@prefix vivo: . @prefix edm: . @prefix ns0: . @prefix dcterms: . @prefix skos: . vivo:departmentOrSchool "Science, Faculty of"@en, "Earth, Ocean and Atmospheric Sciences, Department of"@en ; edm:dataProvider "DSpace"@en ; ns0:degreeCampus "UBCV"@en ; dcterms:creator "Beausoleil, Yvette Léa"@en ; dcterms:issued "2012-10-31T18:08:02Z"@en, "2012"@en ; vivo:relatedDegree "Master of Science - MSc"@en ; ns0:degreeGrantor "University of British Columbia"@en ; dcterms:description """A total of 109 eclogite xenoliths from the Jericho and Muskox kimberlites (Nunavut, Canada) were studied petrographically and mineralogically to constrain their depth distribution within the Northern Slave mantle. The eclogites are dominated by pyrope-almandine and omphacite with accessory rutile, apatite and olivine. Garnet-clinopyroxene thermobaromtry suggests that Northern Slave eclogites formed at 670 -1300 °C and 25 – 70 kbar. Eclogites were classified into Group A, B, or C based on mineral composition and into massive and foliated textural types. Group A Northern Slave eclogites may have formed as cumulates from mantle mafic melts, whereas Group B and C eclogites are interpreted as modified subducted oceanic crust. All Northern Slave eclogites were subjected to partial melting and recrystallization, which produced secondary high-MgO garnet and clinopyroxene, phlogopite, amphibole carbonates and spinel group minerals. The recrystallization was caused by an influx of carbonatitic and hydrous hot fluid. The most recent heating event immediately predating kimberlite eruption resulted in garnet and clinopyroxene zoning. Diamondiferous eclogites from the Northern Slave are always massive and belong mostly to Group A. The majority of diamondiferous eclogites from the Northern Slave occur at shallower depths than those from the Central Slave craton. The criteria for distinguishing diamondiferous eclogites based on high Na₂O content in garnet and high K₂O content in clinopyroxenes can be applied only to Muskox eclogites. The high Mg content in both garnet and clinopyroxene best distinguishes the diamondiferous eclogites from Jericho. A model with multiple subducted slabs of oceanic crust below the Slave craton is proposed. The deepest subducted slab (190 – 210 km) dated at 1.88 – 1.84 Ga below the Central Slave extends to shallower depths of 170 – 185 km below the Northern Slave. Another slab (1.95 – 1.91 Ga) that occurs at 140 – 160 km below the Central Slave may extend to the north where it becomes progressively thicker from imbrication. The shallowest (120 – 130 km) and oldest (2.67 – 2.6 Ga) slab occurs only below the Northern Slave. Eclogites of mantle origin formed in mafic magma chambers, which existed only below the Northern Slave at 135 – 150 km depths."""@en ; edm:aggregatedCHO "https://circle.library.ubc.ca/rest/handle/2429/43556?expand=metadata"@en ; skos:note """Eclogite xenoliths from the Jericho and Muskox kimberlites Nunavut, Canada by Yvette Léa Beausoleil B.Sc., The University of Western Ontario, 2010 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in The Faculty of Graduate Studies (Geological Sciences) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) October 2012 © Yvette Léa Beausoleil, 2012 Abstract A total of 109 eclogite xenoliths from the Jericho and Muskox kimberlites (Nunavut, Canada) were studied petrographically and mineralogically to constrain their depth distribution within the Northern Slave mantle. The eclogites are dominated by pyrope-almandine and omphacite with accessory rutile, apatite and olivine. Garnet- clinopyroxene thermobaromtry suggests that Northern Slave eclogites formed at 670 - 1300 °C and 25 – 70 kbar. Eclogites were classified into Group A, B, or C based on mineral composition and into massive and foliated textural types. Group A Northern Slave eclogites may have formed as cumulates from mantle mafic melts, whereas Group B and C eclogites are interpreted as modified subducted oceanic crust. All Northern Slave eclogites were subjected to partial melting and recrystallization, which produced secondary high-MgO garnet and clinopyroxene, phlogopite, amphibole carbonates and spinel group minerals. The recrystallization was caused by an influx of carbonatitic and hydrous hot fluid. The most recent heating event immediately predating kimberlite eruption resulted in garnet and clinopyroxene zoning. Diamondiferous eclogites from the Northern Slave are always massive and belong mostly to Group A. The majority of diamondiferous eclogites from the Northern Slave occur at shallower depths than those from the Central Slave craton. The criteria for distinguishing diamondiferous eclogites based on high Na2O content in garnet and high K2O content in clinopyroxenes can be applied only to Muskox eclogites. The high Mg content in both garnet and clinopyroxene best distinguishes the diamondiferous eclogites from Jericho. A model with multiple subducted slabs of oceanic crust below the Slave craton is proposed. The deepest subducted slab (190 – 210 km) dated at 1.88 – 1.84 Ga below the Central Slave extends to shallower depths of 170 – 185 km below the Northern Slave. Another slab (1.95 – 1.91 Ga) that occurs at 140 – 160 km below the Central Slave may extend to the north where it becomes progressively thicker from imbrication. The shallowest (120 – 130 km) and oldest (2.67 – 2.6 Ga) slab occurs only below the Northern Slave. Eclogites of mantle origin formed in mafic magma chambers, which existed only below the Northern Slave at 135 – 150 km depths. ii Table of Contents Abstract............................................................................................................................... ii Table of Contents............................................................................................................... iii List of Tables ...................................................................................................................... v List of Figures.................................................................................................................... vi Acknowledgements ............................................................................................................ x Chapter 1: Introduction....................................................................................................... 0 1.1 Objective................................................................................................................... 0 1.2 Geological setting ..................................................................................................... 1 1.2.2 Jericho kimberlite and its xenoliths ................................................................... 4 Chapter 2: Literature review............................................................................................... 8 2.1 Eclogite occurrences................................................................................................. 8 2.2 Types of eclogites ..................................................................................................... 8 2.2.1 Eclogite xenoliths in kimberlite......................................................................... 9 2.2.2 Diamondiferous eclogites ................................................................................ 12 2.3 Theories of eclogite origin...................................................................................... 14 2.3.1 Protolith of subducted oceanic crust................................................................ 16 2.3.2 Protolith of mafic mantle cumulates................................................................ 19 2.3.3 Multiple origins of protoliths........................................................................... 23 Chapter 3: Petrography..................................................................................................... 25 3.1 Analytical methods ................................................................................................. 25 3.2 Jericho thin sections................................................................................................ 25 3.3 Muskox thin sections .............................................................................................. 34 Chapter 4: Mineral chemistry ........................................................................................... 45 4.1 Sample preparation and analytical methods ........................................................... 45 4.2 Northern Slave eclogites......................................................................................... 45 4.2.1 Garnet chemistry.............................................................................................. 47 4.2.2 Clinopyroxene chemistry................................................................................. 49 4.2.3 Rutile chemistry............................................................................................... 54 4.2.4 Mica chemistry ................................................................................................ 54 4.2.5 Amphibole chemistry ...................................................................................... 54 4.2.6 Carbonate chemistry ........................................................................................ 57 4.2.7 Spinel-group mineral chemistry ...................................................................... 58 4.2.8 Olivine chemistry ............................................................................................ 58 4.2.9 Evolution of mineral chemistry due to recrystallization and zoning............... 58 4.3 Central Slave eclogites ........................................................................................... 64 Chapter 5: Bulk chemical composition of Northern and Central Slave eclogites ............ 67 Chapter 6: Geothermobarometry ...................................................................................... 73 6.1 Methodology........................................................................................................... 73 6.2 Jericho..................................................................................................................... 74 6.3 Muskox ................................................................................................................... 80 6.4 Comparison of Jericho and Muskox eclogite xenoliths.......................................... 88 6.5 Central Slave........................................................................................................... 94 Chapter 7: Discussion and conclusions .......................................................................... 101 iii 7.1 Olivine in eclogite ................................................................................................ 101 7.2 Diamond potential of Northern Slave eclogites.................................................... 104 7.3 Origin of Northern Slave eclogite xenoliths......................................................... 106 7.4 Secondary mantle processes recorded by Northern Slave eclogites..................... 112 7.4.1 Partial melting and metasomatism................................................................. 112 7.4.2 Processes forming mineral zonation.............................................................. 118 7.5 Depth distribution of Northern Slave eclogites .................................................... 120 7.6 Comparison of Northern and Central Slave eclogites .......................................... 123 7.7 The Central – Northern Slave mantle cross-section ............................................. 128 7.4 Conclusions .......................................................................................................... 135 References ...................................................................................................................... 137 Appendix A: Macro-specimen descriptions ................................................................... 146 Jericho......................................................................................................................... 147 Muskox ....................................................................................................................... 157 Appendix B: Petrographic descriptions .......................................................................... 165 Jericho......................................................................................................................... 166 Muskox ....................................................................................................................... 174 Appendix C: Major element chemistry of minerals and equilibrium temperatures ....... 178 Jericho……………………………………………………………………………………………….. 182 Muskox…………………………………………………………………………….................................. 265 iv List of Tables Table 4.1 Compositional differences between primary and secondary garnet ………….48 Table 4.2 Compositional differences between primary and secondary clinopyroxene …53 Table 5.1 Bulk chemical compositions of eclogite xenoliths from Jericho........................ 69 Table 5.2 Bulk chemical compositions of eclogite xenoliths from Muskox…………...... 70 Table 5.3 Bulk chemical compositions of eclogite xenoliths from the Central Slave….. 72 Table 7.1 Equilibrium temperatures for metasomatised and zoned eclogites………….114 v List of Figures Figure 1.1 Location and geological map of the North and Central Slave craton ............... 3   Figure 1.2 Cross-section of the Slave Province.................................................................. 3   Figure 1.3 Distribution of the kimberlite cluster in the north-central Slave Craton........... 4   Figure 1.4 Depth distribution of mantle xenoliths from Jericho ........................................ 7   Figure 1.5 Divisions of Groups A, B, and C eclogites from Jericho.................................. 7   Figure 2.1 Constraints of Groups A, B, and C based on garnet composition................... 10   Figure 2.2 Bulk composition of eclogites from the Koidu kimberlite.............................. 12   Figure 2.3 Bulk composition for mantle and orogenic massif eclogites .......................... 12   Figure 2.4 REE pattern of mantle-derived eclogites ........................................................ 14   Figure 2.5 Group I and II for Roberts Victor eclogite xenoliths ...................................... 14   Figure 2.6 Diamondiferous criteria for Jericho eclogites ................................................. 16   Figure 2.7 Stability fields of eclogite, harzburgite, and lherzolite with diamond ............ 16   Figure 2.8 Locality map of the Ronda and Beni Bousera peridotite massifs ................... 16   Figure 2.9 δ18O in garnet from eclogite............................................................................ 19   Figure 2.10 Range of δ13C for eclogitic diamonds ........................................................... 19   Figure 2.11 Ophiolite sequence ........................................................................................ 21   Figure 2.12 Cumulate textures indicative of a mantle origin ........................................... 23   Figure 2.13 Phase diagram of anhydrous mid-ocean ridge basalt .................................... 23   Figure 2.14 Phase diagram of olivine tholeiite................................................................. 24   Figure 3.1 Macrospecimen photos Jericho eclogite ......................................................... 27   Figure 3.2 Macrospecimen photos of Muskox eclogite ................................................... 27   Figure 3.3 Photomicrograph of Jericho eclogite .............................................................. 27   Figure 3.4 Photomicrograph of Jericho eclogite .............................................................. 28   Figure 3.5 Photomicrograph of Jericho eclogite .............................................................. 28   Figure 3.6 Photomicrograph of Jericho eclogite .............................................................. 29   Figure 3.7 Photomicrograph of Jericho eclogite .............................................................. 29   Figure 3.8 Photomicrograph of Jericho eclogite .............................................................. 31   Figure 3.9 Photomicrograph of Jericho eclogite .............................................................. 31   Figure 3.10 Photomicrograph of Jericho eclogite ............................................................ 32   Figure 3.11 Photomicrograph of Jericho eclogite ............................................................ 32   Figure 3.12 Photomicrograph of Jericho eclogite ............................................................ 33   Figure 3.13 Photomicrograph of Jericho eclogite ............................................................ 33   Figure 3.14 Photomicrograph of Jericho eclogite ............................................................ 34   Figure 3.15 Photomicrograph of Jericho eclogite ............................................................ 34   Figure 3.16 Photomicrograph of Jericho eclogite ............................................................ 36   Figure 3.17 Photomicrograph of Muskox eclogite ……………………………………..........36 Figure 3.18 Photomicrograph of Muskox eclogite ……………………………………..........37 Figure 3.19 Photomicrograph of Muskox eclogite ……………………………………..........37 Figure 3.20 Photomicrograph of Muskox eclogite ……………………………………..........39 Figure 3.21 Photomicrograph of Muskox eclogite ……………………………………..........39 Figure 3.22 Photomicrograph of Muskox eclogite ……………………………………..........40 Figure 3.23 Photomicrograph of Muskox eclogite ……………………………………..........40 Figure 3.24 Photomicrograph of Muskox eclogite ……………………………………..........42 Figure 3.25 Photomicrograph of Muskox eclogite ……………………………………..........42 Figure 3.26 Photomicrograph of Muskox eclogite ……………………………………..........43 vi Figure 3.27 Photomicrograph of Muskox eclogite ……………………………………..........43 Figure 3.28 Photomicrograph of Muskox eclogite ……………………………………..........44 Figure 3.29 Photomicrograph of Muskox eclogite ……………………………………..........44 Figure 4.1 FeO vs. MgO of garnet grains from Northern Slave eclogites........................ 47   Figure 4.2 Na2O vs. CaO of clinopyroxene grains from Northern Slave eclogites .......... 47   Figure 4.3 MgO vs. CaO for Jericho garnets from massive and foliated xenoliths ………..49 Figure 4.4 MgO vs. CaO for Muskox garnets from massive and foliated eclogites ...….…49 Figure 4.5 MgO vs. FeO of primary and secondary garnet grains from Northern Slave eclogites ................................................................................................................... 51   Figure 4.6 FeO vs. CaO of primary, foliated garnet grains from Jericho eclogites.......... 51   Figure 4.7 Na2O vs. TiO2 of primary and secondary garnet grains from Northern Slave eclogites .................................................................................................................... 52   Figure 4.8 Na2O + Al2O3 vs. CaO + MgO of primary and secondary clinopyroxene grains from Northern Slave eclogites .................................................................................. 52   Figure 4.9 Al2O3 vs. Na2O for Jericho clinopyroxenes from massive and foliated eclogites ……………………………………………………………………………………...54 Figure 4.10 Al2O3 vs. Na2O for Muskox clinopyroxenes from massive and foliated eclogites ……………………………………………………………………………54 Figure 4.11 K2O vs. Na2O of primary and secondary clinopyroxene grains from Nothern Slave eclogites .......................................................................................................... 56   Figure 4.12 Nb2O5 vs. FeO of primary rutile grains from Northern Slave eclogites........ 56   Figure 4.13 Nb2O5 vs. Cr2O3 of primary rutile grains from Northern Slave eclogites ..... 57   Figure 4.14 Amphibole classification............................................................................... 57   Figure 4.15 MgO + FeO total vs. CaO of carbonate grains from Muskox eclogites ....... 58   Figure 4.16 CaO vs. MnO of olivine grains from Jericho eclogites................................. 60   Figure 4.17 FeO vs. MgO of olivine grains from Jericho eclogites ................................. 60   Figure 4.18 Photomicrograph of primary and secondary garnet grains from Muskox eclogites .................................................................................................................... 62   Figure 4.19 Photomicrograph of primary and secondary clinopyroxene grains from Muskox eclogites ...................................................................................................... 62   Figure 4.20 MgO vs. FeO of primary and secondary garnet grains from Northern Slave eclogites .................................................................................................................... 