawater  0.70540  Sr/86Sr  0.6  87  a 18.570  18.580  Bulk Leachate (solution + particles) Sr content  Pb content  18.590  18.600  OB-93-165 Mont Crozier (Kerguelen Archipelago)  Pb content Sr content  #1  0.4 0.3  0.70535  0.2  0.70533 0.70530  #9  Pb/ Pb 208  0.5  0.70538  Pb content Sr content  18.560  206  0.8 0.7  m  es  oxid  0.1  0.70548 0.70545  #7  0.5  -Mn  h Fe  wit ixing  0.1  Unleached  #2  0.70528 0.70525 18.590  Leached Residue 18.595  #3  18.600  #7 18.605  18.610 206  18.615  Pb/204Pb  18.620  18.625  18.630  b 18.635  Figure 3.6. 87Sr/86Sr versus 206Pb/204Pb diagrams showing the isotopic compositions of each sample fraction obtained throughout the leaching procedure for the Kerguelen basalts compared to the isotopic compositions of possible external contaminants. Pb isotopic compositions and concentrations are from Nobre Silva et al. [2009]. In both diagrams, the size of the symbols is proportional to the Sr (lighter colour) and Pb (darker colour) contents in each sample fraction. The analytical uncertainty of each individual analysis is represented by the black error bars centered on the respective symbol. (a) The relationship between the Sr and Pb isotopic compositions of the leached and unleached powders, individual leaching steps, and bulk leachate solutions of the Northern Kerguelen Plateau basalt (ODP Leg 183, 1140A-31-R1-57-61). The thick red line with 10% increments represents the mixing trend between the leached sample residue and the bulk leachate solutions. Mixing lines are shown between the leached sample residue and Indian Ocean seawater (dashed line) (EA: Faure [1986]; IC: Vlastélic et al. [2001]], Fe-Mn oxides, drilling muds, and volcanic components weathered from older parts of the Kerguelen Plateau (grey arrow) (e.g., CKP: Frey et al. [2002a] and Neal et al. [2002]]. (b) The relationship between the Sr and Pb isotopic compositions of the leached and unleached powders, individual leaching steps, and bulk leachate solutions of the Mont Crozier basalt (OB93-165). The thick red line with 10% increments shows the mixing trend between the leached sample residue (proxy for the fresh sample) and the bulk leachate solutions (proxy for the total alteration removed). The black dashed line passing though the unleached sample (altered basalt) represents the trend that would be expected between a simple binary mixing between the “fresh sample” and the “alteration removed”.  82  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  used by the HSDP (i.e., highly radiogenic in Pb), contamination by the drilling muds cannot be the cause for the observed differences in the isotopic compositions of the unleached and leached powders of sample 1140A-31R-1, 57-61 (Figure 3.6a). Volcanic components released during weathering of older parts of the Kerguelen Plateau (e.g., the ca. 100 Ma Central Kerguelen Plateau), could also potentially be a viable source of contamination by deposition after eruption and emplacement of the Northern Kerguelen Plateau. However, the high Sr and relatively low Pb contents of the Central Kerguelen Plateau basalts, as well as their much less radiogenic Pb isotopic compositions [Frey et al. 2002a; Neal et al., 2002], indicates that they alone are not viable contaminants (Figure 3.6a). The combined information given by the Sr and Pb isotopic compositions of the leaching experiment with sample ODP Leg 183, 1140A-31R-1, 57-61, does not allow the identification the exact nature of its unradiogenic Pb contaminant. As with the other basalts in this study, the most probable scenario is that the source of contamination of this basalt is a composite one. In contrast to the submarine basalts in this study, the unleached rock powder of the subaerial Kerguelen Archipelago basalt from Mont Crozier (sample OB93-165) does not plot along the mixing trend defined by the leached sample residue and the bulk and first leachate solutions (Figure 3.6b). The scatter observed in the Pb isotope values of the individual leachate solutions compared with their Sr isotopic compositions (Figure 3.6a; and chapter 2 Figure 2.2a]) suggests an additional source of variability. This is consistent with the progressive removal at each step of the leaching procedure of a variety of secondary alteration minerals (carbonates, oxides, sulfides, clays, epidote, zeolites [e.g., Nougier et al., 1982; Verdier, 1989; Hanano et al., 2009]). The different Pb isotopic compositions of each leaching step relative to their Sr isotopic compositions suggest that the alteration minerals removed during the acid-leaching procedure may have been formed by more than one alteration process, including surficial chemical and physical weathering, possible interaction with seawater transported by hydrothermal fluids, or interaction with sea spray.  83  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  3.6.2. Effects of Acid Leaching on Trace Element Abundances and Nd-Hf Isotopic Compositions of OIB Both Hawaiian and Kerguelen basalts lose a large relative amount (~90%) of Nd during the acid leaching procedure (Tables 3.2, 3.3, and Figure 3.1). Comparing the trace element abundances of unleached and leached powder splits of the Mont Crozier basalt (sample OB93-165; Table 3.5 and Figure 3.7), the elements that are the most affected (i.e., showing the highest proportion of removal) by the leaching process are Cs, Rb, Th, U, Pb, and the rare earth elements (REE), especially the light REE. The LREE abundances in the leached powder splits are ~5 times less than in the original unleached powder splits, whereas the heavy REE are only a factor of ~1.5 less abundant. This extraction and fractionation of the REE by acid leaching has been previously noted by other authors [e.g., Verma, 1992; Thompson et al., 2008]. A surprising effect of the leaching process is the higher concentrations of Nb and Ta in the leached powder splits than in the unleached splits (Figure 3.7). The elemental concentrations in the leached powder splits are measured on a residue weight of about ~0.2 g (vs. ~0.4 g for the unleached splits) that represents what is left after differential mineral removal by acid leaching. Nb and Ta are high field strength elements and are predominantly concentrated in primary Fe-Ti oxide minerals, such as titanomagnetite or ilmenite. The resistance of these minerals to the acid-leaching process will promote the concentration of Nb and Ta relative to other elements in the final powder residue, hence the leached residues will have higher concentrations of these elements than the unleached powders. The trace element abundances measured on the leached powder splits are ~2–5 times less variable than the abundances measured on the unleached powder splits (e.g., %RSDunl/leach = 4.45 for La; = 1.94 for Lu). This demonstrates the reproducibility of the leaching process and that mineral phases are not randomly removed from the altered crystalline basalt. The acid leached residue of whole rock powders of oceanic basalts is composed mainly of plagioclase and clinopyroxene ± olivine and Fe-Ti oxides [e.g.,  84  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  Table 3.5. ICP-MS Trace Element Abundances in ppm of Unleached and Leached Splits of the Mont Crozier, Kerguelen Archipelago, Basalt Unleached OB93-165  Leached OB93-165  Element  Split 1  Split 2  Split 3  average  SD  Split A  Split B  Split C  average  SD  Unleach./Leach.  Li Sc V Co Ni Cu Zn Ga  4.2 22 292 34 17 61 92 25  4.9 23 298 35 19 71 104 27  5.2 24 339 37 19 72 110 28  4.8 23 310 35 18 68 102 27  0.53 1.0 26 1.8 1.1 6.4 9.2 1.7  4.0 28 146 21 9.3 0.9 51 23  3.9 27 142 21 9.3 0.9 51 23  3.6 26 137 21 9.3 0.9 50 24  3.8 27 141 21 9.3 0.9 51 23  0.19 1.13 4.8 0.19 0.01 0.01 0.81 0.42  1.2 0.85 2.2 1.7 2.0 76 2.0 1.1  Rb Sr Y Zr Nb Mo Cd Sn Cs Ba  25 459 31 252 30 1.9 0.19 1.7 0.07 320  30 483 32 288 31 2.1 0.21 1.7 0.08 351  31 507 34 283 36 2.4 0.22 2.0 0.09 355  29 483 32 275 32 2.1 0.21 1.8 0.08 342  2.9 24 1.5 20 3.0 0.28 0.02 0.17 0.01 19  28 441 16 249 45 1.1 0.12 0.83 0.03 342  28 444 16 241 43 1.0 0.12 0.86 0.03 340  27 441 15 256 42 1.0 0.11 0.89 0.02 358  27 442 16 249 43 1.0 0.12 0.86 0.03 347  0.59 1.76 0.21 7.74 1.94 0.02 0.00 0.03 0.00 9.99  1.0 1.1 2.1 1.1 0.75 2.1 1.8 2.1 3.1 0.99  La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu  34 75 8.8 35 7.5 2.4 7.2 1.1 6.5 1.3 3.4 0.46 2.9 0.43  37 78 9.1 37 7.8 2.5 7.6 1.2 6.7 1.3 3.5 0.48 3.0 0.44  38 83 9.7 39 8.2 2.7 7.9 1.2 7.1 1.4 3.7 0.52 3.2 0.46  37 79 9.2 37 7.8 2.5 7.6 1.2 6.8 1.3 3.6 0.49 3.0 0.44  2.1 4.0 0.45 1.8 0.38 0.14 0.37 0.06 0.33 0.07 0.14 0.03 0.16 0.02  6.9 15 2.0 9.4 2.7 2.2 3.1 0.53 3.4 0.71 2.0 0.29 2.0 0.30  7.1 15 2.0 9.1 2.6 2.2 3.0 0.52 3.3 0.67 1.9 0.28 1.9 0.29  7.1 15 2.0 9.1 2.6 2.3 3.0 0.52 3.3 0.68 1.9 0.29 1.9 0.29  7.0 15 2.0 9.2 2.6 2.2 3.0 0.53 3.3 0.69 1.9 0.29 1.9 0.29  0.09 0.22 0.04 0.18 0.04 0.02 0.08 0.01 0.08 0.02 0.04 0.00 0.04 0.01  5.2 5.1 4.6 4.0 3.0 1.1 2.5 2.2 2.0 1.9 1.8 1.7 1.6 1.5  Hf Ta Pb Th U  6.3 2.0 2.7 4.4 1.1  7.2 2.2 2.8 4.7 1.1  7.1 2.6 2.9 4.8 1.2  6.9 2.3 2.8 4.6 1.1  0.48 0.30 0.12 0.22 0.05  6.2 3.5 1.9 2.4 0.71  5.9 3.4 2.0 2.2 0.71  6.0 3.4 2.0 2.3 0.73  6.0 3.4 1.9 2.3 0.72  0.14 0.04 0.03 0.11 0.01  1.1 0.65 1.4 2.0 1.6  85  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  Mahoney, 1987; Regelous et al., 2003: Hanano et al., 2009]. The chondrite-normalized REE patterns of the leached powder splits of sample OB93-165 (Figure 3.7) are consistent with removal of olivine, pyroxene, and some plagioclase, and with plagioclase being the main mineral left after acid leaching (e.g., positive Eu anomaly). The irregular trace element pattern of the leached powder residues reflects the selective removal of some elements relative to others. This indicates that the elemental abundances measured on the leached residue powders cannot be considered representative of the primary magma elemental concentrations. The REE, given their ionic radii (La: 1.16 Å; Lu: 0.97 Å) and valence (+3), do not readily substitute for other elements in the lattices of early crystallizing minerals (olivine and pyroxene) of a mafic melt [e.g., Krauskopf and Bird, 1995]. Instead, the REE are mostly located in non-structural sites and defects, where the bonds are most easily broken. Hence, the observation that the REE, especially the LREE, are more easily leached out likely reflects their relatively weak bonding in the mineral structures. As a result, the REE are easily remobilized and preferentially concentrated into the leachable secondary mineral phases during alteration [Verma, 1992], which subsequently allows for their differential removal by the acid leaching process. Previous studies have suggested that Nd and Hf isotope ratios of oceanic basaltic rocks are to some extent modified by alteration, generally becoming slightly lower in the altered rock [e.g., Kempton et al., 2002; Thompson et al., 2008]. Despite the large relative amounts of Nd (and other REE) lost during the acid leaching procedure, there is very little effect on the Nd isotopic compositions (Figure 3.3). Seawater has Nd isotopic composition that varies among and within the ocean basins (mean 143Nd/144NdPacific [Piepgras and Wasserburg, 1980]; mean  143  Nd/144NdIndian  Ocean  Ocean  = ~0.5125  = ~0.5123 [Albarède et al.,  1997a]) and that is overall significantly lower than the Nd isotopic composition of oceanic basalts. However, given the very low Nd concentrations in seawater (~2.6 x 10-6 ppm [Faure, 1986]), seawater-basalt interaction will not produce a significant change in the REE content and Nd isotopic compositions of the basalts unless the water/rock ratios are greater than 105 [Ludden and Thompson, 1979]. For each of the four samples in this study, the unleached and leached sample powders, bulk and individual leachate solutions mostly plot within analytical  86  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  Sample/C1 Chondrite  1000  Sample/primitive mantle  100  OB93-195 (Mt. Crozier, KA)  100  Unleached 10  1  Leached  La  Ce  Pr  Nd Pm Sm Eu  Gd Tb  Dy  Ho  Er  Tm Yb  Lu  Unleached 10  Leached  OB93-195 (Mt. Crozier, Kerguelen Archipelago) 1  Cs Rb Ba Th  U  Nb Ta La Ce Pb Pr Nd Sr  Zr  Hf Sm Eu Gd Tb Dy Ho Y  Er Yb Lu Ga  Trace Element Figure 3.7. Extended primitive mantle-normalized trace element diagram showing the results for three unleached powder splits (diamonds) and three leached powders splits (circles) of sample OB93-165 from the Kerguelen Archipelago. The inset diagram shows the chondrite-normalized rare earth element patterns for the same powder splits. Normalizing values are from McDonough and Sun [1995].  87  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  uncertainty of each other, except for a few leachate solutions that present lower isotopic compositions. The cause of the lower isotopic compositions of these odd fractions is unknown, but may be due to some heterogeneity in the secondary alteration phases. Reliable Hf isotopic data could only be obtained for the Kerguelen basalts (Figure 3.4). The Hf isotopic compositions of both the unleached and leached sample powders are the same within analytical uncertainty. The Hf isotopic composition of seawater (mean 176  Hf/177HfIndian Ocean = ~0.28288 [David et al., 2001]) is significantly different from that of  ocean island basalts, being more radiogenic than the Kerguelen Archipelago basalt and less radiogenic than the Northern Kerguelen Plateau basalt. However, given the geochemical nature of Hf (high field strength element) and its low concentration in seawater (~7 x 10–6 ppm [Faure, 1986; Chester, 2003]), no change in the Hf isotopic compositions of the basalts is expected.  3.7. Conclusions The results of this study on the effects of acid leaching of whole-rock powders on radiogenic isotopes (Pb-Sr-Nd-Hf) commonly used to discern and characterize the mantle sources of oceanic basalts demonstrate that accurate multi-isotopic analysis can be acquired on the same sample aliquots after an acid leaching treatment. The Sr isotopic ratios of all leached sample residues (submarine or subaerial) are significantly less radiogenic than their respective unleached powders. The higher  87  Sr/86Sr ratios of the unleached powders are  generally related to seawater interaction/alteration and (in the case of cored samples) drilling mud contamination. Seawater alone cannot explain the Sr and Pb isotopic compositional differences between unleached and leached powders of the submarine tholeiites, and interaction with other external contaminants such as Fe-Mn oxides, deep-sea sediments, and loess is also likely. For all the samples, the Nd and Hf isotopic compositions of leached and unleached powders are within analytical uncertainty of each other, despite the significant losses (~90%) of Nd (and other REE) and to a lesser extent Hf. Given these relatively significant elemental losses, care should be taken to ensure that enough material is available to measure isotopic compositions with the required precision after acid leaching of the  88  CHAPTER 3: Sr-Nd-Hf ISOTOPES OF ACID-LEACHED OIB  samples. Due to the trace element losses and REE fractionation during acid leaching, the elemental abundances measured on leached sample powders are not reliable magmatic elemental signatures. Hence, for calculation of initial isotopic ratios at the time of crystallization, parent/daughter ratios should be determined from elemental concentration ratios measured on unleached sample powders.  89  C HAPTER 4  The Ninetyeast Ridge and its Relation to the Kerguelen, Amsterdam and St. Paul Hotspots in the Indian Ocean  90  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  4.1. Synopsis The Ninetyeast Ridge, a 5000 km long north-south oriented submarine volcanic ridge in the eastern Indian Ocean, is interpreted to have formed from magmatism associated with the deep-seated Kerguelen mantle plume as the Indian plate drifted rapidly northward during the Late Cretaceous. Basalts recovered along the ridge have the characteristic Dupal geochemical signatures of Indian Ocean basalts, but debate concerning the nature and number of components in the mantle source contributing to the formation of these basalts persists. New MC-ICP-MS (Pb, Hf) and TIMS (Sr, Nd) isotopic analyses were determined for tholeiites representative of the ~180 m of basaltic basement recovered from three drill sites (758: 82 Ma; 757: 58 Ma, 756: 43 Ma) along the Ninetyeast Ridge during ODP Leg 121. No systematic isotopic variation is observed along the ridge, which is inconsistent with the hypothesis of an aging mantle plume origin for the ridge. The Pb-Hf-Sr-Nd isotopic compositions are generally intermediate between those of the volcanic products of the Kerguelen and Amsterdam-St. Paul hotspots and define mixing trends between components with relatively enriched and depleted signatures. At least three, possibly four, source components are required to explain the observed isotopic variability along the Ninetyeast Ridge. The unradiogenic isotopic signatures of some Ninetyeast Ridge basalts (e.g., 87Sr/86Sr = 0.70381 – 0.70438) are not related to Indian MORB compositions and are consistent with the presence of a relatively depleted component in a deep mantle source. A similar source component is also identified in other Indian ocean island basalts (e.g., Crozet, Réunion) that are characterized by strong EM-1-like compositions. The Pb-Hf-Sr-Nd isotopic compositions of the Ninetyeast Ridge basalts are consistent with the presence of a mixture of recycled sediments and lower continental crust together with altered oceanic crust in their mantle source, hence supporting a deep origin for the EM-1-like Dupal signatures encountered in ocean island basalts.  4.2. Introduction Hotspot tracks refer to linear, age-progressive ocean island–seamount chains and volcanic aseismic ridges across the ocean floor. They are considered to be the surface  91  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  manifestation of the interaction between focused, relatively fixed mantle melting anomalies and the motion of an overriding lithospheric plate [Wilson, 1963; Morgan, 1971, 1972]. The distinct chemistry (as compared to ocean ridge basalts), longevity (up to 100 Myr; Coffin and Eldholm [1994]; Ernst et al. [2005]) and volume of these intraplate volcanic features appear to result from melting within the conduit (or tail) of deep-seated mantle plumes, most probably originated at the core-mantle boundary [e.g., Morgan, 1971, 1972; Stacey and Loper, 1983; Richards et al., 1989; Farnetani and Samuel, 2005]. The geometry and longevity of hotspot tracks, whether or not hotspots are considered to be fixed [e.g., Morgan, 1971, 1972] or advected by mantle flow [e.g., Steinberger, 2000; O’Neill et al., 2003], are used to calculate models of plate motion [e.g., Morgan, 1972] and estimate cooling rates of the core [e.g., Stacey and Loper, 1984; Davies, 1988; Sleep, 1990]. The geochemistry of hotspot-related volcanic rocks provides information about the compositions and evolution of the mantle regions that are sampled by the mantle plumes [e.g., Wasserburg and DePaolo, 1979; Campbell and Griffiths, 1990; DePaolo et al., 2001; Jellinek and Manga, 2004; Bryce et al., 2005; DePaolo and Weis, 2007]. Located in the eastern Indian Ocean (Figure 4.1), the ~5000 km long Ninetyeast Ridge is the longest linear feature on earth. Together with the Chagos-Laccadive Ridge, which is related to the Réunion hotspot, the Ninetyeast Ridge is the main hotspot track used to define the motion of the Indian Ocean plate [e.g., Morgan 1981; Royer et al., 1991; Weis et al., 1992; Müller et al., 1993]. Its origin has long been subject of debate, with many tectonic settings (e.g., horst structure, overthrusted plate boundary, interaction between a spreading center and transform fault, hotspot track) being proposed for its formation [e.g., Whitford and Duncan, 1978; Royer et al., 1991; and references therein]. Following the DSDP (Legs 22 and 26) and ODP (Leg 121: Sites 756–8) drilling campaigns on the Ninetyeast Ridge during the late 1970s and late 1980s, a general consensus was reached that the ridge formed from enhanced volcanism generated during ~44 Myr of hotspot volcanism related to the Kerguelen mantle plume at the edge of the Indian Plate as the plate moved rapidly northward during the Late Cretaceous to Late Eocene [Whitford and Duncan, 1978; Royer et al., 1991; Duncan, 1991; Frey et al., 1991; Weis and Frey, 1991].  92  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  70ºE  80º   90º   100º   110ºE   -35  00  10ºN   10ºN   758  82 Ma  216  78 Ma  214  59 Ma  Osborn Knoll  757  58 Ma  20º   10ºS   Wharton Basin  20º   253  46 Ma  R WI  756  S  43 Ma  30º   40º   nc h  e Tr  10ºS   a nd Su  CIR  Central Indian Basin  0º   Ninetyeast Ridge  0º   254  38 Ma  Broken Ridge  Amsterdam St. Paul  INDIAN PLATE -300  0  SE  IR  40ºS   -35  00  -3500  Kerguelen Archipelago  50ºS  Heard Island  ANTARCTIC PLATE  Kerguelen Plateau  70ºE   30º   80º   0 90º   100ºE   km  Eastern Indian Ocean  500  1000 110º   50º   E  Figure 4.1. Bathymetry map (500 m interval contours) of the eastern Indian Ocean showing the major topographic features of the ocean basin (drawn using the now extinct online map creator from GEOMAR: www.aquarius.geomar.de). The yellow circles on the Ninetyeast Ridge are the DSDP Legs 22 and 26 drill Site (216, 214, 253 and 254) locations where basement was recovered, and the red stars are the ODP leg 121 drill Site (758, 757, and 756) locations. SEIR, SWIR, and CIR refer to the Southeast Indian Ridge; the Southwest Indian Ridge; and Central Indian Ridge, respectively.  93  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  The identity and number of the mantle sources and respective components involved in the generation of the Ninetyeast Ridge is however still debated. Saunders et al. [1991] and Weis and Frey [1991] interpreted the isotopic compositions of the Ninetyeast Ridge basalts as indicating mixing between at least three mantle components: a depleted mid-ocean ridge basalt (MORB)-like component, an enriched Kerguelen hotspot–type component, and a component comparable to that sampled by Amsterdam-St. Paul volcanic rocks. They interpreted the differences in the isotopic compositions of basalts from each drill site as indicative of different relative proportions of each mantle component due to the changes in tectonic setting, from hotspot-centred volcanism at Site 758 (in the northern end of the ridge) to hotspot-ridge interaction volcanism at Site 756 (in the southern end). A different view of the isotopic compositions of the Ninetyeast Ridge was presented by Class et al. [1993], who interpreted the increase in initial  206  Pb/204Pb with decreasing eruption age, from Site 758  (~82 Ma) to Site 757 (~58 Ma) to Site 756 (~43 Ma), as reflecting the systematic compositional evolution of a single homogeneous plume by secular changes in isotopic ratios resulting from radioactive decay in the source. They proposed that this source is presently best characterized by the low 87Sr/86Sr and high 143Nd/144Nd and  206  Pb/204Pb end-member of  Heard Island volcanic rocks [Barling and Goldstein, 1990]. However, the compositional trend with time detected by Class et al. [1993] is not observed in the Sr and Nd isotopic systems. For example, Site 758 lavas have intermediate Sr and Nd isotopic compositions compared to those of Sites 756 and 757 [Weis and Frey, 1991; Weis et al., 1992; Frey and Weis, 1995]. Since ODP Leg 121 in 1988, when the Ninetyeast Ridge basement was successfully drilled at three Sites (756, 757, 758) and subjected to comprehensive geochemical studies, the precision of Pb and Hf isotopic analyses determined by multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) [e.g., Pb: Belshaw et al., 1998; Rehkämper and Halliday, 1998; White et al., 2000; Weis et al., 2005; Hf: Blichert-Toft et al., 1997; Vervoort and Blichert-Toft, 1999] has provided substantial improvement over the precision achieved by conventional thermal ionization mass spectrometry (TIMS) (ppm vs. ‰ range). In recent years, numerous studies have significantly increased the trace element and isotopic  94  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  datasets for other parts of the Kerguelen large igneous province (the Cretaceous Kerguelen Plateau, after ODP Leg 183; e.g., Frey et al. [2000a]; Neal et al. [2002]; Weis and Frey [2002]; Ingle et al. [2003]; Cenozoic flood basalts on the Kerguelen Archipelago; e.g., Doucet et al. [2005]), Amsterdam–St. Paul hotspot [e.g., Doucet et al., 2004; Graham et al., 2006; Nicolaysen et al., 2007] and Indian Ridges [e.g., Mahoney et al., 2002; Escrig et al., 2004; Hanan et al., 2004; Meyzen et al., 2005]. These results have lead to a better understanding and new interpretations of the genesis of Kerguelen plume and Indian MORB compositions. An “enriched” component with continental geochemical affinities has long been recognized amongst oceanic basalts from the Indian and South Atlantic oceans [Dupré and Allègre, 1983]. The nature of this geographically widespread isotopic domain, characterized by high 87Sr/86Sr, high 207Pb/204Pb and 208Pb /204Pb for a given 206Pb/204Pb and termed Dupal [Hart, 1984], is not well understood. Based on the coincidence of the Dupal maxima with a low seismic velocity zone at the core-mantle boundary, a deep origin for the Dupal anomaly has been advocated [e.g., Castillo, 1988; Wang and Wen, 2004; Wen, 2006]. According to this hypothesis, the surfacing of the Dupal anomaly through the upwelling of long-lived mantle plumes, like the Kerguelen mantle plume [Weis and Frey, 1991; Weis et al., 1991], is responsible for the contamination of the Indian sub-oceanic asthenosphere [Storey et al., 1989; Barling et al., 1994]. In contrast, the results of recent Sr-Nd-Pb-Hf-Os isotopic studies on the Indian ridges argue against the involvement of recycling of subducted oceanic crust and sediments, and instead favor a shallow origin for this regional anomaly by entrainment of lower continental crust into the Indian upper mantle during the rifting of Gondwana [e.g., Escrig et al., 2004; Hanan et al., 2004; Meyzen et al., 2005]. Regelous et al. [2009] proposed a similar model to explain the Dupal anomaly in the South Atlantic. Basalts from the Cretaceous Kerguelen Plateau and Broken Ridge show geochemical evidence of interaction between the basaltic magmas with continental material [Mahoney et al., 1995; Frey et al., 2002a] and fragments of Indian continental crust have been found in some Kerguelen Plateau basalts from Elan Bank [e.g., Weis et al., 2001; Nicolaysen et al., 2001; Frey et al., 2002a; Ingle et al., 2002]. The existing isotopic results for basalts from the Ninetyeast Ridge  95  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  indicate that they exhibit the distinctive Dupal isotopic signature [Dupré and Allègre, 1983; Hart, 1984; Weis and Frey, 1991], however, continental signatures such as the ones identified in the Cretaceous Kerguelen Plateau have not been encountered in the lavas from the Ninetyeast Ridge nor in those from the Kerguelen Archipelago [e.g., Weis and Frey, 1991; Frey et al., 2000a and b; Doucet et al., 2002]. In this paper, we present new high-precision isotope compositions acquired by MCICP-MS (Pb, Hf) and TIMS (Sr, Nd) on acid-leached samples recovered during ODP Leg 121. This is the first comprehensive study to report Hf isotopic data for the Ninetyeast Ridge, although a few Hf results were reported by Barry et al. [2002; 2003]. In light of the new geochemical results that characterize the southeastern Indian Ocean mantle, we use this new multi-isotopic dataset to: 1) test the current interpretations regarding the mantle sources involved in the formation of the Ninetyeast Ridge; and 2) better discern the interaction between the Kerguelen mantle plume and the Southeast Indian Ridge, and the involvement of the Amsterdam-St. Paul hotspot during this period of evolution of the Indian Ocean basin. Finally, together with the geochemistry available from other parts of the Kerguelen large igneous province that has erupted Dupal-like basalts since ~115 Ma, and other Indian Ocean islands, we evaluate the geochemistry of the Ninetyeast Ridge to establish the source and age of the Dupal anomaly.  