63 Figure 4.21 MgO vs. Al2O3 of primary and secondary clinopyroxene grains from Northern Slave eclogites........................................................................................... 63   Figure 4.22 FeO vs. Na2O of primary and secondary clinopyroxene grains from Northern Slave eclogites .......................................................................................................... 64   Figure 4.23 MgO vs. FeO of garnet core and rim analyses from Northern Slave eclogites .................................................................................................................................. 64   Figure 4.24 MgO vs. CaO for garnets from Central Slave eclogites ……………………65 Figure 4.25 Al2O3 vs. Na2O for clinopyroxenes from Central Slave eclogites ……….…67 Figure 4.26 Na2O vs. CaO of clinopyroxene grains from Central Slave eclogite ………67 Figure 6.1 Histogram of equilibration temperatures for texturally massive, foliated, and undetermined eclogites from Jericho........................................................................ 77   Figure 6.2 Equilibrium pressures and temperatures for massive eclogites from Jericho. 77   Figure 6.3 Equilibrium pressures and temperatures for olivine-bearing, massive eclogites from Jericho .............................................................................................................. 78   Figure 6.4 Equilibrium pressures and temperatures for foliated eclogites from Jericho ..78   vii Figure 6.5 Equilibrium pressures and temperatures for olivine-bearing, foliated eclogites from Jericho.............................................................................................................. 79   Figure 6.6 Equilibrium pressures and temperatures for texturally undetermined eclogites from Jericho .............................................................................................................. 79   Figure 6.7 Equilibrium pressures and temperatures for olivine-bearing, texturally undetermined eclogites from Jericho........................................................................ 80   Figure 6.8 Histogram of equilibration pressures for massive, foliated and texturally undetermined eclogites from Jericho........................................................................ 80   Figure 6.9 MgO vs. CaO for Jericho garnets from massive and foliated xenoliths ........ 82   Figure 6.10 Al2O3 vs. Na2O for Jericho clinopyroxenes from massive and foliated eclogites .................................................................................................................... 82   Figure 6.11 Histogram of equilibration temperatures for Group A, B, and C eclogites from Jericho.............................................................................................................. 83   Figure 6.12 Equilibrium pressures and temperatures for Group A eclogites from Jericho .................................................................................................................................. 83   Figure 6.13 Equilibrium pressures and temperatures for Group B eclogites from Jericho .................................................................................................................................. 84   Figure 6.14 Equilibrium pressures and temperatures for Group C eclogites from Jericho .................................................................................................................................. 84   Figure 6.15 Histogram of equilibration pressures for Group A, B, and C eclogites from Jericho....................................................................................................................... 86   Figure 6.16 Histogram of equilibration temperatures for massive and foliated eclogites from Muskox ............................................................................................................ 86   Figure 6.17 Equilibrium pressures and temperatures for massive eclogites from Muskox .................................................................................................................................. 87   Figure 6.18 Equilibrium pressures and temperatures for foliated eclogites from Muskox .................................................................................................................................. 87   Figure 6.19 Histogram of equilibration pressures for massive and foliated eclogites from Muskox ..................................................................................................................... 88   Figure 6.20 MgO vs. CaO for Muskox garnets from massive and foliated eclogites ...... 88   Figure 6.21 Al2O3 vs. Na2O for Muskox clinopyroxenes from massive and foliated eclogites .................................................................................................................... 89   Figure 6.22 Histogram of equilibration temperatures for Group A, B, and C eclogites from Muskox. ........................................................................................................... 90   Figure 6.23 Equilibrium pressures and temperatures for Groups A and B from Muskox eclogites .................................................................................................................... 91   Figure 6.24 Equilibrium pressures and temperatures for Group C eclogites from Muskox .................................................................................................................................. 93   Figure 6.25 Histogram of equilibration pressures for Group A, B, and C eclogites from Muskox ..................................................................................................................... 94   Figure 6.26 Lithological column of texturally massive, foliated, and undetermined eclogites from Jericho............................................................................................... 96   Figure 6.27 Comparison of texturally massive and foliated eclogites from Muskox....... 96   Figure 6.28 Comparison of Group A, B, and C eclogites from Jericho ........................... 97   Figure 6.29 Comparison of Group A, B, and C eclogites from Muskox.......................... 97   Figure 6.30 Histogram of equilibration pressures for eclogites from A154S, A154, Lac de Gras area, and Ekati.................................................................................................. 98   Figure 6.31 MgO vs. CaO for garnets from Central Slave eclogites................................ 98   Figure 6.32 Al2O3 vs. Na2O for clinopyroxenes from Central Slave eclogites............... 100   viii Figure 6.33 Histogram of equilibration temperatures for Group A, B, and C eclogites from Central Slave.................................................................................................. 100   Figure 6.34 Equilibrium pressures and temperatures for Group A eclogites from the Central Slave........................................................................................................... 101   Figure 6.35 Equilibrium pressures and temperatures for Group B eclogites from the Central Slave........................................................................................................... 101   Figure 7.1 MgO/(MgO +FeO total) vs. NiO for olivines in eclogites from Jericho ...... 103   Figure 7.2 SiO2 vs. Na2O + K2O of high-Mg volcanic rocks ......................................... 103   Figure 7.3 Na2O in garnet vs. K2O in clinopyroxene from Northern Slave eclogites .... 106   Figure 7.4 Diamondiferous criteria for eclogites from Jericho ...................................... 106   Figure 7.5 Whole-rock Mg-number for Northern Slave eclogites ................................. 109   Figure 7.6 Whole-rock Cr2O3 vs. MgO for Northern Slave eclogites ............................ 109   Figure 7.7 Chondrite-normalized REE diagram for whole-rock Group A and B eclogites from Jericho............................................................................................................ 110   Figure 7.8 Chondrite-normalized trace elements of clinopyroxenes for Group B and C eclogites from Jericho..................................................................................................... 110   Figure 7.9 Normal mid-ocean ridge basalt normalized incompatible element diagram for Group B and C eclogites from Jericho ............................................................... 11011   Figure 7.10 Equilibrium pressures and temperatures for primary and secondary garnets and clinopyroxenes from Northern Slave eclogites................................................ 115   Figure 7.11 Equilibrium pressures and temperatures for zoned garnets and clinopyroxenes from Northern Slave eclogites...................................................... 120   Figure 7.12 Strain vs. differential stress of omphacite, garnet, and harzburgite ............ 122   Figure 7.13 Depth distribution histogram of Group B and C eclogites from Northern Slave ....................................................................................................................... 122   Figure 7.14 Whole-rock Mg-number for Central Slave eclogite xenoliths .................... 125   Figure 7.15 Whole-rock Cr2O3 vs. MgO for Central Slave eclogites ............................. 125   Figure 7.16 N-MORB normalized trace elements for whole-rock analysis of eclogites from the Central Slave ............................................................................................ 126   Figure 7.17 Depth distribution histogram of Group B and C eclogites from the Central Slave ....................................................................................................................... 127   Figure 7.18 Schematic diagram of a subduction zone in the Central Slave craton ........ 130   Figure 7.19 Schematic diagram of the petrogenesis for Jericho diamondiferous eclogites ................................................................................................................................ 130   Figure 7.21 Schematic lithological cross-section of Group A, B, and C eclogite xenoliths from Central to Northern Slave kimberlites……………………………...…………....... 134 ix Acknowledgements This thesis would not have been possible without the guidance, assistance, leadership, and encouragement of my supervisor, Dr. Maya Kopylova. Her support throughout this entire process has been invaluable and I appreciate all of the knowledge she has bestowed upon me over these past two years. I would also like to thank my other committee members, Kelly Russell and Jim Mortensen for their feedback and contributions. In addition, I would like to thank all of those present in my personal life that have provided me with a social outlet and maintaining my sanity throughout this opportunity. I would especially like to thank my lab mates Chrissy, Evan, and Wren for providing a fun and welcoming research environment. I want to thank my friends and family (near and far) for supporting my decision and providing me with the courage to move across the country in the pursuit of higher education. Finally, I would like to thank Craig for being a true constant in my life. Thank you for putting up with all the late nights, early mornings, cranky afternoons, and everything in between. x Chapter 1: Introduction 1.1 Objective The purpose of this research is to constrain the depth distribution of eclogites beneath the Northern Slave as well as to determine the origin and the criteria for the diamondiferous character for these eclogites. This research is based on eclogite xenoliths from the Jericho and Muskox kimberlites, Nunavut, Canada. Eclogite xenoliths from Jericho have been studied by several workers (Kopylova et al. 1999a; Kopylova et al. 2004; Heaman et al. 2006; De Stefano et al. 2009; Smart et al. 2009), but no eclogite xenoliths from Muskox have ever been analysed. Mantle eclogites entrained by kimberlites in various cratons, are typically studied geochemically, with requisite rare earth elements (REE), trace elements, and isotopic analyses (Shervais et al. 1988; Snyder et al. 1995; Taylor 1993; Jacob et al. 1994; Snyder et al. 1997; Jacob and Foley 1999; Barth et al. 2001; Barth et al. 2002; De Stefano et al. 2009; Smart et al. 2009). Typically, only 5 – 30 eclogite xenoliths per kimberlite pipe are chemically analysed and these analyses are applied to the entire eclogite population. From these studies, conclusions on the origin and depth distribution of the eclogites were made. This study is unique as it concentrates on thermobarometry of eclogites, rather than their trace element geochemistry. Examination of a large quantity of eclogites (109) allows for a more thorough sampling and a better understanding of the eclogite population and a more accurate depth distribution. Eclogites are found in many different parts of the Slave mantle (Kopylova et al. 1999a). The origin and petrology of Northern Slave eclogites have been reported (Kopylova et al. 1999a; Kopylova et al. 2004; Heaman et al. 2006; De Stefano et al. 2009; Smart et al. 2009), as well as those from the Central Slave (Pearson et al. 2004; Aulbach et al. 2007; Schmidberger et al. 2007; Aulbach et al. 2011). However, the relationship between eclogites of these terranes has not been studied. By applying the same thermobarometric techniques to Northern and Central Slave eclogites, I aim at building a single consistent dataset of pressures and temperatures for all Slave eclogites. This is the basis for reconstruction of the depth distribution of eclogite beneath the craton. From the depth distribution of eclogites with a protolith from subducted oceanic crust, the position of ancient subducted slabs in the mantle can be constrained. Correlating these subducted slabs with known high-velocity zones below the Northern 1 and Central Slave (Bostock 1997), a tectonic interpretation of the mantle architecture of the Slave craton can be developed. Furthermore, a petrologically-constrained distribution of eclogites in the Slave mantle is essential for interpretation of geophysical data such as teleseismic P-wave imaging and seismic reflection surveys (Bostock 1997; Cook et al. 1999). This study contributes not only to the fundamental mantle petrology, but also to the development of the diamond exploration techniques. For an exploration company, it is important to forecast the diamond potential of a newly-discovered kimberlite pipe and to prioritize drilling targets. Therefore, diamond explorationists must be able to translate mineral chemistry of mantle rocks into the diamond potential. For this, mineral composition and texture of known diamondiferous rocks should be studied. Diamond potential criteria for eclogites change between cratons and even specific kimberlites. The diamond potential criteria for Muskox eclogites has never been studied and represents one of the goals of this Master of Science project. The contrast or similarity of the criteria for distinguishing diamondiferous eclogites in this pipe and analogous criteria for other Slave kimberlites sheds light on the uniformity of diamond-forming processes craton-wide and globally. 1.2 Geological setting 1.2.1 Northern Slave craton The Jericho and Muskox kimberlites are situated in the north-central area of the Slave craton in Nunavut, Canada (Figure 1.1). The Muskox kimberlite sits approximately 15 km southwest of Jericho. The Slave Structural Province is mainly composed of granite-greenstone with belts of metasedimentary and metavolanic rocks from the Yellowknife Supergroup (2.67 – 2.7 Ga), which were then intruded by 2.58 – 2.63 Ga granite plutons. The Slave craton can be divided into four separate terranes. These terranes (from west to east) are: Snare River, Central Slave Superterrane, Contwoyto, and Hackett River (Figure 1.2) (Helmstaedt et al. 2010). The kimberlites are emplaced in the Hackett River Terrane in the Slave Structural Province, which is within the Contwoyto-Itchen Lake Region (Johnson 2010). 2 Figure 1.1 Location and geological map of the area around Jericho and Muskox kim- berlites. Modified after Hayman et al. (2009) and Heaman et al. (2006). Figure 1.2 Cross-section of the Slave Province modified after Helmstaedt et al. (2010). SRT: Snare River Terrane, CSST: Central Slave Superterrane, CT: Contwoyto Terrane, HRT: Hackett River Terrane. 3 The Hackett River Terrane is located on the easternmost portion of the Slave Province, which completely sutured to the eastern edge of the Contwoyto Terrane 2.65 – 2.63 Ga (Helmstaedt 2009). The Hackett River Terrane is cut by the Malley (2.23 Ga), Mackay (2.21 Ga), Lac de Gras (2.02 Ga), and finally, the Mackenzie (1.27 Ga) diabase dyke swarms (Johnson 2010). There are three Jericho pipes with an age of 173.1 ± 1.3 Ma (Rb-Sr phlogopite megacrysts) that intrude into Archean granites of the Hackett River Terrane (Heaman et al. 2006). The granites were overlain by Devonian-age fossiliferous limestones with minor shale and sandstone more than 750 m thick (Kopylova and Hayman 2008). These sedimentary rocks have been completely eroded and only appear as xenoliths in the kimberlites. The granitoid is now overlain by 10 – 35 m of Quaternary glacial till, which was deposited 6.5 – 18 Ka (Kopylova and Hayman 2008). The single Muskox kimberlite pipe (172.1 ± 2.4 Ma) is circular in plan view with steeply dipping margins tapering at depth; it is emplaced in the granite-granodiorite Contwoyto Batholith (2589 ± 5 Ma) (Hayman et al. 2008, 2009). The Jericho and Muskox kimberlites are associated with a cluster of 15 other kimberlites, which lie 150 km north of the Lac de Gras kimberlite field (Hayman et al. 2009) (Figure 1.3). Figure 1.3 Distribution of the kimberlite cluster in the north-central Slave craton including Jericho and Muskox. Image modified from Manson (2011). 4 1.2.2 Jericho kimberlite and its xenoliths The kimberlite pipes at Jericho are classified as Group 1a indicating that they lack groundmass phlogopite and contain monticellite pseudomorphs (Kopylova and Hayman 2008). Two of the Jericho kimberlite pipes are described by Johnson (2010) – the Jericho and JD-2 pipes. The Jericho pipe is considered to be the most economically significant pipe. It is divided into five different geological units. Four (North, Central – 2 regions, and South lobes) are interpreted as volcaniclastic kimberlite with pyroclastic infill. The North lobe is rich in country rock xenoliths. The South lobe is interpreted to be volcaniclastic to transitional volcaniclastic kimberlite up to 240 m depth. The Central lobe is olivine rich and is split into two units – the first is dominated by well-sorted olivine-rich kimberlite; the second is poorly sorted kimberlite with varying size of olivine macrocrysts. The final geological unit (F1N) is interpreted as a country rock xenolith- rich volcaniclastic kimberlite. Coherent kimberlite is most voluminous along the eastern edge of the Jericho pipe and at depth. The JD-2 kimberlite pipe is dominated by pyroclastic kimberlite, which is well sorted with coarse and abundant olivine – indicating a high potential for coarse diamonds. There also is a lesser amount of coherent kimberlite present. The Jericho kimberlite was in mining operation from 2005 until 2008. The diamond grade was 1.31 carats per tonne (Kopylova and Hayman 2008). 1.2.2.1 Mantle xenoliths Mantle xenoliths associated with the Jericho kimberlite consist of eclogites, coarse to porphyroclastic peridotites, megacrystalline pyroxenites, and ilmenite-garnet wehrlites and clinopyroxenites, which measure up to a maximum diameter of 30 cm (Heaman et al. 2006; Kopylova et al. 1999b). Bimineralic eclogite xenoliths yielded temperatures from 800 to 1300 °C and, based on the 38 mW/m2 geotherm of the Slave craton, originate from depths 90 to 195 km (Kopylova et al. 1998, 1999a, 2004). Geothermobarometric calculations in Kopylova et al. 1999b were applied to coarse and sheared peridotites and ilmenite-garnet wehrlites and clinopyroxenites. Coarse peridotites, which yield temperatures 580 – 990 °C, are considered to be a part of the low-temperature suite formed at depths of 95 – 170 km. Sheared peridotite xenoliths are a part of the high-temperature suite with temperatures ranging from 1000 to 1230 °C corresponding to depths of 170 – 210 km. The ilmenite-garnet wehrlites and clinopyroxenites yield temperatures from 990 to 1189 °C, with depths of formation 170 – 5 195 km. Figure 1.4 shows the depth distribution and lithological column of the mantle xenoliths. 