4.3. Geographic and Geological Setting The Ninetyeast Ridge is a major submarine aseismic volcanic ridge, located in the eastern Indian Ocean along the 90˚E meridian (Figure 4.1). It is ~5000 km long, extending from ~17˚N, where it is buried beneath a ~5 km pile of sediments of the Bengal Fan [e.g., Mukhopadhyay and Krishna, 1995], to 34˚S, where it intersects with Broken Ridge [e.g., Royer et al., 1991; Mukhopadhyay and Krishna, 1995; Krishna et al., 2001]. The Ninetyeast Ridge rises to a maximum height of ~3.5 km above the surrounding ocean floor (Figure 4.2) and has an average width of 200 km, except for a ~700 km segment south of ~7˚S, where it is ~100 km wide [e.g., Peirce et al., 1989; Royer et al., 1991]. Near 15˚S, a circular  96  a  Site 758  Depth (mbsl)  0  Site 757  5º23.049’N  N  Site 756  17º01.458’S  27º21.288’S  S  sea level  1000 2000 3000 4000 5000 10  200 Km  7.5  5  2.5  0º  -2.5  -5  -7.5  -10  -12.5  -15  -17.5  -20  -22.5  -25  -27.5  -30  -32.5  -35  Latitude (ºN) 0  Depth (mbsf)  100 200  Site 758  sea floor  Site 757  Holocene to middle Miocene nannofossil ooze  Site 756 Pleistocene to lower Eocene nannofossil ooze  Basaltic Basement  300  middle Miocene to lower Maastrichtian chalk  upper Paleocene volcaniclastics  400  Campanian calcareous clay  Basaltic Basement  500 600  Campanian tuff  Pleistocene to upper Eocene nannofossil ooze  139 m  372.8 m  498.9 m  Basaltic Basement  700 800  Figure 4.2. (a) Topographic profile along the Ninetyeast Ridge (north–south). Vertical exaggeration = 171x. Profile drawn using GeoMapApp: http://www.geomapapp.org. (b) Schematic summary of the core stratigraphy with depth (in meters below sea floor – mbsf) at Sites 758, 757, and 756 drilled during ODP Leg 121 [Shipboard Scientific Party, 1989]. 97  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  b  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  topographic rise with a radius of ~150 km (Osborn Knoll) stands out to the west of the ridge. This structure is believed to be the result of seafloor spreading [Krishna et al., 1995]. Along its length, the ridge can be divided into distinct morphological segments that reflect its emplacement within the complex tectonic evolution of the Indian Ocean basin [Royer et al., 1991; Krishna et al., 1995, 1999] and the tectonic forces that the ridge has been subjected to [e.g., Subrahmanyam et al., 2008]. North of 2.5˚S, the ridge is formed by a wide series of en echelon blocks separated by areas of abyssal seafloor (Figure 4.1), and is interpreted to have been generated by intraplate volcanism on the Indian plate [Royer et al., 1991]. From 2.5˚S to 15˚S, the ridge becomes narrow, linear and continuous (Figure 4.1), reflecting its emplacement at the transform boundary between the Antarctic (or a short-lived microplate) and Australian plates [Royer et al., 1991]. South of ~15˚S, the ridge again becomes wider, comprised by a slightly less linear succession of shallow volcanic edifices (Figure 4.1), interpreted to have been emplaced near the edge of the Indian plate [Royer et al., 1991]. In addition to these three segments, Krishna et al. [1999] considered another segment, from 11˚S to 17˚S, which, based on the paleomagnetic anomalies and continuity of the ridge at those latitudes, they interpreted as resulting from on-axis volcanism. The Ninetyeast Ridge separates two distinct domains of the Indian Ocean basin (Figure 4.1): the Central Indian Basin to the west, where seafloor ages decrease from north to south, and the Wharton Basin to the east, where seafloor ages decrease from south to north [Duncan, 1978; Royer et al., 1991]. Paleomagnetic, paleontological and radiometric studies on samples recovered during DSDP and ODP cruises, indicate that the Ninetyeast Ridge formed at paleolatitudes near 50˚S (drill Sites 217 to 757) and 43˚S (drill Site 756) [Klootwijk et al., 1991], and that the basement rocks become systematically older in a northward direction, from ~38 Ma at DSDP Site 254 (~30.97˚S) to ~82 Ma at ODP Site 758 (~5.38˚S) [Duncan, 1978; 1991; Royer et al., 1991]. These ages are roughly contemporaneous with those of the adjacent Central Indian basin seafloor, indicating emplacement on very young oceanic crust at/or close to a spreading centre [Royer et al., 1991]. The slight obliquity of the Ninetyeast Ridge relative to the fracture zone pattern of the  98  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  Indian Basin suggests that the location of the ridge was not controlled primarily by the location of the fracture zones [Royer et al., 1991].  4.4. Previous Petrologic and Geochemical Results Petrologic studies on the samples recovered during DSDP and ODP cruises indicate that most of the Ninetyeast Ridge basement rocks consist of enriched tholeiitic basalts and few intermediate differentiated rocks (oceanic andesites at Site 214), affected by variable degrees of post-magmatic, low-temperature alteration [e.g., Thompson et al., 1974; Frey and Sung, 1974; Frey et al., 1991; Saunders et al., 1991; Frey and Weis, 1995]. The effects of late-stage alteration, arising from alkali mobility are strongly reflected on the anomalously high total alkali (Na2O+K2O) content of some ODP Site 757 and 756 samples, as well as on their trace element abundances (Figures 4.3 and 4.4). Although these samples lie in the alkalic field in Figure 4.3, they are in fact altered tholeiitic basalts (see Frey et al. [1991] for more detailed discussion). The trachytic groundmass texture of some Ninetyeast Ridge lavas (DSDP sites) suggests they reached thermal and chemical equilibrium in relatively shallow magma chambers [Thompson et al., 1974; Ludden et al., 1980]. The high abundance of vesicles and cavities in most Ninetyeast Ridge basalts is consistent with eruption in a subaerial environment or at shallow water depths. The exception is Site 758, where all samples are pillow and massive basalts with quenched textures, denoting a submarine eruption environment [Shipboard Scientific Party, 1989; Frey et al., 1991; Saunders et al., 1991]. Although all basalts recovered from the Ninetyeast Ridge basement have been moderately to highly altered (H2O+ contents typically > 1 wt%) in a low-temperature environment under both reducing and oxidizing conditions, many of their geochemical characteristics reflect the original magma compositions [Thompson et al., 1974; Frey et al., 1991; Saunders et al., 1991]. Trace element and isotope compositions of the Ninetyeast Ridge basalts are intermediate between Indian MORB and Indian Ocean island basalts (Kerguelen Archipelago, at ~55˚S, Amsterdam and St. Paul islands, at ~38˚S) [e.g., Hekinian, 1974; Frey and Sung, 1974; Frey et al., 1991; Saunders et al., 1991; Weis and Frey, 1991; Weis et al., 1992; Frey and Weis, 1995]. Together with the large volumes of  99  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  10  Phonotephrite  Foidite  Na2O + K2O (wt %)  8  Basaltic trachyandesite Tephrite or  6  Trachyandesite  Basanite  lic  Trachybasalt  ka Al  4  2  itic  Th  i ole  Andesite Picrobasalt  Basaltic andesite  Basalt  Site 756 Site 757  0 40  Site 758  45  50  55  60  65  SiO2 (wt %) Figure 4.3. Total alkalis versus silica diagram (after LeBas et al. [1986]) showing the compositional range of the Ninetyeast Ridge basalts recovered during ODP Leg 121 at Sites 758, 757, and 756 [Frey et al., 1991]. The tholeiitic-alkalic dividing line is from Macdonald and Katsura (1964). The distinctly higher alkali content of some samples reflects K2O enrichment during postmagmatic alteration and not the true magmatic values of the basalts (see text and discussion of Frey et al. [1991] for details).  100  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  magma involved in the genesis of the Ninetyeast Ridge (~27 x 106 km3, Schubert and Sandwell [1989]), these characteristics are consistent with derivation from enhanced magmatism of enriched composition, such as that associated with a mantle plume source [e.g., Thompson et al., 1974; Ludden et al., 1980; Frey et al., 1991; Weis and Frey, 1991; Saunders et al., 1991; Frey and Weis, 1995; 1996].  4.5. Samples and Methods 4.5.1. Samples Thirty-eight samples of tholeiitic basalts from the Ninetyeast Ridge recovered during ODP Leg 121, previously analyzed and reported by Weis and Frey [1991], were analyzed. These samples encompass basalts representative of the different flow units from ODP drill Sites 756 (n=10), 757 (n=12) and 758 (n=16), and they cover a sampling distance of ~3630 km along the ridge. Each drill site is located on different segments/sections of the ridge (Figure 4.1) and at each site basement was found at different depths (meters below sea floor (mbsf)) reflecting the increasing sediment cover with increasing basement age (Figure 4.2). At Site 758 (~5.23˚N, ~82 Ma) this site, in hole 758A, 178 meters were drilled through basaltic flows, which were first encountered 498.9 mbsf. Twenty-nine flow units were identified; the 16 samples here analyzed are from 14 of those units. At Site 757 (~17.01˚S, ~58 Ma), in hole 757C, basement was encountered 372.8 mbsf and 52 meters of basaltic flows were drilled. Nineteen flow units were identified and the 12 samples here analyzed are from nine of those units. At Site 756 (~27.35˚S, ~43 Ma), in hole 756C, 8.7 meters were drilled through basaltic basement, which was first encountered at 150.2 mbsf. In hole 756D, basement was first encountered 139 mbsf, and 82 meters of basaltic flows were drilled. Two and 14 flow units were identified for each hole, respectively. The 10 samples here analyzed are from nine of those units. A summary of the pertinent geochemical characteristics of these samples is presented in Tables 4.1 and 4.2. Detailed petrographic descriptions can be found in Frey et al. [1991] and their previously analyzed Pb, Sr and Nd isotopic compositions in Weis and Frey [1991].  101  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  Table 4.1. Summary of the Geochemical Characteristics of the Ninetyeast Ridge Basalts Sampled During ODP Leg 121 Sample ID [Core, section, interval (cm)]  Flow Depth within Eruption Ageb core (mbsf) Environment (Ma)  Unita  SiO2 (wt%)  MgO (wt%)  FeOt Na2O K2O Mg#c (wt%) (wt%) (wt%)  A.I.d  Vol.e (wt%)  Zr/Nb  Zr/Y  La/Yb  Site 756 (27º21.253'S; 27º21.288'S) 121-756C-10N-1, 32-36* F1 121-756D-4R-1, 35-39 F1 121-756D-4R-1, 85-89* F2 121-756D-6R-1, 6-10* F4 121-756D-6R-2, 9-13* F5 121-756D-6R-2, 13-17* F5 121-756D-7R-1, 100-104 F6 121-756D-8R-1, 20-23 F7 121-756D-10R-1, 99-103 F12 121-756D-12R-3, 139-143 F14  150.62 139.35 139.85 158.36 159.89 159.93 169.00 177.90 197.99 220.19  subaerial subaerial subaerial subaerial subaerial subaerial subaerial subaerial subaerial subaerial  43 43 43 43 43 43 43 43 43 43  49.24 47.68 47.94 48.60 49.05 48.57 49.22 49.44 49.93 49.21  6.28 4.89 5.53 5.30 6.80 6.92 6.43 6.69 6.71 7.11  11.29 13.12 13.47 12.91 11.16 11.08 9.92 11.26 10.41 10.89  2.79 3.10 2.80 2.82 2.73 2.87 3.75 2.54 2.65 3.00  0.49 0.91 0.51 0.64 0.49 0.23 0.41 0.53 0.52 0.25  0.36 0.27 0.29 0.29 0.38 0.38 0.39 0.37 0.39 0.40  -0.51 0.80 0.00 -0.09 -0.50 -0.44 0.38 -0.79 -0.88 -0.53  1.65 1.65 2.20 3.17 2.68 2.45 2.63 4.12 3.10 1.97  13.33 13.11 13.17 12.57 13.09 12.89 12.46 10.50 12.80 13.22  4.95 5.22 5.37 5.38 4.54 4.25 5.73 4.86 4.58 4.22  3.41 2.95 2.99 3.48 3.19 3.01 3.53 3.84 3.09 2.79  Site 757 (17º01.389'S) 121-757C-9R-1, 105-109* 121-757C-9R-5, 34-38 121-757C-9R-6, 102-105 121-757C-9R-7, 78-82 121-757C-9R-8, 54-59 121-757C-10R-1, 86-90 121-757C-10R-3, 2-6 121-757C-11R-2, 10-14 121-757C-11R-2, 51-56 121-757C-12R-1, 100-104* 121-757C-12R-2, 0-5 121-757C-12R-4, 24-29  F2 F4 F5 F5 F5 F6 F8 F12 F13 F18 F18 F19  383.05 387.11 388.62 389.5 390.64 392.56 394.56 402.95 403.36 412.10 412.45 415.30  subaerial subaerial subaerial subaerial subaerial subaerial subaerial subaerial subaerial subaerial subaerial subaerial  58 58 58 58 58 58 58 58 58 58 58 58  48.76 48.14 48.05 47.64 48.68 48.15 48.20 49.52 50.15 51.80 50.49 51.10  5.88 4.76 5.60 5.14 5.97 5.18 5.28 6.94 5.88 5.18 5.38 5.28  6.14 6.17 6.06 7.32 6.18 5.34 5.25 7.50 8.04 6.85 8.85 8.01  1.93 2.54 1.98 1.82 1.89 2.20 5.15 3.03 4.48 2.80 2.62 4.63  0.14 1.00 1.12 0.64 0.99 1.16 1.67 1.62 1.98 0.77 1.28 2.13  0.49 0.44 0.48 0.41 0.49 0.49 0.50 0.48 0.42 0.43 0.38 0.40  -1.54 0.16 -0.25 -0.74 -0.70 -0.03 3.42 0.76 2.33 -1.17 -0.35 2.28  2.38 4.05 3.62 2.58 2.92 5.70 7.69 7.90 7.15 2.57 2.02 5.13  14.29 13.14 16.25 18.08 13.24 14.17 12.50 12.67 14.75 17.05 16.88 17.01  3.84 4.74 4.77 4.02 4.71 3.95 2.70 4.13 5.00 5.51 5.34 6.32  2.96 3.37 3.34 2.95 2.80 2.83 3.16 3.15 3.09 3.61 3.28 4.09  Site 758 (5º23.049'N) 121-758A-55R-5, 82-86 121-758A-56R-1, 64-68 121-758A-58R-6, 21-25* 121-758A-59R-2, 58-62* 121-758A-60R-1, 122-126 121-758A-62R-1, 130-134 121-758A-62R-3, 80-84 121-758A-65R-1, 51-55 121-758A-67R-4, 31-35 121-758A-69R-3, 53-57 121-758A-70R-1, 71-75 121-758A-70R-2, 129-133 121-758A-71R-1, 127-131 121-758A-71R-3, 64-68 121-758A-73R-1, 82-86 121-758A-73R-4, 105-109  F1 F1 F2 F2 F3 F4 F5 F10 F14 F17 F18 F19 F20 F22 F27 F29  505.42 509.04 534.42 538.49 547.12 565.60 568.01 593.21 616.25 633.39 640.11 642.19 649.87 652.20 668.42 673.08  submarine submarine submarine submarine submarine submarine submarine submarine submarine submarine submarine submarine submarine submarine submarine submarine  82 82 82 82 82 82 82 82 82 82 82 82 82 82 82 82  49.52 49.49 49.97 49.61 49.85 48.75 49.35 49.81 49.88 49.11 49.27 49.49 49.64 49.88 49.90 49.74  7.96 8.18 8.08 8.39 9.00 8.15 9.43 9.11 9.51 9.60 9.60 10.21 9.57 9.26 8.94 8.92  9.49 9.68 14.65 11.27 11.81 7.38 10.98 10.36 10.76 10.14 10.07 10.02 9.67 9.95 9.67 10.78  2.14 2.37 2.78 2.36 2.60 2.01 2.89 2.81 2.47 1.97 2.82 2.36 2.50 2.12 2.13 2.24  0.08 0.12 0.15 0.14 0.09 0.11 0.17 0.10 0.07 0.11 0.09 0.08 0.13 0.05 0.06 0.11  0.46 0.46 0.36 0.43 0.43 0.52 0.46 0.47 0.47 0.49 0.49 0.50 0.50 0.48 0.48 0.45  -1.67 -1.40 -1.13 -1.43 -1.33 -1.48 -0.77 -1.09 -1.49 -1.67 -0.89 -1.44 -1.31 -1.86 -1.85 -1.63  2.06 2.39 4.48 3.29 3.82 3.18 5.38 5.76 5.12 4.35 6.15 6.16 5.24 4.15 5.02 3.92  17.78 18.39 15.59 15.44 14.81 14.71 16.48 15.18 15.69 17.22 15.00 13.08 16.19 15.00 -  2.68 3.33 3.35 2.97 3.42 3.47 3.55 4.40 3.96 2.78 4.24 3.58 3.82 3.32 -  1.42 1.45 1.79 1.66 2.11 1.90 2.37 2.27 1.91 1.77 2.23 2.02 2.06 1.75 1.61 7.45  Defined on-board on the basis of macroscopic and thin-section criteria [Peirce et al., 1989]. Average 40Ar/39Ar ages for each site [Duncan, 1991]. Mg# is molar Mg/(Mg + Fe2+). d A.I. is Alkalinity Index [= (total alkali - 0.37*SiO2 + 14.43)] and represents a measure of the deviation of a sample from the alkalic-tholeiitc boundary defined by MacDonald and Katsura [1964]. Positive values are most probably due to the high alteration degree of those samples and do not reflect the basalt type. See Frey et al. [1991] for details and discussion. e Vol. = Sum of volatiles (H2O+, H2O-, CO2). *samples for which the U content is too low and was recalculated based on a model Th/U = 4 prior to calculation of the initial Pb ratios (see text for details). Major and trace element geochemical data is from Frey et al. [1991]. a  b  c  102  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  Table 4.2. Parent-Daughter Concentrations and Ratios of the Ninetyeast Ridge Basalts Sampled During ODP Leg 121 Sample ID [ Core, section, interval (cm)]  Flow  Th (ppm)  U (ppm)  Pb (ppm)  Th/U  Rb (ppm)  Sr (ppm)  Rb/Sr  Sm (ppm)  Nd (ppm)  Sm/Nd  Lu (ppm)  Hf (ppm)  Lu/Hf  F1 F1 F2 F4 F5 F5 F6 F7 F12 F14  1.26 1.17 1.33 1.70 0.71 0.70 1.01 0.75 0.66 0.68  0.10 0.10 0.08 0.09 0.09 0.38 0.22 0.24 0.17  0.59 0.56 0.40 0.41 0.37 0.80 0.93 0.86 0.79  12.6 13.3 21.3 7.9 7.8 2.7 3.4 2.8 4.0  6.50 10.0 15.0 19.0 33.0 1.10 2.90 13.0 16.0 1.90  188 195 181 166 171 174 200 153 156 163  0.03 0.05 0.08 0.11 0.19 0.01 0.01 0.08 0.10 0.01  4.66 4.81 5.10 5.40 3.97 4.18 4.67 3.93 3.89 4.17  17.20 16.90 16.80 18.70 12.90 14.00 16.80 14.50 13.50 13.40  0.27 0.28 0.30 0.29 0.31 0.30 0.28 0.27 0.29 0.31  0.39 0.48 0.46 0.46 0.37 0.42 0.44 0.38 0.34 0.40  3.69 4.18 4.08 4.23 2.98 3.12 3.83 2.98 2.96 2.91  0.11 0.11 0.11 0.11 0.12 0.13 0.11 0.13 0.11 0.14  F2 F4 F5 F5 F5 F6 F8 F12 F13 F18 F18 F19  0.33 0.24 0.28 0.43 0.24 0.09 0.25 0.35 0.34 1.53 1.48 1.89  0.07 0.13 0.16 0.15 0.11 0.11 0.13 0.19 0.18 0.30 0.45 0.47  0.14 0.37 0.53 0.38 0.34 0.28 0.29 0.49 0.46 0.37 0.79 1.35  4.7 1.8 1.8 2.9 2.2 0.8 1.9 1.8 1.9 5.1 3.3 4.0  1.40 11.0 35.0 18.0 22.0 17.0 17.0 19.0 20.0 7.80 20.0 14.0  176 313 215 181 184 247 559 324 292 164 160 150  0.01 0.04 0.16 0.10 0.12 0.07 0.03 0.06 0.07 0.05 0.13 0.09  1.57 1.35 1.70 1.50 1.44 1.07 1.60 1.82 1.67 3.77 3.70 4.30  5.70 5.20 4.60 5.30 5.10 4.00 5.90 6.10 6.00 14.40 14.30 16.00  0.28 0.26 0.37 0.28 0.28 0.27 0.27 0.30 0.28 0.26 0.26 0.27  0.18 0.15 0.17 0.19 0.17 0.14 0.18 0.21 0.19 0.46 0.51 0.48  1.14 0.98 1.07 1.12 1.14 0.71 0.85 1.35 1.41 3.67 3.45 4.52  0.16 0.15 0.16 0.17 0.15 0.20 0.21 0.16 0.13 0.13 0.15 0.11  F1 F1 F2 F2 F3 F4 F5 F10 F14 F17 F18 F19 F20 F22 F27 F29  0.17 0.23 0.73 0.26 0.43 0.23 0.27 0.33 0.19 0.28 0.22 0.25 0.27 0.33 0.41 0.26  0.07 0.08 0.09 0.04 0.14 0.08 0.20 0.12 0.13 0.10 0.12 0.12 0.11 0.11 0.11 0.11  0.13 0.21 0.97 0.20 0.32 0.56 0.51 0.30 0.75 0.21 0.41 0.33 0.30 0.37 0.43 1.46  2.4 2.9 8.1 6.5 3.1 2.9 1.4 2.8 1.5 2.8 1.8 2.1 2.5 3.0 3.7 2.4  0.40 1.40 1.10 2.20 0.60 0.90 0.70 0.40 0.20 0.70 0.10 0.50 0.80 0.10 0.00 1.50  122 127 143 127 131 139 157 157 144 114 140 125 130 122 121 113  0.00 0.01 0.01 0.02 0.00 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.01 0.00 0.00 0.01  1.79 1.90 4.22 2.44 2.55 1.66 3.40 2.61 2.74 2.40 2.33 2.20 2.10 2.30 2.30 2.50  5.00 5.30 14.20 7.66 9.00 5.50 10.90 9.00 10.40 8.30 8.30 7.80 7.90 7.30 7.00 8.30  0.36 0.36 0.30 0.32 0.28 0.30 0.31 0.29 0.26 0.29 0.28 0.28 0.27 0.32 0.33 0.30  0.28 0.28 0.60 0.36 0.35 0.23 0.37 0.27 0.37 0.33 0.31 0.28 0.27 0.32 0.37 0.36  1.24 1.40 3.03 1.77 1.72 1.25 2.31 2.18 2.40 1.49 1.83 1.68 1.56 1.74 1.81 1.74  0.23 0.20 0.20 0.20 0.20 0.18 0.16 0.12 0.15 0.22 0.17 0.17 0.17 0.18 0.20 0.21  Unit a  Site 756 (27º21.253'S; 27º21.288'S) 121-756C-10N-1, 32-36* 121-756D-4R-1, 35-39 121-756D-4R-1, 85-89* 121-756D-6R-1, 6-10* 121-756D-6R-2, 9-13* 121-756D-6R-2, 13-17* 121-756D-7R-1, 100-104 121-756D-8R-1, 20-23 121-756D-10R-1, 99-103 121-756D-12R-3, 139-143 Site 757 (17º01.389'S) 121-757C-9R-1, 105-109* 121-757C-9R-5, 34-38 121-757C-9R-6, 102-105 121-757C-9R-7, 78-82 121-757C-9R-8, 54-59 121-757C-10R-1, 86-90 121-757C-10R-3, 2-6 121-757C-11R-2, 10-14 121-757C-11R-2, 51-56 121-757C-12R-1, 100-104* 121-757C-12R-2, 0-5 121-757C-12R-4, 24-29 Site 758 (5º23.049'N) 121-758A-55R-5, 82-86 121-758A-56R-1, 64-68 121-758A-58R-6, 21-25* 121-758A-59R-2, 58-62* 121-758A-60R-1, 122-126 121-758A-62R-1, 130-134 121-758A-62R-3, 80-84 121-758A-65R-1, 51-55 121-758A-67R-4, 31-35 121-758A-69R-3, 53-57 121-758A-70R-1, 71-75 121-758A-70R-2, 129-133 121-758A-71R-1, 127-131 121-758A-71R-3, 64-68 121-758A-73R-1, 82-86 121-758A-73R-4, 105-109  Defined on-board on the basis of macroscopic and thin-section criteria [Peirce et al., 1989]. *samples for which the U content is too low and was recalculated based on a model Th/U = 4 prior to calculation of the initial Pb ratios (see text for details). Trace element data is from Frey et al. [1991].  a  103  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  Basalt/Primitive Mantle  1000  Site 756  100  10  1  .1 1000  Cs Rb Ba Th U Nb Ta K La Ce Pb Pr Sr P Nd Zr Sm Eu Ti Dy Y Yb Lu  Basalt/Primitive Mantle  Site 757 100  10  1  .1 1000  Cs Rb Ba Th U Nb Ta K La Ce Pb Pr Sr P Nd Zr Sm Eu Ti Dy Y Yb Lu  Basalt/Primitive Mantle  Site 758 100  10  1  .1  Cs Rb Ba Th U Nb Ta K La Ce Pb Pr Sr P Nd Zr Sm Eu Ti Dy Y Yb Lu  Figure 4.4. Extended primitive mantle-normalized incompatible trace element abundances of the Ninetyeast Ridge basalts collected during ODP Leg 121. Data is from Frey et al. [1991]. Normalizing values are from McDonough and Sun [1995].  104  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  4.5.1.1. Summary of Alteration Features and Secondary Mineralogy of Samples The majority of the basalts recovered at Site 756 are aphyric to sparsely plagioclasephyric, and all have undergone some extent of alteration that led to the pervasive replacement of groundmass by clay minerals and infilling of many of the vesicles (summarized from Shipboard Scientific Party [1989]). The alteration degree is moderate, ranging between 20 and 40% volume of alteration phases. Plagioclase phenocrysts are commonly slightly altered to a pale green clay mineral and the rare olivine phenocrysts are completely replaced by green smectite-vermiculite or red iddingsite. In the groundmass, olive is altered to iddingsite, smectite and vermiculite. Two styles of alteration are found in these basalts [Shipboard Scientific Party, 1989; Frey et al., 1991; Saunders et al., 1991]. The earlier alteration mineral assemblage is bluish to greenish grey in colour and is mainly constituted by saponitic smectites. This non-oxidative alteration resulted in the loss of K and Rb from the bulk rock, which is characteristic of subaerial alteration of basalt and submarine alteration at >150°C. The later, oxidative, alteration mineral assemblage is brown to orange grey in colour and is constituted by calcite, saponite-glauconite and Fe-Mn oxides, which typically affects the uppermost basalts of the oceanic crust. Zeolite minerals are not found in basalts from Site 756. All basalts recovered at Site 757 are moderately (~20 vol%) to highly (40-50 vol%) plagioclase-phyric, vesicular basalts [Shipboard Scientific Party, 1989]. The plagioclase phenocrysts are typically zoned. Alteration to potassium feldspar in several samples reflects addition of K during postmagmatic alteration. Other phenocryst phases include clinopyroxene and olivine, the latter being completely replaced by green smectite with chlorite and iddingsite. The degree of alteration of these basalts varies from moderate to high (up to 80%), which resulted in pervasive replacement of the groundmass by green or brown smectites/chlorites, and Fe-oxides/hydroxides. Veins, cavities and vesicles are filled by calcite, smectites, Fe-oxides/hydroxides, opal or chalcedony, and zeolites (natrolite and analcite), which are more abundant in the lower flow units [Shipboard Scientific Party, 1989; Frey et al., 1991]. The presence of zeolite minerals and chalcedony is indicative of  105  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  hydrothermal alteration of these basalts. The occurrence of oxidative alteration, as observed in Site 756 basalts, is restricted to the uppermost flow units. Basalts from Site 758 are predominantly aphyric to sparcely plagioclase-phyric, and all are highly altered [Shipboard Scientific Party, 1989; Frey et al., 1991; Saunders et al., 1991]. The groundmass is pervasively replaced by brown and green chlorite-smectites, whereas the phenocrysts, when present, are only slightly affected. Black, brown and green smectites, chlorite-smectites, calcite, quartz and ankerite are part of the mineral assemblage filling the vesicles and fractures. In contrast to the basalts from the previous sites, late stage oxidative alteration is not evident in the Site 758 basalts.  