1.2.2.2 Eclogite xenoliths Of all the xenoliths from Jericho, a large proportion (30 %) are bimineralic eclogites; a few percentile of these (2 – 3 %) also contain zircon, apatite, kyanite, rutile, and/or diamond. Eclogite xenoliths from Jericho have been categorized into two groups by Cookenboo (1998) – foliated and massive. The massive eclogites comprise 30 % of all the eclogite xenoliths and can be classified into Groups A and B based on Coleman (1965) classification, contain diamond, and have Mg-rich garnets (Cookenboo 1998) (Figure 1.5). Massive eclogites mainly contain primary minerals (garnet, clinopyroxene, apatite, kyanite, and rutile) with little partial recrystallization (De Stefano et al. 2009; Kopylova et al. 2004). The protolith of the massive eclogites is hypothesized to be a mafic/ultramafic rock (high magnesium, low titanium) with a high pressure cumulate origin (Heaman et al. 2002). However, it is difficult to determine the exact protolith as the geochemical patterns may only show the latest stage of rock formation, which could mask the true origin (De Stefano et al. 2009). The foliated eclogites (70 %) are classified into Groups B and C using the common divisions of Coleman et al. (1965) (De Stefano et al. 2009). Kopylova et al. (2004) determined that foliated eclogites have undergone the most extensive partial recrystallization and developed euhedral mantle metasomatic minerals (garnet, clinopyroxene, amphibole, and phlogopite). These minerals can also show extensive zoning patterns; zoned garnets, for example, have a Ca-depleted core with a Mg-rich rim (De Stefano et al. 2009). Secondary minerals, such as serpentine, chlorite, and epidote, also occur in the rocks. The origin of these eclogites is proposed to be related to subducted oceanic crust at shallow, mid-crustal levels (De Stefano et al. 2009). The protoliths could either be mid-ocean ridge basalts or gabbros from the subducted oceanic crust based on geochemical signatures and ages of the zircon-bearing foliated eclogites (De Stefano et al. 2009). The multimineralic eclogites (garnet and clinopyroxene ± zircon, apatite, kyanite, rutile, diamond) have had a complex history involving two stages of mantle 6 Figure 1.4 Depth distribution of mantle xenoliths modified after Kopylova et al. (1999a). Temperature and pressure of peridotites and pyroxenites were calculated using Brey and Kohler (1990) thermobarometer; eclogite depth distribution was calculated using Ellis and Green (1979) thermometry in conjunction with the peridotite geotherm. Peridotites, pyroxenites, and megacrysts are plotted on PT diagram. Figure 1.5 A) Ternary diagram of garnets from Jericho eclogites; divisions of Groups A, B, and C (Coleman et al. 1965) are shown by dashed lines. B) MgO vs. Na2O (wt. %) of clinopyroxenes from Jericho eclogites, divisions of Groups A, B, and C (Taylor and Neal 1989) are shown by solid lines. Diamond-bearing eclogites (diamonds) cluster at the magnesium-rich end of Group A. Zircon-bearing eclogites (squares) plot within Group C (modified after Heaman et al. 2006). 7 metasomatism, which is outlined in Heaman et al. (2006). The first metasomatic event (1.8 Ga) brought on the enrichment of high field strength elements as well as the growth of zircon and Nb-enriched rutile grains. The second event (1.0 – 1.3 Ga) is responsible for significant apatite growth and enrichment of LREE. These eclogite xenoliths are hypothesized to be derived from metasomatized oceanic crust. Diamond-bearing eclogites could be interpreted as cumulates of mafic or ultramafic sills intruded into the Slave mantle or as metamorphosed olivine gabbros. 1.2.2 Muskox kimberlite and its xenoliths The Muskox kimberlite (172.1 ± 2.4 Ma, Hayman et al. 2009) is classified as a Group 1 kimberlite based on the composition of primary groundmass minerals and lack of phlogopite in the groundmass (Douglas et al. 2006; Hayman et al. 2008). The pipe is dominated by two main facies: a dark, massive hypabyssal magmatic kimberlite facies and a light-coloured, fragmental volcaniclastic kimberlitic breccia facies. The hypabyssal kimberlite facies encompasses 49 % of the kimberlite pipe and has better diamond potential than the volcaniclastic kimberlite breccia facies. The hypabyssal kimberlite is dark grey-black, contain macrocrysts of olivine along with mantle (diamondiferous eclogites) and crustal xenoliths in an aphanitic groundmass made of olivine phenocrysts, opaques, and monticellite. The volcaniclastic kimberlitic breccia facies accounts for 51 % of the pipe volume. The facies is light grey-green in colour (mostly due to carbonate xenoliths). There is a transitional zone between the two facies as they interfinger within the pipe. The Muskox kimberlite pipe has never been mined; its estimated diamond grade is 0.39 carats per tonne (Douglas et al. 2006). Both facies of the kimberlite contain a small percentage of eclogite, harzburgite, lherzolite, and dunite xenoliths (up to 4 %) with a maximum diameter of 10 cm (Hayman et al. 2008). The xenoliths associated with the dark massive facies are fresh, whereas those from the light-coloured fragmental facies show extensive serpentinization overprinting clinopyroxene in eclogites and olivine and pyroxenes in peridotites (Hayman et al. 2009). 8 Chapter 2: Literature review 2.1 Eclogite occurrences Eclogites are high-pressure and high-temperature metamorphic rocks that are primarily composed of coarse garnet and clinopyroxene grains (Winter 2001). Occurrences of eclogite in outcrops are quite rare and thus, their origins have been a source of speculation for many years. Eclogites occur in vastly diverse geological settings under highly variable pressure and temperature conditions. Low-temperature (< 550 °C) eclogites can be found in collisional belts as a part of high-pressure ophiolite and continental margin sequences (Woodland et al. 2002). Eclogites derived from collisional belts have higher degrees of mineral zoning, which is due to their exhumation histories and lower maximum temperature during metamorphism (Woodland et al. 2002). Medium-temperature (550 – 900 °C) eclogites can be found as lenses in low-pressure metamorphic rocks such as gneisses and migmatites (Woodland et al. 2002). The low- and medium-temperature eclogites have been determined to represent basaltic oceanic crust or continental basalt that have experienced high-pressure metamorphism (Woodland et al. 2002). High-temperature (> 900 °C) eclogites occur as layers or lenses within or close to orogenic peridotites; their origin is currently unknown (Woodland et al. 2002). In general, eclogites from massifs (high-pressure orogenic terrains) contain 9.7 wt % MgO with a bulk composition similar to mid-ocean ridge basalts (MORB), thus indicating an origin from subducted oceanic crust (Jacob 2004). 2.2 Types of eclogites Coleman et al. (1965) divided eclogites into three groups, A, B, and C, based on samples from massifs (Figure 2.1). Group A eclogites are considered to have a mantle cumulate origin; these have the highest Mg content in garnet (> 50 % pyrope) and contain the highest diopside component in clinopyroxene. These geochemical signatures are also reflected in eclogites from xenoliths in kimberlites and basalts. Common accessory minerals in Group A eclogites are kyanite, coesite, and diamond. Group A eclogites are formed at higher temperatures (900 – 1600 °C) (Carswell 1990). Group B eclogites occur as bands or lenses in migmatitic gneisses with pyrope composition of 30 – 55 %. Common accessory minerals include quartz, kyanite, zoisite, paragonite, and Ca-amphiboles. Group B eclogites are formed at medium temperatures (550 – 900 °C) (Carswell 1990). Group C eclogites occurs as bands or lenses associated with 9 Figure 2.1 Diagram modified after Coleman et al. (1965) outlining constraints of each eclogite group based on garnet composition. Dots represent garnet analyses from known Group C eclogites (glaucophane schists). Dotted lines represent range in garnet composition form 1) amphibolites, 2) charnockites and granulites, 3) eclogites in gneisses or migmatites, 4) eclogites in kimberlites, 5) eclogite in dunites and peridotites. blueschists. These eclogites have garnets with pyrope content of less than 30 % and contain accessory minerals of epidote, zoisite, quartz, amphibole, phengite, and paragonite. Clinopyroxenes also contain the highest Na2O component of these eclogites. Group C eclogites equilibrated at low temperatures (450 – 550 °C) (Carswell 1990). Groups B and C eclogites are considered fragments of subducted oceanic crust (Coleman et al. 1965). 2.2.1 Eclogite xenoliths in kimberlite Eclogite xenoliths entrained in kimberlites are brought up from the mantle and provide information about cratonic and geodynamic processes from the Archean. The high equilibration temperature from eclogite xenoliths and the presence of diamond indicates a derivation from the upper mantle. Partial melting of mafic protoliths at high pressure creates Archean tonalite–trondhjemite–granodiorite (TTG) suites (Jacob 2004). For example, eclogite xenoliths with low MgO content from the Koidu kimberlite are considered to be a residue from the formation of Archean TTG suites, which comprise continental crust (Jacob 2004). The eclogite xenoliths have been dated at 3.44 – 2.57 Ga, which likely represent the age of emplacement into the lithosphere (Jacob 2004). Eclogites are not normally the dominant xenolith type in kimberlites, as the abundance 10 of eclogite in the upper 200 km of the subcontinental mantle is estimated to be less than 1 % (Schulze 1989; Jacob 2004). Overall, eclogite xenoliths represent less than 10 % of all xenolithic material brought to the surface by kimberlites (Dawson 1980; Schulze 1989). There are a few exceptional kimberlites where eclogites are highly abundant; Roberts Victor kimberlite has 80 – 98 % eclogite xenoliths (Snyder et al. 1997; Jacob 2004). This abundance of eclogite may be due to the local enrichment of eclogite within the subcontinental mantle or due to a preferential disaggregation of garnet peridotites during ascent. Schulze (1989) proposed that mantle metasomatism caused structural weakening of the upper mantle and alteration of orthopyroxene grains to phlogopite (Schulze 1989) resulting in the disaggregation of garnet peridotites during kimberlite ascent, thus leaving behind intact eclogite xenoliths. Since these eclogite xenoliths do not contain orthopyroxene, they are not affected by the metasomatism and therefore did not disaggregate as readily as the peridotite xenoliths (Schulze 1989). Eclogite xenoliths are composed of pristine (with rare inclusions) omphacite and garnet with rare rutile grains. Partial melting, metasomatism, and penetration of host kimberlite material alter all eclogite xenoliths, since the xenoliths are entrained in kimberlite (Jacob 2004; Barth et al. 2001). Eclogite xenoliths in kimberlites typically exhibit a picritic bulk composition with 13.8 wt. % MgO; their bulk composition is dissimilar from MORB because the eclogites represent restites from partial melting of MORB at high pressures (Jacob 2004). The melts in equilibrium with eclogite have silicic to intermediate composition (i.e. tonalites) leaving behind a high-Mg restite (Figure 2.2). This bulk composition overlaps with younger, lower metamorphic grade eclogites from orogenic massifs (Jacob 2004). Jacob (2004) gives a comprehensive literature review of kimberlitic and massif eclogites and how they differ. Eclogites from massifs contain abundant inclusions and coesite/quartz. These eclogites also contain higher amounts of SiO2 and TiO2 and lower MgO than kimberlitic (mantle) eclogites (Figure 2.3). The bulk composition of massif eclogites is closer to MORB than that of eclogite xenoliths from kimberlites. Geochemical signatures found in eclogite xenoliths in kimberlites are the following. Variable degrees of metasomatic overprinting are identified by a heterogeneous rare earth element (REE) pattern (Barth et al. 2002) (Figure 2.4). Garnets have low LREE contents whereas clinopyroxene has higher amounts of LREE (Jacob 2004) (Figure 2.4). Heavy rare earth elements (HREE) are enriched in garnets (Jacob 2004). Clinopyroxene grains show a convex up pattern of REE; garnet and clinopyroxene grains both show a 11 Figure 2.2 Bulk composition of eclogite xenoliths from the Koidu kimberlite (South Africa). The residues from eclogite melting experiments (small diamonds) are shown; the melts from these experiments, at 1 and 3 GPa, contain higher amounts of SiO2 and lower CaO (i.e. trondhjemite-tonalite, granodiorite, and quartz diorite compositions) than the residues (modified after Barth et al. 2001). Figure 2.3 Major element bulk composition for mantle eclogites (black diamonds) com- pared to orogenic massif eclogites (open squares). The fields for komatiites (white), oceanic gabbros (light grey), and MORB (dark grey) are given for comparison purposes (modified after Jacob 2004). 12 positive Eu anomaly (Jacob 2004) (Figure 2.4). 2.2.2 Diamondiferous eclogites Eclogites are the original host rock in which diamonds were first discovered (Bonney 1899). Diamond potential of eclogites correlate with its texture and mineral chemistry. The diamond potential of an eclogite, for example, can be determined by analyzing Na2O content in garnet grains and K2O content in clinopyroxene grains (McCandless and Gurney 1989). Empirically, diamondiferous eclogite contain, on average, 109 ppm Ni, whereas non-diamondiferous eclogites only contain an average of 43 ppm Ni (Jacob 2004). The explanation for this empirical pattern is currently unknown. MacGregor and Carter (1970) separated eclogite xenoliths from the Roberts Victor kimberlite into Group I and Group II. Group I eclogites are diamondiferous and have a higher equilibrium pressure than those from Group II. The garnets are subhedral to rounded in a clinopyroxene matrix and contain higher Na and Mg contents than Group II eclogites. Clinopyroxene grains from Group I have higher K2O, FeO, Cr2O3, CaO, and MnO contents than those from Group II. Group II eclogites have interlocking anhedral garnet and clinopyroxene grains, which are very fresh, and also appear almost gneissic. Group II eclogites are non-diamondiferous. McCandless and Gurney (1989) determined chemical constraints that accurately divide eclogites into Groups I and II. Group I eclogites have ≥ 0.08 wt. % K2O in clinopyroxene or ≥ 0.09 wt. % Na2O in garnet, on average (Figure 2.5). At high pressure, Na2O preferentially is incorporated into garnet, which is why Group I eclogite xenoliths contain more Na2O in garnet than Group II (Gurney and Zweistra 1995). K2O content is high in Group I eclogite xenoliths because they formed where phlogopite is unstable, therefore K2O was incorporated into clinopyroxene. Group II eclogite has lower K2O content in clinopyroxene because K2O was incorporated into phlogopite, which grows in Group II eclogites (McCandless and Gurney 1989). Another indicator of the eclogite diamond potential was proposed for the Jericho eclogite. The diamondiferous eclogite xenoliths from Jericho are richer in Mg compared to worldwide diamond-bearing eclogite xenoliths (Heaman et al. 2006). Eclogite xenoliths from the Jericho kimberlite show that constraints based on MgO content in both clinopyroxene and garnet grains can outline a distinct diamondiferous eclogite field 13 Figure 2.4 A) REE pattern of garnet grains from mantle-derived eclogites worldwide (shaded field). The two black lines represent examples of LREE depletion, a positive Eu anomaly, and a flat HREE pattern (Jacob 2004). B) REE pattern of clinopyroxene grains from mantle-derived eclogites worldwide. The black lines represent examples of both LREE enrichment and depletion patterns (modified after Jacob 2004). Figure 2.5 Na2O in garnet vs. K2O in clinopyroxene graph which shows the separation of Group I and II for Roberts Victor eclogite xenoliths. The red diamond represent Group I eclogites from the Gréau et al. (2011) study and the grey diamonds are Group I eclogites from the literature. The green dots represent Group II eclogites from the Gréau et al. (2011) study and the black dots are Group II eclogites from the literature (modified from Gréau et al. 2011). 14 (Figure 2.6). At Jericho, only the massive eclogites xenoliths with Mg-rich garnets and Na-poor clinopyroxenes are known to contain diamond (De Stefano et al. 2009). Luth (1993) discussed how oxygen fugacity (ƒO2) controls the stability of diamond in eclogites (Figure 2.7). The stability of diamond at high pressures depends on the oxidation state of the minerals in the assemblage, which is monitored by ƒO2. The controlling factors on ƒO2 in the mantle are reactions between diamond and fluid or between silicate, diamond, and carbonates, the presence of sulphur and hydrogen, and/or ferrous and ferric iron in silicates. The reaction of dolomite + 2 coesite ↔ diopside + 2 diamond + 2O2 defines the existence of diamond and carbonate in mantle eclogites. In order for diamondiferous eclogites to coexist with peridotites containing carbonate or carbonate and diamond, the ƒO2 must be one log unit higher (at any given temperature and pressure) than that of diamondiferous peridotites. Diamondiferous eclogites have also been discovered in ultrahigh pressure (UHP) peridotite massifs at two localities in the western Mediterranean: Ronda (Spain) and Beni Bousera (Morocco). These UHP terranes are localized mainly within major continental collision belts in Eurasia and Africa (Figure 2.8) (Liou 1999). The Ronda and Beni Bousera peridotite massif both date an emplacement age of approximately 21.5 Ma (39Ar/40Ar in K-feldspar and Sm/Nd mineral isochron) and are considered tectonically related (Zindler et al. 1983; Pearson et al. 1995; Liou 1999). Both peridotite massifs contain lenses/layers of garnet clinopyroxenite, which also contain graphitized diamonds (Pearson et al. 1995; Liou 1999). The aggregated graphite discovered exhibit cubic and octahedral morphologies. At both localities, the graphite aggregates contain cubo- octahedral faceted inclusions of garnet and clinopyroxene, which are commonly observed on inclusions within natural diamonds (Pearson et al. 1995). The presence of diamond indicates that the garnet clinopyroxenite layer must have crystallized within the diamond stability field. 2.3 Theories of eclogite origin There exist many theories for the origin of eclogites brought up by kimberlites. The first is the metamorphism of subducted oceanic crust to eclogite that may or may not have experienced melting during subduction (Barth et al. 2001). This theory has the most evidence to support it as the majority of mantle eclogites has trace geochemical 15 Figure 2.6 A) MgO vs. CaO (wt. %) of garnets from eclogite xenoliths. B) MgO vs. Na2O (wt. %) of clinopyroxenes from eclogite xenoliths. Fields are outlined for diamondiferous (vertical stripes), barren massive (dash-dot line), and barren foliated (dotted line) eclogite xenoliths from Jericho. Arrows represent mineral chemistry evolution from primary to secondary grains (modified after DeStefano et al. 2009). Figure 2.7 Temperature versus Δlog ƒO2 at 5 GPa showing stability fields of eclogite, harzburgite, and lherzolite with diamond (modified from Luth 1993). Figure 2.8 Locality map of the Ronda (Spain) and Beni Bousera (Morocco) peridotite massifs (modified after Pearson and Nowell 2004). 16 characteristics indicative of a plagioclase-bearing protolith and oxygen isotopic characteristics similar to seawater altered oceanic crust (Jacob 2004). The second theory suggests eclogite originated from mafic cumulates from high-pressure magmas extracted from peridotite (Jacob 2004; Barth et al. 2001). Other possible origins could be a product of metamorphism of a gabbroic-anorthositic protolith during isobaric cooling at high pressure, and relicts from earth’s primary differentiation process (Woodland et al. 2002). Finally, Ringwood (1975) has suggested that there are multiple origins for all eclogites and that they can be formed in a variety of plausible and supported methods. Of the many origin models, only a few have significant amounts of supporting evidence; these will be discussed in further detail below. 2.3.1 Protolith of subducted oceanic crust Ringwood and Green (1966) suggested that with increasing pressure, an oceanic tholeiite would be metamorphosed to a quartz eclogite; quartz would melt during production of a calc-alkaline magma, which would leave behind an eclogitic residue. Schulze and Helmstaedt (1988) first acknowledged that the presence of coesite and sanidine (which exist throughout the entire compositional range of eclogites) suggest a crustal origin; this is based on their study of eclogite xenoliths from the Roberts Victor kimberlite in South Africa. In conjunction with the aforementioned evidence, this model essentially hinges upon the deviation of δ18O values of eclogite xenoliths from those of mantle peridotites – approximately 5.5 ‰ (MacGregor and Manton 1986; Shervais et al. 1988; Neal et al. 1990; Taylor 1993; Snyder et al. 1995; Snyder et al. 1997). The δ18O values in eclogites (5.7 – 12 ‰) are within the range outlined by basalts from ophiolites (MacGregor and Manton 1986). These values are caused by low pressure and temperature fractionation and, therefore, it is hypothesized that seawater has altered the δ18O signature of the oceanic crust. This low-pressure, crustal origin is also supported by the presence of coesite/quartz in some eclogites (Ringwood and Green 1966; Schulze and Helmstaedt 1988; Jacob 2004). 