4.5.2. Analytical Techniques Whole-rock powders of ODP Leg 121 basalts were analyzed for Pb, Hf, Sr, and Nd isotopic compositions at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia. To remove the alteration phases and obtain reproducible Pb isotopic compositions of oceanic basalts [e.g., Hanano et al., 2009], all sample powders were thoroughly acid-leached with 6 M HCl prior to digestion and isotopic analysis, following the sequential acid leaching procedure described in Weis et al. [2005] and in Nobre Silva et al. [2009; 2010]. The leached sample powders were digested in a mixture of concentrated sub-boiled HF and HNO3 acids in sealed Teflon® vessels and then processed through a sequence of chromatographic ion-exchange columns for Pb, Sr, Nd and Hf purification. For all samples, the Pb, Sr, Nd and Hf isotopic compositions were determined on a single dissolution of each sample, using methods similar to those described by Weis et al. [2006; 2007] and the results are reported in Tables 4.4 and 4.5. Pb and Hf isotopic compositions were determined on a Nu Plasma (Nu Instruments Ltd, UK) multiple collector inductively coupled plasma mass spectrometer (MC-ICP-MS), under dry plasma conditions using a membrane desolvator (Nu DSN100) for sample introduction [Weis et al., 2006; 2007]. All analyses were obtained in static multi-collection mode, with interference corrections for  204  Hg on  204  Pb,  176  Lu and  176  Yb on  176  Hf, and  174  Yb  106  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  on  174  Hf. All sample and standard solutions were prepared fresh for each analytical session  and sample analysis followed a modified sample-standard bracketing procedure in which the standard solution (NIST SRM 981 for the Pb analyses, JMC 475 for the Hf analyses) was run after every two samples, to monitor for any systematic in-run drift of the standard value. Pb isotopic ratios were corrected online for instrumental mass fractionation using a Specpure® Tl standard solution with a  205  Tl/203Tl = 2.3885. All Pb sample solutions were  analyzed with similar [Pb]/[Tl] (≈4) as the SRM 981 Pb standard solution, to ensure that sample and standard solutions were matrix-matched with respect to Tl. To accomplish this, the Pb content of a small aliquot of each solution was determined by HR-ICP-MS, on an ELEMENT2 (Thermo Finnigan, Germany) prior to isotopic analysis. Samples were run with an optimal  208  Pb ion beam of 8V and no less than 2V, to limit the significance of the  interference on  204  204  Hg  Pb and ensure a better analytical precision altogether. The fractionation-  corrected Pb isotopic ratios were then normalized off-line to the SRM 981 triple-spike values (206Pb/204Pb = 16.9405 ± 15,  207  Pb/204Pb = 15.4963 ± 16, and  208  Pb/204Pb = 36.7219 ± 44;  Abouchami et al., [2000]), using the ln-ln method as described in White et al. [2000] and Albarède et al. [2004]. During the period of analysis, the SRM 981 Pb standard yielded mean values ± 2SD of 208  206  Pb/204Pb = 16.9414 ± 0.0067,  207  Pb/204Pb = 15.4962 ± 0.0032, and  Pb/204Pb = 36.7121 ± 0.0104 (n = 30), which are within 2SD of the triple spike values  [Abouchami et al., 2000]. Hf isotopic ratios were corrected for instrumental mass fractionation by exponentially normalizing to  179  Hf/177Hf = 0.7325 [Patchett and Tatsumoto, 1984]. The sample results  were then normalized to the  176  Hf/177Hf value of 0.282160 of Vervoort and Blichert-Toft  [1999] using the daily average of the JMC 475 Hf standard. The JMC 475 Hf standard analyzed over the period of analysis gave an average value of  176  Hf/177Hf = 0.282161 ±  0.000010 (n = 112). Two new Hawaiian rock samples used as in-house reference materials, Kil-93 and Ko’olau, collected and made available by Marc Norman (ANU), were processed together  107  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  with the sample set. Kil-93 is a quenched sample from the Pu'u O'o 10 May 1993 eruption. ICP-MS trace element concentrations for this sample have been determined by Eggins et al. [1997] and major element compositions have been reported by Garcia et al. [2000]. Ko'olau is a tholeiitic basalt from a flow near the base of the Makapu'u section on Oahu. Although no geochemical data has been published for this sample, it should be comparable to flows 1, 2, 3 of Frey et al. [1994]. In Table 3 we report the first Pb, Sr, Nd, Hf isotopic compositions for these samples. These values are in good agreement with the values acquired over three years at the PCIGR for these samples (Table 4.3). All Sr and Nd isotopic ratios were measured on a TRITON (Thermo Finnigan) thermal ionization mass spectrometer (TIMS) in static mode with relay matrix rotation on single Ta (Sr) or double Re-Ta (Nd) filaments, respectively. Sr and Nd isotopic compositions were corrected for mass fractionation using  86  Sr/86Sr = 0.1194 and  146  Nd/144Nd = 0.7219  using an exponential law. The data was then normalized using the average of the corresponding standard (NIST SRM 987 for Sr and La Jolla for Nd) in the barrel, relative to the values of  87  Sr/86Sr = 0.710248 and  143  Nd/144Nd = 0.511858 [Weis et al., 2006]. During  the course of these analyses, the average value of the SRM 987 Sr standard was 0.710249 ± 0.000015 (n = 23) and the La Jolla Nd standard was 0.511851 ± 0.000003 (n = 28). For more accurate comparison between our data and the literature isotopic data shown and discussed here, all literature data have been renormalized to the respective accepted standard values used in this study [Vervoort and Blichert-Toft, 1999; Abouchami et al., 2000; Weis et al., 2006].  4.5.3. Age-Correction of the Isotopic Ratios of the Ninetyeast Ridge Samples To account for in-situ decay of  87  Rb,  147  Sm,  176  Lu,  238-235  U and  232  Th since  crystallization, age corrections were applied to the measured (present-day) Sr, Nd, Hf, and Pb isotopic ratios from Sites 756 (43 Ma), 757 (58 Ma) and 758 (82 Ma) (ages from Duncan [1991]). For Rb, Sm and Lu, we consider the elemental abundances measured on unleached  108  Table 4.3. Pb-Sr-Nd-Hf Isotopic Compositions by MC-ICP-MS and TIMS of Hawaiian Basalt Reference Materials Analyzed in this Study Reference Materials  5 9  % Weight Loss 42.06 53.98 41.12  Pb/204Pb  206  18.4107 17.8396 17.8413  Error (2SE) 0.0011 0.0007 0.0008  Pb/204Pb  207  15.4749 15.4364 15.4384  Error (2SE)  Pb/204Pb  208  Error (2SE)  87  Sr/86Sr  Error (2σ)  Nd/144Nd  143  Error (2σ)  Hf/177Hf  176  Error (2SE)  0.0010 0.0006 0.0008  38.0712 37.7458 37.7517  0.0029 0.0018 0.0022  0.703590 0.704103 0.704082  8 8 8  0.512976 0.512759 –  6 6 –  0.283108 0.282961 0.282953  5 4 5  0.0036 0.0028  38.0667 37.7451  0.0094 0.0096  0.703589 0.704101  15 22  0.512975 0.512758  20 9  0.283104 0.282959  7 19  Average Isotopic Compositions and 2SD Over Three Years (2007 - 2010) at the PCIGR Kil-93 (n = 10) Ko'olau (n = 6)  10 10  35.32 50.35  18.4083 17.8405  0.0050 0.0058  15.4737 15.4364  109  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  Kil-93 Ko'olau Ko'olau dup.  Acid Leaching Steps 10  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  powders to be more representative of the primary elemental concentrations than those measured on leached samples [Nobre Silva et al., 2010]. For the Rb-Sr, Sm-Nd and Lu-Hf isotope systems, the elemental abundances measured on the unleached powders [Frey et al., 1991] were used for the age-correction calculations, as the parent:daughter concentration ratios for these samples are typical of values expected for ocean island basalts. The U, Th, and Pb elemental concentrations in the Ninetyeast Ridge basalts have been disturbed by the extensive post-eruptive alteration [e.g., Frey et al., 1991; Saunders et al., 1991]. The measured ratios of some samples are very high (up to 21.3, see Tables 4.1 and 4.2), and most likely result from U loss during hydrothermal alteration, as other trace element ratios not involving U are less affected (Figure 4.5). For this reason, age-correction of Pb isotope ratios using the U abundances from unleached sample powders of these samples is likely to be unreliable. This assumption is supported by the tight linear trends of measured 208Pb/206Pb for basalts from each site (Figure 4.6a and 4.6b), which become widely dispersed after correction to initial 208Pb/206Pb values (Figure 4.6c and 4.6d). For the samples exhibiting high Th/U (samples indicated by * in Tables 4.1, 4.2 and 4.4, and surrounded by dashed field in Figures 4.5c, 4.5d, 4.6c and 4.6d), we estimated U abundances assuming that Th/U = 4, based on the Kerguelen and ocean island basalts average values for this elemental ratio [O’Nions and McKenzie, 1993; Weis et al., 1998; Doucet et al., 2002]. The subsequent age-corrected Pb isotopic data is more coherent, as outlying samples now plot within the main trends of each site (Figures 4.6e and 4.6f). Previous studies where age-corrections were made using the measured U concentrations showed age progressions in initial Pb isotope ratios between the three sites [Weis et al., 1991; Class et al., 1993]. These variations with age are not seen in the data in this study. To properly compare the new (MC-ICP-MS) and old (TIMS) data sets, we applied the same rationale to the TIMS Pb isotope data (i.e., recalculated the initial Pb isotopic ratios for the samples exhibiting high Th/U; Figure 4.6).  110  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  Th (ppm)  2.0  Th (ppm)  2.0  Site 756  1.8  Lower Site 757  1.4  1.0  0.8  0.8  0.6  0.6  0.4  0.4  a  0.2 0  50  U (ppm)  100  150  Zr (ppm)  200  250  0.0  0.4  0.4  0.3  0.3  0.1  Nb (ppm)  100  150  Zr (ppm)  4  6  8  La (ppm)  10  200  12  250  0.0  14  U loss during alteration  0.1  c 50  2  0.2  U loss during alteration  0  0  U (ppm)  0.5  0.2  b  0.2  0.5  0.0  Site 758  1.2  1.0  0.0  Site 757  Lower Site 757  1.4  Site 758  1.2  121-756D-8R-1,20-23  1.6  Site 757  1.6  Site 756  1.8  121-756D-8R-1,20-23  d 0  2  4  Zr (ppm)  6  8  La (ppm)  10  12  14  250  16.0 14.0  200  12.0 10.0  150  8.0 100  6.0 4.0  0.0  50  e  2.0 0  50  100  150  Zr (ppm)  200  250  0  f 0  2  4  6  8  La (ppm)  10  12  14  Figure 4.5. Incompatible trace element concentration variations for the ODP Leg 121 basalts. (a) Th versus Zr; (b) Th versus La; (c) U versus Zr; (d) U versus La; (e) Nb versus Zr; (f) Zr versus La. White-, grey-, and black-filled diamonds represent basalts from Site 756, 757 and 758 respectively. Grey circles with black outlines represent basalts from the lower flow units of Site 757, which are characterized by higher trace element concentrations. The white circle with black outline represents a basalt from Site 756 that has distinctive geochemical compositions in comparison to other basalts from the same drill site. Note the significant loss of U due to alteration of some of the Ninetyeast Ridge basalts relative to other alteration resistant incompatible elements (Th, Nb, Zr, La). Samples enclosed by the dashed fields (diagrams c and d) correspond to the samples that exhibit high Th/U (samples indicated by * in Tables 1 and 2) and for which age-correction calculations were performed using the estimated U concentrations based on Th/U = 4 (see text for details).  111  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  4.6. Results 4.6.1. Site 758 (~5.38˚N; ~82 Ma) Samples from the 178 m of basaltic basement penetrated in drill hole 758A yield the least radiogenic and the largest variation of Pb isotopic compositions observed within the three Ninetyeast Ridge drill sites (206Pb/204Pbm = 18.326 – 18.686, 18.533;  207  Pb/204Pbm = 15.542 – 15.585,  38.618 –38.974,  207  206  Pb/204Pbi = 18.049 –  Pb/204Pbi = 15.521 – 15.568;  208  Pb/204Pbm =  208  Pb/204Pbi = 38.280 – 38.822; where the subscripts m and i refer to the  measured and initial ratios, respectively) (Table 4.4, Figures 4.6, 4.8 and 4.9). Their Hf isotopic compositions are also the most variable amongst the Ninetyeast Ridge basalts (176Hf/177Hfm = 0.283029 – 0.283132,  176  Hf/177Hfi = 0.28299 – 0.28308) (Table 4.5, Figure  4.7b and 4.7d). In contrast, very little variation is observed in the Sr (87Sr/86Srm = 0.704318 – 0.704502,  87  Sr/86Sri = 0.70431 – 0.70449) and Nd isotopic compositions (143Nd/144Ndm =  0.512854 – 0.512888,  143  Nd/144Ndi = 0.51275 – 0.51278) throughout the basement  stratigraphy of hole 758A (Table 4.5, Figure 4.7a and 4.7c). Based on Pb and Hf isotopic variations, three distinct groups of flows can be identified in hole 758A (Figure 4.8c). The lowermost flows (units 29 to 14) show the least variable Pb isotopic compositions and have some of the least radiogenic Hf isotopic values (units 29 and 19) and the most radiogenic Nd isotope ratios (units 29, 27, and 22) measured within this core (Figure 4.8a). The middle section (flow units 14 to 3) is characterized by a systematic increase of Pb and Sr isotopic ratios upwards with stratigraphic position, whereas Hf and Nd isotopic compositions evolve to less radiogenic compositions. The upper section (flow units 3 to 1) shows larger Pb, Hf and Nd isotopic variations than the underlying flow units (Figure 4.8a). The Sr isotope values increase slightly upwards until flow unit 5, and then decrease in the upper part of the core, toward the average 87Sr/86Sr value (0.70441) for this drill hole. Four samples (121-758A-58R-6, 21-25 cm; 121-758A-55R-5, 82-86 cm; 121-758A56R-1, 64-68 cm; 121-758A-59R-2, 58-62 cm) have considerably lower 208  206  Pb/204Pb and  Pb/204Pb values than the rest of Site 758 core samples (Figures 4.6 and 4.8a). Sample 121-  758A-58R-6, 21-25 cm is highly enriched in Fe2O3, TiO2, P2O5, V, Zn, Y and most of the  112  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  Table 4.4. Pb Isotopic Compositions by MC-ICP-MS from the Ninetyeast Ridge Basalts ODP Leg 121 Sample ID Site 756 121-756C-10N1, 32-36* 121-756C-10N1, 32-36 dup.*a 121-756D-4R1, 35-39 [ag.]b 121-756D-4R1, 35-39 [ag.] dup. 121-756D-4R1, 35-39 [wc]b 121-756D-4R1, 35-39 [wc] dup. 121-756D-4R1, 85-89* 121-756D-4R1, 85-89 rep.*c 121-756D-6R1, 6-10* 121-756D-6R2, 9-13* 121-756D-6R2, 9-13 dup.* 121-756D-6R2, 13-17* 121-756D-7R1, 100-104 121-756D-8R1, 20-23 121-756D-10R1, 99-103 121-756D-12R3, 139-143  Acid Leaching Steps  % Weight Loss  206  Error Pb/204Pbd (2SE)  207  Error Pb/204Pbd (2SE)  208  Error Pb/204Pbd (2SE) (206Pb/204Pb)ie (207Pb/204Pb)ie (208Pb/204Pb)ie  Pb*/204Pb*f  208  17 17 17 19 14 19 15 19 13 13 13 14 17 13 16  59.92 50.23 70.51 71.86 84.26 80.40 78.17 60.98 71.02 71.81 64.30 82.31 69.48 64.57 71.90  18.6697 18.6704 18.7110 18.7140 18.7186 18.7138 18.6918 18.6922 18.6630 18.7011 18.6972 18.6958 18.6859 18.7232 18.6924 18.6626  0.0007 0.0011 0.0011 0.0009 0.0017 0.0010 0.0016 0.0017 0.0023 0.0019 0.0019 0.0016 0.0016 0.0013 0.0016 0.0024  15.5468 15.5475 15.5534 15.5559 15.5510 15.5529 15.5457 15.5468 15.5444 15.5533 15.5522 15.5506 15.5536 15.5756 15.5481 15.5444  0.0007 0.0010 0.0010 0.0010 0.0015 0.0008 0.0018 0.0014 0.0025 0.0014 0.0016 0.0013 0.0013 0.0013 0.0015 0.0015  38.9793 38.9811 38.9823 38.9887 38.9803 38.9813 38.9691 38.9684 38.9636 39.0452 39.0394 39.0284 38.9931 39.2427 38.9941 38.9423  0.0023 0.0030 0.0025 0.0029 0.0042 0.0022 0.0040 0.0038 0.0078 0.0042 0.0045 0.0039 0.0037 0.0036 0.0043 0.0048  18.4406 18.4412 18.4370 18.4374 18.2072 18.5151 18.5111 18.4926 18.4819 18.6212 18.5726 18.5703  15.5360 15.5368 15.5337 15.5349 15.5230 15.5446 15.5435 15.5411 15.5440 15.5709 15.5425 15.5400  38.6780 38.6797 38.6339 38.6332 38.3640 38.8005 38.7947 38.7611 38.8148 39.1283 38.8858 38.8209  1.0150 1.0151 1.0109 1.0112 1.0098 1.0105 1.0115 1.0114 1.0141 1.0186 1.0184 1.0174 1.0147 1.0372 1.0141 1.0118  68.50 68.32 83.18 75.94 53.17 70.93 65.97 91.61 68.35 83.77 51.97 57.48 69.79 83.96  18.7011 18.6986 18.7749 18.7213 18.7155 18.6941 18.7946 19.1841 18.7584 18.9097 18.8259 18.7548  0.0039 0.0015 0.0058 0.0021 0.0008 0.0076 0.0016 0.0067 0.0045 0.0049 0.0011 0.0015  15.5804 15.5796 15.5965 15.5798 15.5809 15.5785 15.5921 15.6122 15.5898 15.5906 15.5774 15.5687  0.0033 0.0016 0.0049 0.0023 0.0008 0.0051 0.0018 0.0053 0.0037 0.0040 0.0010 0.0012  38.9499 38.9476 38.9921 38.9177 38.9496 38.9295 38.9820 39.1105 39.0155 39.0220 39.0713 39.0489  0.0090 0.0050 0.0117 0.0085 0.0022 0.0166 0.0050 0.0137 0.0094 0.0099 0.0027 0.0033  18.3594 18.3570 18.5709 18.5463 18.4866 18.5065 18.5664 18.9218 18.5331 18.6819 18.2246 18.4240  15.5643 15.5634 15.5869 15.5715 15.5701 15.5696 15.5813 15.5999 15.5792 15.5798 15.5490 15.5531  38.5009 38.4986 38.8683 38.8170 38.7340 38.7951 38.9207 38.9448 38.8792 38.8806 38.2809 38.6913  1.0085 1.0085 1.0051 1.0029 1.0069 1.0071 1.0019 0.9754 1.0093 0.9941 1.0080 1.0132  121-757C-12R2, 0-5 dup. 121-757C-12R4, 24-29  10 11 17 10 9 12 13 13 10 19 9 10 10 10  18.7587 18.8101  0.0014 0.0031  15.5709 15.5744  0.0014 0.0028  39.0466 39.1964  0.0036 0.0073  18.4278 18.6062  15.5553 15.5648  38.6890 38.9284  1.0126 1.0229  Site 758 121-758A-55R5, 82-86 121-758A-56R1, 64-68 121-758A-58R6, 21-25* 121-758A-59R2, 58-62* 121-758A-60R1, 122-126 121-758A-60R1, 122-126 dup. 121-758A-62R1, 130-134 121-758A-62R3, 80-84 121-758A-65R1, 51-55 121-758A-67R4, 31-35 121-758A-69R3, 53-57 121-758A-70R1, 71-75 121-758A-70R2, 129-133 121-758A-71R1, 127-131 121-758A-71R3, 64-68 121-758A-73R1, 82-86 121-758A-73R4, 105-109  10 13 7 10 16 10 13 14 14 18 12 12 10 17 11 13 10  48.48 52.40 62.41 57.72 63.08 56.91 59.76 72.57 63.05 73.96 62.19 59.65 65.54 63.64 42.74 63.29 63.99  18.4879 18.4691 18.3261 18.3307 18.6862 18.6833 18.6421 18.6334 18.5804 18.4967 18.5748 18.5264 18.4821 18.5240 18.4952 18.5197 18.5943  0.0030 0.0017 0.0015 0.0015 0.0017 0.0019 0.0022 0.0042 0.0012 0.0023 0.0032 0.0019 0.0024 0.0023 0.0013 0.0033 0.0031  15.5421 15.5467 15.5689 15.5688 15.5852 15.5840 15.5706 15.5729 15.5635 15.5535 15.5598 15.5620 15.5652 15.5721 15.5659 15.5626 15.5591  0.0027 0.0013 0.0014 0.0014 0.0019 0.0017 0.0021 0.0037 0.0011 0.0018 0.0032 0.0019 0.0026 0.0017 0.0009 0.0025 0.0038  38.7290 38.6945 38.6178 38.6269 38.9741 38.9657 38.9323 38.8938 38.8710 38.7987 38.8016 38.8358 38.7936 38.8112 38.8528 38.7785 38.8064  0.0076 0.0037 0.0044 0.0044 0.0053 0.0039 0.0056 0.0105 0.0027 0.0051 0.0114 0.0058 0.0079 0.0062 0.0026 0.0068 0.0102  18.0485 18.1584 18.1731 18.0664 18.3268 18.3240 18.5250 18.3119 18.2528 18.3551 18.1853 18.2870 18.1850 18.2242 18.2521 18.3106 18.5327  15.5212 15.5319 15.5616 15.5562 15.5681 15.5668 15.5650 15.5575 15.5479 15.5468 15.5412 15.5506 15.5510 15.5579 15.5543 15.5526 15.5561  38.3788 38.4014 38.4170 38.2799 38.6119 38.6036 38.8217 38.7514 38.5754 38.7308 38.4437 38.6918 38.5905 38.5697 38.6135 38.5228 38.7586  1.0078 1.0062 1.0136 1.0141 1.0127 1.0121 1.0130 1.0098 1.0131 1.0145 1.0062 1.0152 1.0155 1.0128 1.0205 1.0098 1.0046  Site 757 121-757C-9R1, 105-109* 121-757C-9R1, 105-109 dup.* 121-757C-9R5, 34-38 121-757C-9R6, 102-105 121-757C-9R7, 78-82 121-757C-9R8, 54-59 121-757C-10R1, 86-90 121-757C-10R3, 2-6 121-757C-11R2, 10-14 121-757C-11R2, 51-56 121-757C-12R1, 100-104* 121-757C-12R2, 0-5  Pb isotopic ratios normalized to the SRM 981 Pb standard solution TS-TIMS reference values of Abouchami et al. [2000]; the 2σ error is the absolute error value of the individual sample analysis (internal error). Initial isotopic ratios calculated for leached samples on the basis of the 40Ar/39Ar eruption age measured for basalts from Sites 756 (43 Ma), 757 (58 Ma) and 758 (82 Ma) [Duncan, 1991]. c208 Pb*/206Pb* defined as (208Pb/204Pbsample - 29.475)/(206Pb/204Pbsample - 9.037) by Galer and O’Nions [1985]. *samples for which the U content is too low and was recalculated based on a model Th/U = 4 prior to calculation of the initial Pb ratios (see text for details). d dup. = full procedural duplicate analysis of the same starting sample powder. e [ag] = samples crushed in agate mill, [wc] = samples crushed in tungsten-carbide mill. f rep. = replicate analysis of the same sample solution on the MC-ICP-MS. a  b  113  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  208  Pb/204Pbm  39.40  207  MC-ICP-MS Site 756  39.30  Fields =TIMS (Weis and Frey, 1991)  121-756D-8R-1,20-23  39.20  Site 757  39.10  Site 757  Site 758  38.90  121-757C10R-3,2-6  Site 758  Lower Site 757  Site 757  Site 758  15.60  121-757C10R3,2-6  Site 758  121-757C11R-2,51-56  15.58  Site 756  15.54  121-758A-59R2,58-62 & 58R6,21-25  38.50  Site 757  15.56  121-758A55R5,82-86 121-758A-56R1,64-68  38.60  Fields =TIMS (Weis and Frey, 1991)  121-756D-8R-1,20-23  15.62  121-757C11R-2,51-56  38.80 38.70  MC-ICP-MS Site 756  15.66 15.64  Site 756  Lower Site 757  39.00  Pb/204Pbm  15.68  TIMS 2SD  a  15.52  121-758A-59R2,58-62 & 58R6,21-25  TIMS 2SD  b  38.40 15.50 18.20 18.30 18.40 18.50 18.60 18.70 18.80 18.90 19.00 19.10 19.20 18.20 18.30 18.40 18.50 18.60 18.70 18.80 18.90 19.00 19.10 19.20  206  208  Pb/ Pbi 204  39.40  Pb/204Pbm  Lower Site 757  121-757C11R-2,51-56  121-758A56R1,64-68  38.60  208  121-757C10R3,2-6  18.40  206  Pb/204Pbi  39.40  18.60  Pb/204Pbi  18.80  19.00  Fields =TIMS (Weis and Frey, 1991)  Lower Site 757  38.80 38.60  121-757C11R-2,51-56  121-758A56R1,64-68  121-757C10R3,2-6  18.00  18.00  18.20  18.40  206  Pb/204Pbi  18.60  Pb/204Pbi  MC-ICP-MS Site 756  15.66  18.20  18.40  206  e 18.60  Pb/204Pbi  18.80  19.00  15.52  19.00  19.20  121-756D-8R-1,20-23  Site 757  Lower Site 757  Site 758  15.60  15.54  121-756D6R1,6-10  18.80  Fields =TIMS (Weis and Frey, 1991)  Site 757 121-757C10R3,2-6  Site 758  15.56  121-758A59R2,58-62  38.00 17.80  c  15.58  38.40 38.20  15.50 19.20 17.80  15.62  Site 758  121-757C11R-2,51-56  121-758A55R5,82-86  15.64  Site 757  121-757C10R3,2-6  Site 756  15.52  207  121-756D-8R-1,20-23  39.00  Site 757 Site 758  15.68  MC-ICP-MS Site 756  39.20  15.54  c 18.20  Site 758  15.58  121-756D6R1,6-10  121-758A55R5,82-86  18.00  Lower Site 757  15.60  15.56  38.40  38.00 17.80  Site 757  15.62  Site 758  38.80  121-756D-8R-1,20-23  15.64  Site 757  Pb/204Pbm  MC-ICP-MS Site 756  15.66  121-756D-8R-1,20-23  39.00  Pb/ Pbi 204  15.68  MC-ICP-MS Site 756  39.20  38.20  206  207  121-757C11R-2,51-56 121-758A55R5,82-86  15.50 19.20 17.80  18.00  Site 756  18.20  18.40  206  d 18.60  Pb/204Pbi  18.80  19.00  19.20  Figure 4.6. Diagrams of 208Pb/204Pb and 207Pb/204Pb versus 206Pb/204Pb showing the comparison between the MC-ICP-MS results from this study (symbols) and published TIMS results (shaded grey fields) from Weis and Frey [1991] for the Ninetyeast Ridge, ODP Leg 121 basalts. (a) and (b) show the present-day (measured) Pb isotope values. Note that the analytical uncertainty of all MC-ICP-MS measurements is smaller than the symbol sizes. The 2SD uncertainty of the TIMS measurements is shown at the bottom right corner of the plots. (c) and (d) show the initial ratios, where the age-corrections are 82 Ma (Site 758), 58 Ma (Site 757), and 43 Ma (756). Dashed fields enclose the samples that exhibit high Th/U and consequently show unreliable age-corrected values. (e) and (f) show the initial ratios calculated using the estimated U concentrations for the samples with high Th/U (see text for details). In all diagrams, samples with distinct compositions relative to the compositional average of each drill core (outliers) are labeled.  114  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  rare earth elements (REE), and has very low abundances of Al2O3, CaO, Ni and Cr, compared to other Site 758 core samples [Frey et al., 1991]. In contrast, the other three samples are not distinctive in either major element and/or trace element characteristics, but yield slightly higher Hf isotopic values (Figures 4.7 to 4.9).  4.6.2. Site 757 (~17˚S; ~58 Ma) Within the ~50 m of basalts recovered in hole 757C, Pb and Sr isotopic compositions show relatively little variation throughout the stratigraphy of the hole (206Pb/204Pbm = 18.694 – 18.826,  206  Pb/204Pbi = 18.225 – 18.922;  207  Pb/204Pbm = 15.569 – 15.597,  207  Pb/204Pbi =  15.549 – 15.587; 208Pb/204Pbm = 38.918 – 39.196, 208Pb/204Pbi = 38.281 – 38.928; 87Sr/86Srm = 0.704396 – 0.704886,  87  Sr/86Sri = 0.70422 – 0.70466) (Figures 4.6, 4.7, and 4.8b). In  contrast, the Hf and Nd isotopic compositions are more variable (143Nd/144Ndm = 0.512788 – 0.512822, 176  143  Nd/144Ndi = 0.51272 – 0.51276;  176  Hf/177Hfm = 0.282994 – 0.283034,  Hf/177Hfi = 0.28296 – 0.28301) (Figures 4.6, 4.8 and 4.9) and two stratigraphic groups  with distinct Hf isotope values can be identified (Figure 4.9b). The lower group encompasses samples from flow units 19 to 12 and display higher average  εHf and εNd values (+9.2 and  3.4, respectively), whereas the upper group encompasses samples from flow units 8 to 2 and yields lower average εHf and εNd values (+8.4 and 3.1, respectively). Site 757 basalts display the widest range of Sr and Nd isotope compositions among the Ninetyeast Ridge basalts and their Nd and Hf isotopes are the least radiogenic of the three drill sites (Table 4.5, Figure 4.7). Two samples from Site 757 (121-757C-10R-3, 2-6 cm and 121-757C-11R-2, 51-56 cm) have distinctly higher Pb and Sr isotopic values (Tables 4.4 and 4.5), extending considerably the range of Pb and Sr isotopic variations for this site. These samples are located in the middle to lower part of the core (flow units 8 and 13, respectively) and are strongly altered (H2O+ up to 7.2 wt%), highly plagioclase-phyric basalts, where the plagioclase megacrysts show K-feldspar and albite reaction rims [Frey et al., 1991]. We  115  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  Table 4.5. Sr-Nd-Hf Isotopic Compositions by TIMS from the Ninetyeast Ridge Basalts ODP Leg 121 Sr/86Sra  Error (2σ)  Site 756 121-756C-10N1, 32-36 121-756C-10N1, 32-36 dup.e 121-756D-4R1, 35-39 [ag.]f 121-756D-4R1, 35-39 [ag.] dup. 121-756D-4R1, 35-39 [wc]f 121-756D-4R1, 35-39 [wc] dup. 121-756D-4R1, 85-89 121-756D-6R1, 6-10 121-756D-6R2, 9-13 121-756D-6R2, 9-13 dup. 121-756D-6R2, 13-17 121-756D-6R2, 13-17 rep.g 121-756D-7R1, 100-104 121-756D-8R1, 20-23 121-756D-10R1, 99-103 121-756D-12R3, 139-143  0.703868 0.703858 0.703907 0.703916 0.703904 0.703899 0.703899 0.703864 0.703895 0.703899 0.703896 0.703951 0.704379 0.703891 0.703813  9 8 8 6 7 6 8 8 7 8 7 7 7 10 7  Site 757 121-757C-9R1, 105-109 121-757C-9R1, 105-109 dup. 121-757C-9R5, 34-38 121-757C-9R6, 102-105 121-757C-9R7, 78-82 121-757C-9R8, 54-59 121-757C-10R1, 86-90 121-757C-10R3, 2-6 121-757C-10R3, 2-6 dup. 121-757C-11R2, 10-14 121-757C-11R2, 51-56 121-757C-12R1, 100-104 121-757C-12R1, 100-104 rep. 121-757C-12R2, 0-5 121-757C-12R2, 0-5 dup. 121-757C-12R4, 24-29 121-757C-12R4, 24-29 dup.  0.704396 0.704415 0.704525 0.704654 0.704501 0.704653 0.704707 0.705775 0.705781 0.704776 0.705589 0.704427 0.704518 0.704526 0.704878 0.704886  Site 758 121-758A-55R5, 82-86 121-758A-56R1, 64-68 121-758A-58R6, 21-25 121-758A-59R2, 58-62 121-758A-60R1, 122-126 121-758A-60R1, 122-126 dup. 121-758A-62R1, 130-134 121-758A-62R3, 80-84 121-758A-65R1, 51-55 121-758A-67R4, 31-35 121-758A-69R3, 53-57 121-758A-70R1, 71-75 121-758A-70R2, 129-133 121-758A-71R1, 127-131 121-758A-71R3, 64-68 121-758A-73R1, 82-86 121-758A-73R4, 105-109  0.704405 0.704416 0.704426 0.704425 0.704481 0.704482 0.704475 0.704502 0.704425 0.704440 0.704390 0.704425 0.704351 0.704345 0.704335 0.704318 0.704343  Sample ID  87  Nd/143Nda  Error (2σ)  εNd  0.703807 0.703797 0.703816 0.703825 0.703813 0.703808 0.703753 0.703662 0.703554 0.703558 0.703885 0.703925 0.704229 0.703710 0.703792  0.512902 0.512894 0.512893 0.512896 0.512903 0.512891 0.512902 0.512904 0.512893 0.512886 0.512896 0.512878 0.512820 0.512890 -  6 6 7 7 7 8 6 6 7 7 6 7 8 7 -  5.2 5.0 5.0 5.0 5.2 4.9 5.2 5.2 5.0 4.8 5.0 4.7 3.6 4.9  0.512856 0.512848 0.512845 0.512848 0.512855 0.512843 0.512851 0.512855 0.512841 0.512834 0.512845 0.512831 0.512774 0.512841 -  8 7 8 7 7 7 5 8 8 8 7 7 8 9 9 8  0.704377 0.704396 0.704584 0.704266 0.704264 0.704368 0.704543 0.705702 0.705708 0.704636 0.705426 0.704314 0.704220 0.704228 0.704655 0.704663  0.512799 0.512811 0.512792 0.512794 0.512788 0.512802 0.512799 0.512805 0.512808 0.512811 0.512818 0.512820 0.512822 0.512809 0.512810  6 7 7 7 6 7 8 6 6 7 7 5 5 7 4  3.1 3.4 3.0 3.1 2.9 3.2 3.1 3.3 3.3 3.4 3.5 3.6 3.6 3.3 3.4  7 8 6 9 7 10 8 8 8 8 9 7 8 7 9 7 8  0.704394 0.704379 0.704400 0.704366 0.704466 0.704466 0.704453 0.704487 0.704416 0.704435 0.704369 0.704423 0.704338 0.704324 0.704332 0.704310  0.512888 0.512876 0.512854 0.512857 0.512865 0.512871 0.512871 0.512867 0.512886 0.512886 0.512876  7 6 6 6 6 5 6 6 8 7 6  4.9 4.7 4.2 4.3 4.4 4.6 4.6 4.5 4.8 4.8 4.6  87  Sr/86Srib  143  c  143  Hf/177Hfa Error (2SE)  εHf  5.3 5.2 5.1 5.2 5.3 5.1 5.2 5.3 5.0 4.9 5.1 4.8 3.7 5.0 -  0.283070 0.283071 0.283058 0.283065 0.283062 0.283068 0.283082 0.283076 0.283060 0.283064 0.283069 0.283064 0.283045 0.283016 0.283057 0.283074  4 8 5 7 8 7 8 5 7 6 6 5 14 6 8 6  0.512736 0.512748 0.512732 0.512730 0.512724 0.512741 0.512737 0.512743 0.512740 0.512748 0.512758 0.512761 0.512763 0.512748 0.512749  3.4 3.6 3.3 3.2 3.1 3.5 3.4 3.5 3.4 3.6 3.8 3.9 3.9 3.6 3.6  0.282997 0.283001 0.283013 0.283009 0.283018 0.283006 0.282994 0.283010 0.282995 0.283033 0.283033 0.283033 0.283029 0.283030 0.283034 0.283025 -  0.512772 0.512760 0.512751 0.512766 0.512764 0.512777 0.512778 0.512781 0.512784 0.512779 0.512778  4.7 4.4 4.3 4.5 4.5 4.8 4.8 4.9 4.9 4.8 4.8  0.283132 0.283117 0.283034 0.283130 0.283064 0.283062 0.283103 0.283035 0.283055 0.283061 0.283073 0.283060 0.283038 0.283087 0.283075 0.283097 0.283029  Nd/143Ndib  εNd  i  d  176  Hf/177Hfib  εHf  10.5 10.6 10.1 10.4 10.3 10.5 11.0 10.7 10.2 10.3 10.5 10.3 9.7 8.6 10.1 10.7  0.283057 0.283058 0.283044 0.283052 0.283049 0.283054 0.283069 0.283063 0.283046 0.283049 0.283054 0.283048 0.283032 0.283001 0.283043 0.283058  11.1 11.1 10.6 10.9 10.8 11.0 11.5 11.3 10.7 10.8 10.9 10.7 10.2 9.1 10.6 11.1  7 6 8 7 7 11 7 16 6 7 12 6 5 5 5 10 -  8.0 8.1 8.5 8.4 8.7 8.3 7.9 8.4 7.9 9.2 9.2 9.2 9.1 9.1 9.3 8.9 -  0.282972 0.282976 0.282989 0.282984 0.282991 0.282982 0.282963 0.282976 0.282961 0.283008 0.283012 0.283013 0.283010 0.283006 0.283011 0.283008 -  8.4 8.5 9.0 8.8 9.0 8.8 8.1 8.5 8.0 8.0 9.7 9.8 9.7 9.6 9.8 9.7 -  10 6 7 10 5 7 6 11 8 14 7 15 12 9 5 9 11  12.7 12.2 9.3 12.7 10.3 10.3 11.7 9.3 10.0 10.2 10.6 10.2 9.4 11.1 10.7 11.5 9.1  0.283081 0.283072 0.282990 0.283084 0.283018 0.283017 0.283061 0.282999 0.283027 0.283026 0.283023 0.283022 0.283000 0.283048 0.283034 0.283051 0.282983  12.8 12.5 9.6 12.9 10.6 10.5 12.1 9.9 10.9 10.9 10.7 10.7 9.9 11.6 11.1 11.7 9.3  c  176  d  i  a Measured Sr isotopic ratio normalized to the NBS-987 standard solution mean of the wheel: 87Sr/86Sr = 0.710256; the 2σ error is the absolute error value of the individual sample analysis (internal error) and reported as x106. Measured Nd isotopic ratio normalized to the La Jolla standard solution mean of the wheel:143Nd/144Nd = 0.511852; the 2σ error is the absolute error value of the individual sample analysis (internal error) and reported as x106. Measured Hf isotopic ratio normalized to the JMC475 standard solution, based on the mean of the daily analyses: 176Hf/177Hf = 0.282160; the 2σ error is the absolute error value of the individual sample analysis (internal error) and reported as x106. b Initial Sr, Nd and Hf isotopic ratios calculated for leached samples on the basis of the 40Ar/39Ar eruption age measured for basalts from Sites 756 (43 Ma), 757 (58 Ma) and 758 (82 Ma) [Duncan, 1991]. c εNd and εHf are calculated using present-day 143Nd/144Nd and 176Hf/177Hf values for CHUR = 0.512638 and 0.282772, respectively. d εNdi and εNdi are calculated using both sample and CHUR 143Nd/144Nd and 176Hf/177Hf values corrected for the age of eruption. e dup. = full procedural duplicate analysis of the same starting sample powder. f [ag] = samples crushed in agate mill, [wc] = samples crushed in tungsten-carbide mill. g rep. = replicate analysis of the same sample solution on the MC-ICP-MS.  116  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  143  Nd/144Ndm  176  Hf/177Hfm  0.28314  0.51292  Site 756 Site 757 Site 758  0.51290  121-756D8R-1,20-23 Lower Site 757  0.51288  Site 756 Site 757 Site 758  0.28312  0.51286  0.28310  121-758A58R2,58-62  121-756D8R-1,20-23 Lower Site 757  0.28308  121-758A55R5,82-86 121-758A56R1,64-68  0.28306 0.51284  121-757C10R3,2-6  0.51282  0.28302  0.51280  121-757C10R3,51-56  0.51278 0.7035  143  0.7040  0.7045  87  Nd/144Ndi  0.7050  Sr/86Srm  0.7055  a  176  Site 756 Site 757 Site 758  (1)  0.51280  0.51282  0.51280  (3 & 4?)  0.51278  121-757C10R3,2-6  0.51276  0.51272  121-757C10R3,51-56  (2) 0.7040  87  0.7045  Sr/86Sri  0.7050  0.7055  Site 756 Site 757 Site 758  0.28306  c  143  0.51286  0.51288  Nd/144Ndm  (3)  0.51290 0.51292  (1)  121-756D8R-1,20-23 Lower Site 757  0.28304 0.28302  (4?)  0.28300 0.28298  0.51274  0.51284  Hf/177Hfi  0.28308  121-756D8R-1,20-23 Lower Site 757  0.51282  0.7035  b  0.28310  0.51284  0.51270 0.7030  0.28300  0.28298 0.51278 0.7060  0.51288 0.51286  0.28304  0.28296  (2)  0.28294 0.7060 0.51270  d 0.51273  0.51276  143  0.51279  0.51282  Nd/144Ndi  0.51285  0.51288  Figure 4.7. Diagrams of 143Nd/144Nd versus 87Sr/86Sr and 176Hf/177Hf versus 143Nd/144Nd for the Ninetyeast Ridge, ODP Leg 121 basalts. (a) and (b) show the measured ratios by MC-ICP-MS and TIMS; (c) and (d) show the calculated initial ratios, respectively. Note that the analytical uncertainty of all measurements is smaller than the symbol sizes. In all diagrams, the samples with distinct isotopic compositions relative to the compositional average of each drill core (outliers) are labeled. Numbers (1, 2, 3, 4) refer to the inferred components necessary to explain the isotopic variability within the Ninetyeast Ridge basalts and the arrows point towards the possible direction of such components (see text for details).  117  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  suspect that these particular secondary alteration phases may be more resistant to acid leaching and were not fully removed by the sequential acid-leaching procedure. Even excluding these samples, Site 757 basalts have the most radiogenic  87  Sr/86Sr compositions  observed amongst the three Ninetyeast Ridge sites.  4.6.3. Site 756 (~27.35˚S; ~43 Ma) The samples representing the 82 m of basaltic flows recovered at Site 756 display little variation in their Pb, Hf, Sr and Nd isotopic compositions throughout the stratigraphy of the drill hole (Figures 4.6 to 4.9). Their Pb isotopic ratios form a cluster with very narrow variation (206Pb/204Pbm = 18.663 – 18.719,  206  Pb/204Pbi = 18.207 – 18.573;  207  Pb/204Pbm =  15.544 – 15.577, 207Pb/204Pbi = 15.523 –15.545; 208Pb/204Pbm = 38.942 – 39.045, 208Pb/204Pbi = 38.364 – 38.886) of intermediate compositions between those of basalts from Sites 757 and 758 (Table 4.4, Figure 4.6). Their Hf isotope compositions (176Hf/177Hfm = 0.283045 – 0.283082,  176  Hf/177Hfi = 0.28303 – 0.28307) overlap mostly with those of basalts from Site  758 (Table 4.5, Figures 4.7 to 4.9). The  87  Sr/86Sr ratios of Site 756 basalts show limited  variation (87Sr/86Srm = 0.703813 – 0.703951, 87Sr/86Sri = 0.70355 – 0.70393) and are the least radiogenic values measured within basalts from the Ninetyeast Ridge (Table 4.5, Figures 4.7 to 4.9), whereas their Nd isotope values (143Nd/144Ndm = 0.512878 – 0.512904,  143  Nd/144Ndi  = 0.51283 – 0.57286) are amongst the most radiogenic, together with some of the Site 758 basalts (Table 4.5, Figures 4.7 to 4.9). Overall, three shifts in isotopic compositions are identified for this drill hole. From flow unit 14 (at the bottom of the hole) to flow unit 7 (~180 mbsf), there is a slight increase in the Sr isotope ratios coupled with a decrease of Hf and Nd isotopes. These trends are then reversed upwards to flow unit 4, shifting once again to values closer to the average compositions for this site, until flow unit 1. Sample 121-756D-8R-1, 20-23 cm (flow unit 7), recovered at ~180 mbsf, has distinct isotopic compositions compared to the rest of Site 756 basalts. It has considerably higher 87  Sr/86Srm (0.704379), similar to Site 758 samples, yields lower 143Nd/144Ndm (0.512820) and  176  Hf/177Hfm (0.283016), similar to some Site 757 samples, and has higher  207  Pb/204Pb and  118  0  Depth (mbsl)  100  0  a  Site 756 Site 757 Site 758  b  100 121-756D-8R-1,20-23  0 100  c  Site 756 Site 757 Site 758  0  200  300  300  300  400  400  400  500  500  500  600  600  600  600  700  700  700  700  200 121-757C10R3,51-56  300 400  121-757C10R3,2-6  500  800 18.2  18.4  18.6 206  18.8  19.0  Pb/204Pbm  19.2  19.4  Site 756 Site 757 Site 758  800 800 0.7035 0.7040 0.7045 0.7050 0.7055 0.7060 5.0 87  Sr/86Srm  7.0  9.0  11.0  εHf  m  13.0  Site 756 Site 757 Site 758  800 15.0 0.51276 0.51280 0.51284 0.51288 0.51292 143  Nd/144Ndm  Figure 4.8. Comparison of the measured (a) 206Pb/204Pbm; (b) 87Sr/86Srm; (c) εHfm; and (d) 143Nd/144Ndm variations as a function of drilling depth in meters below sea level (mbsl) between the three Ninetyeast Ridge ODP Leg 121 drill sites. Symbols and colour coding as described in previous Figures. Note that the analytical uncertainty of all measurements is smaller than the symbol sizes.  119  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  200  200  d  100  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  208  Pb/204Pb values for a given  206  Pb/204Pb than the other Ninetyeast Ridge samples (Figures  4.6 and 4.8c). This sample was interpreted by Frey et al. [1991] as being derived from a compositionally different parent magma than the other Site 756 samples.  4.6.4. Comparison Between Sites Whether considering the measured or the initial Pb-Hf-Sr-Nd isotopic ratios, basalts from Site 756 display the smallest ranges of variations compared to basalts from the other Ninetyeast Ridge sites. They generally have intermediate compositions to those of Sites 758 and 757, whereas basalts from Site 757 present the greater range of variation and some of the most extreme isotopic signatures within the Ninetyeast Ridge (Figures 4.6 to 4.9). In contrast to the previous TIMS dataset, which led Class et al. [1993, 1996] to propose a single homogeneous “aging” plume source model for the Ninetyeast Ridge, reanalysis of the tholeiites representative of the ~180 m of basaltic basement recovered during ODP Leg 121 shows that there are no systematic short-term (intra-site) or long-term (inter-site) isotopic evolution trends (Figures 4.9 and 4.10). The isotopic compositions of the Ninetyeast Ridge basalts do not correlate with stratigraphic position throughout the cores at each individual site (i.e., eruption age), nor with distance (i.e., emplacement age) along the ridge. The absence of a temporal trend in the isotopic compositions of the Ninetyeast Ridge basalts is consistent with the involvement of distinct mantle components, intrinsic or not to the plumesource, throughout the volcanic history of the ridge [Saunders et al., 1991; Weis and Frey, 1991; Frey and Weis, 1995; 1996]. The considerable isotopic differences between basalts from the Ninetyeast Ridge sites likely reflect: 1) variability inherent to the source of the magmas erupting at each volcanic centre at a given time, or 2) interaction and mixing of source derived magmas with extraneous material. In multi-isotopic diagrams, basalts from each drill site define distinct isotopic compositional fields with very little overlap and each site trends toward a different end-member composition (Figures 4.6 to 4.8). Simple binary mixing relations cannot explain the distribution of the isotopic compositions observed amongst the Ninetyeast Ridge basalts  120  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  a. Site 756 120  120  120  140  140  140  160  160  180  180  200  200  220  220  Depth (mbsf)  Youngest F1  160  F4  180  F7  200 220  F14  240 18.64  18.66  b. Site 757  Oldest 18.68  206  18.70  18.72  Pb/204Pbm  18.74  380  F2  385  Depth (mbsf)  390 F5  395  F6  F8  400 F12  405  F13  410 415  F19  240  8.0  206  Depth (mbsf)  560  F2  F3  600  640  11.0  εHfm  87  385  385  390  390  395  395  400  400  405  405  410  410  415  415  Pb/ Pbm  F1  10.0  380  7.5  8.0  204  480  520  9.0  240 0.7036 0.7038 0.7040 0.7042 0.7044 0.7046 12.0  380  420 420 7.0 18.30 18.50 18.70 18.90 19.10 19.30  c. Site 758  Relative Age  8.5  εHfm  9.0  9.5  Sr/86Srm  Youngest  Relative Age  Oldest  420 10.0 0.7035 0.7040 0.7045 0.7050 0.7055 0.7060 87  480  480  520  520  560  560  600  600  640  640  680  680  Sr/86Srm  Youngest  Relative Age  F14 F19  680  F17  F29  18.30 18.40 18.50 18.60 18.70 18.80 206  Pb/204Pbm  7.0  9.0  11.0  εHfm  13.0  15.0  0.7042  Oldest 0.7043  87  0.7044  0.7045  Sr/86Srm  0.7046  Figure 4.9. Variations of the measured 206Pb/204Pb, εHf, and 87Sr/86Sr as a function of drilling depth in meters below sea floor (mbsf) for each individual drill site. (a) Site 756; (b) Site 757; (c) Site 758. In all plots, the dark grey areas correspond to the ± 2SD of the mean of the respective data set, and the light grey areas correspond to the ± 2SD of the mean minus outliers of the respective data set. Labels F1, F2, etc. refer to the flow units the samples belong to (see Table 4.1).  121  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  and at least three distinct components are required. To account for the isotopic compositions of most of Site 756 and some of Site 758 basalts, a relatively depleted component with unradiogenic  87  Sr/86Sr values and  207  Pb/204Pb, intermediate  206  Pb/204Pb and  208  Pb/204Pb  values, and relatively radiogenic Nd and Hf isotopic compositions is required. An enriched component with radiogenic Hf isotopic compositions, unradiogenic 208  206  Pb/204Pb and  Pb/204Pb values, and intermediate Sr and Nd isotopic compositions is necessary to account  for most of the isotopic compositions of Site 758 samples, the sample from flow unit 7 of Site 756 and part of the Site 757 samples. A third component, relatively enriched with unradiogenic Hf and Nd isotopic compositions and radiogenic Sr and Pb isotopic compositions, is required to account for the isotopic compositions of some of Site 757 lavas. The isotopic compositional fields defined by the samples from each site appear to converge toward a common, shared composition, with 143  206  Pb/204Pbi ≈ 18.55,  87  Sr/86Sri ≈ 0.70425,  Nd/144Ndi ≈ 0.51278, 176Hf/177Hfi ≈ 0.28315 (e.g., Figure 4.7 and 4.9). This may reflect an  additional end-member composition present within the Ninetyeast Ridge source region or may be an artifact resulting from mixing of the other distinct components [e.g., Douglas and Shilling, 2000].  4.7. Discussion 4.7.1. Defining the Mantle Source Components of the Ninetyeast Ridge Basalts Previous studies interpreted the isotopic variations in the Ninetyeast Ridge basement samples to reflect mixing between at least three components [Saunders et al., 1991; Weis et al., 1991]. Saunders et al. [1991] and Weis and Frey [1991] recognized that two of these components were best represented by the compositions attributed to the Kerguelen and Amsterdam-St. Paul mantle plumes. However, whereas Weis and Frey [1991] considered the third component to be a Indian MORB-like depleted component, Saunders et al. [1991] argued that there was no obvious contribution from a MORB source component to the Ninetyeast Ridge basalts. The isotopic compositions of the Ninetyeast Ridge basalts from these previous studies did not define clear mixing arrays, which indicated that they were not derived from simple mixing between any of the identified components. The new high-  122  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  Figure 4.10. Variation of initial isotopic compositions of the Ninetyeast Ridge drilled basalts as a function of latitude. (a) 206Pb/204Pbi; (b) εHfi; (c) 87Sr/86Sri; and (d) εNdi. Large black, grey and white diamond symbols with black outlines represent the ODP Leg 121 data from this study. The other diamond symbols without outlines correspond to literature data for the DSDP samples [Frey and Weis, 1995]. Data for other nearby Indian Ocean island basalts (OIB) and MORB are also shown for context, at their appropriate latitude: Amsterdam and St. Paul islands [Doucet et al., 2004]; Mont Crozier: (D. Weis, unpublished data); Site 1138 [Neal et al., 2002; Ingle et al., 2003]; Indian MORB [Chauvel and Blichert-Toft, 2001; Mahoney et al., 2002; Nicolaysen et al., 2007]. For convenience, the Kerguelen plume derived volcanic products Mont Crozier (which represents the present composition of the Kerguelen plume tail mantle source) and Site 1138 (which represents the composition in the Kerguelen plume head mantle source) are shown at the same latitude. Dashed lines mark the different segments along the Ninetyeast Ridge as defined by Royer et al. [1991] and Krishna et al. [1999].  123  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  17.50  18.00  Pb/204Pbi 18.50  19.00  19.50  a  Site 758  Site 214  Site 757  segment 3  Latitude (º N)  -10  segment 2  Site 216  0  -20 Site 253  Site 756  -30  -40  20.00  segment 1  206  17.00 10  Site 216  segment 2  segment 1  c  Site 214  Site 757  segment 3  Latitude (º N)  22.5  b  Site 757  -20  Site 756  St. Paul  Site 1138 (CKP)  -50  -20 Site 253  Site 756  -2.5  Amsterdam  εNdi  0.0  2.5  5.0  0  Site 1138 (CKP)  Mt. Crozier (KA)  10.0  12.5  d  Site 216  -10  Site 214  Site 757  -20 Site 253  Site 756 Site 254  Amsterdam St. Paul  -40  St. Paul  7.5  Site 758  -30  Site 254  Indian MORB  Indian MORB  -50  17.5  -10  Sr/86Sri  0  -40  12.5  0  Mt. Crozier (KA)  Mt. Crozier (KA)  Site 758  -30  i  Site 758  0.7025 0.7030 0.7035 0.7040 0.7045 0.7050 0.7055 0.7060 -5.0 10 10  -10  εHf  -40  St. Paul  87  7.5  Amsterdam  Amsterdam  Indian MORB  -50  2.5  -30  Site 254  Site 1138 (CKP)  -2.5 10  -50  Mt. Crozier (KA)  Site 1138 (CKP)  Indian MORB  Figure 4.10.  124  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  precision Pb-Hf-Sr-Nd isotope dataset also indicates the presence of three, possibly four, components contributing to the compositions of the Ninetyeast Ridge basalts. However, in contrast to the older TIMS dataset, the newly determined Pb isotopic compositions from Site 758 and 757 basalts define linear arrays that may be interpreted as binary mixing arrays, and the compositions for Site 756 show relatively little variation (Figures 4.6 and 4.7). In light of the new isotopic dataset of the Ninetyeast Ridge basalts, the question arises as to how the components identified compare to the more recent end-member compositions now attributed to the Kerguelen [Neal et al., 2002; Ingle et al., 2003] and Amsterdam mantle plume sources [Doucet et al., 2004] and MORB. 4.7.1.1. Is the Relatively Depleted Ninetyeast Ridge Component Indian MORB? The tholeiitic basalts of the Ninetyeast Ridge erupted during a period characterized by higher spreading rates (18–20 cm/yr; e.g., Müller et al., [2008]) when the Kerguelen hotspot was considered to be relatively close to the spreading axis of the newly forming Southeast Indian Ridge [e.g., Frey et al., 1991; Royer et al., 1991; Frey and Weis, 1995]. Any contribution from depleted asthenospheric mantle, resultant from plume-ridge interaction as volcanism on the Ninetyeast Ridge evolved from an intraplate setting to nearaxis and on axis settings, can be discriminated by the isotopic compositions of the basalts forming at the different segments of the Ninetyeast Ridge. Basalts from Site 758, together with DSDP Site 216 basalts, erupted in the northern segment of the ridge (Figure 4.10) that formed as intraplate volcanism on the Indian plate [Royer et al., 1991; Krishna et al., 1999]. As a result, basalts from this segment of the ridge should yield the strongest enriched plume signatures within the Ninetyeast Ridge, as the MORB influence would be smaller or nonexistent [e.g., Frey and Weis, 1995]. Site 757 basalts erupted during the period when the Kerguelen hotspot was coincident with the Wharton spreading center (60 to 54 Ma; Krishna et al. [1999]). These basalts should show a more diluted plume signature, given the higher degree of possible interaction with the surrounding depleted astenospheric mantle. Site 756, located on the southern segment of the Ninetyeast Ridge, is thought to have formed on the edge of the Indian plate [Royer et al., 1991; Krishna et al., 1999]. Basalts forming at this  125  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  tectonic environment should yield the most variable isotopic signatures, reflecting the variable degrees of mixing between the enriched plume material and the depleted mantle. Linear regressions through the Pb isotopic compositions of each Ninetyeast Ridge site produce three linear arrays of slightly different slopes (Figure 4.11). These arrays are best interpreted as binary mixing trends between source components that lie along the radiogenic and unradiogenic extensions of the arrays [e.g., Abouchami et al., 2000; Eisele et al., 2003]. When comparing the initial Pb isotopic compositions of the Ninetyeast Ridge basalts and other Indian Ocean basalts (Figure 4.11a), the arrays defined by basalts from Site 757 and Site 756 intersect the Indian MORB compositional field, represented by compositions of basalts from the Southeast Indian Ridge (excluding the segments on the Amsterdam-St. Paul Plateau), Central Indian Ridge, and Southwest Indian Ridge. However, the same is not true when considering the measured Pb isotopic compositions. For example, all Ninetyeast Ridge basalts have higher 208Pb/206Pb for a given 207Pb/206Pb than MORB, and none of the arrays intersect the Indian MORB field (Figure 4.11b). In diagrams of 87Sr/86Sr, 143  Nd/144Nd, and 176Hf/177Hf versus 206Pb/204Pb, the arrays (not shown) defined by each of the  Ninetyeast Ridge sites also do not trend towards Indian MORB. Instead, they are mostly parallel to the MORB compositional field (Figures 4.12 and 4.13). Additionally, together with other Indian Ocean island basalts, the Ninetyeast Ridge basalts plot at higher 176Hf/177Hf for a given  143  Nd/144Nd compared to the mantle array [Vervoort and Blichert-Toft, 1999]  (Figure 4.13b), which reflects derivation from a source with higher time-integrated Lu/Hf relative to Sm/Nd than the MORB source. The depleted isotopic signatures observed in the Ninetyeast Ridge hence cannot be accounted for by mixing with depleted Indian asthenospheric mantle, in contrast to basalts from the Northern Kerguelen Plateau that formed ~50 km away from the southeast Indian Ridge at >34 Ma and whose compositions reveal incorporation of asthenospheric mantle by the Kerguelen plume [Weis and Frey, 2002]. Although the ridge formed close to a spreading axis, MORB was not entrained by plume-derived material to form the Ninetyeast Ridge. This is in good agreement with fluid dynamical models that predict the absence of upper mantle  126  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  Figure 4.11. Diagrams of (a) initial 208Pb/204Pb versus 206Pb/204Pb, and (b) measured 208Pb/206Pb versus 207Pb/206Pb for the Ninetyeast Ridge basalts compared to other Indian Ocean basalts. Dashed lines represent the regression lines through the compositions from each Ninetyeast Ridge site; The small grey circles represent basalts from all the Indian Ocean spreading centres: SEIR [Chauvel and Blichert-Toft, 2001; Mahoney et al., 2002; Nicolaysen et al., 2007]; CIR [Rehkämper and Hofmann, 1997; Nauret et al., 2006]; SWIR [Mahoney et al., 1992; Janney et al., 2005; Meyzen et al., 2005]. Red arrows represent two cases of 100 Ma evolution (tick marks represent 20 Ma increments) by radiogenic decay of a compositionally homogeneous source considering compositions similar to those of Site 1138 (proxy for the Kerguelen plume head composition) and of Mont Crozier (proxy for the Kerguelen plume tail composition), respectively. Black arrow represents a similar evolution trajectory for an average Indian MORB source. Data sources for the Indian Ocean island basalts compositional fields: Amsterdam and St. Paul islands [Doucet et al., 2004]; Kerguelen Archipelago [Weis et al., 1998; Frey et al., 2002b; Doucet et al., 2005; Xu et al., 2007]; Mont Crozier (D. Weis unpublished data, 2001); ODP Sites 747, 749, 750 on the Central and South Kerguelen Plateau and Broken Ridge [Mahoney et al., 1995; Frey et al., 2002b]; ODP Sites 1136 and 1138 on the South and Central Kerguelen Plateus [Neal et al., 2002; Ingle et al., 2003]; Heard Island [Barling et al., 1990; 1994]; Crozet [Mahoney et al., 1996; Salters and White, 1998]; Réunion [Vlastélic et al., 2007, 2009; Bosh et al., 2008]; DSDP Sites 214, 216, and 253 [Frey and Weis, 1995]. Global isotopic end-members DMM, EM-1 and EM-2 are from Zindler and Hart [1986], C is from Hanan et al. [1996; 2004] and Fozo is from Stracke et al. [2005]. NRHL refers to the Northern Hemisphere Reference Line as defined by Hart [1984].  127  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  208  40.00  Pb/204Pbi Site 756 Site 757  39.50  Site 758  Elan Bank  Site 216  Site 254  60  60  DMM  17.00  RL  a  NH  17.50  18.00  Pb/206Pbm  2.175  Site 1138 (CKP) 20 Ma  206  2.200  Réunion (PDN)  7  36.50 16.50  te  75  Si  te  75  6  37.00  Site 750 (SKP)  8  75  FOZO  100 Ma  Si  e Sit  20  100  Site 749 (SKP)  37.50  “C”  St. Paul  EM1 Site 747 (CKP)  EM2  60  100 Ma  Réunion (PDF)  Site 1136 (SKP)  38.00  Crozet  20 Ma  Broken Ridge  38.50  208  Mt. Crozier  Site 214  39.00  Amsterdam  Heard Island  Kerguelen Archipelago  Pb/ Pbi  18.50  19.00  19.50  204  Site 756 Site 757  te Si  Site 758  2.150  6  75  te  Si  7  75  Site  EM1 758  2.125 L  NHR  2.100 2.075 2.050 2.025 2.000 0.81  DMM EM2 “C”  b 0.82  0.83  0.84  0.85 207  0.86  0.87  0.88  0.89  0.90  Pb/206Pbm  Figure 4.11.  128  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  entrainment by plume heads and consequent absence of MORB contribution in plume-related basalts [Farnetani et al., 2002; Farnetani and Samuel, 2005]. Thus, the relatively depleted component responsible for the more depleted signatures within the Ninetyeast Ridge basalts is likely intrinsic to the heterogeneous upwelling mantle source. 4.7.1.2. Are the Ninetyeast Ridge Enriched Components Related to the Kerguelen and Amsterdam-St. Paul Mantle Plumes? The geochemical similarities between basalts from the Ninetyeast Ridge and those derived from the geographically proximal Kerguelen and Amsterdam–St. Paul hotspots led to the interpretation that they must be genetically related [e.g., Saunders et al., 1991; Weis and Frey, 1991; Frey and Weis, 1995; 1996]. Geochronological studies and plate tectonic reconstructions provide temporal and spatial evidence that relate the Ninetyeast Ridge to the long-lived (~120 m.y.) Kerguelen mantle plume [e.g., Duncan, 1991; Royer et al., 1991; Klootwijk et al., 1991; Duncan and Storey, 1992]. Association of the Amsterdam–St. Paul hotspot to the Ninetyeast Ridge is supported by the lower paleolatitudes measured on the basaltic cores of the drill sites at the southern end of the ridge compared to the other drill sites on the ridge [Klootwijk et al., 1991], and the existence of a volcanic chain connecting the southern end of the ridge and the northeast part of the Amsterdam–St. Paul plateau (Figure 4.1). This volcanic trail may be an older trace of the Amsterdam–St. Paul hotspot [e.g., Royer and Schlich, 1988]. The activity of the Amsterdam–St. Paul hotspot is considerably more recent than that of Kerguelen [Nicolaysen et al., 2000]. Although volcanism on the Amsterdam–St. Paul plateau is younger than 5 Ma [Scheirer et al., 2000], it is possible that volcanic activity of Amsterdam–St. Paul started as early as ~40–45 Ma, a time period when the newly formed Southeast Indian Ridge separated Broken Ridge from the Kerguelen Plateau, and when the southern end of the Ninetyeast Ridge formed [e.g., Mutter and Cande, 1983; Munschy et al., 1992; Frey and Weis, 1996]. A deep-seated origin for the long-lived Kerguelen mantle plume is supported by recent tomographic studies [e.g., Montelli et al., 2004, 2006]. Recent geochemical studies have shown a compositional distinction between the volcanic products of the Kerguelen  129  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  plume head and tail [e.g., Mattielli et al., 2002; Neal et al., 2002; Ingle et al., 2003; Weis and Frey, 2002]. The Kerguelen plume head source is, at present, best represented by the compositions of the Cretaceous basalts recovered on Site 1138, ODP Leg 183, [Neal et al., 2002; Ingle et al., 2003], whereas the Kerguelen plume tail source is best represented by the compositions ca. 24 Ma mildly alkalic basalts from Mont Crozier on the Courbet Peninsula of the Kerguelen Archipelago [Mattielli et al., 2002; Weis and Frey, 2002; Ingle et al., 2003; D. Weis, unpublished data]. The distinct isotopic compositions of these components are not related by radiogenic ingrowth in the source (red arrows; Figures 4.11 to 4.13). Instead they reflect distinct heterogeneities intrinsic to the Kerguelen plume source [e.g., Neal et al., 2002; Ingle et al., 2003]. Similarly, the Amsterdam–St. Paul mantle plume is likely to be compositionally heterogeneous in space and time [Doucet et al., 2004; Nicolaysen et al., 2007]. Evidence for this is based on the isotopic differences between the young Amsterdam and St. Paul volcanoes that were interpreted as resulting from sampling of different chemical domains present within the Amsterdam–St. Paul mantle plume itself [Doucet et al., 2004]. The isotopic compositions of the Ninetyeast Ridge basalts generally plot between the compositional fields defined by basalts from the Southern and Central Kerguelen Plateaus and the Kerguelen Archipelago / Amsterdam and St. Paul islands (Figures 4.11 to 4.13). The Pb isotope ratios of the ~82 Ma and ~58 Ma basalts from Sites 758 and 757, respectively, overlap for the most part with those of the (29 to <1 Ma) basalts from the Kerguelen Archipelago [e.g., Weis et al., 1998; Doucet et al., 2002; Frey et al., 2002b] and <1 Ma basalts from Heard Island [Barling and Goldstein, 1990, Barling et al., 1994]. Their Sr, Hf, and Nd isotopic compositions are mostly intermediate between the more enriched compositions of the Kerguelen Archipelago and the compositions observed today on basalts from Amsterdam Island [Doucet et al., 2004]. The ~43 Ma old basalts from Site 756 have Pb-Sr-Hf-Nd isotopic compositions that cluster close to those of the recent (<0.04 Ma) basalts from St. Paul island [Doucet et al., 2004] (Figures 4.11 to 4.13). The intermediate isotopic compositions of the Ninetyeast Ridge basalts relative to the volcanic products of the Kerguelen mantle plume and Amsterdam-St. Paul hotspot are consistent with the Ninetyeast Ridge being related to volcanic activity of these two hotspot systems.  