2.3.1.1 Geochemical and petrological evidence Major and trace elements from eclogites with subducted origin exhibit positive Eu, Nb, and Sr anomalies, depleted LREE, unradiogenic Sr, enrichment in Al and Na, low Mg numbers ( molar MgO/(MgO + FeO) < 71, and a flat HREE signature in the bulk composition (including quartz-bearing, kyanite-bearing, and bimineralic eclogite 17 samples) (Taylor and Neal 1989; Snyder et al. 1997; Jacob and Foley 1999; Barth et al. 2001; Barth et al. 2002; Jacob 2004). The positive Sr anomaly is highly indicative of low-pressure plagioclase fractionation, as plagioclase also exhibits a positive Sr anomaly (Barth et al. 2002). In concert with the Sr anomaly, the positive Eu anomaly indicates plagioclase-bearing protolith that may have formed in a magma chamber within the oceanic crust (Jacob and Foley 1999). There also exist several supporting lines of evidence for this model from isotopic data. In oceanic crust, the changing fractionation between seawater and basalt causes opposing δ18O trends at various temperatures. At high temperature and greater depth δ18O decreases, at low temperature and shallower depth δ18O increases (Figure 2.9) (Jacob and Foley 1999; Jacob 2004; Barth et al. 2001). In particular, garnet grains from kyanite-bearing eclogite xenoliths tend to exhibit low δ18O values, which indicate high temperature alteration in deep oceanic crust; this is consistent with a plagioclase-rich protolith (Barth et al. 2001). The broad range of δ13C from eclogitic diamonds (-41 – 0 ‰) (Figure 2.10) is consistent with a contribution from organic carbon that may have accumulated in sedimentary rocks on the ocean floor (Snyder et al. 1997; De Stefano et al. 2009). The δ13C values for crustal organic matter range from -35 to -20 ‰ (Kirkley et al. 1991; Palot et al. 2009). Some petrological evidence for a subducted oceanic crust as the protolith for eclogite xenoliths in kimberlites is the presence of kyanite and corundum. It indicates metamorphic growth from a plagioclase-rich protolith (Jacob and Foley 1999). 2.3.1.2 Geological implications Eclogite xenoliths that exhibit geochemical evidence for subduction origin were formed from residues of the Archean continental crust (Barth et al. 2001). The subducted oceanic crust underwent partial melting to produce a TTG suite leaving behind an eclogitic residue, which was then emplaced into the lithosphere (Snyder et al. 1997; Barth et al. 2001). The geochemical evidence supporting this theory is the chemistry of the TTG suite. Trace element signatures of TTG are complimentary to those in the eclogite xenoliths (Snyder et al. 1997; Barth et al. 2001). As partial melting of the subducted oceanic crust occurred, the residue was transported deeper into the mantle. The buoyant SiO2-rich melts rose and interacted with the peridotite mantle creating the 18 Figure 2.9 A) δ18O vs. FeO (wt. %) in garnet from eclogite, which exhibits the pattern resulting from seawater alteration. The δ18O values increase at low temperature and decrease at high temperature alteration. B) δ18O vs. La/Lu in garnet from eclogite, which shows the resultant REE patterns after seawater alteration. The dashed line in both graphs represents δ18O values of unaltered MORB (modified after Jacob and Foley 1999). Figure 2.10 Range of δ13C (‰) for eclogitic diamonds. Orange and black bars represent eclogitic, websteritic, peridotitc, and unknown diamonds from Jericho. The range spans the values accepted for both mantle and crustal δ13C (modified after DeStefano et al. 2009). 19 TTG suites of the Archean. The eclogitic residue left behind was then later sampled by kimberlites ascending to the surface. By comparing the δ18O and 87Sr/86Sr with an ancient ophiolite analog, it can be determined in which part of the ophiolite sequence different eclogites originated (McCulloch et al 1981; Jacob et al. 1994; Snyder et al 1997) (Figure 2.11). Samples with lower δ18O and 87Sr/86Sr values correspond to lower position in the sequence and to cumulate gabbros. Conversely, samples with high δ18O and 87Sr/86Sr values correspond to upper position in the sequence, to sheeted dykes and pillow lavas (Snyder et al. 1997). In the Udachnaya kimberlite, there exist eclogite xenoliths that span the entire range of 87Sr/86Sr and δ18O and thus could potentially represent an intact Archean ophiolite sequence (Snyder et al. 1997). Known kimberlites with eclogite xenoliths that exhibit the aforementioned geochemical and petrological evidence include Mir (Siberia) and Koidu (Sierra Leone). 2.3.1.3 Contradictory evidence The subducting oceanic crust origin for eclogites is well justified and supported when a suite of eclogites is considered. However, it is contested when individual samples are studied. Single eclogite samples can show δ18O values that are similar to unaltered mantle (Jacob 2004). Some samples show no evidence of plagioclase accumulation (Jacob 2004). It should be noted that this lack of a plagioclase signature in the geochemical data does not completely exclude an oceanic origin for eclogite protoliths. 2.3.2 Protolith of mafic mantle cumulates O’Hara and Yoder (1967) originally proposed that eclogites are derived from high- pressure igneous fractionation and represent mafic cumulates and evolved liquids. This theory suggests that eclogitic xenoliths from kimberlites have a purely mantle origin forming as mafic cumulates at high pressure (Barth et al. 2002). The evidence of high- pressure crystal fractionation of the mafic melt and the absence of olivine and orthopyroxene from the eclogite xenoliths indicate formation between 2 – 3 GPa (Barth et al. 2002). Eclogites that correspond to Group A eclogites are considered to be mantle derived (Coleman et al. 1965). 20 Figure 2.11 Ophiolite sequence showing where eclogites originated based on O and Sr isotopic data (modified after Winter 2001). 21 2.3.2.1 Geochemical and petrological evidence Eclogites originated as mafic cumulates have a broader range of compositions than those interpreted to have a basaltic protolith. Major and trace element contents from these eclogite xenoliths exhibit enrichments in Cr2O3, MgO, Al2O3, and NiO (493 – 659 ppm), depleted V2O5, and high Mg-numbers (> 74) (Barth et al. 2001; Barth et al. 2002). These eclogites have a fractionated HREE signature and an enriched LREE clinopyroxene signature (Caporuscio and Smyth 1990; Barth et al. 2002). Coleman et al. (1965) determined that the Group A eclogites are derived from mantle cumulates. These eclogites have a high MgO/FeO, low Na2O in clinopyroxene, a depletion in incompatible elements, δ18O within mantle values range, low 87Sr/86Sr and 143Nd/144Nd, and enriched Cr2O3 in both garnet and clinopyroxene grains. There also exist some isotopic evidence for the cumulate origin. The δ18O values from eclogite xenoliths are very similar to the mantle – approximately 5.5 ‰ δ18O (Snyder et al. 1997; Barth et al. 2001). The δ13C value for the mantle is identified as approximately -5 ‰ (Deines 2002). Petrological evidence supporting the cumulate origin includes mineralogical layering (banding) in the xenoliths, cumulate textures (accumulation of minerals in liquid, orthocumulate, adcumulate, and heteroadcumulate), and exsolution of garnet and kyanite from clinopyroxene grains (Snyder et al. 1997; Barth et al. 2001) (Figure 2.12). This exsolution feature indicates cooling near the solidus and at pressures greater than 3 GPa (Snyder et al. 1997). Known kimberlites that contain eclogites with cumulate origin include Udachnaya and Obnazhennaya (Siberia), Kuruman, Bobbejaan, and Roberts Victor (South Africa), and Koidu (Sierra Leone). 2.3.2.3 Contradictory evidence There are several lines of evidence that contradict an origin of eclogites from mantle cumulates. First, at the high pressures typical of eclogite equilibration (30 – 70 kbar), mafic melts cumulate garnet, clinopyroxene, and coesite (Figure 2.13). Therefore, it is unlikely to crystallize only garnet and clinopyroxene. However, it is possible to crystallize garnet and clinopyroxene together at lower pressures (15 – 30 kbar) (Figure 2.14). These cumulates would need to be transported to greater depths in order to generate the high pressures of equilibrium. Another contradictory piece of evidence is that garnet may have crystallized from a melt with different bulk composition and not from olivine tholeiite (Barth et al. 2002; Jacob 2004). 22 Figure 2.12 Cumulate textures of igneous rock, typical for eclogites. A) Crystals (white rectangles) accumulate by settling and intercumulus liquid (red) in-fills interstices. B) Inter- cumulus liquid crystallizes to form rims around crystals and new crystals in the interstices. C) Open system where exchange occurs between the intercumulus liquid and the magma chamber; crystals fill in most void space and new minerals escape. D) New, large minerals nucleate and envelope the original crystals (modified after Winter 2001). Figure 2.13 Phase diagram of anhydrous mid-ocean ridge basalt (modified after Yasuda et al. 1994). 23 Figure 2.14 Phase diagram of olivine tholeiite based on the experimental work of Green and Ringwood (1967). The grey field shows where garnet and clinopyroxene coexist with the melt and could cumulate together (modified after Jacob 2004). Moreover, the presence of kyanite-bearing eclogites contradicts a high-pressure mantle cumulate protolith. Kyanite eclogites do not form at high pressures typical for cratonic eclogites (Barth et al. 2001) (Figure 2.14). Kyanite reacts with olivine at pressures within the diamond stability field to form clinopyroxene and garnet (Jacob 2004. 2.3.3 Multiple origins of protoliths Ringwood (1975) suggested that eclogites may form from a variety of ways, both from the mantle and from subducted oceanic crust. This hypothesis is supported by the fact that some eclogite xenoliths have clear indications of being either mantle-derived or residues of partially melted oceanic crust (e.g. Obnazhenaya and Mir (Siberia), respectively). Also, some eclogite xenoliths do not give clear evidence for either origin, but a combination of both (e.g. Udachnaya (Siberia)). This occurs when there exists an inconsistency in the patterns from major and trace element patterns along with isotopic compositions that vary above and below accepted mantle and/or crustal values (Snyder et al. 1997). In addition, there also can exist two separate groups of eclogite xenoliths from 24 the same kimberlite where one group is distinctly from oceanic crust and the other of mantle derivation (e.g. Koidu, South Africa). Brueckner (1977) suggested that eclogites may have a heterogeneous nature where they may be formed from different protoliths in different tectonic settings at different times resulting in a number of origins. 25 Chapter 3: Petrography 3.1 Analytical methods A total of 76 Jericho eclogites (1 – 15 cm in diameter) and 33 Muskox eclogites (2 – 20 cm in diameter) were examined. Observations were made based on mineralogy, texture, and alteration products. From this, the eclogites were classified into two textural groups, massive or foliated (Figures 3.1 and 3.2). Samples were described (Appendix A) using a Fisher Scientific stereomicroscope in the Department of Earth, Ocean and Atmospheric Sciences at the University of British Columbia. All petrography was performed in the Department of Earth, Ocean and Atmospheric Sciences at the University of British Columbia. Optical petrography was performed on a research grade Mecatron Precision Leica polarizing microscope. All photomicrographs were taken using a Nikon camera mounted onto the microscope. Potentially diamondiferous eclogite xenoliths from the Jericho kimberlite were selected to be made into thin sections (Chapter 3.3). These samples were chosen based on massive texture, high-MgO content in garnet, and low-Na2O content in clinopyroxene. Thin sections from the Muskox kimberlite had previously been prepared and show both massive and foliated textures (Chapter 3.5). 3.2 Jericho thin sections Petrographic studies have been conducted on seven thin sections of eclogite xenoliths from the Jericho kimberlite. These samples are all composed of a very similar bimineralic aggregate of garnet (42 – 45 vol. %), clinopyroxene (35 – 60 vol. %) with common accessory minerals of opaques (1 vol. %), rutile (1%), and phlogopite (1 vol. %) (Appendix B). All of the garnet (3.4 mm average) and clinopyroxene (3.9 mm average) grains exhibit hypidioblastic texture and are usually heavily fractured. Garnet grains are rounded octahedrons and contain few inclusions consisting of opaques, clinopyroxene, apatite, and primary biotite laths (Figures 3.3 and 3.4). Kelyphitic rims are uncommon, but mainly occur where a garnet grain is in contact with carbonate alteration or veining (Figure 3.5). Clinopyroxene grains are rounded prisms and also contain few inclusions consisting of garnet, opaques, biotite, and rarely rutile and other clinopyroxene grains (Figures 3.6 and 3.7). Twin lamellae are common along cleavage planes (Figure 3.6). In 26 Figure 3.1 Macrospecimen photos of A) massive Jericho eclogite LGS1 Mx1; B) foliated Jericho eclogite JD35 Mx6. Figure 3.3 Sample LGS08 Mx13 shows garnet with inclusion of clinopyroxene and opaque (black). Serpentine alteration occurs in fractures (PPL, 5X, FOV: 4.35mm). Figure 3.2 Macrospeciment photos of A) massive eclogite xenolith MOX 206.9 and B) foliated eclogite xenolith MOX7 53.9. Red scale bar corresponds to 1 cm. 27 Figure 3.4 Sample LGS035 Mx4 shows apatite inclusion in a garnet grain. Alteration along grain boundary is composed of hornblende, phlogopite, opaques, and serpentine (PPL, 5X, 4.35 mm) Figure 3.5 Sample LGS08 Mx13 shows a garnet grain with thick kelyphitic rims near a carbonate vein (PPL, 5X, FOV: 4.35 mm). 28 Figure 3.6 Sample LGS025 Mx11 shows a clinopyroxene grain with lamellae and a garnet inclusion. Partial melting also occurs in fractures of the grain (XPL, 5X, FOV: 4.35 mm). Figure 3.7 Sample LGS025 Mx11 shows a rutile grain surrounded by serpentine altera- tion and a garnet grain following the grain shape; all are as an inclusion in clinopyroxene (PPL, 20X, FOV: 1.05 mm). 29 one sample the cleavage traces on clinopyroxene grains contains brown clay alteration, which is present on all grains in the xenolith (Figure 3.8). Significant partial melting also occurs in fractures and along grain boundaries of three xenolith samples; it is most extensive on touching grains of clinopyroxene (Figures 3.6, 3.9, and 3.10). The margins of these touching grains are dark grey to black (Figure 3.9). Partial melting is evidenced by growth of fine euhedral grains of clinopyroxene. Deformation is also prevalent in these samples as undulatory extinction is common in clinopyroxene and accessory phlogopite grains (Figure 3.11). Sample LGS025 Mx11 exhibits an unusual texture of rounded garnet blebs as inclusions within clinopyroxene grains (Figure 3.11). Sample LGS035 Mx4 contains 5 vol. % primary apatite, which are distributed evenly throughout the sample (Figure 3.4). These grains are idioblastic, hexagonal, and have an average size of 0.4 mm. Inclusions of opaques are uncommon. Common accessory minerals are opaques, rutile, and phlogopite. Opaque grains (0.27 mm average) occur in all samples as rounded, hypidioblastic inclusions in garnet and clinopyroxene grains (Figure 3.3). Phlogopite grains (0.76 mm average) occur as hypidioblastic laths as inclusions in clinopyroxene (Figure 3.8). Rutile grains (2.36 mm average) are only present in three samples often as a cluster of hypidioblastic needles (Figure 3.7). One sample exhibits partial melting along the grain boundaries (Figure 3.12). Secondary alteration commonly occupies 4 – 12 vol. %; a few samples have 20 – 80 vol. % alteration. The main secondary minerals are serpentine, phlogopite, carbonate, spinel, and opaques. This suite of minerals occur in fractures and on grain boundaries of garnet and clinopyroxene (Figure 3.13). Alteration patches occur mainly in garnet with some occurrences in clinopyroxene, which are mainly composed of serpentine, carbonate, biotite, and opaques (Figure 3.14). Serpentine occurs in two different forms: green and yellow/colourless. The green serpentine occurs on the outer edges of fractures and alteration patches, with yellow/colourless serpentine in the centre; both types form fibrous, radiating needles (Figure 3.15). Phlogopite generally forms hypidioblastic plates in the alteration suite; where phlogopite is interstitial, it appears more xenoblastic (Figures 3.4, 3.12, and 3.14). Phlogopite is generally deformed as evidenced by undulatory extinction. Where carbonate is present it occurs in the very centre of alteration veins or patches as hypidioblastic plates (Figures 3.13, 3.14, and 3.15). Most 30 Figure 3.8 Sample LGS08 Mx13 shows a fractured clinopyroxene grain with clay altera- tion along cleavage planes and a primary hypidioblastic phlogopite grain (PPL, 5X, FOV: 4.35 mm). Figure 3.9 Sample LGS1 Mx1 shows partial melting along grain boundaries of touch- ing clinopyroxene grains and the grey to black margins of the grains (PPL, 5X, FOV: 4.35 mm). 31 Figure 3.10 Sample LGS016 Mx12 shows late stage carbonate veining cutting through clinopyroxene grains (PPL, 5X, FOV: 4.35 mm). Figure 3.11 Sample LGS025 Mx11 shows garnet blebs in a clinopyroxene grain. Phlogo- pite alteration occurs around edges of garnet. The clinopyroxene grain shows deformation by undulatory extinction (XPL, 5X, FOV: 4.35 mm). 32 Figure 3.12 Sample JD40 Mx103 shows a rutile grain with partial melting along the margin surrounded by carbonate and phlogopite alteration (PPL, 5X, FOV: 4.35 mm). Figure 3.13 Sample JD35 Mx9 shows the alteration suite of green, fibrous serpentine, colourless serpentine, carbonate grains, opaques overprinting serpentine, and spinel grains (PPL, 20X, FOV: 1.05 mm). 33 Figure 3.14 Sample JD35 mx9 large alteration patch within a garnet grain consisting of spinel, clinopyroxene, carbonate, and phlogopite (PPL, 20X, FOV: 1.05 mm). Figure 3.15 Sample LGS035 Mx4 shows secondary hornblende alteration with an opaque inclusion along garnet grain boundary with serpentine and carbonate (PPL, 20X, FOV: 1.05 mm). 34 samples contain late stage carbonate veining that cuts through most primary minerals (Figure 3.10). Opaque and spinel grains occur on the outer edges of alterations, usually overprinting serpentine as hypidioblastic, rounded grains (Figure 3.13). Secondary opaques minerals are very finely crystalline. Secondary hornblende only occurs in two samples as deformed, hypidioblastic plates containing inclusions of spinel, opaques, phlogopite, and serpentine (Figures 3.4 and 3.15). Hornblende occurs in fractures, along grain boundaries, and in alteration patches of garnet grains (Figures 3.4 and 3.15). Chlorite only appears in two samples as either hypidioblastic plates or fibrous, radiating needles as part of alteration suites replacing garnet and clinopyroxene in fractures and grain boundaries (Figure 3.16). Chlorite most often appears in areas of larger amounts of alteration – usually between grains. 3.3 Muskox thin sections Petrographic studies have been conducted on four thin sections of eclogite xenoliths from the Muskox kimberlite. Three of these samples exhibit a massive texture and the other is foliated. The massive eclogite samples have a hypidioblastic texture and are composed of a similar bimineralic aggregate of garnet (50 – 75 vol. %) and clinopyroxene (20 – 45 vol. %) with accessory minerals of opaques (1 – 3 vol. %) and rutile (1 vol. %) (Appendix B). All of the garnet (5.3 mm average) and clinopyroxene (2.7 mm average) grains are heavily fractured (Figure 3.17). Garnet grains are rounded octahedrons and contain rare inclusions of opaques, phlogopite, and clinopyroxene (Figure 3.17). Clinopyroxene grains are rounded to subangular prisms and also contain inclusions of opaques, rutile, and garnet grains (Figure 3.18). Twin lamellae are present along cleavage planes of some grains. Clay alteration occurs along cleavage planes of clinopyroxene grains in one xenolith (similar to Figure 3.19). Partial melting is significant (10 – 90 vol. %) along grain boundaries and in fractures of both garnet and clinopyroxene grains in two of the massive xenoliths (Figure 3.18). Very little deformation occurs in these xenoliths as only a few grains of clinopyroxene show undulatory extinction. 35 Figure 3.16 Sample LGS025 Mx11 shows chlorite in an alteration patch within a clinopy- roxene grain associated with serpentine and phlogopite (PPL, 20X, FOV: 1.05 mm). Figure 3.17 Sample MOX28 308.4 shows hypidioblastic and fractured garnet and clino- pyroxene grains. Inclusions of a clinopyroxene and a hypidioblastic opaque grain are in a garnet grain. Secondary opaques within fractures and along grain boundaries are seen in both garnet and clinopyroxene grains (PPL, 2.5X, FOV: 8.7 mm). 36 Figure 3.18 Sample MOX28 308.4 shows a clinopyroxene grain with partial melting and an inclusion of rutile (PPL, 5X, FOV: 4.35 mm). Figure 3.19 Sample MOX24 206.9 shows a xenoblastic rutile grain altered to ilmenite and opaques. This rutile grain is surrounded by a partially melted clinopyroxene grain (PPL, 5X, FOV: 4.35 mm). 37 Common accessory minerals are opaques and rutile. Primary, hypidioblastic opaques are common in sample MOX28 308.4 and occur as inclusions in clinopyroxene and garnet grains. Sample MOX24 206.9 contains primary, xenoblastic rutile, which has been altered to either opaques or ilmenite (Figure 3.19). Secondary alteration typically occupies 5 – 7 vol. %; one sample shows 37 vol. % alteration. Main secondary minerals are carbonate, phlogopite, opaques, rutile, and chlorite, which crystallized through non-metamorphic processes. These minerals do not exhibit the typical morphology of metamorphic minerals, as the grain boundaries are not straight and do not form triple junctions between multiple grains (Figure 3.20 and 3.21). This suite of minerals occurs in the fractures and along the grain boundaries of garnet and clinopyroxene (Figure 3.20). The morphology of these minerals shows crystallization of euhedral to subhedral phlogopite grains from the outer edges of the vein toward the centre with carbonate grain infilling the space between phlogopite grains in the centre of the vein (Figures 3.20 and 3.21); this is consistent with hydrothermal crystallization or crystallization from mantle fluid. In two samples, very fine-grained carbonate replaces garnet and clinopyroxene in fractures, along grain boundaries, in cleavage traces, and as large veins (Figure 3.20). Carbonate is associated with phlogopite and opaques in all samples (Figure 3.20). In sample MOX28 308.4, carbonate occurs as patches, which are rimmed by opaques, chlorite, and rare phlogopite (Figure 3.21). These patches average 1.6 mm wide and contain carbonate, which formed as subhedral to anhedral plates (0.6 mm average) (Figure 3.21). Phlogopite forms subhedral to anhedral plates and laths (0.4 mm average) (Figure 3.21). When phlogopite is associated with carbonate alteration, it too, is altered by the carbonate. Phlogopite is deformed in one sample as evidenced by undulatory extinction. Subhedral opaques (0.07 mm average) are most often associated with carbonate alteration and occur as inclusions in garnet and clinopyroxene grains (Figure 3.17). Chlorite alteration occurs in two samples as very fine-grained, subhedral plates and needles replacing garnet and clinopyroxene grains (Figure 3.22). Chlorite is always associated with carbonate and phlogopite alteration (Figure 3.22). The formation of chlorite likely occurred at low temperatures in the presence of water and thus represents a subsurface, crustal process. Secondary rutile (0.8 mm average) is present in sample MOX28 308.4 as rounded, subhedral grains between the grain boundaries of garnet and clinopyroxene (Figures 3.18 and 3.19). 38 Figure 3.20 Sample MOX24 206.9 shows a large fracture through a garnet grain that is infilled with carbonate, phlogopite, and opaque grains. This garnet grain also exhibits partial melting along fractures (PPL, 2.5X, FOV: 8.7 mm). Figure 3.21 Sample MOX 28 308.4 shows garnet grains replaced by a patch of carbon- ate, phlogopite, and opaque grains. Hypidioblastic carbonate grains are shown as well as hypidioblastic plates and laths of phlogopite grains (PPL, 2.5X, FOV: 8.7 mm). 39 Figure 3.22 Sample MOX24 206.9 shows a garnet grain replaced by carbonate, chlo- rite, and opaque grains. The chlorite grains shown are hypidioblastic needles (PPL, 20X, FOV: 1.05 mm). Figure 3.23 Sample MOX7 53.9 shows elongated and fractured clinopyroxene and garnet grains. The clinopyroxene grain shown exhibits some partial melting in fractures and along grain boundaries (PPL, 2.5X, FOV: 8.7 mm). 40 The foliated sample is composed of a bimineralic aggregate of clinopyroxene (75 vol. %), garnet (15 vol. %), and accessory rutile (7 vol. %) (Appendix B). All of these primary grains are xenoblastic to hypidioblastic and show elongation in the same direction (Figure 3.23). All clinopyroxene (2.3 mm average) and garnet (2 mm average) grains are fractured and contain inclusions rutile (Figure 3.23). Only clinopyroxene grains show little partial melting along grain boundaries and in fractures (Figure 3.23). Deformation is prevalent as undulatory extinction is present in some clinopyroxene and the majority of phlogopite grains. Garnet grains also contain inclusions of clinopyroxene (Figure 3.24) and are present as inclusions in phlogopite (Figure 3.25). Secondary melting of the eclogite is exhibited by a magmatic texture in some areas of the sample (Figures 3.24 and 3.26). These magma patches contain anhedral, curvilinear, blebs of garnet and clinopyroxene, which appear as inclusions in garnet in clinopyroxene grains (Figure 3.25). Primary rutile grains (2 mm average) occur as inclusions in garnet and clinopyroxene grains; larger grains are present between grains of clinopyroxene and/or garnet along the foliation (Figure 3.27). Secondary alteration spans approximately 7 vol. % of the sample. The main secondary minerals are phlogopite, carbonate, and opaques. This suite of non- metamorphic minerals occurs on grain boundaries and fractures of garnet and clinopyroxene grains and comprises the veins cutting through the sample (Figures 3.24, 3.27, and 3.28). These minerals do not exhibit the typical morphology of metamorphic minerals, as the grain boundaries are not straight and do not form triple junctions between multiple grains (Figure 3.28). The morphology of these minerals shows crystallization of subhedral phlogopite grains from the outer edges of the vein toward the centre with subhedral carbonate grain infilling the space between marginal phlogopite (Figure 3.28); this is consistent with hydrothermal crystallization or crystallization from mantle fluid. In the larger veins and fractures, carbonate infills the centre; phlogopite occurs further to the edge of the vein and the opaques crystallize on the outer-most edges away from the centre (Figure 3.28). Phlogopite grains (0.6 mm average) form subhedral 41 Figure 3.24 Sample MOX7 53.9 shows an inclusion of clinopyroxene in a garnet grain. The magmatic texture of intergrown garnet and clinopyroxene grains is also depicted. A replacement vein cuts through the garnet and clinopyroxene grains. It is comprised of carbonate grains in the centre and opaques on the outer edge (PPL, 2.5X, FOV: 8.7 mm). Figure 3.25 Sample MOX7 53.9 shows a garnet grain surrounded by phlogopite (PPL, 5X, FOV: 4.35 mm). 42 Figure 3.26 Sample MOX7 53.9 shows a magmatic texture of intergrown garnet and clinopyroxene grains. Some partial melting of the clinopyroxene grains shown occur along grain boundaries and in fractures (PPL, 2.5X, FOV: 8.7 mm). Figure 3.27 Sample MOX7 53.9 shows an elongated opaque grain between grains of elongated clinopyroxene. A vein of carbonate grains replaces clinopyroxene grains as it runs through their grain boundaries. The upper clinopyroxene grain shows some partial melting along fractures (PPL, 2.5X, FOV: 8.7 mm). 43 Figure 3.28 Sample MOX7 53.9 shows a replacement vein cutting through a garnet grain. Carbonate grains occur in the centre of the vein with overprinting, euhedral spinel grains. Phlogopite grains occur further to the outer edge of the vein. Opaque grains occur on the outermost edge of the vein (PPL, 5X, FOV: 4.35 mm). Figure 3.29 Sample MOX7 53.9 shows secondary amphibole, carbonate, opaques, and phlogopite altering garnet (PPL, 5X, FOV: 4.35 mm). 44 plates that encompass garnet and opaque grains (Figure 3.26). Carbonate grains are elongated and occur only in veins and fractures through garnet and clinopyroxene grains (Figures 3.24, 3.27, and 3.28). Carbonate (0.2 mm average) is always associated with phlogopite and opaques (Figures 3.24, 3.27, and 3.28). Rare spinel grains are present in large veins. The spinel grains occur as euhedral inclusion in larger, poikilitic carbonate grains (Figure 3.28). Spinel grains are only associated with carbonate alteration veins and occur only in the centre of these veins (Figure 3.28). Secondary amphibole is very rare and occurs only within garnet grains as anhedral plates in areas where the kimberlite has infiltrated the xenolith (Figure 3.29). Significant mineralogical differences between the massive and foliated eclogite xenoliths are the presence of chlorite in the massive samples and amphibole and spinel in the foliated sample. Also, the foliated sample contains a much greater volume percentage of clinopyroxene (75 vol. % vs. 20 – 45 vol. %) than the massive samples. Differences in the sequential formation of massive and foliated eclogites include the additional phlogopite development event in the massive eclogites. The amount of partial melting of the massive eclogites (average of 50 vol. %) is much greater than that of the foliated eclogite (5 vol. %). Also, only the foliated eclogite xenolith shows a secondary melting event of the garnet and clinopyroxene grains producing the magmatic texture in Figures 3.24 and 3.26. Kimberlite infiltration is only restricted to the foliated eclogite. 45 Chapter 4: Mineral Chemistry 4.1 Sample preparation and analytical methods A total of 76 eclogite xenoliths (1 cm to 15 cm) from Jericho were cut and crushed; an average of four fresh grains of garnet, clinopyroxene, and olivine were selected of each mineral for each sample. A total of 33 eclogite xenoliths (2 cm to 20 cm) from Muskox were cut and broken. For each eclogite, an average of three chips of each xenolith, where garnet and clinopyroxene grains are touching, were selected for analysis. These touching grains allowed for the chemical analyses of the core and rim for each mineral. The mineral grains were mounted onto a glass slide attached with a double-sided adhesive strip. A transoptic ring was placed around the grains and filled with Miapoxy 100TM resin. Once hardened, the ring was removed from the glass slide using a thin blade. The surface of the mount was cleaned of any remaining adhesive using ethanol. The mounts were then polished by hand using BulherTM polishing cloths and diamond pastes. A total of 24 mounts were produced (13 from Jericho and 11 from Muskox). Quantitative chemical analysis was conducted using a fully automated CAMECA SX-50 electron microprobe at the University of British Columbia in the Department of Earth, Ocean and Atmospheric Sciences. Analyses of all elements were completed using a beam current of 20 nA, acceleration voltage of 15 kV, and peak count time of 20 s. Minimum detection limits are shown in Appendix C – Table C1. 4.2 Northern Slave eclogites Major element chemistry of both garnet and clinopyroxene from the Muskox and Jericho eclogites is very similar. Figure 4.1 shows a comparison of the negative correlation between FeO and MgO of garnet from Muskox and Jericho. Figure 4.2 shows a comparison of the negative correlation between Na2O and CaO of clinopyroxene from Muskox and Jericho. It is evident from these plots that compositions of minerals in Muskox and Jericho eclogite xenoliths could be discussed together as Northern Slave eclogites. Mineral compositions that are the basis for these descriptions are listed in all tables from Appendix C. 46 Figure 4.1 FeO total vs. MgO (wt. %) of garnet from Muskox and Jericho eclogite xenoliths. Figure 4.2 Na2O vs. CaO (wt. %) of clinopyroxene from Muskox and Jericho eclogite xenoliths. 47 4.2.1 Garnet chemistry Garnet compositions from Northern Slave eclogites are typically dominated by pyrope (Mg3Al2Si3O12) with lesser almandine (Fe3Al2Si3O12) and grossular (Ca3Al2Si3O12). Percentage amounts of each component are calculated based on cation amount normalized to a total of 8 cations using total FeO content. Table 4.1 shows the changes in composition between primary and secondary garnets from massive, foliated, texturally undetermined, and diamondiferous eclogites. All secondary garnet grains have a greater pyrope content than the primary garnet grains. Table 4.1 Compositional differences between primary and secondary garnet Type Texture Composition Primary Massive Almandine24-62Pyrope21-46Grossular17-29 Foliated Almandine23-64Pyrope23-42Grossular13-35 Undetermined Almandine26-57Pyrope27-43Grossular15-30 Diamondiferous Pyrope43-48Almandine35-25Grossular22-26 Secondary Massive Pyrope50-73Grossular11-27Almandine15-22 Foliated Pyrope52Almandine29Grossular19 Undetermined Pyrope70Almandine20Grossular10 Diamondiferous Pyrope67-72Almandine17-22Grossular10-11 Major element chemistry of the Jericho eclogites has been employed to subdivide the xenolith suite using classification by Coleman (1965) and Taylor and Neal (1989). Figure 4.3 shows the subdivisions of garnets from massive (barren and diamondiferous), foliated, and texturally undetermined eclogite xenoliths. Barren and diamondiferous massive eclogites generally have higher MgO content than the foliated eclogites. Group A samples have a massive (barren and diamondiferous) texture with two samples being texturally undetermined. All diamondiferous eclogites plot within Group A, except one, which plots within Group B. Also, the foliated eclogites have a wider range of CaO content than the massive xenoliths. Major element chemistry of the Muskox eclogites has been employed to subdivide the xenolith suite in the same manner as eclogites from Jericho. Figure 4.4 shows the subdivisions of garnets from both massive (barren and diamondiferous) and foliated eclogite xenoliths. Massive eclogites generally have higher MgO content than the foliated eclogites. The only Group A sample has a barren, massive texture. The 48 Figure 4.3 MgO (wt. %) vs. CaO (wt. %) for Jericho garnet from massive and foliated xenoliths. Group A > 16.25 wt. % MgO; Group B 10.5 – 16.25 wt. % MgO; Group C < 10.5 wt. % MgO. Includes data from Heaman et al. (2006), Smart et al. (2009), Kopylova et al. (1999a), and Kopylova et al. (2004). Figure 4.4 MgO (wt. %) vs. CaO (wt. %) for Muskox garnet from massive and foliated xenoliths. Group A > 16.25 wt. % MgO; Group B 10.5-16.25 wt. % MgO; Group C < 10.5 wt. % MgO. A B C 49 diamondiferous eclogites plot within Group B. Also, the massive eclogites have a wider range of CaO than the foliated xenoliths. Major and minor elements of garnet show some correlations. There are positive correlations between MnO and FeO, MgO and Al2O3, and as well between TiO2 and Na2O. Negative correlations exist between Al2O3 and FeO, Al2O3 and MnO, as well as between FeO and MgO, which is depicted in Figure 4.5. The negative correlation between FeO and MgO is due to the solid solution between pyrope and almandine, controlled by the substitution of FeO for MgO (and vice versa) in the dodecahedral site. Figure 4.5 also shows that the primary garnet from foliated samples deviate somewhat from the correlation of FeO and MgO defined by the primary garnet in massive samples. The majority of these primary garnets in foliated samples are from Jericho eclogites. These primary garnets from foliated sampleshave a strong positive correlation between FeO and MnO and a less pronounced negative correlation between FeO and CaO (Figure 4.6). The Na2O and TiO2 content in garnets can identify megacrystal garnets and distinguish them from eclogitic (Figure 4.7). The majority of garnet analyses do not plot within the megacryst field because their TiO2 content is lower. 4.2.2 Clinopyroxene chemistry All clinopyroxene from Northern Slave eclogites have an omphacitic composition with 20 – 80 mol. % CaMgSi2O6 – CaFeSi2O6 (diopside – hedenbergite) and 80 – 20 mol. % jadeite (NaAlSi2O6). Percentage amounts of each component are calculated based on cation amount normalized to a total of 4 cations using total FeO content. Table 4.2 shows the changes in composition between primary and secondary clinopyroxenes from massive, foliated, texturally undetermined, and diamondiferous eclogites. Primary, foliated grains dominate the clinopyroxene population with a jadeite component of 45 % or greater (Figure 4.8; Na2O + Al2O3 ≥ 18.5 wt. %). Analyses of secondary clinopyroxene grains show an overall increase in diopside component at the expense of the jadeite component. 50 Figure 4.5 MgO vs. FeO total (wt. %) of primary and secondary analyses of garnet from Northern Slave eclogite xenoliths. Figure 4.6 FeO total vs. CaO (wt. %) of primary, foliated analyses of garnet from Jericho eclogite xenoliths. 51 Figure 4.7 Na2O vs. TiO2 (wt. %) of primary and secondary garnet from Northern Slave eclogite xenoliths. The dotted lines are based on empirical observations (564 grains analysed by McCandless and Gurney 1989; 124 grains analysed by Sobolev and Lavrent’ev 1971). Modified from Cookenboo and Grütter (2010). Figure 4.8 Na2O + Al2O3 vs. CaO + MgO (wt. %) of primary and secondary clinopyrox- ene from Northern Slave eclogite xenoliths. 52 Table 4.2 Compositional differences between primary and secondary clinopyroxene Type Texture Composition Primary Massive Diopside45-76Jadeite24-55 Foliated Diopside47-71Jadeite29-53 Undetermined Diopside61Jadeite39 Diamondiferous Diopside69-73Jadeite27-31 Secondary Massive Diopside78-94Jadeite6-22 Foliated Diopside74-86Jadeite14-26 Undetermined Diopside84-89Jadeite11-16 Diamondiferous Diopside79-91Jadeite9-21 Clinopyroxene grains from the Jericho eclogites are also classified into Groups A, B, and C based on Na2O and Al2O3 contents (Figure 4.9). Massive (barren and diamondiferous) eclogites generally have a lower Na2O than the foliated xenoliths. Most of the Group A xenoliths are massive (barren and diamondiferous), two xenoliths are foliated, and four are texturally undetermined. All diamondiferous eclogites plot within Group A, except one, which plots within Group B. Clinopyroxene grains from the Muskox eclogites are also classified based on Na2O content (Figure 4.10). Massive (barren and diamondiferous) eclogites have higher Na2O than the foliated xenoliths. The only Group A sample has a barren, massive texture and the only Group C samples have a foliated texture. Diamondiferous eclogites plot only in Group B. Massive eclogites also have a wider range of Al2O3 than the foliated eclogites. Major element chemistry of clinopyroxene shows a positive correlation between Al2O3 and Na2O, Cr2O3 and MnO, Cr2O3 and MgO, Cr2O3 and CaO, and as well between MgO and CaO, which implies the residence of these elements in the jadeite and diospide end-members, respectively. Extensive solid solution exists between jadeite and diopside, which produces a negative correlation between Na2O and CaO. There also exist negative correlations between MgO and Al2O3, Al2O3 and CaO, Cr2O3 and Al2O3, Cr2O3 and Na2O, and as well between MgO and Na2O. The overall Cr2O3 content of clinopyroxene grains ranges from 0.01 to 1.24 wt. %; TiO2 content ranges from 0.03 to 0.65 wt. %. No correlations exist between TiO2 and any other element analysed. Figure 4.8 shows a negative correlation between Na2O and Al2O3 with MgO and CaO. The K2O content in clinopyroxenes may also be used to determine diamond potential (McCandless and Gurney 1989), therefore we analysed studied clinopyroxenes 53 Figure 4.9 Al2O3 (wt. %) vs. Na2O (wt. %) for Jericho clinopyroxene from massive and foliated eclogite xenoliths. Group A has the lowest jadeite content and Group C has the highest jadeite content. Includes data from Heaman et al. (2006), Smart et al. (2009), Kopylova et al. (1999a), and Kopylova et al. (2004). Figure 4.10 Al2O3 (wt. %) vs. Na2O (wt. %) for Muskox clinopyroxene from massive and foliated eclogite xenoliths. Group A has the lowest jadeite content and Group C has the highest jadeite content. A B C 54 for K2O. Figure 4.11 compares K2O with Na2O. All of the primary clinopyroxenes from diamondiferous eclogites, except one, plot at K2O concentrations higher than 0.08 wt. %. The one that does not is the Group B eclogite from Smart et al. (2009). Very few clinopyroxenes from non-diamondiferous eclogites plot above 0.08 wt. % K2O. 4.2.3 Rutile chemistry Rutile grains are only analysed in Muskox eclogites from this study and contain TiO2 (89 – 94 wt. %), Al2O3 (0.03 – 3.44 wt. %), Cr2O3 (0 – 0.08 wt. %), FeO (2.43 – 6.22 wt. %), MnO (0 – 0.07 wt. %), MgO (0.04 – 0.58 wt. %), and Nb2O5 (0.14 – 1.93 wt. %) (Tables C109 and C112). Positive correlations occur between Cr2O3 and FeO, Cr2O3 and Nb2O5, as well as FeO and MgO. Figure 4.12 shows the relationship between rutile grains from massive and foliated eclogites. Rutile grains from the massive Muskox eclogite contain greater amounts of Nb2O5 than those from the foliated eclogite. However, rutiles from Jericho eclogites show the opposite pattern (Kopylova et al. 1999a) (Figure 4.12). Figure 4.13 shows the positive correlation between Cr2O3 and Nb2O5 for Muskox eclogites, but a negative correlation for the Jericho eclogites. 