130  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  Figure 4.12. Variations of (a) 87Sr/86Sri and (b) 176Hf/177Hfi with 206Pb/204Pbi for the Ninetyeast Ridge basalts compared to other Indian Ocean basalts. Data sources for compositional fields are as indicated in Figure 4.11. plus: Heard Island (Barling et al. unpublished, 2002); Réunion [Albarède et al., 1997b; Pietruszka et al., 2009]. In both diagrams, grey lines with arrows represent the mixing trajectories between a common mantle component of compositions intermediate to those of C and Fozo and enriched mantle compositions produced by incorporation of: 1) clastic sediments derived from upper continental crust, to produce compositions similar to EM-2 (after Chauvel et al. [1992]); 2) lower continental crust to produce compositions similar to EM-1 (after Escrig et al. [2004], and Meyzen et al. [2005]); 3) 1.5–2 Ga altered oceanic crust (90%) plus pelagic sediments (10%) to produce compositions similar to EM-1 (after Rehkämper and Hofmann [1997], and Kempton et al. [2002]); and 4) a mixture of altered oceanic crust and pelagic sediments (90%) plus lower continental crust (10%) deep into the mantle source of OIB (after Willbold and Stracke [2006; 2010]). Data sources for the used compositions are as follows: old altered oceanic crust, Rehkämper and Hofmann [1997], and Stracke et al. [2003]; clastic sediments, Plank and Langmuir [1998], and Chauvel et al. [2008]; pelagic sediments, Rehkämper and Hofmann [1997], and Kempton et al. [2002]; lower continental crust (Proterozoic Eastern Ghats Belt, India), Rickers et al. [2001]. Tick marks on mixing trends correspond to 10% increments.  131  CHAPTER 4  87  Sr/86Sri  0.7085  UCC  Heard Island  0.7075 LCC  0.7065  EM1  (2)  Site 747 (CKP)  0.5  Site 756 Site 757  EM2  (1)  Site 758  Broken Ridge  Mt. Crozier  Site 216  0.7055 0.7045 0.7035  Site 750 (SKP)  (3)  alt OC (4)  60 100 Ma  Site 1138 (CKP)  0.3  + LCC  Kerguelen Archipelago  Site 1136 (SKP)  alt OC + seds  Site 214  0.1  0.7025  Amsterdam Réunion  Site 749 (SKP)  0.2  Crozet Isl.  20 Ma  Marion  C  O  FOZ  St. Paul  20 Ma  100 Ma 60  0.1  0.1  DMM  0.7015 16.50  17.00  17.50  a  18.00  18.50  206  19.00  19.50  20.00  Pb/204Pbi  Hf/177Hfi  176  0.2837  Site 756 Site 757 Site 758  0.2835  DMM 0.2833  0.2831  0.2829  (4)  alt  Amsterdam  Site 750 (SKP)  + OC  LCC  0.2  Site 1138 (CKP)  0.6  LCC  (2)  17.00  0.1  Site 749 (SKP)  60  Broken Ridge  0.2827  0.2825 16.50  St. Paul  0.2  alt OC + seds (3)  20  60  100  EM1  17.50  Crozet Isl.  Archipelago 100 20  0.5  Site 1136 (SKP)  Site 747 (CKP)  Elan Bank  Heard Island  EM2  Mt. Crozier  (1)  b  UCC  18.00 206  FOZO  Réunion  20 Kerguelen  0.5  C  18.50  19.00  19.50  20.00  Pb/ Pbi 204  Figure 4.12.  132  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  However, despite the compositional similarities between the Ninetyeast Ridge basalts and the volcanic products of both Kerguelen and Amsterdam-St. Paul mantle plumes, simple mixing of the compositions measured to date within these hotspot systems cannot explain the isotopic variability of the Ninetyeast Ridge basalts. The calculated mixing arrays do not indicate direct mixing between any of the potential components (Figures 4.11 to 4.13). Instead they indicate mixing between components of intermediate compositions. This may be because the mantle source(s) sampled by the Ninetyeast Ridge are compositionally more complex than previously assumed. This complexity is also evidenced by the distinct isotopic compositions of the volcanic products of varying age associated with the Kerguelen and Amsterdam-St. Paul mantle plumes. Hence, to assume a Kerguelen and/or Amsterdam–St. Paul mantle plume origin for the Ninetyeast Ridge implies that these mantle plumes are compositionally heterogeneous in time and space, having components that are common and others that are not.  4.7.2. Implications for the Source of Indian Ocean Basin Volcanic Rocks and the Dupal Anomaly The geochemical heterogeneity observed among ocean island basalts (OIB) is commonly assumed to derive from melting of distinct mantle components or some mixture of these [e.g., White, 1985; Zindler and Hart, 1986; Hart et al., 1992; Hanan and Graham, 1996; Stracke et al., 2005; Willbold and Stracke, 2006; 2010]. Basalts from the Ninetyeast Ridge show a marked Dupal signature (high 87Sr/86Sr, and high  207  Pb/204Pb and  208  Pb /204Pb  for a given 206Pb/204Pb), characteristic of oceanic basalts from the Indian Ocean [Hart, 1984]. Relative to Indian MORB, the ~82 – 43 Ma basalts from the Ninetyeast Ridge are enriched in 208Pb/204Pb, 207Pb/204Pb and 87Sr/86Sr, and depleted in 176Hf/177Hf and 143Nd/144Nd (Figures 4.11 to 4.13). Their Pb-Hf-Sr-Nd isotopic compositions are very similar to those of other ocean island basalts within the Indian Ocean (Figures 4.11 to 4.13). The compositional trends defined by the individual Ninetyeast Ridge sites are to some extent part of larger trends containing other Indian Ocean island basalts, which suggests that any local mixing is part of a broader regional scale of mixing between several distinct components in the Indian Ocean  133  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  143  Nd/144Ndi  0.5134  Site 758  DMM  Site 757  0.5132  Site 756  St. Paul  FO ZO  0.5130  Crozet Isl. Site 750 (SKP) Site 1136 (SKP)  C  HIMU  0.5128  Amsterdam Réunion  Marion  Broken Ridge  Site 749 (SKP)  0.5126  BSE Mt. Crozier  0.5124  Site 216  Site 1138 (CKP)  Site 747 (CKP)  0.5120 0.7020  0.7030  0.7040  0.7050  87  0.2835  Heard Island  EM1  0.5122  176  EM2  Kerguelen Archipelago  a  0.7060  Sr/ Sri  0.7070  0.7080  86  Hf/177Hfi Site 758  DMM  Site 757  0.2833  Site 756  Amsterdam  ZO  FO  Site 1138 (CKP)  0.2829  0.2827  0.2825 0.5120  Mt. Crozier Kerguelen Archipelago  Site 747 (CKP)  0.5122  M  C  Site 750 (SKP)  0.2831  y  ra  Ar  a  St. Paul Réunion  le nt  HIMU  Crozet Isl.  Site 749 (SKP)  EM2 BSE  EM1  0.5124  Heard Island  0.5126 143  b 0.5128  Nd/ Ndi  0.5130  0.5132  0.5134  144  Figure 4.13. (a) 87Sr/86Sri versus 143Nd/144Ndi and (b) 143Nd/144Ndi versus 176Hf/177Hfi of the Ninetyeast Ridge basalts compared to other Indian Ocean basalts and to the global isotopic end-members. Data sources for compositional fields are as described in Figures 4.11. and 4.12. Mantle array as defined by Chauvel et al. [2008].  134  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  mantle. Specifically, the close isotopic similarity of the Ninetyeast Ridge basalts to those from Réunion, Crozet, Amsterdam and St. Paul islands, indicates derivation from compositionally similar deep sources. Together with other Indian Ocean basalts, the Ninetyeast Ridge samples converge at isotopic compositions that are intermediate between the values for the common components “C” [Hanan and Graham, 1996; Hanan et al., 2004] and FOZO [Hart et al., 1992; Stracke et al., 2005]. From these common compositions, Indian Ocean OIB form arrays that fan out towards enriched mantle (EM) compositions that are mostly intermediate to those of EM-2 (high 143  87  Sr/86Sr, variable  Nd/144Nd and  176  208  Pb/204Pb, intermediate and almost constant  Hf/177Hf) and EM-1 (high  87  Sr/86Sr and  208  206  Pb/204Pb and low  Pb*/206Pb*, low  206  Pb/204Pb,  and lower 143Nd/144Nd and 176Hf/177Hf) (Figures 4.11 to 4.13). Recycling of upper continental crust material in the form of clastic marine sediments together with altered oceanic crust via subduction is considered to be the mechanism responsible for the trace element and isotopic compositions of EM-2 basalts [Weaver, 1991; Chauvel et al., 1992; Willbold and Stracke, 2006; Jackson et al., 2007; Porter and White, 2009]. However, the type of recycled material leading to EM-1, Dupal-like, source characteristics is the subject of considerable debate. Any plausible source capable of generating EM-1-like isotopic compositions must have had high time-integrated Rb/Sr, Th/U and low U/Pb, Sm/Nd and Lu/Hf. Possible candidates of recycled material in the EM-1 source include: pelagic sediment with oceanic crust [e.g., Weaver, 1991; Chauvel et al., 1992; Eisele et al., 2002], incorporation and dispersal of delaminated metasomatized subcontinental lithospheric mantle prior to continental rifting [e.g., Storey et al., 1992; Mahoney et al., 1996; Douglass et al., 1999; Borisova et al., 2001; Kempton et al., 2002; Janney et al., 2005; Geldmacher et al., 2008; Goldstein et al., 2008], recycled lower continental crust by delamination [Escrig et al., 2004; Hanan et al., 2004; Meyzen et al., 2005] or by subduction erosion [Willbold and Stracke, 2006; 2010]. The isotopic compositions of most Ninetyeast Ridge basalts are distributed close to the EM-1 trend together with basalts from the Kerguelen and Amsterdam Plateaus and the Afanasy-Nikitin Rise. However, the intermediate character of the enriched end-member of  135  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  the Indian Ocean OIB array formed by basalts from Heard Island, Kerguelen Archipelago, Réunion, Crozet, and Amsterdam islands, led Barling and Goldstein [1990] to question the compositional individuality of the EM components. The authors proposed that EM-1 and EM-2 represent extreme compositions of a continuous spectrum of isotopic compositions that reflect mantle contaminated by recycled continental crust material of different compositions into the OIB source. Models that assume a lithospheric mantle or lower continental crust material contribution to the EM-1 component advocate for its shallow incorporation into the asthenospheric mantle. Alternatively, Willbold and Stracke [2006, 2010] provide a model where upper and lower continental crust are concomitantly recycled together with oceanic lithosphere, deep into the OIB source, at erosive plate margins, and thus generate the multiple EM-type trace element and isotopic signatures observed in OIB. In the Indian Ocean basin, EM-1-like compositions are found in both OIB and MORB, leading to the question of whether this ubiquitous isotopic anomaly in Indian Ocean basalt results from a deep or shallow origin. Additional questions are whether or not the EM1-like signatures of Indian Ocean OIB and MORB result from the presence of the same contaminant into their respective sources; and if they are related to the same process(es). If lower continental crust eroded during subduction is the material responsible for EM-1-like signatures [Willbold and Stracke, 2006; 2010], and assuming that this material is likely to be recycled into the upper mantle as well as into the deeper mantle [e.g., Clift and Vannuchi, 2004], EM-1 signatures in MORB should be randomly distributed in other oceans and not be restricted to the Indian Ocean, South Atlantic Ocean, and the Gakkel Ridge in the Arctic Ocean [e.g., Hart, 1984; Escrig et al., 2004; Regelous et al., 2009; Goldstein et al., 2008]. This suggests that although the EM-1 characteristics in OIB and MORB in the Indian Ocean may be related to the same type of material, they are generated by different processes. A shallow origin, by delamination of subcontinental lithospheric mantle and lower continental crust may explain the EM-1-like compositions of MORB in young oceans [Escrig et al., 2004; Hanan et al., 2004; Meyzen et al., 2005; Janney et al., 2005; Geldmacher et al., 2008; Goldstein et al., 2008], whereas a deep origin by deep recycling of upper and lower  136  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  continental crust may explain the EM characteristics of OIB [Willbold and Stracke, 2006; 2010]. There is evidence for interaction and incorporation of continental material at shallow depth by basalts related to the Kerguelen plume head at the onset of continental rifting. Depletion of Nb, Ta and Th relative to primitive mantle concentrations is found in some samples from the Kerguelen Plateau and Broken Ridge, which together with their extreme isotopic compositions indicates contamination by subcontinental lithospheric mantle or lower continental crust [e.g., Mahoney et al., 1995; Frey et al., 2002a]. In addition, garnet-biotite gneiss and fluvial conglomerates were recovered at Elan Bank, Site 1137 [Weis et al., 2001; Ingle et al., 2002]. In contrast, basalts formed in intraplate settings (Ninetyeast Ridge and Kerguelen Archipelago; Frey et al. [2002b]; Weis et al. [2001]) lack such evidence of shallow-level contamination. Although the isotopic signatures of the Ninetyeast Ridge basalts could be consistent with some lower crust contribution (Figures 4.11 to 4.13), they do not display the diagnostic Nb and Ta depletions [e.g., Frey et al., 1991; Frey and Weis, 1995]. According to Late Cretaceous plate reconstructions, the Ninetyeast Ridge did not form close to continental plate margins [Royer et al., 1991; Müller et al., 1993; Steinberger, 2000]. This provides a geological explanation for the lack of shallow-level contamination and indicates that any isotopic continental “flavour” in the Ninetyeast Ridge basalts is inherited from its deep source. The Pb, Hf, Sr and Nd isotopic compositions of the Ninetyeast Ridge basalts are best explained by contribution of a mixture of pelagic sediments and lower continental crust recycled together with altered oceanic crust into its deep and compositionally heterogeneous mantle source.  4.8. Conclusions High-precision Pb-Hf-Sr-Nd isotopic analysis of the basalts recovered from the 5000 km long submarine Ninetyeast Ridge during ODP Leg 121 (Sites 756, 757 and 758) define tight compositional fields for each site and are inconsistent with an age-progressive isotopic evolution along the ridge. The distinct compositional fields defined by the Ninetyeast Ridge basalts reflect mixing between components with relatively depleted and enriched signatures.  137  CHAPTER 4: ISOTOPIC SYSTEMATICS OF THE NINETYEAST RIDGE  Indian asthenospheric mantle does not contribute to the depleted component, which instead is likely to be inherited from a deep mantle source. The Ninetyeast Ridge basalts have isotopic compositions intermediate to those of the volcanic products of the Kerguelen and Amsterdam-St. Paul hotspots, yet simple mixing of the compositions measured to date within these hotspot systems cannot explain this isotopic variability. A Kerguelen and/or Amsterdam–St. Paul mantle plume origin for the Ninetyeast Ridge implies that the plume sources are compositionally heterogeneous in time and space. At a regional scale, the isotopic variability of the Ninetyeast Ridge basalts, together with other Indian OIB, is part of larger mixing trends between several distinct components in the Indian Ocean. Most of the isotopic variability of Indian OIB can be accounted by mixing between the common mantle component (C or Fozo) and enriched mantle compositions (mostly between EM-1 and EM2). The EM–1, Dupal-like characteristics of Indian MORB and Indian OIB that formed at the onset of continental rifting can be ascribed to shallow level contamination by lower continental crust and/or subcontinental lithospheric mantle and some upper crust material. In contrast, no evidence for such shallow contamination is found in the Ninetyeast Ridge basalts and other OIB formed in intraplate setting. The current isotopic and trace element dataset of the Ninetyeast Ridge basalts is not consistent with both the entrainment of Indian mantle asthenosphere into its source, and the presence of upper continental crustal material, which rules out the possibility of shallow contamination. Instead, the EM-1-like characteristics of the Ninetyeast Ridge basalts are best explained by some contribution of lower continental crust and pelagic sediments in their deep mantle source.  138  C HAPTER 5  Pb-Sr-Nd Systematics of the Early Mauna Kea Shield Phase and Insight Into the Deep Mantle Beneath the Pacific Ocean  139  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  5.1. Synopsis In this study, we present high-precision Pb, Sr and Nd isotopic compositions of 40 basalts recovered during the final drilling phase of the Hawai’i Scientific Drilling Project (HSDP2-B and -C). The samples are from the last 408 metres of the 3500 metre-long drill core and represent ages >650 ka in the stratigraphic record of Mauna Kea volcano. Two sample groups can be distinguished based on their isotopic variability compared to the rest of the core. Over a depth interval of 210 m (3098.2 to 3308.2 m) the samples show very restricted Pb, Sr and Nd isotopic variation (e.g., 5x and 3x smaller variation for and  86  Pb/204Pb  Sr/87Sr, respectively, than observed for younger samples). Samples from the bottom  170 metres show the largest range of  206  Pb/204Pb (18.304–18.693) and  208  Pb/204Pb (37.923–  38.270) compared to the rest of the core. These older lavas extend the 208  206  206  Pb/204Pb and  Pb/204Pb record of Mauna Kea both to significantly more radiogenic values, similar to the  isotopic compositions shown by “ancestral” Kilauea lavas, and to significantly less radiogenic values, similar to those observed in Mauna Kea and Kohala post-shield lavas. The two sample groups identified also define two tight arrays in diagrams of 206  208  Pb/204Pb -  Pb/204Pb. The majority of these older Mauna Kea basalts plot along the “Kea-mid8” array  [Eisele et al., 2003] that encompasses ~75% of the Mauna Kea drill core. Some samples from the two groups, characterized by low-SiO2 lie within the “Kea-hi8” array, indicating that a component characterized by high  208  Pb*/206Pb*, was already being sampled early in  Mauna Kea’s history. Compared to the previous 3100 m of drill core, the more restricted Pb, Sr and Nd isotopic compositions of the samples recovered during HSDP2-B indicate sampling of a more homogeneous source domain during the time interval represented by the eruption of these basalts. Similar to Mauna Loa, the earlier stages (older lavas) of Mauna Kea volcanism are isotopically more variable than subsequent stages. Overall, the isotopic heterogeneity in Mauna Kea shield lavas can be explained by mixing variable proportions of four isotopically distinct components intrinsic to the Hawaiian mantle plume. These components are: the more radiogenic “Kea” component, a high  208  Pb*/206Pb* (producing  low-SiO2 basalts) component similar to Loihi, and two components with less radiogenic Pb isotopic compositions. Besides being a common and long-lived component in the Hawaiian  140  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  plume, the isotopic similarities of “Kea” to the common mantle component “C”, and the fact that it is positioned at the convergence between other Pacific Ocean island basalt groups, suggests that “Kea” is a common composition in the deep mantle beneath the Pacific Ocean basin.  5.2. Introduction The age-progressive Hawaiian-Emperor volcanic chain in the Pacific Ocean is the classic example of intraplate hotspot volcanism attributed to a deep-seated mantle plume [e.g., Morgan, 1971; Courtillot et al., 2003; DePaolo and Weis, 2007]. This simple tectonic setting, together with the distant location from plate margins and large rates of magma flux, make Hawaiian volcanoes one of the best places to study the chemical evolution and structure of mantle plumes and consequently of the deep mantle [e.g., DePaolo et al., 2001; Bryce et al., 2005]. As the Pacific plate moves across the Hawaiian plume, at a velocity of 9– 10 cm/yr, individual volcanoes grow and evolve through several stages (pre-shield, shield, post-shield, and a much later one, rejuvenated) as they sample different areas of the meltingregion of the plume [e.g., Clague and Dalrymple, 1987]. The geochemistry of continuous eruptive sequences of individual volcanoes hence provides a record of the geochemical timeseries of the melting region of the plume [e.g., Hauri et al., 1996; Lassiter et al., 1996; DePaolo et al., 2001; Bryce et al., 2005]. Isotopic compositional variations occur over various time scales during the eruptive life of a Hawaiian volcano. Both short-term (decadal to centennial) and long-term (millennial) isotopic variations have been recognized in the shield lavas of Hawaiian volcanoes [e.g., Kilauea, Pietruska and Garcia, 1999; Marske et al., 2007; 2008], Mauna Loa, DePaolo et al., 2001, Mauna Kea, Eisele et al., 2003; Kurz et al., 2004, Kauai, Salters et al., 2006; Fekiacova et al., 2007], all within the larger time scale (≥1Myr) of volcano growth and compositional evolution [e.g., Frey et al., 1990; Garcia et al., 2006]. The ~3500 km long volcanic core recovered by the Hawaii Scientific Drilling Project (HSDP) constitutes the longest stratigraphically controlled record of the magmatic output of a single  141  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  long-lived volcano sampled thus far. The analysis of the temporal geochemical covariations within the first ~3100 m of core has allowed for the identification of compositional heterogeneities within the Hawaiian plume as well as their maximum and minimum sizes [e.g., Blichert-Toft et al., 2003; Eisele et al., 2003; Abouchami et al., 2005; Bryce et al., 2005]. In addition, direct comparison with lavas from other Hawaiian volcanoes erupted at equivalent stages of volcano growth has led to the formulation of new models for the chemical structure (i.e., distribution of heterogeneities within the plume conduit) of the Hawaiian plume and its deep mantle source [e.g., DePaolo et al., 2001; Blichert-Toft et al., 2003; Kurz et al., 2004; Abouchami et al., 2005; Bryce et al., 2005; Farnetani and Hofmann, 2010] The last ~400 m of the HSDP sampled a series of submarine tholeiitic basalts erupted in the early shield-building stage of Mauna Kea. In this study, we use the Pb, Sr, and Nd isotopic analysis of 40 whole rock samples to identify short-term isotopic fluctuations that reflect compositional heterogeneities sampled in the early phase of growth of Mauna Kea. We examine the >650 kyr isotopic record of the magmatic history of Mauna Kea to evaluate both the short- and long-term isotopic variations within the core, and provide constraints on the chemical structure of the Hawaiian mantle plume and deep Pacific mantle.  5.3. Hawaii Scientific Drilling Project: Geological Setting and Core Stratigraphy The Hawaii Scientific Drilling Project was a multidisciplinary international scientific effort to test the models of growth and chemical evolution of Hawaiian volcanoes as they drift over the mantle plume by systematically sampling a continuous stratigraphic sequence of lavas from a large Hawaiian volcano [Stolper et al., 1996; DePaolo et al., 2001]. The HSDP drill sites were located on the northeast side of the island of Hawaii, on the east flank of the Mauna Kea volcano, near Hilo Bay in an abandoned rock quarry adjacent to the Hilo International Airport (Figure 5.1). Mauna Kea was the targeted volcano as it is the youngest Hawaiian volcano that has completed its life cycle of major growth stages (pre-shield, shield, 142  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  160°W  158°W  156°W  Pacific Ocean  KAUA’I  22°N NI’IHAU  MOLOKA’I O’AHU  MAUI  22°N  Kea  LANA’I KAHO’OLAWE  HAWAI’I  20°N  156°W  155°W 20´  154°W 40´  19°N  Loa LOHI’I  20°N  19°N  500  00 10  20°N  Sub  Ko  50  300  00  0  Ma u na  Ke a  S  160°W  159°W  156°W  ho  0  0 10  me rg ed  reline  MK  00  Hu  HSDP2  0 00  2  20  19°N 40´  3000  ML  Ki  4000  19°N 20´  1000  00  50  1000  19°N  2000  Lo  4000  0  156°W  155°W 20´  20  40 km  154°W 40´  Figure 5.1. Location maps of the Hawaiian Islands and HSDP2 drill site. (a) Overview map of the Hawaiian Islands, showing the Loa and Kea volcano trends. (b) Enlarged view of the topography and bathymetry (500 m interval contours) of the island of Hawai’i (maps drawn using GeoMapApp: http://www.geomapapp.org). The boundaries of the five volcanoes (Ko, Kohala; Hu, Hualalai; MK, Mauna Kea; ML, Mauna Loa; Ki, Kilauea) are shown, as well as the submerged shoreline of Mauna Kea. The location of the Hawai’i Scientific Drilling Project deep hole (HSDP2) drilled during 1999 and 2004-2007 is identified by the red star, near Hilo Bay. The Loihi seamount (Lo) occurs to the southeast of the Big Island.  