4.2.4 Mica chemistry The composition of mica in all of the eclogite xenoliths is predominantly phlogopite (KMg3(AlSi3O10)(F,OH)2) (72 – 88 mol. %) with some annite (KFe3(AlSi3O10)(F,OH)2) component (12 – 28 mol. %) (Tables C110 – 112). There is no correlation between textural position of phlogopite in thin section and major element chemistry. The formula for phlogopite ranges from (K0.8Na0.1Ca0.01)(Mg1.9Fe0.5Mn0.01Ti0.1Al0.1)(Si2.8Al1.1)O10(OH)2 to (K0.9Na0.1Ca0.01)(Mg2.2Fe0.8Mn0.01Ti0.2Al0.3)(Si2.8Al1.3)O10(OH)2. Substitution between Al2O3 and SiO2 is very limited and the amount of eastonite-siderophyllite (KMg2Al3Si2O10(OH)2 – KFe2+2Al3Si2O10(OH)2) is 0.1 to 0.3 mol. %. 4.2.5 Amphibole chemistry Amphibole is only present in Muskox eclogite MOX7 53.9 (Table C109). Both analyses are from amphibole grains that are adjacent to carbonate patches replacing garnet grains. The first analysis is a calcic amphibole with the formula (Ca1.2Mg0.8)(Fe3.1AlMg0.7Mn0.1)(Si5Al3)O22(OH)2 and is classified as ferrotschermakite by Leake et al. (1997) (square symbol in 4.14a) . The second analysis is a sodic-calcic amphibole with the formula 55 Figure 4.11 K2O vs. Na2O (wt. %) of primary and secondary clinopyroxene from Nothern Slave eclogite xenoliths. The dotted line is based on empirical observation and recom- mended as being suitable for assessing diamond potential. Diamondiferous eclogites contain clinopyroxenes with > 0.08 wt. % K2O (Gurney and Zweistra 1995). Figure 4.12 Nb2O5 vs. FeO total (wt. %) of primary rutile from Northern Slave eclogite xenoliths. 56 Figure 4.13 Nb2O5 vs. Cr2O3 (wt. %) of primary rutile from Northern Slave eclogite xenoliths. Figure 4.14 Diagram of amphibole classification modified after Leake et al. (1997). A) Group 2: calcic amphiboles, square symbol represents ferrotschermakite; B) Group 3: sodic-calcic amphiboles, diamond represents taramite. 57 (Na0.9K0.1)(Ca1.7Mg0.3Na0.1)(Fe2.3Mg1.8Al0.8Ti0.1)(Si6Al2)O22(OH)2 and is classified as taramite by Leake et al. (1997) (diamond symbol in Figure 4.14b). 4.2.6 Carbonate chemistry Several different carbonate minerals are present in the eclogites analyzed (Tables C109 – C112). These include calcite (CaCO3), siderite (FeCO3), magnesite (MgCO3), calcite-siderite, and dolomite-siderite (Figure 4.15). A negative correlation exists between MgO with FeO and CaO (Figure 4.15). This correlation exists due to the solid solution between calcite and siderite as well as between calcite and magnesite. CaO substitutes for MgO as well as for FeO and vice versa. Figure 4.15 MgO + FeO total vs. CaO (wt. %) of carbonate within veins, fractures, and in patches from Muskox eclogite xenoliths. These carbonate grains occur in veins, fractures, and patches, all of which replace garnet and clinopyroxene grains. There are no systematic differences between chemistry and textural position of carbonate in the rock or chemistry and whether the mineral replaces garnet or clinopyroxene. Composition of carbonate depends only on the sample. 58 Eclogite xenolith MOX28 308.4 only contains calcite, and magnesite is only found in eclogite MOX25 207. 4.2.7 Spinel-group mineral chemistry Spinel-group minerals are magnetite (Fe2+0.97Fe3+1.83Ti0.07Mg0.07Mn0.04Al0.02O4) and spinel-magnetite (Fe2+0.91Fe3+1.18Al0.46Mg0.26Ti0.17Mn0.01O4) solid solution in composition (Table C112). The spinel group minerals in the solid solution are likely magnetite (FeO·Fe2O3), spinel (MgAl2O4), and ulvöspinel (TiFe2O4). Magnetite and spinel-magnetite are associated with carbonate patches that replace garnet and clinopyroxene grains. The magnetite contains TiO2 (2.62 wt. %), Al2O3 (0.36 wt. %), Cr2O3 (0.09 wt. %), FeO (94.9 wt. %), MnO (1.3 wt. %), MgO (1.36 wt. %), and CaO (0.16 wt. %). The grain that shows extensive solid solution between spinel and magnetite contains TiO2 (6.73 wt. %), Al2O3 (11.73 wt. %), Cr2O3 (0.08 wt. %), FeO (75.04 wt. %), MnO (0.4 wt. %), MgO (5.23 wt. %), and CaO (0.13 wt. %). 4.2.8 Olivine chemistry Olivine is only present in eclogite xenoliths from Jericho and all olivine grains are primary (Tables C15, C18, C25, C31, C47, and C48). Major element chemistry shows that all olivine grains are dominated by forsterite (Mg2SiO4) with a lesser fayalite (Fe2SiO4) component. Olivine grains from massive eclogites show a range in compositions Forsterite86-93Fayalite7-14. Olivines from foliated and texturally undetermined eclogite xenoliths are mostly magnesian and both have a very limited composition of Forsterite92Fayalite8. A positive correlation exists between Cr2O3 and MgO as well as between FeO and MnO. A negative correlation occurs between CaO and MnO (Figure 4.16), FeO and CaO, MnO and MgO, as well as between FeO and MgO, which is shown in Figure 4.17. FeO and MgO show a negative correlation due to the extensive solid solution between forsterite and fayalite and substitution of Fe for Mg and vice versa. 4.2.9 Evolution of mineral chemistry due to recrystallization and zoning Mineral compositions that are the basis for these descriptions are listed in Appendix C (Tables C102 – C105). The chemical evolution of primary to secondary garnet and clinopyroxene grains, as a result of partial melting, has been analysed for samples MOX24 206.9, MOX7 53.9, MOX25 207, JDF6N, 52-5, 55-4, 47-2, and 47-8. 59 Figure 4.16 CaO vs. MnO (wt. %) of olivine from Jericho eclogites. Dashed lines represent minimum detection limits. Figure 4.17 FeO total vs. MgO (wt. %) of olivine from Jericho eclogites. 60 Figures 4.18 and 4.19 show textural relationships between primary and secondary garnet and clinopyroxene grains, respectively. Garnet grains exhibit the same pattern shown in Figure 4.5, where secondary garnets show a significant increase in MgO and decrease in FeO content compared to the primary grains (Figure 4.20). Samples JDF6N, 52-5, and 47-8 and grains 25-1, 25-3, and 24-9 from samples MOX25 207 and MOX24 206.9 all exhibit a decrease in CaO content from primary to secondary garnet. Grain 25-1 from eclogite MOX25 207 and samples JDF6N and 52-5 show significant increase in Al2O3 content from primary to secondary garnets. However, grain 25-3 from MOX25 207 and sample 47-8 show a decrease in Al2O3 content. These patterns are shown in Figure 4.21. Secondary clinopyroxene is always characterized by decreased jadeite component. Figure 4.22 shows the trend of increasing MgO and decreasing Al2O3 content from primary to secondary grains. With respect to the FeO evolution, however, secondary clinopyroxenes differ. Eclogite MOX24 206.9 shows an increasing trend of FeO content from primary to secondary clinopyroxene in grains 24-3 and 24-11 as well as in samples JDF6N, 55-4, 47-2, and 47-8; however, samples MOX7 53.9 and MOX25 207 show a decrease in FeO content from primary to secondary clinopyroxene in grain 7- 6. Theses contrasting trends are shown in Figure 4.23. The chemical evolution of garnet and clinopyroxene from the core to the rim were analysed for the Muskox sample MOX28 308.4 and the Jericho samples LGS8 Mx3 and 26-6. Clinopyroxene showed a difference between the core and the rim analyses only for sample 26-6. The omphacitic composition of clinopyroxene remains constant from the core to the rim with equal diopside and jadeite components for sample MOX28 308.4 and composition Diopside75Jadeite25 for the Jericho sample LGS8 Mx3. The zoned clinopyroxenes from sample 26-6 show increases in Al2O3, FeO, MgO, and Na2O as well as decreases in Cr2O3 and CaO from the core to the rim. The core analyses of this sample have a composition of Diopside85Jadeite15. The analyses on the margin of the grain show an increase in Na2O content with a composition of Diopside84Jadeite16. Garnet grains show a significant increase in MgO and decrease in FeO from the core to the rim of each grain (except 26-6) (Figure 4.24). There is also a pronounced increase of CaO from the core to the rim of grain 28-5. Margins of primary garnet from eclogite MOX28 308.4 show zoning (Figure 4.24). 61 Figure 4.18 Photomicrograph of primary and secondary garnet from sample MOX25 207 (PPL, 20X, FOV: 1.05 mm). Figure 4.19 Photomicrograph of primary and secondary clinopyroxene from sample MOX7 53.9 (XPL, 20X, FOV: 1.05 mm). 62 Figure 4.20 MgO vs. FeO total (wt. %) show the chemical evolution from primary to secondary garnet from Muskox eclogite xenoliths MOX24 206.9 and MOX25 207 as well as Jericho eclogites 47-8, JDF6N, and 52-5. Tie-lines connect analyses from primary to secondary garnet. Figure 4.21 CaO vs. Al2O3 (wt. %) show the chemical evolution from primary to secondary garnet from Muskox xenoliths MOX24 206.9 and MOX25 207 as well as Jericho eclogites 47-8, JDF6N, and 52-5. Tie-lines connect analyses from primary to secondary garnet. 63 Figure 4.23 MgO vs. FeO total (wt. %) show the evolution of garnet from the core to the rim of Muskox eclogite xenolith MOX28 308.4 and Jericho eclogite xenolith LGS8 Mx3. Sample numbers represent different garnet in the eclogite. Tie-lines connect analyses from the core to the rim of the same grain. Figure 4.22 FeO total vs. Na2O (wt. %) shows the evolution from primary to secondary clinopyroxene from the Muskox eclogite xenoliths MOX24 206.9, MOX7 53.9, and MOX25 207 and the Jericho xenoliths JDF6N, 55-4, 47-2, and 47-8. Tie-lines connect analyses from primary to secondary clinopyroxene. 64 Figure 4.24 FeO total vs. Na2O (wt. %) shows the evolution from primary to secondary clinopyroxene from the Muskox eclogite xenoliths MOX24 206.9, MOX7 53.9, and MOX25 207 and the Jericho xenoliths JDF6N, 55-4, 47-2, and 47-8. Tie-lines connect analyses from primary to secondary clinopyroxene. The core analyses have a composition of Almandine20-46Pyrope30-68Grossular12-23. The analyses on the margin of each grain show an increase in Mg content with a composition of Almandine20-56Pyrope31-69Grossular11-23. Sample 26-6 does not show significant differences between core and rim analyses and is thus not included in Figure 4.24. However, there does exist a slight increase in Mg and Ca content along the margin. 4.3 Central Slave eclogites Literature data of eclogite xenoliths from several kimberlites in the Central Slave have been selected for comparative analysis with Jericho and Muskox eclogite xenoliths. Kimberlite pipes in the Lac de Gras region are examined in the following discussion. Chemical analyses from these pipes are selected from Aulbach et al. (2007) (“A154S”), Schmidberger et al. (2007) (“A154”), Pearson et al. (2004) (“Lac de Gras”), and Aulbach et al. (2011) (“Ekati”). The textures of all these eclogites have not been reported in the literature. 65 Major element chemistry of the Central Slave eclogites has been employed to subdivide the xenolith suite using classification by Coleman (1965) and Taylor and Neal (1989). Figure 4.25 shows the subdivisions of garnet grains from eclogite xenoliths from A154S, A154, Lac de Gras area, and Ekati. Barren A154 and Lac de Gras area eclogites generally have higher MgO content than the eclogites from A154S, diamondiferous A154, and Ekati. Group A samples are from A154S, barren A154, and the Lac de Gras area. All diamondiferous eclogites plot within Groups B and C. Also, the barren A154 eclogites have the widest range of CaO content. Eclogites from A154S, A154, and Lac de Gras area generally have a lower Na2O than those from A154 and Ekati. The Group A xenoliths are from A154S, barren A154, and the Lac de Gras area. All diamondiferous eclogites plot within Groups B and C. 66 Figure 4.25 MgO (wt. %) vs. CaO (wt. %) for eclogites from the Central Slave garnets. Group A >16.25 wt. % MgO; Group B 10.5-16.25 wt. % MgO; Group C <10.5 wt. % MgO. Analyses from “A154S” selected from Aulbach et al. (2007), “A154” selected from Schmidberger et al. (2007), “Lac de Gras” selected from Pearson et al. (2004), and “Ekati” selected from Aulbach et al. (2011). Figure 4.26 Al2O3 (wt. %) vs. Na2O (wt. %) eclogites from the Central Slave clinopy- roxenes. Group A has the lowest jadeite content and Group C has the highest jadeite content. Analyses from “A154S” selected from Aulbach et al. (2007), “A154” selected from Schmidberger et al. (2007), “Lac de Gras” selected from Pearson et al. (2004), and “Ekati” selected from Aulbach et al. (2011). 67 Chapter 5: Bulk chemical composition of Northern and Central Slave eclogites Bulk chemical compositions of eclogite xenoliths from the Jericho and Muskox kimberlites have been selected from the literature and calculated for samples from this study. Smart et al. (2009) calculated bulk eclogite composition for Jericho eclogites using an estimated mode of 50 % garnet and 50 % clinopyroxene and electron micro- probe analyses. Russell et al. (2001) analysed bulk eclogite composition for Jericho eclogites using x-ray fluorescence. Bulk chemical compositions of 34 eclogite xenoliths from this study have been calculated (seven from Jericho, 27 from Muskox) (Tables 5.1 and 5.2). Calculations are based on normalized modes of minerals and electrion probe micro-analyses of minerals. Modes were estimated from thin section analyses with a petrographic microscope and macro-specimens with a 10X hand lens and electron microprobe analyses. Margin of error for thin section mode estimations is ± 3 – 5 vol. % and ± 10 vol. % for macro- specimens. Similar calculations were repeated for the Central Slave xenoliths. Bulk major element composition for 48 eclogite xenoliths studied by Schmidberger et al. (2007) were calculated (Table 5.3). Calculations were conducted using modal percentage estimations and the corresponding EMP analyses for each eclogite xenolith reported by Schmidberger et al. (2007). 68 69 70 71 72 73 Chapter 6: Geothermobarometry 6.1 Methodology Temperature estimates for Jericho and Muskox eclogite xenoliths are calculated using the Ellis and Green (1979) (EG) and Nakamura (2009) geothermometers. Temperatures calculated with EG are used for comparison purposes only and can be found in Appendix C. The Nakamura (2009) temperatures are used in the following discussions in this chapter and Chapter 7 (Appendix C). EG is calibrated for rocks with a mafic composition approximated by the Ca, Mg, Fe, Al, Si system, and can be used accurately between 750 – 1300 ºC and 24 – 30 kbar. This thermometer is based on the Fe-Mg exchange between coexisting high-Mg garnet and clinopyroxene. EG also incorporates the Ca2+ content in garnet to account for the Ca-effect (non-ideal substitution of Ca-Mg in garnet), which was not successfully accounted for in earlier thermometers. Previous work on the Slave craton eclogites has used EG to calculate temperatures and pressures (Heaman et al. 2006; Smart et al. 2009; Kopylova et al. 1999a; Kopylova et al. 2004). In the calculation of the distribution coefficient (KD), only total iron as FeO, for both garnet and clinopyroxene, is considered because Fe3+ measurements in clinopyroxene are highly uncertain and often overestimated. This results in the calculated temperatures to be unrealistically low along the P-T array at any given pressure. Temperature estimates for Jericho and Muskox eclogite xenoliths are also calculated using the Nakamura (2009) geothermometer. Nakamura (2009) can be used accurately between 800 – 1820 ± 74 ºC and at given pressures of 15 – 75 kbar. This thermometer is also based on the Fe-Mg exchange between coexisting high-MgO garnet and clinopyroxene. However, Nakamura (2009) determined that EG overestimates temperatures ≤ 1000 ºC by 20 – 100 ºC when applied to natural eclogites. The equation provided by Nakamura (2009), account for Fe2+ and Fe3+ in both garnet and clinopyroxene. However, since Fe3+ content was not accurately measured, in our samples Fe2+ in the equation have been replaced by FeO total. Temperatures are calculated at 40, 50, and 60 kbar for each sample creating a P-T line. The intersection point of these data with the Brey and Köhler 1990 (BK) peridotitic P-T array (Kopylova et. al. 1999a) yields the equilibrium pressure and temperature. The BK thermobarometer for peridotites is a 74 combination between a two-pyroxene thermometer and an aluminium barometer both of which are calibrated for the CaO-MgO-Al2O3-SiO2 system. BK is can be used accurately between 900 – 1400 ºC and 25 – 60 kbar. This technique is based on the assumption that eclogites are thermally equilibrated with peridotites and that both are similar in age. Each point on the geotherm is accurate to ± 90 oC (Brey and Köhler 1990; Nakamura 2009) and ± 2.2 kbar (Brey and Köhler 1990). The majority of samples, 181 grains from 74 xenoliths from Jericho and 124 grains from 24 xenoliths from Muskox, are homogeneous. Multiple grain analyses from each xenolith were averaged to produce only one analysis for garnet and clinopyroxene to use with EG or Nakamura thermometers. Analyses with major element totals closest to 100%, those with the best Si4+ totals (closest to 3.000 for garnet and 2.000 for clinopyroxene), and garnet analyses with divalent cation totals (Ca2+, Fe total, Mn2+, Mg2+) closest to 3.000 were selected for geothermometric calculations. The remaining samples, 16 grains from four xenoliths from Jericho and 39 grains from four xenoliths from Muskox, show significant zoning. The zoning in eclogites from Jericho is expressed as significant differences in Na, Ca, Mg, and Al in clinopyroxene. The zoning in eclogites from Muskox is mainly expressed as significant differences in Ca, Mg, and Fe in garnet; one xenolith exhibits zoning in clinopyroxene as significant differences in Ca, Mg, Na, Al, and Fe. Consequently, there should be two sets of pressure and temperature conditions for these heterogeneous samples. 6.2 Jericho Eclogite xenoliths from the Jericho kimberlite have been separated into massive, foliated, and undetermined textural groups. Massive eclogites are further divided into barren and diamondiferous. Data from Jericho eclogite xenoliths from Heaman et al. (2006), Smart et al. (2009), Kopylova et al. (1999a), and Kopylova et al. (2004) have been included in the following figures and discussion. These eclogites, at the estimated equilibration pressure of 50 kbar, record temperatures from 850 to 1250 ºC (Figure 6.1). Foliated eclogites exhibit a wider range of temperatures than the diamondiferous, barren, and undetermined xenoliths. Equilibrium pressure and temperature estimates for both massive and foliated eclogite xenoliths from Jericho are calculated using the intersection of univariant PT lines 75 from the Nakamura (2009) thermometer and the peridotitic geotherm based on the thermobarometer by Brey and Kohler (1990). There are a total of 59 (25 from this study and 31 from the literature) massive eclogite xenoliths, which range from 732 to 1305 ºC and from 29 to 71 kbar (Figure 6.2). There are 39 barren (25 from this study and 14 from the literature), massive eclogites, which span the entire range of temperatures and pressures; 26 of these xenoliths plot within the diamond stability field. There are 20 diamondiferous eclogites (all from the literature), which range from 908 to 1100 ºC and from 41 to 55 kbar (Figure 6.2); 10 of these xenoliths plot within the diamond stability field. Some of the Jericho eclogites are olivine-bearing (all from this study). Figure 6.3 shows the distribution of the three olivine-bearing eclogites that exhibit a massive (all are barren) texture. Two of the olivine-bearing eclogite xenoliths plot within the diamond stability field. There are 59 foliated eclogite xenoliths (47 from this study and 12 from the literature), which range from 775 to 1230 °C and from 32 to 65 kbar (Figure 6.4). Of these eclogites, 25 plot within the diamond stability field. Figure 6.5 shows the distribution of the two olivine-bearing eclogites that exhibit a foliated texture. One of the olivine-bearing, foliated eclogite xenoliths plot within the diamond stability field. Finally, there are 10 samples for which a texture cannot be determined (four from this study and six from the literature), which range from 828 to 1125 °C and from 35 to 57 kbar (Figure 6.6). From the xenoliths in this study, seven are too small to accurately determine if the eclogite is massive or foliated. One sample appears to have been partially retrograded to the amphibolite facies. Four of these samples plot within the diamond stability field. Figure 6.7 shows the distribution of the only olivine-bearing eclogite that is texturally undetermined. The olivine-bearing, texturally undetermined eclogite xenolith does not plot within the diamond stability field. A comparison of the equilibration pressures for massive, foliated, and texturally undetermined eclogites from Jericho is shown in Figure 6.8. The massive and foliated eclogites span approximately the same range of pressures. The massive eclogites have a more even spread of xenoliths from 30 to 60 kbar, whereas the foliated xenoliths are most abundant at 40 to 50 kbar with fewer at higher and lower pressures. The foliated eclogite xenoliths expand a larger pressure range than both the barren, diamondiferous, and texturally undetermined xenoliths. 76 Figure 6.1 Histogram showing frequencies of equilibration temperatures at 50 kbar for texturally massive (barren and diamondiferous), foliated, and undetermined eclogites from Jericho. Includes data from Heaman et al. (2006), Smart et al. (2009), Kopylova et al. (1999a), and Kopylova et al. (2004). Figure 6.2 Equilibrium pressures and temperatures for massive (barren and diamondifer- ous) eclogite xenoliths from Jericho. The graphite-diamond constraint is from Kennedy and Kennedy (1976). Includes data from Heaman et al. (2006), Smart et al. (2009), Kopylova et al. (1999a), and Kopylova et al. (2004). Graphite Diamond 77 Graphite Diamond Figure 6.3 Equilibrium pressures and temperatures for olivine-bearing, massive eclogite xenoliths from Jericho. The graphite-diamond constraint is from Kennedy and Kennedy (1976). Includes data from Heaman et al. (2006), Smart et al. (2009), Kopylova et al. (1999a), and Kopylova et al. (2004). Figure 6.4 Equilibrium pressures and temperatures for foliated eclogite xenoliths from Jericho. The graphite-diamond constraint is from Kennedy and Kennedy (1976). Includes data from Heaman et al. (2006), Smart et al. (2009), Kopylova et al. (1999a), and Kopylova et al. (2004). Graphite Diamond 78 Graphite Diamond Figure 6.5 Equilibrium pressures and temperatures for olivine-bearing, foliated eclogite xenoliths from Jericho. The graphite-diamond constraint is from Kennedy and Kennedy (1976). Includes data from Heaman et al. (2006), Smart et al. (2009), Kopylova et al. (1999a), and Kopylova et al. (2004). Figure 6.6 Equilibrium pressures and temperatures for texturally undetermined eclogite xenoliths from Jericho. The graphite-diamond constraint is from Kennedy and Ken- nedy (1976). Includes data from Heaman et al. (2006), Smart et al. (2009), Kopylova et al. (1999a), and Kopylova et al. (2004). Graphite Diamond 79 Graphite Diamond Figure 6.7 Equilibrium pressures and temperatures for olivine-bearing, texturally undetermined eclogite xenoliths from Jericho. The graphite-diamond constraint is from Kennedy and Kennedy (1976). Includes data from Heaman et al. (2006), Smart et al. (2009), Kopylova et al. (1999a), and Kopylova et al. (2004). Figure 6.8 Histogram showing frequencies of equilibration pressures for massive, foliated and texturally undetermined eclogites from Jericho. Includes data from Heaman et al. (2006), Smart et al. (2009), Kopylova et al. (1999a), and Kopylova et al. (2004). 80 Figure 6.9 exhibits Groups A, B, and C eclogite xenoliths (based on garnet data only), at the assumed equilibration pressure of 50 kbar, record temperatures from 850 to 1250 °C. Groups B and C eclogites show higher temperatures than Group A eclogites overall. Group C eclogites show the largest range in temperature. Equilibrium pressure and temperature estimates for Groups A, B, and C (based on the data from garnet only) have been calculated in the same way as the massive and foliated eclogites above. Figure 6.10 shows the depth distribution of Group A eclogites. There are 30 Group A samples, which range from 908 to 1305 ºC and from 41 to 71 kbar; 18 of these xenoliths plot within the diamond stability field. Figure 6.11 shows the equilibrium pressures and temperatures for Group C eclogite xenoliths from Jericho. There are 39 Group B eclogites, which range from 732 to 1300 ºC and from 29 to 70 kbar; 20 of these xenoliths plot within the diamond stability field. Figure 6.12 shows the equilibrium pressures and temperatures for Group C eclogite xenoliths from Jericho. There are 57 eclogites, which range from 775 to 1230 ºC and from 32 to 65 kbar; 33 of these xenoliths plot within the diamond stablility field. A comparison of the equilibration pressures for Groups A, B, and C eclogites from Jericho is shown in Figure 6.13. Groups A and B exhibit higher pressures than Group C. Group C eclogites show a greater range in pressure than Groups A and B, with Group A exhibiting the narrowest range. Group B eclogites tend to exhibit a bimodal distribution. The first group clusters from 35 to 45 kbar depth. Within this depth range, the majority of foliated eclogites also occur with fewer barren massive eclogites. This is due to the low pressure, subducted oceanic crustal origin of the foliated eclogite xenoliths. The second group clusters from 50 to 60 kbar, which is mostly composed of barren massive eclogites with fewer foliated eclogites. This is explained by the mantle cumulate origin of some of the barren massive eclogites, which typically occur at greater pressures. 6.3 Muskox Eclogite xenoliths from the Muskox kimberlite have been separated into two textural groups: massive and foliated. Massive eclogites are further divided into barren and diamondiferous. These eclogites, at the estimated equilibration pressure of 50 kbar, record temperatures from 800 to 1200 °C (Figure 6.14). Barren, massive eclogites 81 Figure 6.9 Histogram showing frequencies of equilibration temperatures at 50 kbar for Groups A, B, and C eclogites from Jericho. Includes data from Heaman et al. (2006), Smart et al. (2009), Kopylova et al. (1999a), and Kopylova et al. (2004). Figure 6.10 Equilibrium pressures and temperatures for Group A from Jericho eclogite xenoliths. The graphite-diamond constraint is from Kennedy and Kennedy (1976). Includes data from Heaman et al. (2006), Smart et al. (2009), Kopylova et al. (1999a), and Kopylova et al. (2004). Graphite Diamond 82 Figure 6.11 Equilibrium pressures and temperatures for Group B from Jericho eclogite xenoliths. The graphite-diamond constraint is from Kennedy and Kennedy (1976). Includes data from Heaman et al. (2006), Smart et al. (2009), Kopylova et al. (1999a), and Kopylova et al. (2004). Figure 6.12 Equilibrium pressures and temperatures for Group C from Jericho eclogite xenoliths. The graphite-diamond constraint is from Kennedy and Kennedy (1976). Includes data from Heaman et al. (2006), Smart et al. (2009), Kopylova et al. (1999a), and Kopylova et al. (2004). Graphite Diamond Graphite Diamond 83 Figure 6.13 Histogram showing frequencies of equilibration pressures for Groups A, B, and C eclogites from Jericho. Includes data from Heaman et al. (2006), Smart et al. (2009), Kopylova et al. (1999a), and Kopylova et al. (2004). Figure 6.14 Histogram showing frequencies of equilibration temperatures at 50 kbar for massive (barren and diamondiferous) and foliated eclogites from Muskox. 84 exhibit a much wider range of temperatures than the foliated xenoliths. Equilibrium pressure and temperature estimates for both massive and foliated eclogite xenoliths from Jericho are calculated using the intersection of univariant PT lines from the Nakamura (2009) thermometer and the peridotitic geotherm based on the thermobarometer by Brey and Kohler (1990). There are 15 barren, massive eclogites, which range from 670 to 1287 °C and from 25 to 69 kbar (Figure 6.15). Of these xenoliths, 13 plot within the diamond stability field. There are 2 diamondiferous eclogites, which range from 1072 to 1085 °C and from 53 to 54 kbar; both of these xenoliths plot within the diamond stability field. The remaining 11 xenoliths are foliated; these range from 871 to 1215 °C and from 38 to 64 kbar (Figure 6.16). Nine of these xenoliths plot within the diamond stability field. A comparison of the equilibration pressures for massive and foliated eclogites from Muskox is shown in Figure 6.17. The foliated eclogites span narrower range of pressures than the massive eclogites. The massive eclogites from Muskox expand a larger temperature and pressure range than the foliated eclogites. The majority of both the massive and foliated eclogites plot within the diamond stability field. Figure 6.18 exhibits Groups A, B, and C eclogite xenoliths (based on garnet data only), at the assumed equilibration pressure of 50 kbar, record temperatures from 800 to 1200 °C. Group B eclogites show generally lower temperatures than Groups A and C eclogites overall. Group B exhibits the largest range in temperatures. Equilibrium pressure and temperature estimates for Groups A, B, and C (based on the data from garnet only) have been calculated in the same way as the massive and foliated eclogites above. Figure 6.19 shows the depth distribution of Groups A and B. There is only one Group A sample, which is calculated at 1181 °C and 61 kbar and plots within the diamond stability field. There are 11 Group B eclogites, which range from 670 to 1287 ºC and from 25 to 69 kbar. Of these xenoliths, five plot within the diamond stability field. Figure 6.20 shows the equilibrium pressures and temperatures for Group C eclogite xenoliths from Muskox. There are 16 eclogites, which range from 960 to 1222 ºC and from 45 to 64 kbar; all of these eclogites plot within the diamond stability field. A comparison of the equilibration pressures for Groups A, B, and C eclogites from Muskox is shown in Figure 6.21. Groups B and C eclogites show very similar range in pressure. 85 Figure 6.15 Equilibrium pressures and temperatures for massive Muskox eclogite xenoliths. The graphite-diamond constraint is from Kennedy and Kennedy (1976). Figure 6.16 Equilibrium pressures and temperatures for foliated Muskox eclogite xenoliths. The graphite-diamond constraint is from Kennedy and Kennedy (1976). 86 Figure 6.17 Histogram showing frequencies of equilibration pressures for massive and foliated eclogites from Muskox. Figure 6.18 Histogram showing frequencies of equilibration temperatures at 50 kbar for Groups A, B, and C eclogites from Muskox. 87 Figure 6.19 Equilibrium pressures and temperatures for Groups A and B from Muskox eclogite xenoliths. The graphite-diamond constraint is from Kennedy and Kennedy (1976). Figure 6.20 Equilibrium pressures and temperatures for Group C from Muskox eclogite xenoliths. The graphite-diamond constraint is from Kennedy and Kennedy (1976). 88 Figure 6.21 Histogram showing frequencies of equilibration pressures for Groups A, B, and C eclogites from Muskox. 6.4 Comparison of Jericho and Muskox eclogite xenoliths The proportions of massive and foliated eclogites at Jericho are almost equal (47 % massive, 53 % foliated). For Muskox, the majority (61 %) of the samples are texturally massive. Figures 6.22 and 6.23 exhibit the temperature, pressure, and depth distribution of the Muskox and Jericho eclogite xenoliths discussed below. Massive eclogite xenoliths from Muskox and Jericho exhibit the same temperature and pressure range. However, the majority of massive Jericho eclogites appear to have a bimodal distribution, from 850 to 998 °C and from 37 to 47 kbar and also from 1040 to 1100 °C and from 50 to 55 kbar. The majority of massive eclogites from are Muskox are more evenly distributed from 865 to 1085 °C and from 38 to 54 kbar. The overall depth distribution of the massive eclogites shows more Jericho xenoliths from 120 to 150 km than Muskox eclogites. However, from 200 to 230 km depths there are more Muskox eclogites than Jericho. Most massive eclogite xenoliths from both kimberlites plot within the diamond stability field; proportionally, more 89 Figure 6.22 Comparison of texturally massive, foliated, and undetermined eclogite xenoliths from the Jericho kimberlite. The lithological column (left) is constructed based on the depth distribution of massive, foliated, and undetermined eclogites. Boundaries between spinel perido- tite, spinel-garnet peridotite, garnet peridotite, lithosphere, and asthenosphere are from Kopylova and Caro (2004) and Kopylova et al. (1999a). The depth distribution of the pyroxenite intrusions in based on data from Kopylova and Caro (2004). The boundary between graphite and diamond is based on the intersection of the graphite-diamond constraint with the geotherm from the PT plot (centre). The following figures (6.27 – 6.29) have been constructed in the same manner. 90 Figure 6.23 Comparison of texturally massive and foliated eclogite xenoliths from the Muskox kimberlite. 91 xenoliths from Muskox plot within the diamond stability field than those from Jericho. Diamondiferous eclogites from Muskox range from 1072 to 1085 °C and from 53 to 54 kbar. The majority of diamondiferous eclogites from Jericho range from 908 to 970 °C and from 41 to 45 kbar. Also, only the Jericho kimberlite contains any olivine-bearing eclogites, which range from 882 to 1155 °C and from 39 to 59 kbar. Foliated eclogite xenoliths from the Muskox kimberlite cover a very narrow range of pressures and temperatures in comparison with those from Jericho. The majority of Muskox eclogites plot from 960 to 1032 °C and from 45 to 50 kbar. The Jericho foliated eclogites span a much wider range with the majority from 825 to 1050 °C and from 35 to 51 kbar. The majority of foliated eclogites from Muskox plot within the diamond stability field, whereas less than half of the eclogites from Jericho plot within this area. The foliated, olivine-bearing eclogites from Jericho range from 845 to 970 °C and from 36 to 45 kbar. Figures 6.24 and 6.25 exhibit the temperature, pressure, and depth distribution of the Muskox and Jericho eclogite xenoliths divided into Groups A, B, and C for which the following discussion is based. The majority of xenoliths from Jericho and Muskox are classified as Group C. In datasets from both kimberlites, Group A makes up the smallest proportion. The majority of Group A garnets from Jericho plot from 908 to 1015 °C and from 41 to 49 kbar; there is only one Group A garnet from Muskox, which plots at 1181 °C and 61 kbar. The overall depth distribution of these xenoliths shows over half plot at 160 km depth. The majority of Group A xenoliths from Muskox and Jericho plot within the diamond stability field. There appears a bimodal distribution of Group B garnets from Jericho. One cluster plots from 785 to 960 °C and from 32 to 45 kbar, whereas the higher temperature and pressure cluster plots from 1042 to 1128 °C and from 50 to 57 kbar. The majority of Group B eclogites from Muskox plot from 865 to 942 °C and from 38 to 43 kbar. The depth distribution for the majority of Group B eclogites from Muskox and Jericho occur from 120 to 185 km. The majority of Group C garnets from Jericho plot within the range from 840 to 1050 °C and from 36 to 51 kbar; approximately half of these xenoliths plot within the diamond stability field. The majority of Group C garnets from Muskox plot from 960 to 1045 °C and from 45 to 51 kbar; all of these eclogites plot within the diamond stability field. The depth distribution of the majority of the Group C 92 Figure 6.24 Comparison of Groups A, B, and C eclogite xenoliths from the Jericho kimberlite. 93 Figure 6.25 Comparison of Groups A, B, and C eclogite xenoliths from the Muskox kimberlite. 94 eclogites from both kimberlites occur from 120 to 175 km. Overall, the majority of all eclogite xenoliths from Muskox occur at greater depths than eclogites from Jericho. Muskox eclogites plot from 135 to 215 km, whereas the majority of Jericho eclogites are found from 120 to 180 km. 6.5 Central Slave Equilibrium pressure and temperature estimates for eclogite xenoliths from Central Slave were calculated using the Nakamura (2009) thermometer and an estimate of the Central Slave geotherm from Menzies et al. (2003). These eclogites, at the estimated equilibration pressure of 50 kbar, record temperatures from 700 to 1350 ºC (Figure 6.26). Eclogites from A154S exhibit a wider range of temperatures than those from A154, Lac de Gras, and Ekati. There are a total of 97 eclogite xenoliths from the Central Slave, which range from 590 to 1428 ºC and 30 to 68 kbar (Figures 6.27, 6.28, and 6.29). There are three eclogites from Lac de Gras, which span from 1040 to 1188 ºC and 50 to 57 kbar (Figure 6.27); all of these xenoliths plot within the diamond stability field. There are seven eclogites from Ekati, which range from 1155 to 1225 ºC and 55 to 59 kbar (Figure 6.27); all of these xenoliths plot within the diamond stability field. There are 54 eclogite xenoliths from A154 (48 barren and 8 diamondiferous). The barren eclogites range from 760 to 1330 °C and 37 to 64 kbar and the diamondiferous eclogites range from 863 to 1365 °C and 42 to 65 kbar (Figure 6.28). Of the A154 eclogites, 52 plot within the diamond stability field. There are 33 eclogite xenoliths from A154S, which range from 590 to 1428 °C and 30 to 68 kbar (Figure 6.29). Of the A154 eclogites, 27 plot within the diamond stability field. A comparison of the equilibration pressures for eclogites from the A154S, A154, Lac de Gras area, and Ekati kimberlites is shown in Figure 6.30. The diamondiferous eclogites from A154 have the highest pressures, ranging from 60 to 70 kbar. The eclogites from A154S have a more even spread of xenoliths from 40 to 60 kbar, whereas the diamondiferous xenoliths from A154 and Ekati are most abundant at 60 to 70 kbar. The A154S eclogite xenoliths span a larger temperature and pressure range than the A154, Lac de Gras area, and Ekati xenoliths. Figure 6.31 exhibits Groups A, B, and C eclogite xenoliths (based on garnet data only), at the assumed equilibration pressure of 50 kbar, record temperatures from 700 – 95 Figure 6.26 Histogram showing frequencies of equilibration temperatures at 50 kbar for eclogites from A154S, A154, Lac de Gras, and Ekati. Analyses from “A154S” selected from Aulbach et al. (2007), “A154” selected from Schmidberger et al. (2007), “Lac de Gras” selected from Pearson et al. (2004), and “Ekati” selected from Aulbach et al. (2011). Figure 6.27 Equilibrium pressures and temperatures for eclogite xenoliths from Ekati and the Lac de Gras area. The graphite-diamond constraint is from Kennedy and Kennedy (1976). Analyses from “Lac de Gras” selected from Pearson et al. (2004), and “Ekati” selected from Aulbach et al. (2011). 96 Figure 6.28 Equilibrium pressures and temperatures for eclogite xenoliths (barren and diamondiferous) from A154. The graphite-diamond constraint is from Kennedy and Kennedy (1976). Figure 6.29 Equilibrium pressures and temperatures for eclogite xenoliths from A154S. The graphite-diamond constraint is from Kennedy and Kennedy (1976). Analyses from “A154S” selected from Aulbach et al. (2007). 97 Figure 6.30 Histogram showing frequencies of equilibration pressures for eclogites from A154S, A154, Lac de Gras area, and Ekati. Analyses from “A154S” selected from Aulbach et al. (2007), “A154” selected from Schmidberger et al. (2007), “Lac de Gras” selected from Pearson et al. (2004), and “Ekati” selected from Aulbach et al. (2011). Figure 6.31 Histogram showing frequencies of equilibration temperatures at 50 kbar for Group A, B, and C eclogites from Central Slave. Temperatures are calculated based on the Nakamura (2009) geothermometer. 98 1350 °C. Generally, Group B and C eclogites show higher temperatures than Group A eclogites overall. Group A eclogites show the largest range in temperature. Equilibrium pressure and temperature estimates for Groups A, B, and C (based on the data from garnet only) have been calculated in the same way as the Diavik, Lac de Gras area, and Ekati eclogites above. Figure 6.32 shows the depth distribution of Group A eclogites. There are 18 Group A samples, which range from 774 to 1428 ºC and 38 to 68 kbar; 15 of these xenoliths plot within the diamond stability field. Figure 6.33 shows the equilibrium pressures and temperatures for Group B eclogite xenoliths from Central Slave. There are 50 Group B eclogites, which range from 841 to 1365 ºC and 41 to 65 kbar; 49 of these xenoliths plot within the diamond stability field. Figure 6.34 shows the equilibrium pressures and temperatures for Group C eclogite xenoliths from Central Slave. There are 31 eclogites, which range from 590 to 1245 ºC and 30 to 59 kbar; 23 of these xenoliths plot within the diamond stability field. A comparison of the equilibration pressures for Groups A, B, and C eclogites from Central Slave is shown in Figure 6.35. Generally, Groups B and C exhibit higher pressures than Group A. Group A eclogites show a greater range in pressure than Groups B and C. 99 Figure 6.32 Equilibrium pressures and temperatures for Group A eclogites from the Central Slave. The graphite-diamond constraint is from Kennedy and Kennedy (1976). Figure 6.33 Equilibrium pressures and temperatures for Group B eclogites from the Central Slave. The graphite-diamond constraint is from Kennedy and Kennedy (1976). 100 Figure 6.34 Equilibrium pressures and temperatures for Group C eclogites from the Central Slave. The graphite-diamond constraint is from Kennedy and Kennedy (1976). Figure 6.35 Histogram showing frequencies of equilibration pressures for Groups A, B, and C eclogites from Central Slave. 