143  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  post-shield) of a typical Hawaiian volcano [e.g., Clague and Dalrymple, 1987; Stolper et al., 1996; Garcia et al., 2007; Stolper et al., 2009]. At this location, almost 250 metres of Mauna Loa lavas overlie Mauna Kea lavas and thus also allowed for a comparative study of the lavas of the two consecutive volcanoes. The first phase of the project (HSDP1) was carried out in 1993 with a “pilot hole” to a depth of 1052 metres below sea level (mbsl) to evaluate the feasibility of the project. The success of the HSDP1 [e.g., Stolper et al., 1996] led to the deep drilling of the HSDP2, a ~3500 metre long core that was recovered in two phases over a total period of almost five years. The main drilling phase of the HSDP2 was carried out throughout 1999, during which time the hole reached 3110 metres below sea level (mbsl) [Garcia et al., 2007]. The second drilling phase was accomplished in two stages. Starting in 2003 with the enlargement of the hole’s diameter, coring began in late 2004 and proceeded until early 2005 (phase 2-B). Coring restarted in late 2006 and reached completion in early 2007 at 3508 mbsl (phase 2-C) [Stolper et al., 2009]. On the basis of observed contacts and/or variations in mineralogical, lithological, and structural features, a total of 389 lithological units were identified within the HSDP2 core [Stolper et al., 2009]. Five depth zones were recognized (Figure 5.2): (1) subaerial Mauna Loa lavas (surface to 246 mbsl), mostly composed of tholeiitic flow units, all with Mauna Loa geochemical characteristics, and some interspersed sedimentary units [Rhodes and Vollinger, 2004; Garcia et al., 2007, Stolper et al., 2009]; (2) subaerial Mauna Kea lavas (246 to 1079 mbsl), composed of interbedded alkalic and tholeiitic flow units belonging to Mauna Kea’s Hamakua volcanic series formed during its post-shield phase (down to ~340 mbsl), and by tholeiitic flow units interspersed with some sedimentary units, marking the end of the shield phase of Mauna Kea’s volcanic cycle [Rhodes and Vollinger, 2004]; (3) shallow submarine Mauna Kea lavas (1079 to 1984 mbsl), mostly comprising hyaloclastite debris flows and few massive tholeiitic flow units; (4) deep submarine Mauna Kea lavas (1984 to 3098 mbsl), mostly composed of pillow lavas and few hyaloclastite and massive flow units, all of tholeiitic composition; and (5) submarine Mauna Kea lavas from the final phase of  144  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  0 HSDP-1 (1993)  246 500  Depth (mbsl)  HSDP-2 (1999)  Mauna Kea Subaerial (including alkalic lavas)  ML MK  Mauna Kea Subaerial tholeiite  ash or soil  1000 1079 1500  Subaerial Submarine  rotary drilled intervals massive hyaloclastite  1984 2000  2500  HSDP-2B & C (2003-2007)  Mauna Loa Subaerial tholeiite  3000  1st intrusive 1st pillow  pillow  intrusive  3098 3500  Figure 5.2. Simplified lithologic column of the HSDP2 drill core (adapted from Stolper et al. [2009]). The different lithologies are represented by different patterns as indicated. The depth scale is in metres below sea level (mbsl) and key depths are identified: boundary between Mauna Loa (ML) and Mauna Kea (MK) lavas; transition between subaerial and submarine MK lavas; first occurrence of intrusive rocks and pillow lavas. The dashed box highlights the stratigraphic levels drilled during phases B and C of the HSDP2, from where the samples analyzed in this study were recovered.  145  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  drilling (3098 to 3508 mbsl). In this final phase of drilling (HSDP2–B and –C), 44 flow and intrusive units were identified, consisting primarily of submarine pillow lavas (~60%), with lesser amounts of hyaloclastites (~17%), massive volcanic units (~12%), pillow breccias (~10%), and intrusive units (~1%) [Stolper et al., 2009]. The youngest dated Mauna Kea sample in the core of the HSDP2 main phase, an alkalic basalt at ~277 mbsl, has a  40  Ar/39Ar age of 236 ± 16 kyr and the deepest dated  sample, at a depth of 2789 mbsl, has a  40  Ar/39Ar age of 683 ± 82 kyr [Sharp and Renne,  2005]. Assuming a linear fit to the 40Ar/39Ar ages, Sharp and Renne [2005] determined mean accumulation rates of ~9 m/kyr and ~0.9 m/kyr for the shield and post-shield sequences, respectively, over the 400 kyr of volcanism recorded by the ~2.7 km thick section of Mauna Kea basalts. Based on the linear shield lavas accumulation rates, the bottom of the main phase of the HSDP2 drill core was estimated to be ~635 kyr [Sharp and Renne, 2005]. No ages have been determined on samples from the last drilling phase. Assuming the same shield lavas accumulation rates, the ages of these deeper samples are inferred to range between ~647 and ~694 kyr (Table 5.1).  5.4. Previous Geochemical Results from the HSDP2 The samples within the Mauna Kea section of the HSDP2 core show a bimodal distribution of their SiO2 contents (<50 wt% and ≥50 wt%), evident both from whole rock XRF compositions [Rhodes and Vollinger, 2004] and from glass electron microprobe analysis [Stolper et al., 2004; 2009]. Different correlations between SiO2 and glass volatile content, incompatible trace element concentrations and isotope ratios led to the division of the HSDP core into distinct geochemical groups [Huang and Frey, 2003; Rhodes and Vollinger, 2004; Stolper et al., 2004]. Samples in the high-SiO2 group are tholeiitic basalts and picrites. They are distributed throughout the Mauna Kea shield section, representing the majority (60 %) of basalts from Mauna Kea [Huang and Frey, 2003; Rhodes and Vollinger, 2004; Stolper et al., 2004; 2009]. High-SiO2 samples typically have high Zr/Nb values (11– 13), corresponding to the type-1 magmas of Rhodes and Vollinger [2004]. They are  146  Table 5.1. Geochemical Characteristics of Basalts Sampled from the Last Drilling Phases (-B and -C) of the HSDP2 Core Depth  Sample [Run-Interval-Unit-Core Box] - 2A - 3 - 2B - 2D - 4 - 5 - 7 - 7 - 9 - 10 - 11 - 13 - 13 - 13 - 14 - 14 - 14 - 15 - 17 -18A - 19B - 20B -  HSDP-2c R149 - 1.2 - 1.6 - 22 R154 - 2.1 - 2.95 - 23 R155 - 2.7 - 3.5 - 23 R158 - 1.75 - 2.10 - 24 R162 - 1.55 - 2.3 - 25 R163 - 2.1 - 2.6 - 26 R165 - 0.75 - 1.45 - 29 R168 - 3.7 - 4.15 - 30 R176 - 5.93 - 6.5 - 30 R184 - 1.15 - 2.1 - 32 R187 - 0.9 - 1.8, 2.05 - 2.5 - 33 R192 - 4.4 - 5.4 - 35 R198 - 0.5 - 1.9 -37A R204 - 1.1 - 1.8 - 37E - 40 R205 - 7.5 - 8.15 R210 - 3.3 - 3.95 - 43 R219 - 5.55 - 6.2, 7.25 - 7.7 - 44 R222 - 8.0 - 8.8 - 45  Inferred Ageb  Lithologyc  b  Typed  SiO2 (wt%)  MgO (wt%)  Na2O (wt%)  K 2O (wt%)  P2O5 (wt%)  LOIe (wt%)  A.I.f  Mg#g  Zr/Nb  Zr/Y  1 1 3 6 7 8 11 18 26 30 31 34 47 56 60 67 72 75 81 83 85 87  3097.7 3098.2 3102.4 3109.4 3111.5 3115.2 3122.5 3141.2 3160.8 3171.1 3173.4 3181.2 3215.5 3234.7 3244.8 3263.9 3277.0 3285.8 3300.1 3305.9 3308.2 3313.5  647.3 647.4 647.9 648.7 648.9 649.4 650.2 652.4 654.7 655.8 656.1 657.0 661.0 663.2 664.4 666.6 668.1 669.2 670.8 671.5 671.8 672.4  pillows massive intrusive pillows pillows pillow breccia pillows pillows pillows pillows pillow breccia pillows pillows pillows pillows massive volcanic massive volcanic massive volcanic pillows pillows massive intrusive pillows massive intrusive  MK 1 MK 3 MK 1 MK 1 MK 1 MK 1 MK 1 MK 1 MK 1 MK 1 MK 1 MK 1 MK 1 MK 1 MK 1 MK 1 MK 1 MK 1 MK 1 MK 1 MK 4 MK 1  50.3 47.3 50.4 50.4 50.4 50.2 50.3 50.3 50.4 50.2 50.2 50.4 50.3 50.1 50.3 50.7 50.2 50.4 50.8 50.4 50.4 49.8  7.75 12.7 7.96 7.71 7.72 7.77 8.01 8.16 8.03 7.86 7.91 8.14 8.00 7.97 8.10 7.42 7.91 8.11 6.60 7.90 6.94 11.7  2.39 1.91 2.33 2.37 2.35 2.49 2.35 2.40 2.46 2.27 2.28 2.28 2.35 2.35 2.28 2.41 2.35 2.40 2.65 2.49 2.42 2.19  0.48 0.95 0.31 0.39 0.37 0.33 0.31 0.30 0.36 0.34 0.33 0.37 0.32 0.33 0.40 0.46 0.41 0.28 0.34 0.33 0.31 0.30  0.25 0.20 0.24 0.25 0.25 0.25 0.25 0.24 0.25 0.25 0.25 0.24 0.23 0.24 0.23 0.26 0.23 0.23 0.27 0.24 0.24 0.20  0.75 5.65 0.91 0.72 0.86 -4.46 0.92 1.10 0.80 0.86 1.04 0.65 0.70 0.75 0.60 0.34 0.45 0.79 0.61 0.90 1.14 0.36  -1.30 -0.19 -1.54 -1.43 -1.46 -1.32 -1.49 -1.44 -1.37 -1.53 -1.51 -1.55 -1.50 -1.40 -1.47 -1.42 -1.37 -1.52 -1.34 -1.37 -1.47 -1.46  0.58 0.68 0.59 0.58 0.58 0.58 0.58 0.59 0.58 0.58 0.58 0.59 0.58 0.58 0.59 0.56 0.58 0.59 0.54 0.58 0.55 0.69  11.1 10.1 11.2 11.1 11.1 11.2 11.2 11.1 11.5 11.1 11.2 11.5 11.2 11.1 11.3 11.3 11.3 11.3 11.1 11.2 10.5 12.8  6.0 5.6 5.9 5.9 5.9 5.9 6.0 5.9 5.9 5.9 5.9 5.8 5.8 5.8 5.8 5.9 5.6 5.7 6.0 5.9 6.1 5.6  - 94 - 96 - 97 - 99 - 101 - 102 - 103 - 106 - 112 - 121 - 123 - 127 - 133 - 140 - 142 - 147 - 157 - 159  3333.3 3339.5 3341.4 3348.3 3353.9 3357.0 3359.8 3366.3 3381.3 3400.9 3405.3 3418.1 3435.2 3453.4 3458.4 3472.2 3500.9 3505.7  674.7 675.4 675.6 676.4 677.1 677.4 677.7 678.5 680.2 682.5 683.0 684.5 686.5 688.6 689.2 690.8 694.1 694.7  pillows massive volcanic massive volcanic pillow breccia pillows hyaloclastite pillows pillows pillows massive intrusive pillows massive volcanic pillows pillows pillows hyaloclastite pillows pillows  MK 4 MK 1 MK 1 MK 4 MK 4 MK 4 MK 4 MK 4 MK 4 MK 3 ML! MK 1 MK 4 MK 4 MK 4 ML! ML! MK 4  50.7 48.3 47.9 50.8 50.7 49.6 49.2 49.6 49.5 47.6 48.6 47.1 47.1 45.9 45.5 50.4 50.2 50.0  6.75 15.6 17.4 6.83 6.85 9.66 9.72 9.61 9.55 12.3 15.7 20.3 20.8 24.2 25.4 9.67 9.88 6.89  2.13 1.78 1.37 1.64 2.10 1.44 1.87 1.91 1.93 2.06 1.80 1.36 1.32 1.24 1.09 2.20 1.57 2.54  0.36 0.27 0.10 0.30 0.37 0.31 0.28 0.31 0.34 0.14 0.16 0.20 0.19 0.15 0.12 0.30 0.33 0.35  0.25 0.16 0.18 0.25 0.24 0.23 0.22 0.22 0.22 0.21 0.18 0.17 0.15 0.13 0.12 0.26 0.27 0.26  0.46 1.10 2.26 0.79 0.76 0.57 1.18 0.55 0.35 1.82 0.63 0.32 0.35 0.29 0.67 0.66 0.87 0.56  -1.83 -1.35 -1.79 -2.40 -1.83 -2.17 -1.61 -1.68 -1.57 -0.94 -1.56 -1.41 -1.45 -1.13 -1.15 -1.69 -2.22 -1.14  0.55 0.73 0.75 0.55 0.55 0.63 0.63 0.63 0.62 0.67 0.74 0.78 0.79 0.81 0.82 0.64 0.64 0.55  10.9 13.0 12.6 10.8 10.9 9.7 10.1 10.0 10.2 10.6 13.0 11.2 10.9 10.8 11.0 15.1 13.3 10.6  6.3 5.6 5.9 6.2 6.3 6.3 6.3 6.3 6.3 5.9 5.8 6.2 5.9 5.8 5.8 5.8 6.3 6.3  mbsl = meters below sea level. Age of the samples infered from the model age curve of Sharp and Renne [2005]. c Five lithologic types identified by the on-side logging team [Garcia et al., 2007; Stolper at al., 2009]. d Chemical Types defined by Rhodes and Vollinger [2004], based on SiO2 content and Zr/Nb. d A.I. = alkalinity index (AI = total alkalis - (SiO2 x 0.37 - 14.43)) [Rhodes, 1996]. e LOI = weight loss-on-ignition after 30 min at 1020ºC. f Mg# = Mg/(Mg+Fe) after adjusting Fe3+/total Fe to 0.1 [Rhodes, 1996; Rhodes and Vollinger , 2004]. All geochemical data presented in this table is from Rhodes et al. (in preparation). a  Chemical  147  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  HSDP-2b R6 - 0.85-1.2 R6 - 2.15-3.0 R8 - 0.15-0.7 R10 - 4.35-4.7 R11 - 5.3-5.7 R13 - 0.1-0.5 R16 - 5.2-6.0 R30 - 4.25-5.2 R51 - 2.4-3.6 R56 - 1.0-1.5 R57 - 5.0-5.9 R60 - 7.6-8.4 R80 - 2.8-3.5 R93 - 1.4-2.0 R101 - 4.4-4.9 R108 - 0.0-0.7 R112 - 7.1-7.7 R116 - 6.25-7.2 R122 - 1.85-2.65 R125 - 4.9-5.3 R127 - 1.4-2.2 R129 - 5.8-6.5  (mbsl)a  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  characterized by high 208Pb/204Pb [Eisele et al., 2003], 176Hf/177Hf [Blichert-Toft et al., 2003], and 143Nd/144Nd [Bryce et al., 2005], and low 3He/4He [Kurz et al., 2004] and 87Sr/86Sr values [Bryce et al., 2005] relative to low-SiO2 samples. However, some high-SiO2 basalts are characterized by low Zr/Nb values and have been considered a distinct group, the type-4 magmas of Rhodes and Vollinger [2004]. Samples with low SiO2 contents are found at different levels in the stratigraphy of Mauna Kea, forming discrete groups that differ mostly in elemental abundances and isotopic compositions. The intercalated alkalic and tholeiitic basalts from the post-shield group, at the top of Mauna Kea section, are characterized by low SiO2 and high TiO2 contents, high incompatible element abundances, and high La/Yb, Sm/Yb, low Zr/Nb [Huang and Frey, 2003; Rhodes and Vollinger, 2004]. Their isotope compositions are generally less radiogenic, except for  143  Nd/144Nd which define the high end of range of values [Bryce et al., 2005].  Within the shield section, low-SiO2 tholeiitic basalts are found near the top (above 850 mbsl) and deeper in the core section (between 1974 and 3098 mbsl). Both stratigraphic groups have identical isotopic compositions, but are discriminated by the higher Zr/Nb values of the younger basalts. These correspond to the type-2 and type-3 magmas of Rhodes and Vollinger [2004], respectively. The geochemical differences between these distinct groups of samples have been inferred to reflect changes in melt production, depth of melting and differences in the source components sampled during the ~400 kyr of eruptive history of Mauna Kea as it crossed ~40 km over the Hawaiian plume [Feigenson et al., 2003; Huang and Frey, 2003; Rhodes and Vollinger, 2004; Stolper et al., 2004]. Correlations between the isotopic and trace element compositions of the HSDP tholeiites require that the high and low-SiO2 groups formed from compositionally distinct source components, similar to those of Kilauea and Loihi, respectively [Huang and Frey, 2003; Rhodes and Vollinger, 2004]. The high-SiO2 tholeiites (type-1 and -4) are inferred to derive from melting of depleted peridotite, whereas the low-SiO2 tholeiites, located deeper in the core (type-3), are consistent with partial melting (30–40 kbar) of an isotopically enriched source, such as lherzolite [Feigenson et al., 2003;  148  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  Rhodes and Vollinger, 2004; Stolper et al., 2004]. The late-stage (type-2) and post-shield low-SiO2 samples represent a transition to more alkalic compositions and are considered to derive from low degrees of partial melting of an isotopically depleted peridotite source, similar to the source of the high-SiO2 samples, at higher pressures within the garnet stability field [Feigenson et al., 2003; Stolper et al., 2004]. Basalts from Mauna Kea have higher εNd, εHf, 3  206  Pb/204Pb, and lower  87  Sr/86Sr and  He/4He compared to Mauna Loa basalts, indicating they are derived from a more depleted  source than that of Mauna Loa basalts [Lassiter et al., 1996; DePaolo et al., 2001; BlichertToft et al., 2003; Kurz et al., 2004; Bryce et al., 2005]. The systematic differences in the isotopic compositions of Mauna Kea basalts compared to those from Mauna Loa, or between the “Kea-trend” and “Loa-trend” volcanoes [Dana, 1849; Jackson et al., 1972], have been interpreted to reflect the compositional structure of the Hawaiian mantle plume. The Sr, Nd, Os and He isotopic differences between the Mauna Kea and Mauna Loa basalts recovered by the HSDP are consistent with a concentrically zoned melting region for the Hawaiian mantle plume [Lassiter et al., 1996; DePaolo et al., 2001; Kurz et al., 2004; Bryce et al., 2005]. However, the Pb isotopic differences are best explained by a bilaterally zoned melting region [Abouchami et al., 2005]. Additional support for an axisymmetric zoning is given by the Pb isotope compositions of post-shield lavas of Hawaiian volcanoes [e.g., Fekiacova et al., 2007; Hanano et al., 2010]. In addition to horizontal zoning, a vertical component of heterogeneity is required in the structure of the Hawaiian plume to explain the short-term fluctuations in isotopic compositions throughout the HSDP core. Different models have been proposed to explain how the vertical heterogeneities are distributed within the plume conduit. These include: vertical streaks of at least several tens of km in length within the Hawaiian plume conduit [Eisele et al., 2003], pulses of magma with different isotopic compositions, corresponding to heterogeneities within the hotter plume core derived from the base of the mantle [Kurz et al., 2004; Bryce et al., 2005], and heterogeneous packets of melts distributed vertically within the plume that are sampled consecutively by the volcano [Blichert-Toft et al., 2003].  149  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  5.5. Samples Forty Mauna Kea basalts from the whole rock reference suite of the deeper section of the HSDP cored during the last two drilling phases [HSDP-2B (2003–2005) and HSDP-2C (2006–2007)] were analyzed for Pb-Sr-Nd isotope compositions. A summary of the relevant geochemical characteristics of these samples is presented in Table 5.1. All basalts are tholeiitic and cover a wide range of MgO contents (6.6–25.4 wt%). The majority has high SiO2 contents (>50 wt%) and based on their trace element contents belong to the type-1 (high SiO2, high Zr/Nb) and type-4 (high SiO2, low Zr/Nb) magma groups [Rhodes and Vollinger, 2004; Rhodes unpublished data]. Exceptions are samples R6-2.15-3.0 and R184-1.15-2.1, at 3098.2 and 3400.9 mbsl, respectively, which are most similar to type-3 samples; and samples R210-3.3-3.95 and R219-5.55-6.2, 7.25-7.7, at 3472.2 and 3500.9 mbsl, respectively, which have compositions most similar to those of Mauna Loa [Rhodes and Vollinger, 2004; Rhodes unpublished data].  5.6. Analytical Techniques All isotopic analyses were performed on whole rock powders, with the exception of sample 26-102-R163-2.1-2.6, which was in glass chip form. The sample powders were prepared from rock chips that were repeatedly rinsed in deionized water and pulverized following the crushing and washing procedures described in Rhodes [1996] and Rhodes and Vollinger [2004]. Given the time gap between these two phases of sample recovery, the isotopic measurements presented in this study were obtained in two analytical sessions (mid2006 for the 22 samples from phase -2B and early 2008 for the 18 samples from phase -2C). Chemical purification and mass spectrometric analyses of the 40 Mauna Kea samples were carried out in Class 100 and Class 10,000 clean laboratories, respectively, at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia. For all samples, the Pb, Sr and Nd isotopic compositions were determined on the same sample solution, following the sequential chromatographic purification methods  150  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  described in Weis et al. [2006]. Given the importance of efficiently removing alteration phases and any extraneous Pb contaminant to get accurate and reproducible Pb isotopic compositions of oceanic basalts [Hanano et al., 2009; Chapter 2, Nobre Silva et al., 2009], all sample powders and glass chips were thoroughly acid leached with 6 M HCl prior to digestion and isotopic analysis, following the sequential acid leaching procedure of Weis et al. [2005] and Nobre Silva et al. [2009; 2010; Chapters 2 and 3]. The leached sample powders and chips were digested in a mixture of concentrated HF and HNO3 acids in sealed Teflon® vessels and processed twice on Pb anion exchange columns for Pb purification. The sample fractions that were washed out from the Pb columns, containing all other sample matrix elements, were then processed on other chromatographic ion exchange columns for Sr and Nd purification [Weis et al., 2006]. All Pb isotopic compositions were determined on a Nu Plasma MC-ICP-MS (Nu Instruments Ltd, UK), under dry plasma conditions using a membrane desolvator (Nu DSN100) for sample introduction. All analyses were obtained in static multi-collection mode, with masses 202 to 208 measured in collectors L2 to H4, respectively, following the procedures detailed in Weis et al. [2006]. Instrumental mass fractionation was monitored and corrected on-line using a Specpure® Tl standard solution with potential  204  Hg isobaric interference on the  204  205  Tl/203Tl = 2.3885. The  Pb ion beam was monitored at mass 202 and,  when necessary, was corrected for assuming natural abundances (202Hg/204Hg = 4.35) adjusted for instrumental mass fractionation. All sample solutions were analyzed with the same [Pb]/[Tl] (≈ 4) as the NIST SRM 981 Pb standard solution, to ensure that sample and standard solutions were matrix-matched as much as possible. To accomplish this, the Pb content of a small aliquot of each solution was determined by HR-ICP-MS, on an ELEMENT2 (Thermo Finnigan, Germany), prior to isotopic analysis. All sample and standard solutions were prepared fresh for each analytical session by adding the corresponding amount of Tl standard to the respective solution and diluting with 0.05M HNO3 to obtain an optimal 208Pb ion beam of ~8V (and no less than 2V).  151  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  Sample analysis followed a modified sample-standard bracketing protocol in which the SRM 981 Pb standard was run after every two samples. The fractionation-corrected isotopic ratios were then normalized off-line to the triple-spike values (206Pb/204Pb = 16.9405,  207  Pb/204Pb = 15.4963, and 208Pb/204Pb = 36.7219; [Abouchami et al., 2000]), using  the ln-ln method as described in White et al. [2000] and Albarède et al. [2004]. During the two analytical sessions (2006, n = 140; and 2008, n = 36), analyses of the SRM 981 Pb standard yielded mean values of 207  206  Pb/204Pb = 16.9406 ± 0.0013 and 16.9420 ± 0.0016,  Pb/204Pb = 15.4964 ± 0.0025 and 15.4987 ± 0.0020, and  208  Pb/204Pb = 36.7138 ± 0.0084  and 36.7156 ± 0.0062, respectively, which are within 2SD of each other and of the triple spike values [Abouchami et al., 2000]. United States Geological Survey (USGS) reference material BHVO-2 and Hawaiian rock Kil-93 (sample collected in 1993 from Kilauea’s summit Pu`u `O`o eruption by M. O. Garcia), processed together with the sample sets, yielded Pb isotopic ratios of 207  206  Pb/204Pb = 18.6451 ± 0.0011 and 18.4066 ± 0.0008,  Pb/204Pb = 15.4890 ± 0.0013 and 15.4715 ± 0.0007, and 208Pb/204Pb = 38.206 ± 0.004 and  38.061 ± 0.002, respectively, and are in agreement with published values for leached residues of BHVO-2 [Weis et al., 2006] and with the in-house values of Kil-93 [Chapter 4, Table 4.2]. All Sr and the first batch (HSDP-2B) of Nd isotopic ratios were measured on a TRITON (Thermo Finnigan) thermal ionization mass spectrometer (TIMS) in static mode with relay matrix rotation on a single Ta and double Re-Ta filament, respectively. Sr and Nd isotopic compositions were corrected for mass fractionation using 146  86  Sr/88Sr = 0.1194 and  Nd/144Nd = 0.7219 using an exponential law. The data was then normalized to the average  of the corresponding standard solution (NIST SRM 987 for Sr and La Jolla for Nd) analyzed during the analytical sessions, relative to the values of 87Sr/86Sr = 0.710248 and 143Nd/144Nd = 0.511858 [Weis et al., 2006]. During the course of these analyses, the average  87  Sr/86Sr  value of the SRM 987 Sr standard was 0.710256 ± 0.000015 (n = 10) in 2006 and 0.710246 ± 0.000004 (n = 9) in 2008, and the average 143Nd/144Nd values of the La Jolla Nd standard was 0.511852 ± 0.000010 (n = 10). USGS reference material BHVO-2 and Hawaiian rock Kil-93 yielded Sr isotopic ratios of 0.703462 ± 0.000007 and 0.703582 ± 0.000009,  152  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  respectively, and Nd isotopic compositions of 0.512979 ±0.000007 and 0.512972 ± 0.000007, respectively. These are in agreement with the published values for BHVO-2 [Weis et al., 2006] and with the in-house values of Kil-93 [Chapter 4, Table 4.2]. The Nd isotopic compositions of the second batch of samples (HSDP-2C) were analyzed on the Nu Plasma MC-ICP-MS under dry plasma conditions in static multicollection mode. The analysis procedure followed that detailed in Weis et al. [2006], which showed a good comparison between the Nd isotopic compositions of several USGS reference materials covering a wide range of compositions obtained on the TRITON TIMS and on the Nu Plasma MC-ICP-MS at the PCIGR. Masses 140 to 150 were measured in collectors L3 to H5, respectively. Potential isobaric interferences on masses 144, 148, 150 and 142 were monitored at masses 147 (Sm) and 140 (Ce) and were, when necessary, corrected for assuming natural isotopic abundances (144Sm = 0.030734,  147  Sm = 0.149934,  148  Sm =  0.112406, 150Sm = 0.073796, 140Ce = 0.88449, 142Ce = 0.11114 [Rosman and Taylor, 1998]), adjusted for instrumental mass discrimination using an exponential law as monitored by the 146  Nd/144Nd ratio. Nd isotope ratios were normalized internally for fractionation to  146  Nd/144Nd = 0.7219 and then to the daily average value of the Rennes Nd standard solution,  relative to the value of  143  Nd/144Nd = 0.511973 [Weis et al., 2006]. These analyses were  acquired over two nonconsecutive days, and for each the average value for the Rennes Nd standard was 0.511986 ± 0.000009 (n = 12), and 0.512014 ± 0.000009 (n = 28), respectively. The Hawaiian Kil-93 (n = 2), processed together with the sample batch, yielded an average Nd isotopic ratio of 0.512972 ± 0.000008, which is in agreement with the value obtained on the TIMS during the first batch of HSDP2-B samples and with the in-house values of Kil-93 [Chapter 4, Table 4.2].  5.7. Results 5.7.1. Stratigraphic Variations in Pb-Sr-Nd Isotope Compositions Basalts from the last ~400 metres (phases B and C) of the HSDP2 core yield 206  204  Pb/ Pb = 18.3033 – 18.6936,  207  Pb/204Pb = 15.4707 – 15.4993,  208  Pb/204Pb = 37.924 –  153  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  Table 5.2. Pb Isotopic Compositions by MC-ICP-MS of Mauna Kea Samples From the Last Drilling Phases (-B and -C) of the HSDP2 Core Depth  Sample HSDP-2b R6 - 0.85-1.2 R6 - 2.15-3.0 R8 - 0.15-0.7 R10 - 4.35-4.7  (mbsl)a  206  Pb/204Pbb  2SE  Pb/204Pbb  207  2SE  Pb/204Pbb  208  2SE  Pb/206Pb  207  Pb/206Pb  208  Pb*/206Pb*c  208  rep. 7.1-7.7 6.25-7.2 1.85-2.65 4.9-5.3 1.4-2.2 rep. R129 - 5.8-6.5  3097.7 3098.2 3102.4 3109.4 3109.4 3111.5 3115.2 3122.5 3122.5 3141.2 3160.8 3171.1 3171.1 3173.4 3181.2 3181.2 3215.5 3215.5 3234.7 3244.8 3263.9 3263.9 3277.0 3285.8 3300.1 3305.9 3308.2 3308.2 3313.5  18.5913 18.4953 18.5934 18.5949 18.5920 18.5853 18.5868 18.5866 18.5857 18.5912 18.5936 18.5865 18.5888 18.5931 18.5811 18.5796 18.5833 18.5815 18.5819 18.5853 18.5935 18.5938 18.5845 18.5765 18.5960 18.5920 18.5907 18.5910 18.4286  0.0009 0.0006 0.0009 0.0006 0.0010 0.0008 0.0007 0.0009 0.0012 0.0005 0.0012 0.0010 0.0010 0.0007 0.0012 0.0010 0.0011 0.0030 0.0010 0.0008 0.0007 0.0007 0.0010 0.0010 0.0009 0.0012 0.0009 0.0008 0.0007  15.4854 15.4728 15.4871 15.4882 15.4850 15.4871 15.4881 15.4888 15.4880 15.4859 15.4883 15.4875 15.4838 15.4883 15.4881 15.4866 15.4894 15.4885 15.4879 15.4854 15.4864 15.4868 15.4853 15.4876 15.4887 15.4882 15.4985 15.4993 15.4725  0.0009 0.0006 0.0008 0.0005 0.0009 0.0008 0.0008 0.0009 0.0011 0.0005 0.0014 0.0011 0.0007 0.0008 0.0013 0.0011 0.0012 0.0038 0.0011 0.0006 0.0007 0.0007 0.0009 0.0011 0.0008 0.0013 0.0007 0.0007 0.0007  38.1905 38.1869 38.1962 38.2017 38.1903 38.1885 38.1939 38.1940 38.1934 38.1939 38.2005 38.1926 38.1869 38.2003 38.1867 38.1824 38.1939 38.1914 38.1900 38.1891 38.1954 38.1959 38.1883 38.1858 38.2020 38.1989 38.2018 38.2042 38.0685  0.0026 0.0019 0.0023 0.0013 0.0026 0.0024 0.0025 0.0024 0.0033 0.0016 0.0045 0.0031 0.0024 0.0019 0.0040 0.0036 0.0036 0.0126 0.0031 0.0016 0.0018 0.0020 0.0022 0.0037 0.0022 0.0041 0.0018 0.0021 0.0019  0.8329 0.8366 0.8329 0.8329 0.8329 0.8333 0.8333 0.8333 0.8333 0.8330 0.8330 0.8333 0.8330 0.8330 0.8335 0.8335 0.8335 0.8335 0.8335 0.8332 0.8329 0.8329 0.8332 0.8337 0.8329 0.8331 0.8337 0.8337 0.8396  2.0542 2.0647 2.0543 2.0544 2.0541 2.0548 2.0549 2.0549 2.0550 2.0544 2.0545 2.0549 2.0543 2.0545 2.0551 2.0551 2.0553 2.0553 2.0552 2.0548 2.0542 2.0542 2.0548 2.0556 2.0543 2.0546 2.0549 2.0550 2.0657  0.9386 0.9480 0.9390 0.9395 0.9385 0.9390 0.9394 0.9395 0.9395 0.9390 0.9395 0.9393 0.9385 0.9395 0.9393 0.9389 0.9398 0.9397 0.9395 0.9391 0.9389 0.9390 0.9391 0.9396 0.9394 0.9395 0.9399 0.9401 0.9420  HSDP-2c R149 - 1.2 - 1.6 R154 - 2.1 - 2.95 R155 - 2.7 - 3.5 R158 - 1.75 - 2.10 R162 - 1.55 - 2.3 R163 - 2.1 - 2.6 R165 - 0.75 - 1.45 dup. R168 - 3.7 - 4.15 R176 - 5.93 - 6.5 R184 - 1.15 - 2.1 R187 - 0.9 - 1.8, 2.05 - 2.5 R192 - 4.4 - 5.4 rep. R198 - 0.5 - 1.9 R204 - 1.1 - 1.8 R205 - 7.5 - 8.15 R210 - 3.3 - 3.95 dup. dup. rep. R219 - 5.55 - 6.2, 7.25 - 7.7 R222 - 8.0 - 8.8  3333.3 3339.5 3341.4 3348.3 3353.9 3357.0 3359.8 3359.8 3366.3 3381.3 3400.9 3405.3 3418.1 3418.1 3435.2 3453.4 3458.4 3472.2 3472.2 3472.2 3500.9 3505.7  18.6305 18.5035 18.5488 18.6296 18.6292 18.6885 18.6922 18.6898 18.6936 18.6916 18.5289 18.5929 18.6438 18.6473 18.6297 18.5935 18.6424 18.3041 18.3056 18.3033 18.3059 18.4836  0.0007 0.0007 0.0010 0.0007 0.0007 0.0009 0.0008 0.0008 0.0009 0.0007 0.0006 0.0007 0.0013 0.0007 0.0023 0.0016 0.0008 0.0009 0.0007 0.0006 0.0008 0.0008  15.4906 15.4780 15.4803 15.4911 15.4897 15.4941 15.4947 15.4933 15.4969 15.4938 15.4788 15.4869 15.4889 15.4910 15.4894 15.4984 15.4933 15.4707 15.4733 15.4712 15.4733 15.4729  0.0007 0.0006 0.0010 0.0007 0.0006 0.0011 0.0009 0.0006 0.0008 0.0007 0.0006 0.0007 0.0010 0.0006 0.0020 0.0014 0.0007 0.0010 0.0006 0.0007 0.0006 0.0008  38.2233 38.1296 38.1501 38.2242 38.2195 38.2588 38.2651 38.2601 38.2710 38.2624 38.2058 38.1756 38.2103 38.2167 38.2193 38.2043 38.2347 37.9340 37.9412 37.9341 37.9241 38.1172  0.0017 0.0030 0.0030 0.0024 0.0020 0.0033 0.0026 0.0018 0.0021 0.0018 0.0017 0.0020 0.0026 0.0017 0.0053 0.0035 0.0025 0.0027 0.0016 0.0019 0.0016 0.0027  0.8315 0.8365 0.8346 0.8315 0.8315 0.8291 0.8289 0.8290 0.8290 0.8289 0.8354 0.8329 0.8308 0.8307 0.8314 0.8335 0.8311 0.8452 0.8453 0.8453 0.8453 0.8371  2.0517 2.0607 2.0567 2.0518 2.0516 2.0472 2.0471 2.0471 2.0473 2.0470 2.0620 2.0532 2.0495 2.0495 2.0515 2.0547 2.0510 2.0724 2.0727 2.0725 2.0717 2.0622  0.9382 0.9410 0.9386 0.9384 0.9379 0.9362 0.9365 0.9362 0.9370 0.9363 0.9466 0.9369 0.9355 0.9358 0.9379 0.9399 0.9382 0.9401 0.9407 0.9402 0.9388 0.9417  R11 R13 R16  - 5.3-5.7 - 0.1-0.5 - 5.2-6.0  R30 R51 R56  - 4.25-5.2 - 2.4-3.6 - 1.0-1.5  R57 R60  - 5.0-5.9 - 7.6-8.4  R80  - 2.8-3.5  R93 - 1.4-2.0 R101 - 4.4-4.9 R108 - 0.0-0.7 R112 R116 R122 R125 R127  rep.d  dup.e  dup. dup. rep.  -  mbsl = meters below sea level. Pb isotopic ratios by MC-ICP-MS, normalized to the SRM 981 TS-TIMS reference values of Galer and Abouchami [1998]; the 2SE is the absolute error value of the individual sample analysis (internal error). c208 Pb*/206Pb* defined as (208Pb/204Pbsample - 29.475)/(206Pb/204Pbsample - 9.037) by Galer and O’Nions [1985]. d rep = replicate analysis of the same sample solution by MC-ICP-MS. e dup = full procedural duplicate analysis of the same sample. a  b  154  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  38.271, 87Sr/86Sr = 0.703513 – 0.703631, and 143Nd/144Nd = 0.512968 – 0.513011 (Tables 5.2 and 5.3, Figure 5.3). The Pb and Nd isotopic values are in good agreement with those of Blichert-Toft and Albarède [2009] (see Appendix A) and are, for the most part, similar to those of the preceding, younger lavas (above ~3098 metres; Figure 5.3) [e.g., Eisele et al., 2003; Blichert-Toft et al., 2003; Bryce et al., 2005]. Between 3097.7 and 3308.2 metres depth, the isotopic variability of these older basalts is very restricted (206Pb/204Pb = 18.4953 – 18.5960, 87  Sr/86Sr = 0.703595 – 0.703631, and  143  208  Pb/204Pb = 38.187 – 38.204,  Nd/144Nd = 0.512968 – 0.512990). Type-1 basalts  are the most abundant in this interval and the observed range of isotopic compositions correlates with the limited variation in major and trace element contents of these basalts (Table 5.1, Rhodes unpublished data). These 210.5 metres represent the longest interval (corresponding to ~10 kyr) of restricted geochemical variation observed within the HSDP2 core; the relative variation in  206  Pb/204Pb,  86  Sr/87Sr, and  143  Nd/144Nd is ~11x, ~4x, and ~3x,  respectively, smaller than that observed for younger basalts. The exception is sample R62.15-3.0, a massive intrusive rock at 3098.2 metres depth with low SiO2 and Zr/Nb that has isotopic compositions identical to other type-3 samples recovered during the main drill phase of the HSDP2. In contrast, basalts from the last 192 metres of drill core (3313.5 to 3505.7 metres depth) are most similar to type-4 basalts and show significantly larger Pb isotopic variations (Figure 5.3, Tables 5.2 and 5.3). This group of 19 samples yields the largest range of Pb isotopic compositions (206Pb/204Pb = 18.3033 – 18.6936,  208  Pb/204Pb = 37.924 – 38.291)  compared to basalts from the entire ~3200 metres of core above. These deeper basalts extend the Pb isotopic range (206Pb/204Pb and  208  Pb/204Pb) of Mauna Kea to compositions that are  both significantly more and less radiogenic than those previously observed for this volcano [e.g., Abouchami et al., 2000; Blichert-Toft et al., 2003; Eisele et al., 2003]. This distinction is less obvious in the Sr and Nd isotopic compositions (Figure 5.3). Collectively, basalts within the the lowermost 192 meters of the core present (~2x) smaller Sr and Nd isotopic variations compared to the variations among the younger drill core samples [Bryce et al.,  155  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  Table 5.3. Sr and Nd Isotopic Compositions by TIMS and MC-ICP-MS of Mauna Kea Samples From the Last Drilling Phases (-B and -C) of the HSDP2 Core Sample HSDP-2b R6 - 0.85-1.2 R6 - 2.15-3.0 R8 - 0.15-0.7 R10 - 4.35-4.7 R11 - 5.3-5.7 dup.e R13 R16 R30 R51 R56 R57 R60 R80 R93 R101 R108 R112 R116  -  0.1-0.5 5.2-6.0 4.25-5.2 2.4-3.6 1.0-1.5 5.0-5.9 7.6-8.4 2.8-3.5 1.4-2.0 4.4-4.9 0.0-0.7 7.1-7.7 6.25-7.2  R122 R125 R127 R129  -  1.85-2.65 4.9-5.3 1.4-2.2 5.8-6.5  dup.  HSDP-2c R149 - 1.2 - 1.6 R154 - 2.1 - 2.95 R155 - 2.7 - 3.5 R158 - 1.75 - 2.10 rep.f R162 - 1.55 - 2.3 R163 - 2.1 - 2.6 R165 - 0.75 - 1.45 dup. R168 - 3.7 - 4.15 R176 - 5.93 - 6.5 R184 - 1.15 - 2.1 R187 - 0.9 - 1.8, 2.05 - 2.5 R192 - 4.4 - 5.4 R198 - 0.5 - 1.9 R204 - 1.1 - 1.8 R205 - 7.5 - 8.15 R210 - 3.3 - 3.95 dup. R219 - 5.55 - 6.2, 7.25 - 7.7 R222 - 8.0 - 8.8  Depth (mbsl)a  Sr/86Srb  87  2SE  143  Nd/144Ndc  2SE  εNd  3097.7 3098.2 3102.4 3109.4 3111.5 3111.5 3115.2 3122.5 3141.2 3160.8 3171.1 3173.4 3181.2 3215.5 3234.7 3244.8 3263.9 3277.0 3285.8 3285.8 3300.1 3305.9 3308.2 3313.5  0.703582 0.703595 0.703577 0.703573 0.703565 0.703571 0.703551 0.703564 0.703591 0.703568 0.703564 0.703572 0.703575 0.703574 0.703572 0.703587 0.703569 0.703561 0.703580 0.703579 0.703565 0.703585 0.703588 0.703631  0.000010 0.000007 0.000006 0.000006 0.000007 0.000008 0.000007 0.000007 0.000008 0.000006 0.000008 0.000006 0.000007 0.000006 0.000009 0.000008 0.000009 0.000010 0.000010 0.000008 0.000008 0.000006 0.000006 0.000007  0.512984 0.512968 0.512983 0.512980 0.512977 0.512977 0.512972 0.512974 0.512990 0.512982 0.512975 0.512982 0.512987 0.512983 0.512980 0.512980 0.512972 0.512978 0.512969 0.512972 0.512984 0.512980 0.512988 0.512971  0.000007 0.000006 0.000005 0.000007 0.000006 0.000006 0.000005 0.000006 0.000006 0.000008 0.000005 0.000007 0.000007 0.000006 0.000007 0.000007 0.000005 0.000005 0.000008 0.000006 0.000006 0.000005 0.000006 0.000008  6.7 6.4 6.7 6.7 6.6 6.6 6.5 6.5 6.9 6.7 6.6 6.7 6.8 6.7 6.7 6.7 6.5 6.6 6.5 6.5 6.7 6.7 6.8 6.5  3333.3 3339.5 3341.4 3348.3  0.703559 0.703551 0.703564 0.703566  0.000008 0.000007 0.000007 0.000007  0.512983 0.512996 0.512981 0.512979  0.000008 0.000005 0.000005 0.000009  6.7 7.0 6.7 6.6  3348.3 3353.9 3357.0 3359.8 3359.8 3366.3 3381.3 3400.9 3405.3 3418.1 3435.2 3453.4 3458.4 3472.2 3472.2  0.703560 0.703604 0.703562 0.703563 0.703556 0.703557 0.703513 0.703587 0.703529 0.703559 0.703570 0.703583 0.703554 0.703567  0.000008 0.000008 0.000010 0.000010 0.000008 0.000008 0.000010 0.000007 0.000009 0.000009 0.000009 0.000007 0.000009 0.000009  0.512980 0.512986 0.5129786 0.512987 0.512981 0.512977 0.512975 0.512993 0.512968 0.512981 0.512988 0.512984 0.512989 0.513000 0.513011  0.000008 0.000007 0.000007 0.000010 0.000006 0.000009 0.000006 0.000006 0.000006 0.000006 0.000007 0.000007 0.000006 0.000007 0.000009  6.7 6.8 6.6 6.8 6.7 6.6 6.6 6.9 6.4 6.7 6.8 6.7 6.9 7.1 7.3  3500.9 3505.7  0.703596 0.703534  0.000007 0.000009  0.513000 0.512994  0.000007 0.000006  7.1 6.9  d  mbsl = meters below sea level. b Sr isotopic ratios measured by TIMS, normalized to the average value of the SRM 987 standard solution of the wheel for each analytical session; the 2σ error is the absolute error value of the individual sample analysis (internal error) reported as x106. c Nd isotopic ratios measured by TIMS (HSDP-2b samples) and MC-ICP-MS (HSDP-2c samples), normalized to the mean value of the La Jolla standard solution during the TIMS analytical session (for the HSDP-2b samples) or to the mean value of the Rennes Nd standard solution run during the analytical session with the MC-ICP-MS (for the HSDP-2c samples); the 2σ error is the absolute error value of the individual sample analysis (internal error) reported as x106. d εNd values are calculated using the 143Nd/144Nd CHUR value = 0.512638 [Jacobsen and Wasserburg, 1984]. e dup = full procedural duplicate analysis of the same sample. f rep = replicate analysis of the same sample solution by MC-ICP-MS. a  156  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  206  0  Pb/204Pb  208  18.0 18.1 18.2 18.3 18.4 18.5 18.6 18.7 18.8 ML MK  400 800  Depth (mbsl)  1200  subaerial submarine  1600 2000 2400  zone 1 zone 2 zone 3  2800 3200  87  0.7034  400  ML MK  0.7035  0.7037  Depth (mbsl)  2400  3600  38.0  38.1  208  38.2  800  1200  1200  1600  1600  2000  2000  2400  2400  2800  2800  3200  3200  3600  3600  1200 1600  zone 1 zone 2 zone 3  0  6.0  εNd 6.5  7.0  7.5  Pb*/206Pb*  0.925 0.930 0.935 0.940 0.945 0.950 0.955 0.960  800  800 subaerial submarine  38.3  400  400  2800 3200  Pb/204Pb  400  0.7038 0.7039 5.5 0  1600 2000  37.9  Sr/86Sr  0.7036  800 1200  37.8  zone 4  3600  0  0  37.7  HSDP2 Mauna Loa Mauna Kea “kea-lo8” “kea-mid8” “kea-hi8” HSDP2-B & C Mauna Kea  2000 2400 2800  zone 4  3200 3600  Figure 5.3. Pb, Sr, and Nd isotopic variations with depth (meters below sea level, mbsl) in the HSDP2 drill core. Red filed symbols at the bottom of the core indicate the samples analyzed in this study. Other HSDP2 data sources are as follows: Mauna Loa, DePaolo et al. [2001] and Blichert-Toft et al. [2003]; Mauna Kea, Blichert-Toft et al. [2003], Eisele et al. [2003] and Bryce et al. [2005]. Also indicated are the Mauna Loa/Mauna Kea and subaerial/submarine boundaries, as well as the four zones identified by Stolper et al. [2004]. All literature data were normalized to the same standard values, i.e., triple spike values of SRM 981 [Abouchami et al., 2000], La Jolla 143Nd/144Nd = 0.511858 and SRM 987 = 0.710248 [Weis et al., 2006].  157  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  2005] and show a systematic progression of increasing  87  Sr/86Sr and increasing  eNd  with  depth (Figure 5.3). In this deeper section of the core, few samples have major and trace element compositions that trend towards Mauna Loa compositions (Table 5.1, Rhodes unpublished data). However, their Pb, Sr and Nd isotope compositions are distinct from those of Mauna Loa, especially their Nd isotopic compositions that are most similar to those of the late shield and post-shield groups found in the upper Mauna Kea sections of the drill core [e.g., Bryce et al., 2005; Hanano et al., 2010].  5.7.2. Isotope Correlations The Sr, Nd and Pb isotopic compositions of the tholeiites from phases -B and -C of the HSDP2 drill core show some degree of correlation, consistent with the trends formed by other Hawaiian volcanoes. With respect to Sr-Nd isotopes, these older tholeiites lie within the compositional range of Mauna Kea, at the depleted end (low 87Sr/86Sr, high 143Nd/144Nd) of the array formed by the Hawaiian Islands (Figure 5.4). Compared to other Mauna Kea basalts, the  87  Sr/86Sr and 143Nd/144Nd of HSDP2-B and -C basalts are displaced towards the  lower and higher limits, respectively, of the isotopic ranges. In Sr-Pb and Nd-Pb isotope diagrams, these basalts extend from the compositional trend defined by other Mauna Kea and Kea-like shield basalts, towards slightly lower 87Sr/86Sr and higher  143  Nd/144Nd values, most  similar to the compositions of the late stage, post-shield lavas from Mauna Kea and Kohala [Holcomb et al., 2000; Eisele et al., 2003; Bryce et al., 2005; Hanano et al., 2010] (Figures 5.5 and 5.6). The HSDP2-B and -C basalts form two distinct Pb isotope arrays that intersect at the radiogenic end (Figure 5.7), extending the range of 206Pb/204Pb and 208Pb/204Pb of Mauna Kea to significantly more radiogenic values, similar to those of “ancestral” Kilauea lavas [Kimura et al., 2006]. One Pb isotope array falls within the main Pb isotopic compositional field for Mauna Kea, best represented by the “Kea-mid8” array defined by the younger HSDP2 samples [Eisele et al., 2003]. The second array is defined by samples R6-2.15-3.0 and R1841.15-2.1 (two massive intrusive units with low SiO2 and Zr/Nb at depths of 3098.2 and  158  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  143  Nd/144Nd  0.51305  DMK  0.51300  Kohala Post-shield  Kohala Late-Shield  Loa Volcanoes Loihi Hualalai Mauna Loa  (0.7031, 0.5130)  LOIHI  0.51295  0.51290  KEA  Mauna Kea Post-shield  Kea Volcanoes  0.51285  0.51280 0.7033  Kohala Kilauea Hilina Bench Mauna Kea “kea-lo8” “kea-mid8” “kea-hi8” HSDP2-B&C  (0.  70  0.7035  0.7037 87  0.7039  42  EM  7,  K  0.5  12  65  )  0.7041  Sr/86Sr  Figure 5.4. Diagram of 143Nd/144Nd versus 87Sr/86Sr for the HSDP2 Mauna Kea basalts compared to shield basalts from other volcanoes on the Big Island of Hawaii (Kohala, Hualalai, Mauna Loa, Kilauea, Loihi) and to the post-shield lavas of Mauna Kea and Kohala. The analytical uncertainty (2s error) is smaller than the symbol sizes. For reference, the “Kea”, depleted Makapuu (DMK), and enriched Makapuu (EMK) end-members from Tanaka et al. [2002] and the “Loihi” end-member from Tanaka et al. [2008] are shown (open hexagons). Data sources are as follows: Mauna Kea, Bryce et al. [2005]; Kilauea, Pietruszka and Garcia [1999], Abouchami et al. [2005], Marske et al. [2007]; Hilina bench, Chen et al. [1996], Kimura et al. [2006]; Kohala, Hofmann et al. [1987], M. O. Garcia (unpublished data, 2001), and D. Weis (unpublished data, 2001); Hualalai, Yamasaki et al. [2009]; Mauna Loa, DePaolo et al. [2001] and D. Weis (in preparation, 2011); Loihi, Garcia et al. [1993]; Mauna Kea post-shield, Bryce et al. [2005]; Hanano et al. [2010]; Kohala post-shield, Holcomb et al. [2000]; Hanano et al. [2010]. Literature data were normalized to the same standard values as noted in Figure 5.3.  159  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  3400.9 metres, respectively), which plot together with other low-SiO2 samples higher in the core within the “Kea-hi8” array [Eisele et al., 2003]. Samples R210-3.3-3.95 and R219-5.556.2; 7.25-7.7, at the bottom of the core, extend the Pb isotopic compositions of Mauna Kea to significantly less radiogenic values. Although these samples have 208Pb/204Pb that are within the range of Mauna Loa values, their  206  Pb/204Pb and  207  Pb/204Pb (not shown) are higher,  comparable to those for lavas from Kohala, Kilauea and post-shield stage of Mauna Kea (Figure 5.7).  5.8. Discussion 5.8.1. HSDP2: A Record of the Evolution of a Single Volcano or the Output of Different Volcanoes? Hawaiian volcanoes evolve through several growth stages (pre-shield, shield, postshield) marked by changes in composition and eruption rates as they are carried by the Pacific plate across the magma-production region of the Hawaiian mantle plume [e.g., Chen and Frey, 1983; Clague and Dalrymple, 1987; Moore and Clague, 1992]. Oceanic lithosphere is considered to not significantly influence the isotopic compositions of Hawaiian lavas [e.g., Lassiter et al., 1996; Fekiacova et al., 2007; Marske et al., 2007; Hanano et al., 2010]. The geochemical variations within the stratigraphic sequence of lavas derived from a single volcano can therefore be interpreted as reflecting the temporal geochemical variations within the magma source [e.g., Lassiter et al., 1996; Rhodes, 1996; Pietruska and Garcia, 1999; DePaolo et al., 2001; Abouchami et al., 2005; Bryce et al., 2005; Marske et al., 2007, 2008]. As different volcanoes grow to form the Hawaiian Islands, their lavas overlap and inter-layer, with older volcanoes becoming partially covered by lavas from younger volcanoes [e.g., Moore and Clague, 1992; DePaolo and Stolper, 1996]. Mauna Kea is built on top of ~6 km thick Cretaceous oceanic crust plus pelagic and clastic sediments, and on the flank of the adjacent older Kohala volcano [e.g., Moore and Clague, 1992; DePaolo and Stolper, 1996].  160  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  87  0.7045  Sr/86Sr Loa Volcanoes  0.7043  Loihi Hualalai Mauna Loa  KOOLAU  0.7041  Kea Volcanoes Kohala Kilauea Hilina Bench Mauna Kea “kea-lo8” “kea-mid8” “kea-hi8” HSDP2-B&C  0.7039 Kohala Late-Shield Hilo Ridge  DMK  0.7035  (17.78, 0.7031)  0.7037  0.7033 17.70  LOIHI  17.90  18.10  18.30 206  KEA  Kohala Mauna Kea Post-shield Post-shield  18.50  18.70  18.90  Pb/204Pb  Figure 5.5. Diagram of 87Sr/86Sr versus 206Pb/204Pb for the HSDP2 Mauna Kea basalts compared to shield basalts from the other volcanoes on the Big Island of Hawaii (Kohala, Hualalai, Mauna Loa, Kilauea, Loihi) and to post-shield lavas of Mauna Kea and Kohala. Symbols are as in Figure 5.4. Data sources for Pb isotopes are as follows: Mauna Kea, Blichert-Toft et al. [2003], and Eisele et al. [2003]; Kilauea, Pietruszka and Garcia [1999], Abouchami et al. [2005], and Marske et al. [2007]; Hilina bench, Chen et al. [1996], Abouchami et al. [2005], and Kimura et al. [2006]; Kohala, Abouchami et al. [2005], M. O. Garcia (unpublished data, 2001), and D. Weis (unpublished data, 2001); Hualalai, Yamasaki et al. [2009]; Mauna Loa, Blichert-Toft et al. [2003] and D. Weis (in preparation, 2011); Loihi, Abouchami et al. [2005]; Mauna Kea post-shield, Hanano et al. [2010]; Kohala post-shield, Holcomb et al. [2000], and Hanano et al. [2010]. Symbols and standard normalization values are as noted in Figure 5.3.  161  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  It has been suggested that the lavas encountered in the deeper section of the HSDP2 core (characterized by low SiO2, high 208Pb*/206Pb*) may not represent the variable output of the Mauna Kea volcano, but instead that of Kohala [Holcomb et al., 2000] or of another unknown volcano [e.g., Stolper et al., 2004; Blichert-Toft and Albarède, 2009]. The isotopic similarities between basalts dredged along the Hilo Ridge below ~1100 mbsl and subaerial lavas from Kohala volcano (Pololu and Hawi volcanic rocks) suggest that the deeper section of the Hilo Ridge may be part of Kohala’s southeast rift zone rather than belonging to Mauna Kea [Holcomb et al., 2000]. Thus, older Kohala lavas should be encountered at the HSDP2 drill site location at ~3400 mbsl [e.g., Stolper et al., 2004]. Based on the elemental and isotopic distinction between the HSDP2 samples shallower than 3098 mbsl and known Kohala samples, Rhodes and Vollinger [2004] and Stolper et al. [2004] argued against the presence of Kohala lavas within this section of HSDP2 core, although they did not dismiss the possibility of encountering Kohala lavas at deeper levels. The last phase of drilling of the HSDP2 extended the core to ~3500 mbsl. Within the deeper part of the core stratigraphy, the abundance of type-4 basalts (low Zr/Nb and high SiO2) increases down-section, whereas type-1 basalts are mostly absent in the last 170 m of the core (Table 5.1, Rhodes personal communication). Some of the basalts at the stratigraphic base of the core do show isotopic similarities to late shield (Pololu) and postshield (Hawi) lavas from Kohala, as suggested by Holcomb et al. [2000], as well as to postshield lavas of Mauna Kea [e.g., Eisele et al., 2003; Bryce et al., 2005; Hanano et al., 2010] (Figures 5.4 to 5.7). To assume that these samples in question are derived from Kohala would imply that ~650 thousand years ago the volcano was reaching the end of its evolution. This is not supported by growth models for Kohala, which at this period of time should have been in its vigorous tholeiitic shield-building stage [Moore and Clague, 1992], nor it is supported by the age range of subaerially exposed Kohala post-shield lavas (~175–450 ka; Clague and Dalrymple [1987]; Aciego et al., [2009]). Although little is known about the history and interactions between Hawaiian volcanoes [Stolper et al., 2004], it is difficult to geometrically reconcile the existence of an unknown volcano older than Mauna Kea [e.g., Baker et al., 2003]. The overall isotopic consistency of the basalts deeper in the core  162  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  143  Nd/144Nd  0.5131  Kohala Late-Shield  0.5130  Mauna Kea Post-shield  Kohala Post-shield Hilo Ridge  DMK  KEA  0.5129  0.5128  Kea Volcanoes  0.5127  Loa Volcanoes Loihi Hualalai Mauna Loa  EMK  0.5126 17.70  17.90  18.10  18.30  206  Pb/204Pb  18.50  Kohala Kilauea Hilina Bench Mauna Kea “kea-lo8” “kea-mid8” “kea-hi8” HSDP2-B&C  18.70  18.90  Figure 5.6. Diagram of 143Nd/144Nd versus 206Pb/204Pb for the HSDP2 Mauna Kea basalts compared to shield basalts from the other volcanoes on the Big Island of Hawaii (Kohala, Hualalai, Mauna Loa, Kilauea, Loihi) and to post-shield lavas of Mauna Kea and Kohala. Symbols, data sources and standard normalization values are as in previous figures.  163  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  stratigraphy with those from the preceding younger Mauna Kea lavas (Figures 5.4 to 5.7) attests to the continuity of the Mauna Kea section, and for the relative homogeneity of the Mauna Kea source throughout its evolution. The ~3263.7 m long Mauna Kea section of the HSDP2 core is thus interpreted to represent a continuous record of the last ~650 kyr of the volcanic activity of the Mauna Kea volcano as it crossed ~60–80 km over the melting region of the Hawaiian mantle plume [e.g., Stolper et al., 2009]. Assuming a model ~1.5 Myr lifetime for typical Hawaiian volcanoes [e.g., Garcia et al., 2006], it is unlikely that the early growth history (i.e., pre-shield stage) of Mauna Kea was sampled by the deep HSDP2 core. The fact that the deeper section of the core is constituted solely of tholeiitic basalts supports this hypothesis. Nevertheless, the duration of individual volcanoes is likely to vary, depending on the proximity of the volcano track to the center of the hotspot’s melting region, which influences the amount of magma supplied to each volcano [e.g., DePaolo and Stolper, 1996; DePaolo et al., 2001; Baker et al., 2003]. According to the model of DePaolo and Stolper [1996], Mauna Kea is likely to have started to grow ~1.050 Myr ago. This would place the bottom of the HSDP2 core (>650 kyr) close to the transition between the pre-shield and shield growth stages. The older basalts recovered from deeper in the stratigraphy of the core show greater isotopic variability compared to the preceding younger tholeiitic basalts, and are isotopically similar, especially in  206  Pb/204Pb,  87  Sr/86Sr and  143  Nd/144Nd, to post-shield stage lavas of  Mauna Kea (Figures 5.3 to 5.7). Together with the low Zr/Nb of these basalts, which are characteristic of Kilauea and Loihi [Rhodes and Vollinger, 2004], this suggests that the deeper samples of the HSDP2 core may represent the very early shield stage of Mauna Kea’s growth history. At this time, the degrees of melting would have increased enough to continuously produce tholeiitic compositions, but the volcano capture zone was still close enough to the edge of the melting region of the plume to sample a similar compositional domain to that sampled by the post-shield lavas.  164  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  208  Pb/204Pb  38.45 38.35  Loa Volcanoes  KEA  Loihi Hualalai Mauna Loa  38.25  LOIHI  38.15  i8  Kea-h  38.05 37.95  Kohala Late-Shield  37.65 17.75  LO 17.85  17.95  A  2.100  KE  18.05  18.15  206  18.25  2.090 204  Pb/ Pb  208  18.45  18.55  18.65  18.75 18.85  Pb/206Pb  Ke  2.080  18.35  8  A  a  Kohala Kilauea Hilina Bench Mauna Kea “kea-lo8” “kea-mid8” “kea-hi8” HSDP2-B&C  hi  37.75  EMK  Kea Volcanoes  a-  37.85  Hilo Ridge Kohala Post-shield Mauna Kea Post-shield  2.070  8  id  -m  a Ke  o8  a-l  Ke  HSDP2 Mauna Loa Mauna Kea “kea-lo8” “kea-mid8” “kea-hi8”  2.060 Mauna Kea Post-shield  2.050  HSDP2-B & C Mauna Kea  b  2.040 0.825  0.830  0.835  0.840 207  0.845  0.850  0.855  0.860  Pb/ Pb 206  Figure 5.7. (a) 208Pb/204Pb versus 206Pb/204Pb and (b) 208Pb/206Pb versus 207Pb/206Pb diagrams for the HSDP2 Mauna Kea basalts compared to shield basalts from other volcanoes on the Big Island of Hawaii (Kohala, Hualalai, Mauna Loa, Kilauea, Loihi) and to the post-shield lavas of Mauna Kea and Kohala. The thick black line represents the Pb isotopic division for Loa and Kea trend volcanoes as defined by Abouchami et al. [2005]. The thin black lines represent the HSDP2 Pb isotopic arrays “Kea hi8”, “Kea-mid8”, and “Kea-lo8” defined by Eisele et al. [2003]. Symbols, data sources and standard normalization values are as in previous figures. The dashed line represents the linear regression though the “Kea-mid8” and Mauna Loa Pb isotopic compositions, indicating the involvement of Loa-like compositions, best defined by the EMK component in the Mauna Kea source region.  165  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  5.8.2. HSDP2 Isotope Variability and the Hawaiian Source Components The isotopic heterogeneity among Hawaiian shield basalts has been interpreted as reflecting mixing between at least three isotopically distinct source components [e.g., Staudigel et al., 1984; Eiler et al., 1996; Hauri et al., 1996]. These include: a relatively “depleted” component (with low 87Sr/86Sr, 3He/4He, 208  Pb/204Pb,  143  Nd/144Nd, and  176  207  Pb/204Pb, δ18O, and high  206  Pb/204Pb,  Hf/177Hf), best observed in basalts from Kilauea (especially  in the Hilina bench) and Mauna Kea, and referred to as the “Kea” component; a “modestly depleted” component (with low higher  208  87  Sr/86Sr, high 3He/4He,  Nd/144Nd, and  176  Hf/177Hf, and  Pb/204Pb) best expressed in basalts from Loihi, hence referred to as the “Loihi”  component; and an “enriched” component (with high 206  143  87  Sr/86Sr,  207  Pb/204Pb, δ18O, and low  Pb/204Pb, 208Pb/204Pb, 143Nd/144Nd, and 176Hf/177Hf) best recognized in basalts from Koolau  and Lanai, and referred to as “Koolau” component [e.g., Eiler et al., 1996]. Given the large isotopic variability among shield basalts from Koolau volcano, recent studies have proposed that two end-member compositions contribute to the Koolau component [e.g., Tanaka et al., 2002; 2008; Fekiacova et al., 2007]. The enriched Koolau end-member, which is best observed in the Makapuu stage lavas, is widely referred to as the “enriched Makapuu component” (EMK). However, two different isotopic compositions have been attributed to the depleted Koolau end-member. Fekiacova et al. [2007] considered the depleted Koolau end-member to be best represented by the Kahili stage lavas and referred to it as the “Kahili” component, whereas Tanaka et al. [2008] considered the depleted Koolau end-member to also include rejuvenated-stage lavas and termed it the “depleted Makapuu component” (DMK). The isotopic variability observed at the scale of growth of a single volcano can be explained by similar mixing relations involving systematically different proportions of these components. Koolau (or enriched Makapuu) and Loihi components are interpreted to contribute to most of the Loa-like volcano variability, and Kea is considered the major component in Kea-like volcanoes [e.g., Eiler et al., 1996; Kimura et al., 2006; Tanaka et al., 2008]. Based on the Pb isotope variations within the first 3100 m of the HSDP2 core, Eisele et al. [2003] recognized the necessity of four distinct components in the Mauna Kea source  166  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  to explain the geometry of the three “Kea” Pb arrays. The Pb isotope variability of the basalts recovered in the bottom 400 m of the core is consistent with this isotopic end-member scenario, involving a radiogenic Pb end-member, most similar to the “Kea” component, and three other distinct components of unradiogenic Pb isotopic compositions with distinct 208  Pb/204Pb, most similar to “Loihi”, EMK, and DMK (Figure 5.7). Whereas “Kea” is a  common component throughout the long-term evolution of Mauna Kea, “Loihi”-, EMK-, and DMK-like components were involved at different stages of the volcano’s eruptive history and each contributed to the shorter-term isotopic variability of Mauna Kea as represented by the different “Kea-hi8”, “-mid8”, and “-lo8” arrays of Eisele et al. [2003], respectively (Figure 5.7). Some basalts in the deeper stratigraphic section of the core, characterized by low-SiO2 contents, plot within the “Kea-hi8” array defined by a restricted group of basalts higher in the stratigraphic section of Mauna Kea [Eisele et al., 2003]. This implies that the component responsible for producing low-SiO2–high  208  Pb*/206Pb* lavas was sampled early  in Mauna Kea’s history and not only for a short time period (~45 kyr, corresponding to a depth interval of ~900 m) of its evolution.  5.8.3. Chemical Structure of the Hawaiian Plume During the Growth of Mauna Kea Hawaiian volcanoes younger than 5 Ma form two subparallel chains, termed Kea and Loa [Dana, 1849; Jackson et al., 1972] that are systematically distinct chemically and isotopically [e.g., Frey and Rhodes, 1993; Abouchami et al., 2005]. The geochemical differences between volcanoes along the two chains are interpreted to be related to the structure of the Hawaiian plume. Two main models of plume structure are invoked to explain the geochemical differences between Kea and Loa volcanoes: a concentrically zoned plume [e.g., Hauri et al., 1996; Lassiter et al., 1996; DePaolo et al., 2001] and a bilaterally zoned plume [Abouchami et al., 2005]. Both purely concentric and bilateral asymmetrical models are however unable to fully explain all the geochemical variability observed in Hawaiian basalts, and a diversity of variations of these structures incorporating vertical heterogeneities within the upwelling plume have been proposed, [e.g., Frey and Rhodes, 1993; DePaolo et  167  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  al., 2001; Blichert-Toft et al., 2003; Kurz et al., 2004; Abouchami et al., 2005; Huang and Frey, 2005; Marske et al., 2007; Xu et al., 2007; Blichert-Toft and Albarède, 2009; Hanano et al., 2010; Farnetani and Hofmann, 2010]. The similarity of the Sr and Nd isotopic compositions of basalts from the deeper section of the HSDP2 core to those of lavas erupted during the post-shield stage indicates that during both the early and late stages of Mauna Kea evolution the volcano sampled a compositionally similar domain within the melting region of the Hawaiian plume. This would be consistent with a concentrically zoned chemical structure of the Hawaiian plume [e.g., Lassiter et al., 1996; DePaolo et al., 2001; Bryce et al., 2005]. However, the Pb isotopic compositions of the deeper basalts of the Mauna Kea HSDP2 core differ from the respective post-shield lavas, having higher  208  Pb/206Pb (Figure 5.7). This indicates that the  early shield stage sampled a domain of the plume characterized by higher Th/U than the post-shield stage lavas. In a comparative study of the geochemistry of post-shield and shield lavas from consecutive Loa and Kea volcano pairs, Hanano et al. [2010] showed that the post-shield lavas retain their Loa- and Kea-like Pb isotope signatures, which also does not support a concentrically zoned plume structure. The isotopic similarity of the old Mauna Kea basalts from the HSDP2 core to prehistoric and to young Kilauea basalts [Kimura et al., 2006; Marske et al., 2007; 2008] supports the proposals that heterogeneities within the Hawaiian plume are sampled by melting regions of consecutive Kea volcanoes [Abouchami et al., 2005; Farnetani and Hofmann, 2010]. Vertical heterogeneity appears to be an intrinsic feature of the Hawaiian plume and is superimposed on radial heterogeneity derived from the thermal structure of the plume [e.g., Hauri et al., 1996]. The Sr and Nd isotopic compositions of Hawaiian basalts follow the concentric thermal structure of the Hawaiian plume as the potential temperature of the plume varies from the periphery to the core of the plume [e.g., DePaolo et al., 2001; Bryce et al., 2005]. As each volcano moves across the melting region of the plume the range of  87  Sr/86Sr and  143  Nd/144Nd compositions sampled increases and decreases, respectively,  from the periphery to the centre of the plume. In contrast, the Pb isotopic compositions  168  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  sampled during the lifetime of Hawaiian volcanoes support an aspect of bilateralism in the distribution of compositional heterogeneities in the plume. These heterogeneities may be restricted to the “core zone” of the plume [Bryce et al., 2005] or distributed through the entire plume radius [e.g., Abouchami et al., 2005; Farnetani and Hofmann, 2010].  5.8.4. The Nature of the “Kea” Component and the Deep Pacific Mantle The enriched nature of the Koolau component is considered to result from incorporation of ancient subducted oceanic crust and sediments into the Hawaiian plume [e.g., Lassiter and Hauri, 1998; Blichert-Toft et al., 1999; Tanaka et al., 2008], however the depleted nature of the “Kea” component continues to be a matter of debate. On the basis of Sr and Nd isotopic compositions, “Kea” was first interpreted to result from entrainment of depleted asthenospheric mantle [Lassiter et al., 1996], but O, Os, and Pb isotopic studies precluded this hypothesis and instead support assimilation of Pacific lithosphere [Eiler et al., 1996], or the presence of recycled oceanic lithosphere [Lassiter and Hauri, 1998] or “young HIMU” material (recycled oceanic crust younger than 1.5 Ga) in the Hawaiian plume source [Thirlwall, 1997; Eisele et al., 2003]. Some of the basalts recovered in the bottom 400 m of the HSDP2 core, together with the pre-historical Kilauea basalts from the Hilina bench [Kimura et al., 2006], have the most “depleted” Sr, Nd and Hf isotopic characteristics and the most radiogenic Pb compositions of all Hawaiian shield basalts. This suggests that during the earlier phase of shield building of a Kea trend volcano the “Kea” component is sampled in higher proportions compared to later periods of volcano growth. The compositions of these basalts erupted early in the growth history of a Kea trend volcano thus provide insight into the nature of the “Kea” component. The correlation between high 143Nd/144Nd,  208-7-6  Pb/204Pb, and low 87Sr/86Sr of “Kea”  compositions reflects derivation from a source that developed high Sm/Nd and (U,Th)/Pb with lower Th/U and Rb/Sr over time. Such geochemical characteristics are normally attributed to a HIMU-like source, generated by recycling of ancient subduction-modified oceanic lithosphere [e.g., Hofmann and White, 1982; Zindler and Hart, 1986; Chauvel et al.,  169  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  87  Sr/86Sr  0.710  MORB Hawaii Galapagos Samoa Society Is. Austral-Cook Marquesas Pitcairn-Gambier Easter Juan Fernandez  0.709 EM II  0.708  “Kea”  0.707 0.706  “Kea” end-member  EM I  0.705 0.704  C  HIMU  0.703 0.702 17.5  εNd 14  a 18.0  18.5  19.0  19.5  206  20.0  20.5  Pb/ Pb 204  21.0  21.5  22.0  22.5  “Kea”  12 10 8  C  6 4 HIMU  2 0 -2 -4 -6 -8  EM I 17.5  18.0  b  EM II 18.5  19.0  19.5  206  20.0  20.5  Pb/ Pb 204  21.0  21.5  22.0  22.5  Figure 5.8. Comparison of the isotopic compositions of Hawaiian basalts and of the “Kea” component to those of other groups of Pacific Ocean island basalts. (a) 87Sr/86Sr versus 206Pb/204Pb, and (b) 143Nd/144Nd versus 206 Pb/204Pb diagrams. Data compiled from the GEOROC database (http://www.georoc.mpch-mainz.gwdg.de). End-members are from Zindler and Hart [1986] and Hanan and Graham [1996].  170  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  1992; Stracke et al., 2005; Willbold and Stracke, 2006]. Compared to “Kea”, HIMU-like compositions have much more radiogenic Pb isotopic signatures, which indicates that “Kea” must be derived from a source with significantly lower µ (238U/204Pb) values to produce 206  Pb/204Pb ratios below 19. Throughout the volcanic activity related to the Hawaiian mantle plume, the “Kea”  component has been a prevalent composition. When comparing the Pb-Sr-Nd isotopic compositions of Hawaiian basalts to other Pacific Ocean island basalts, the “Kea” component occupies an intermediate position towards which general trends (from EM-1, EM-2 and HIMU) converge in binary diagrams (Figure 5.8). The isotopic characteristics of the Kea component are not very different from those of the common mantle component “C” [Hanan and Graham, 1996]. The Hawaiian plume is one of the most productive mantle plumes on Earth, having erupted ~7x106 km3 of volcanic material over the formation of the HawaiianEmperor chain [Vidal and Bonneville, 2004] and Kea-like compositions have been erupted throughout its life [e.g., Regelous et al., 2003; Abouchami et al., 2005; Tanaka et al., 2008]. This may imply that the Kea composition is itself a common and widespread composition within the Pacific mantle.  5.9. Conclusions High-precision Sr, Nd, and Pb isotopic compositions of basalts from the bottom 400 m of the ~3500 m long HSDP2 drill core on Mauna Kea reveal compositional continuity with the overlying basalts and the presence of two distinct sample groups. Basalts from the top 210 m form the longest interval (~10 kyr) of limited variation within the sampled output record of Mauna Kea. This indicates the sampling of a more homogeneous domain within the magma source region of Mauna Kea during the time interval represented by those basalts. In contrast, basalts from the bottom 192 m show the largest range of variation in their Pb isotopic compositions. These older basalts extend the isotopic compositions of Mauna Kea to significantly more radiogenic values, similar to those of “ancestral” Kilauea lavas, and also to significantly less radiogenic values, similar to those of late shield and post-shield lavas  171  CHAPTER 5: ISOTOPIC COMPOSITIONS OF THE EARLY SHIELD PHASE OF MAUNA K EA  (<400 kyr) from Mauna Kea and Kohala. Based on their older ages (> 650 kyr) and compositional characteristics, these basalts likely erupted in the very early shield phase of Mauna Kea, rather than representing part of the output of Kohala or another unknown volcano. The Pb isotopic compositions of the HSDP2 basalts are consistent with the presence of four source components during the growth of Mauna Kea and include the “Kea”, “Loihi”, EMK, and DMK components. “Kea” is the prevailing component throughout the evolution of Mauna Kea, whereas the remaining three components appear to have been involved in different stages of volcano development and contribute to the short-term isotopic variability of Mauna Kea basalts. In Pb-Sr-Nd isotope binary diagrams, “Kea” occupies an intermediate position towards which the general trends formed by other Pacific Ocean island groups (from EM-1, EM-2 and HIMU) converge. This indicates that the “Kea” component is not only the common composition within the Hawaiian mantle plume, but also that it is a common composition within the deep Pacific mantle.  172  C HAPTER 6  Conclusions  173  CHAPTER 6: CONCLUSIONS  6.1 Summary and Conclusions For geochemists, oceanic basalts related to hotspot activity represent the best samples to access and characterize the composition of regions of the deep mantle, in contrast to midocean ridge basalts that sample the relatively shallow asthenospheric mantle. The research reported in this dissertation focused on deciphering the nature and relative proportions of mantle source components involved in the formation of two major hotspot tracks on Earth, the Ninetyeast Ridge [Luyendyk, 1977; Royer et al., 1991] and Hawaii [Dana, 1849; Jackson et al., 1972]. These two hotspot tracks are associated with volcanic activity of the deep and long-lived Kerguelen and Hawaiian mantle plumes, respectively [e.g., Morgan, 1971; Sleep, 1990; Weis et al., 1991; Montelli et al., 2004, 2006; Bryce et al., 2005]. To ensure that the measured isotopic compositions (e.g., Pb-Sr-Nd-Hf) of ocean island basalts are indeed representative of their mantle sources, an analytical investigation of the efficiency of acid leaching in removing post-magmatic alteration phases and extraneous contaminants was first undertaken. Scientific drilling (e.g. Deep Sea Drilling Project [DSDP], Ocean Drilling Program [ODP], Integrated Ocean Drilling Program [IODP] and International Continental Scientific Drilling Program [ICDP]) is an essential sampling method for recovering deep ocean basalt. Drilling provides a continuous sequence of samples that can be studied and interpreted in the context of shallow-level magmatic processes and deeper mantle evolution with time. As part of two major international research programs (IODP and ICDP), this project provided a detailed study of Pb, Hf, Sr and Nd isotope variations on key stratigraphically controlled samples from the Ninetyeast Ridge, the longest linear feature on Earth, and Hawaii. These results were then used to understand the chemical evolution of the Kerguelen and Hawaii hotspot systems and place constraints on the composition of distinct deep mantle domains beneath the Indian and Pacific Ocean basins, respectively.  174  CHAPTER 6: CONCLUSIONS  6.1.1. Leaching Systematics of Ocean Island Basalts The Pb and Sr isotopic compositions of oceanic basalts can be significantly modified by post-magmatic, low-temperature seawater alteration, hindering their use as geochemical tracers of mantle source compositions (Chapters 2 and 3). In contrast, the Nd and Hf isotopic compositions are not significantly affected by alteration. Additional disturbances to the isotopic compositions of basalts can be introduced by assimilation of external contaminants such as Fe-Mn oxides, deep-sea sediments, or loess, and by contamination during sample collection, crushing and grinding. To overcome such chemical disturbances and to obtain the most accurate and reproducible isotopic compositions on basalts required to decipher mantle source components, the samples must be subjected to acid-leaching treatment prior to isotopic analysis. The studies presented in Chapters 2 and 3 show that the elemental concentrations are greatly affected by acid leaching and thus are not reliable magmatic elemental signatures. In contrast, comparison of the isotopic differences between several unleached and acid-leached basalts of different ages, from the Hawaii and Kerguelen hotspots, indicate that reproducible high-precision isotopic compositions, representative of the magma source of oceanic basalts, can be achieved by adequate sample treatment, encompassing both thorough acid leaching and careful matrix elimination.  6.1.2. Identifying Mantle Components in Basalts from Major Hotspot Tracks on Earth High-precision isotopic analysis of ODP Leg 121 basalts from the Ninetyeast Ridge and of basalts from the deeper stratigraphic section of the HSDP2 on Hawaii were obtained following a careful sample preparation and analysis using state-of-the-art analytical instruments such as the Nu Plasma MC-ICP-MS. These results have allowed for enhanced resolution of the source components involved in the Ninetyeast Ridge and in the Mauna Kea volcano. The isotopic compositions of the Ninetyeast Ridge, a 5000 km-long linear aseismic ridge located in the Indian Ocean, are consistent with the involvement of at least three  175  CHAPTER 6: CONCLUSIONS  distinct components intrinsic to the mantle source of the ridge (Chapter 4). Contrary to earlier studies [e.g., Weis and Frey, 1991], the new and more precise isotopic data reveal that the depleted Indian asthenospheric mantle does not take part in its genesis. The isotopic compositions of the Ninetyeast Ridge are intermediate to those of the volcanic products of both the Kerguelen and Amsterdam-St. Paul hotspots. Simple binary mixing between the two cannot account for the isotopic variability observed among the Ninetyeast Ridge basalts. This implies that the mantle sources of the ridge are compositionally more heterogeneous in time and space than previously assumed. The ~3500 metre-long HSDP2 core represents the longest record of the magmatic output of a single volcano yet, covering >600 kyr of the evolution of Mauna Kea. Integration of the isotopic compositions of the older basalts deeper in the stratigraphy of the HSDP2 core obtained during this research (Chapter 5) with those of the younger basalts higher in the stratigraphic section of Mauna Kea confirms the presence of four source components during the growth of this Hawaiian volcano. The “Kea” component is the prevailing component throughout the evolution of Mauna Kea, whereas the remaining three components (“Loihi”, Enriched Makapuu [EMK] and Depleted Makapuu [DMK]) were involved in different stages of volcano development and contributed to the short-term isotopic variability of Mauna Kea basalts.  6.1.3. Fingerprinting Deep Mantle Heterogeneities Beneath the Indian and the Pacific Ocean Basins At the broader, regional scale, isotopic compositions of basalts from both the Ninetyeast Ridge and Hawaiian reflect sampling of a mantle source with geochemical characteristics that are intrinsic their respective mantle domains (Chapters 4 and 5). Within the Indian Ocean basin, the Ninetyeast Ridge, together with other Indian Ocean island basalts, has isotopic characteristics typical of the Dupal isotopic domain [Hart, 1984]. These characteristics are interpreted to result from mixing of a common mantle source of composition similar to “C” or “FOZO” [Hanan and Graham, 1996; Stracke et al., 2005] and  176  CHAPTER 6: CONCLUSIONS  an enriched mantle source component. In contrast to the early stages of evolution of the Kerguelen Plateau that show evidence for contamination by continental crust [e.g., Weis et al., 2001; Frey et al., 2000a, 2002a], the isotopic and trace element compositions of the Ninetyeast Ridge do not indicate incorporation of continental material at shallow depths. This implies that the enriched EM-1-like, Dupal compositions observed in the Ninetyeast Ridge basalts are inherited from its deep source. The compositions are best explained by a contribution of a mixture of pelagic sediments and lower continental crust recycled together with altered oceanic crust in their deep mantle source (Chapter 4). This is in agreement with models proposed for the generation of enriched mantle compositions [e.g., Willbold and Stracke, 2010] and argues for a deep-seated origin the Dupal isotopic domain [e.g., Castillo, 1988; Weis et al., 1991]. Relatively “depleted” isotopic signatures inherent to the mantle plume source have been observed in a number of hotspot settings, including Galapagos, Iceland, Kerguelen, and Hawaii [e.g., Blichert-Toft and White, 2001; Doucet et al., 2002; Fitton et al., 2003; Mukhopadyay et al., 2003; Frey et al., 2005]. Throughout the volcanic activity related to the Hawaiian mantle plume (~7x106 km3 of volcanic material over ~80 Myr; Vidal and Bonneville [2004]), the “Kea” component has been a prevalent composition. Within the Pacific Ocean basin, the relatively “depleted” isotopic compositions of the Hawaiian “Kea” component represent an end-member shared by other Pacific OIB groups (Chapter 5). The isotopic compositions of the “Kea” component are not very different from those of the common mantle component “C” [Hanan and Graham, 1996]. This may imply that the Kea composition is itself a common and widespread composition within the Pacific mantle.  6.2. Directions for Future Research 6.2.1. Broader Leaching Studies The capabilities of analytical instrumentation continue to advance and the level of precision and accuracy required to make progress in our understanding of mantle geochemistry continues to improve. Careful sample preparation and chemical separation protocols are thus inevitable for reliable, accurate, and precise isotopic analyses and  177  CHAPTER 6: CONCLUSIONS  elemental concentrations. The present study focused on a step-by-step characterization of how Pb, Sr, Nd, and Hf isotopes and elemental concentrations in oceanic basalts of tholeiitic to mildly alkalic compositions behave during acid leaching treatment and matrix elimination. To fully understand the effects of seawater alteration on other types of volcanic rocks in or near the ocean environment, it would be beneficial to undertake a similar study using samples of more alkalic compositions such as trachybasalts and basanites, as well as more evolved rocks such as trachytes and phonolites. 6.2.2. Additional Sampling on the Ninetyeast Ridge The results reported in Chapter 4 provide new insight into the mantle source components of the Ninetyeast Ridge. However, the genetic relationship of the Kerguelen and Amsterdam-St. Paul hotspots and the timing of interaction between the two to form the Ninetyeast Ridge are still not clearly understood. Stratigraphically controlled samples from the Ninetyeast Ridge for geochronological and geochemical analyses are sparse and may not be fully representative of the compositions of the main phase of volcanism of the ridge. This is evidenced by the relatively shallow (< 200 m) basaltic basement penetrations at only seven sites during both DSDP and ODP drilling campaigns in the late 1980s. After the IODP site survey in June-July of 2007, the sampling density along the Ninetyeast Ridge increased significantly. Dredging represents a good complement to drilling as it allows for sampling larger amounts of material and recovers a greater range of lithologies from a more diverse sampling region. Although this is a good technique for reconnaissance, it does not allow for stratigraphically-controlled geochemical studies, which are essential for characterizing the temporal geochemical variations that occur at the volcanic edifice scale and for constraining the compositional variability within the plume source region. Additional constraints to the Ninetyeast Ridge source variability would benefit from deepening of the existing drill holes (using the IODP Chikyu research vessel, which has deeper drilling capabilities), as well as from sampling at new drill sites of interest based on the preliminary geochemical results obtained on the dredged samples (Appendix B).  178  CHAPTER 6: CONCLUSIONS  A better understanding of the relation between the Kerguelen and Amsterdam-St. Paul hotspots and the formation of the Ninetyeast Ridge would benefit from sampling the seamount chains between the southeastern end of the Ninetyeast Ridge and the AmsterdamSt. Paul Plateau and between the southeastern part Amsterdam-St. Paul Plateau and the northeastern part of the Kerguelen Plateau (Figure 1.3).  6.2.3. Drilling of a Loa Trend Volcano? The temporal and spatial chemical variations of basalts erupted by Hawaiian volcanoes provide information about the physical and chemical structure of one of the most productive and long-lived mantle plumes on Earth [e.g., Sleep, 1990]. To test models for the temporal and spatial distribution of compositional heterogeneities within the Hawaiian plume conduit, recent studies, including the one presented in Chapter 5, have focused on the highresolution of temporal chemical variations within a single volcano as observed in subaerial stratigraphic lava successions [e.g., Pietruszka and Garcia, 1999; Marske et al., 2007; 2008], deep drilling cores [e.g., HSDP on Mauna Kea, DePaolo et al., 2001; Blichert-Toft et al., 2003; Bryce et al., 2005; SOH on Kilauea, Quane et al., 2000; KSDP on Koolau, Huang and Frey, 2005; Salters et al., 2006; Fekiacova et al., 2007], or submarine stratigraphic successions sampled by ROV [e.g., Mauna Loa SWRZ, Weis unpublished data, 2002] . The geochemical differences between basalts from the Loa and Kea trend volcanoes are interpreted to reflect sampling of two different areas of the Hawaiian mantle plume [e.g., Lassiter et al., 1996; Abouchami et al., 2005; Bryce et al., 2005; Weis, 2010 in preparation]. The recovery of volcanic rocks up to a depth of ~3500 metres below sea level during the HSDP, which span most of Mauna Kea’s life cycle, has allowed for the characterization and quantification of the geochemical evolution of a single Kea trend volcano as it moved across the melting source region and has provided important information about plume conduit dynamics [e.g., Abouchami et al., 2005; Bryce et al., 2005; Farnetani and Hofmann, 2010]. Additional knowledge regarding the Hawaiian plume structure would be gained by a similar deep drilling study on a Loa trend volcano. Although scientific drilling has been conducted in Koolau, which is a Loa trend volcano, the shallow penetration depth (~632 mbsl) allowed  179  CHAPTER 6: CONCLUSIONS  only for characterization of the later portions of volcano evolution (late-shield, post-shield and rejuvenated stages). Koolau also has some of the most extreme isotopic compositions observed in Hawaii and is not representative of the main Loa compositions. The most suitable target volcano for scientific drilling and studying the temporal geochemical evolution of a Loa volcano, and subsequently of the Loa source region, would be Hualalai. This is the third youngest and third-most historically active volcano on the Island of Hawaii. Hualalai has gone through all the major growth stages of the life cycle of a Hawaiian volcano. 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