101 Chapter 7: Discussion and conclusions 7.1 Olivine in eclogite Although olivine associated with garnet and clinopyroxene is stable at the high pressure and temperature conditions of eclogite-facies metamorphism, it is unusual and rare to occur in eclogites because the protoliths are often not ultramafic enough to contain olivine. Olivine was found in powders of six crushed eclogite xenoliths from Jericho. Other olivine-bearing eclogites have been analysed from Bellsbank and Kimberley, South Africa (Shervais et al. 1988; Jacob et al. 2009, respectively). Figure 7.1 shows the comparison between composition of olivine in these eclogites and the range of Mg- number for macrocrystal olivine from kimberlites. This comparison is important, as it is possible that eclogite powders were contaminated by olivine, Fo90-94, which is abundant in the host kimberlite (Scott Smith 1996; Price et al. 2000). The olivines from Jericho that have Mg-number > 90 are found in foliated, massive, and texturally undetermined eclogites. Foliated eclogites are typically more felsic than massive eclogites (Figures 4.3, 6.9, and 6.10); thus, the probability of these eclogites containing high-Mg olivine is low. The eclogite xenoliths from this study that have olivine grains with Mg-number < 90 are all massive. To exclude possible contamination from kimberlite, the olivine grains that have Mg-number > 90 will not be discussed further, as they may have been derived from the kimberlite surrounding these eclogite xenoliths. The olivine from Shervais et al. (1988) has a Mg-number of 93; since individual grains were selected from these eclogites instead of analysing the olivine in thin section, this olivine may also be from the host kimberlite. The amount of NiO in olivine can be another parameter to determine if olivine originated in mafic eclogites or ultramafic kimberlites. The average NiO content of olivine in mantle peridotite and therefore, in xenocrysts in kimberlites, is 0.4 ± 0.05 wt. % (Sato 1977; Winter 2001). However, the average NiO content of ocean-floor basalt ranges from 0.005 to 0.035 wt. % (Sato 1977). Based on these data, we conclude that concentrations of NiO > 0.35 wt. % classify olivine as peridotitic and a contaminant from kimberlite (Figure 7.1). All of the olivine-bearing eclogites from this study plot below the macrocrystal olivine range of Mg-number, and all eclogites containing olivine from 102 Figure 7.1 Molar MgO/(MgO + FeO total) vs. NiO (wt. %) for olivine in eclogite xenoliths. The dashed line represents the lower limit for Mg-number for macrocrystal olivine from kimberlite (Fo90-94, Scott Smith 1996; Price et al. 2000). The solid line represents the lower limit for NiO in olivine in mantle peridotite (0.4 ± 0.05 NiO, wt. % (Sato 1977; Winter 2001)). Figure 7.2 SiO2 vs. Na2O + K2O wt. % of high-Mg volcanic rocks modified after Le Bas 2000. The diamond and circle symbols represent the bulk composition of Jericho and Muskox eclogite xenoliths, respectively 103 Shervais et al. (1988) and Jacob et al. (2009) contain high MgO garnets and are classified as Group A eclogites. The high Mg-number for these eclogites may indicate olivine accumulation from a picritic protolith (Jacob 2004; De Stefano et al. 2009). The Group A eclogites from Bellsbank have geochemical characteristics consistent with a protolith of a mantle cumulate, which also typically contain accessory olivine (Shervais et al. 1988). However, these high Mg-numbers may also be the result of metasomatism of the eclogites. Recrystallization of garnet and clinopyroxene to more magnesian end- members is related to mantle metasomatism and partial melting (De Stefano et al. 2009). The textural position of olivine in eclogites cannot be determined as no thin sections containing olivine are available. Thus, metasomatism and partial melting cannot be discounted as olivine-producing processes. The olivine-bearing eclogites studied by Jacob et al. (2009) display an enriched LREE pattern, which is indicative of cryptic metasomatism. Olivine is only found in Jericho eclogites, but is absent from Muskox eclogites. Through analysis of the average bulk MgO content of Jericho and Muskox eclogites, it is evident that Jericho eclogites are more magnesian (Figure 7.2). This average bulk MgO content is calculated based on the bulk chemical compositions of several eclogites from Tables 5.1 – 5.3. Jericho eclogites have an average bulk MgO content of 15.2 ± 2.9 wt. %, whereas Muskox eclogites have bulk MgO of 9.8 ± 2.9 wt. %. This indicates that the Jericho eclogites may be derived from a picritic protolith, whereas Muskox eclogites may be derived from a basaltic protolith. Figure 7.2 shows the average bulk composition of eclogite xenoliths from Jericho and Muskox plotted on the classification of high-Mg volcanic rocks (Le Bas 2000). The Jericho eclogite xenoliths have a bulk composition similar to picrites due to their high average bulk MgO content. The Muskox eclogite xenoliths have a bulk composition similar to basalts due to their lower average bulk MgO (Figure 7.2). Muskox and Jericho petrography could give clues to the origin of olivine, either due to ultramafic composition of the protolith or to late metasomatism and recrystallization. Recrystallization and partial melting is also present in eclogite xenoliths from Muskox. However, these eclogites failed to produce any olivine. Therefore, the primary reason for the presence of olivine is the original bulk composition of the protolith, not metasomatism. 104 7.2 Diamond potential of Northern Slave eclogites This section summarizes the applicability of criteria commonly used for distinguishing diamondiferous and barren eclogites to the Northern Slave eclogite xenoliths. Garnets with > 0.07 wt. % Na2O are considered to be potentially diamondiferous based on empirical observation (Gurney and Zweistra 1995). Another criteria for distinguishing diamondiferous eclogites is proposed by Cookenboo and Grütter (2010), which is based on the Na2O and TiO2 contents in garnets (Figure 4.5). Figure 4.5 shows the positive correlation between Na2O and TiO2. All of the primary garnets from diamondiferous eclogites from the Northern Slave, except one, plot within the diamond-inclusion field. The one that does not is the Group B eclogite from Smart et al. (2009). However, some of the garnets from the non-diamondiferous samples also plot above 0.07 wt. % Na2O. This was also demonstrated by Grütter and Quadling (1999) with graphite-bearing eclogites that show significant overlap with the diamond-inclusion field. Therefore, matching garnet analyses with the high-Na, low-Ti field of garnet equilibrated with diamond works for Northern Slave eclogite in the same imperfect way as for other worldwide eclogites. McCandless and Gurney (1989) determined based on empirical observations that the majority of clinopyroxenes from diamondiferous eclogites contain > 0.08 wt. % K2O (Figure 4.7). However, some clinopyroxenes from diamondiferous eclogites plot below this line and some clinopyroxenes from barren eclogites plot above this line. The same is noted for other kimberlite-derived eclogites (McCandless and Gurney 1989) indicating that this method of determining diamond potential can be applied for clinopyroxenes from Northern Slave eclogite xenoliths in the same manner as for the other eclogites worldwide. Figure 7.3 shows the distribution of Jericho and Muskox eclogites (barren and diamondiferous) superimposed on the plot for Type I (diamondiferous) and Type II (barren) South African eclogites. The diamondiferous eclogites from Muskox plot within the Type I area; however, the majority of barren Muskox samples also plot within the Type I field. This indicates that this is not an acceptable method of discerning diamondiferous from barren eclogites from Muskox. However, the principle that was the 105 Figure 7.3 Na2O in garnet vs. K2O in clinopyroxene from Northern Slave eclogites. Solid diamonds represent diamondiferous eclogites from Jericho; solid squares represent diamondiferous eclogites from Muskox. Type II eclogites are defined by ≤ 0.07 wt. % Na2O in garnet and ≤ 0.08 wt. % K2O in clinopyroxene (modified after McCandless and Gurney, 1989; Gréau et al. 2011). Dashed lines represent new criteria for diamondiferous eclogites from Muskox. Figure 7.4 A) MgO vs. Ca (wt. %) of garnet from Northern Slave eclogites; B) MgO vs. Na2O (wt. %) of clinopyroxene from Northern Slave eclogites. The grey field in both diagrams represents diamondiferous eclogites as determined by De Stefano et al. (2009). 106 basis for these criteria may be applicable to Muskox eclogites, as the diamondiferous eclogites indeed have the highest Na2O content in garnet and K2O content in clinopyroxene. The boundary between barren and diamondiferous eclogites for Muskox lies at > 0.17 wt. % K2O in clinopyroxene and > 0.11 wt. % Na2O in garnet. Only one of the diamondiferous eclogites from Jericho plots within the Type I area as well as numerous barren eclogites. So, these criteria also do not apply to eclogites from Jericho. Textural criteria were also proposed by McCandless and Gurney (1989) for Type I and Type II eclogites. Type I eclogites contain subhedral to euhedral garnets whereas Type II eclogites contain anhedral garnets. Eclogite xenoliths with euhedral to subhedral garnets from Jericho and Muskox plot in both the Type I and Type II fields. Eclogite xenoliths with anhedral garnets from Jericho and Muskox also plot in both the Type I and Type II fields. This indicates that these criteria do not apply to eclogites xenoliths from the Northern Slave. However, other textural criteria apply to Northern Slave samples. Only massive eclogites are known to contain diamond at both Jericho and Muskox. New criteria were proposed specifically for Jericho, where diamondiferous eclogites are typically Mg-rich in comparison to other diamond-bearing eclogites worldwide (De Stefano et al. 2009). Eclogite xenoliths from the Jericho kimberlite show that constraints based on MgO content in both clinopyroxene and garnet grains can outline a distinct diamondiferous eclogite field (Figure 7.4). The grey field represents diamondiferous eclogites from Jericho as determined by De Stefano et al. (2009). In addition, in both of the diamond-bearing fields, barren eclogites from both Jericho and Muskox also plot within this area. This indicates that this is an imperfect indicator for diamond association. 7.3 Origin of Northern Slave eclogite xenoliths Eclogites from the Muskox and Jericho kimberlites exhibit very similar chemical compositions and will thus be discussed together as Northern Slave eclogites. Eclogites xenoliths from the Northern Slave kimberlites appear to exhibit two distinct origins. The majority of massive (barren) and all foliated xenoliths are classified as Groups B and C, which are typically attributed to an oceanic crustal origin (Coleman et al. 1965). The majority of barren, massive eclogites correspond to Group B, whereas the majority of foliated eclogites correspond to Group C. There are several lines of chemical and 107 petrographical evidence for Group B and C eclogites that indicate an oceanic crustal origin. These eclogites may have formed in the subducting slab from oceanic basalts and gabbros. The model of the crustal origin of Group B and C Northern Slave eclogites fits well with geophysical and geochronological data. Several Jericho zircon-bearing eclogites have been dated by Heaman et al. (2006) and exhibit similar ages to easterly subduction under the Western Slave Craton (~1.88 – 1.84 Ga). Cook et al. (1999) geophysically mapped Proterozoic age subducted oceanic crust beneath the Great Bear magmatic arc. The major indicator of a subducted oceanic crust protolith is the presence of a plagioclase-rich precursor. The following facts support this. A. Major element analysis of Group B and C eclogites typically exhibit low (< 71) bulk Mg number (Figure 7.5). The Group B eclogite from Jericho studied by Smart et al. (2009) has Mg number of 52. The majority of Group B and C eclogites from Northern Slave eclogites analyzed in this study yield Mg numbers from 42 to 79. These Mg-numbers match those from mafic crustal rocks (Figure 7.5). B. Major element analysis of Group B and C eclogites exhibits relatively low Cr2O3 and MgO (Figure 7.6). Group B eclogite from Smart et al. (2009) shows depletion of Cr2O3 and MgO. C. A positive Eu anomaly is often found in Jericho eclogites. Studies of Jericho eclogites by De Stefano et al. (2009), Smart et al. (2009), and Heaman et al. (2002) all showed a positive Eu anomaly for the Group B eclogite xenoliths (Figure 7.7). Eu commonly substitutes for Ca in plagioclase and the presence of anomaly is the sign of equilibration with plagioclase (Jacob and Foley 1999; Winter 2001) (Figure 7.7). D. Jericho Group B and C eclogites studied by De Stefano et al. (2009) showed a positive Sr anomaly (Figure 7.8). A positive Sr anomaly is also common for Group B and C eclogites in general (Jacob 2004). This is due to Sr preferentially incorporating into plagioclase (Jacob 2004). 108 Figure 7.5 Whole-rock Mg-number for Northern Slave eclogite xenoliths. The purple field represents crustal values (Barth et al. (2001); red field represents mantle values (Barth et al. 2002). The green ellipse represents Group A eclogites from the Northern Slave; yellow ellipse represents Group B and C eclogites from the Northern Slave. Data points from Smart et al. (2009), Russell et al. (2001), and the rest are calculated bulk composition from this study (see Chapter 5). There exists an overlap between the textural eclogite data (massive and foliated) and the Group A, B, and C eclogite data as the two are not mutually exclusive. Figure 7.6 Whole-rock Cr2O3 vs. MgO (wt. %) for Northern Slave eclogites. The purple field represents crustal values and the red field represents mantle values (Barth et al. 2002). The green field represents Group A eclogites from the Northern Slave; yellow field represents Group B and C eclogites from the Northern Slave. Data points from Smart et al. (2009) and the rest are calculated bulk composition from this study (see Chapter 5). There exists an overlap of between the textural eclogite data (massive and foliated) and the Group A, B, and C eclogite data as the two are not mutually exclusive. 109 Figure 7.7 Chondrite-normalized REE diagram for whole-rock values of Jericho Group A eclogites (all JDE and 44-9) and the Group B eclogite. Data for Group B eclogites (Smart et al. 2009) are shown as a grey bound band. Modified after Smart et al. 2009. Figure 7.8 Chondrite-normalized trace element pattern for clinopyroxene for Group B and C eclogites from Jericho. Samples 47-8P and 47-8S are primary and secondary (respectively) clinopyroxene grains from foliated eclogites studied by De Stefano et al. (2009). Other symbols represent clinopyroxene inclusions in diamonds. Dashed lines are minimum detection limits. Modified after De Stefano et al. (2009). 110 The origin of Northern Slave eclogites due to subduction of oceanic crust is also supported by the following evidence: A. Group B and C eclogites from Jericho studied by Heaman et al. (2002, 2006) showed a positive Nb anomaly (Figure 7.9). Positive Nb anomalies are common in subducted oceanic crust protoliths because Nb remains in the residue of the subducted slab after partial melting (Barth et al. 2001). Rutile, which is a very common accessory mineral in eclogite, has a high partition coefficient of Nb between eclogite and the felsic melt, and incorporates Nb into the crystal structure (Winter 2001). Rutile is present in approximately half of the eclogite thin sections from Northern Slave kimberlites from this study. Figure 7.9 Normal mid-ocean-ridge basalt normalized incompatible element diagram. All symbols represent Group B and C eclogites from Jericho. The grey field represents island-arc basalt and andesites formed in subduction zones – given for comparison. Modified after Heaman et al. (2002). B. A flat HREE signature is shown in Group C eclogites from Heaman et al. (2002) and Group B and C eclogites from De Stefano et al. (2009) (Figure 7.7). This is an indication of the protolith forming above the garnet-bearing mantle (Figure 7.7). If garnet is not present to incorporate HREE then the HREEs will not be fractionated from the melt (Winter 2001). 111 C. Group B and C eclogites from Smart et al. (2009) exhibit depletions of LREE (Figure 7.7). A depleted LREE pattern is caused by partial melting or dehydration of the subducting oceanic crust (Barth et al. 2001). The other origin was inferred for the remaining massive xenoliths, barren and diamondiferous . These analyses correspond to Group A eclogites, which are attributed to a mantle cumulate origin (Coleman et al. 1965; Shervais et al. 1988). These xenoliths generally occur deeper than those from Groups B and C, ranging from 150 to 230 km. There are several lines of geochemical evidence from Group A eclogites that point towards a mantle cumulate origin. A. Group A Jericho eclogites from Smart et al. (2009) exhibit Mg-numbers from 87 to 88. Mg numbers > 74 coincide with primary, peridotite-derived melts (Barth et al. 2002) (Figure 7.5). B. Group A eclogites from both De Stefano et al. (2009) and Smart et al. (2009) exhibit enriched bulk Cr2O3 and MgO contents (Figure 7.6). Cr2O3 and MgO accumulate in mantle cumulates because they are compatible elements and are easily removed from the magma (Winter 2001). C. Group A Jericho eclogites from Heaman et al. (2006) and Smart et al. (2009) show enrichement of LREE (Figure 7.7). Trace element analyses for Group A tend to have enriched LREE because mantle metasomatism increases LREE (Barth et al. 2002). D. Group A Jericho eclogites from Smart et al. (2009) show fractionated HREE patterns (Figure 7.7). A fractionated HREE pattern is indicative of a deep mantle cumulate origin because garnet preferentially incorporates HREE. The protolith must have originated in the garnet zone of the mantle where garnet crystallizes and, due to its high partition coefficient, incorporates HREE creating a fractionated HREE pattern (Winter 2001). E. Some of the eclogitic Jericho diamonds studied by De Stefano et al. (2009) exhibit δ13C values of -5 ‰, which is considered to be equal to that of mantle carbon (see Figure 1.11 from Chapter 1). 112 7.4 Secondary mantle processes recorded by Northern Slave eclogites 7.4.1 Partial melting and metasomatism Eclogites from Jericho and Muskox record partial melting expressed in recrystallization of garnet and clinopyroxene into secondary, fine-grained aggregates along grain boundaries. Secondary garnet shows a slight enrichment in MgO and TiO2 and depletion of Al2O3; secondary clinopyroxene shows an increase in CaO and MgO and a decrease in Al2O3 and Na2O content compared to respective primary minerals. However, it has also been determined that secondary garnet and clinopyroxene are not only depleted in magmaphile elements, but enriched in incompatible trace elements, which indicates that partial melting is not the only process to produce the recrystallized eclogite (De Stefano et al. 2009). Also, the presence of secondary phlogopite, rutile, amphibole, carbonate, and spinel implies the influx of Fe, Ca, Na, K, Nb, Ti, CO2, and H2O. The temperatures of equilibrium of primary garnet and clinopyroxene pairs from Muskox and Jericho are much lower than those from secondary garnet and clinopyroxene pairs when calculated at the same pressure (Table 7.1). This increase in temperature that accompanies partial melting is constrained by the increase in MgO content in the secondary minerals. Figure 7.10 shows the pressure and temperature evolution that accompanied recrystallization and partial melting, on the assumption that the pressure remained constant. During partial melting and metasomatism, the geotherm is perturbed and so the equilibrium pressure for the recrystallized eclogites cannot be determined on the premise that it corresponds to the geotherm. The dotted lines represent the multitude of temperatures and pressures for which the secondary minerals may have crystallized. The Jericho eclogites typically show temperatures cooler than those from Muskox. 113 Table 7.1 Equilibrium temperatures for garnet and clinopyroxene grains Temperatures, oC, (Nakamura 2009) calculated for Location Sample Grains Garnet core vs. CPX core Garnet rim vs. CPX core Primary garnet vs. primary CPX Secondary garnet vs. secondary CPX Samples with recrystallized minerals Muskox MOX24 206.9 24-9/24-3 - - 1048 1173 24-10/24-11 - - 1076 1170 Muskox MOX 25 207 25-3 - - 1071 1260 Jericho 47-8 - - - 962 1101 Jericho 42-3a - - - 851 869 Jericho 55-4 - - - 1076 1161 Jericho 52-5b - - - 998 1043 Jericho 47-2a - - - 831 867 Jericho JDF6N - - - 887 907 Samples with zoned minerals Muskox MOX28 308.4 28-3 859 959 - - 28-4 882 920 - - 28-5 861 918 - - Jericho 26-6 - 978 1061c Jericho LGS8 Mx3 - 627 747 - - a calculated with primary garnet and secondary clinopyroxene b calculated with secondary garnet and primary clinopyroxene c calculated with garnet rim and clinopyroxene rim 114 Figure 7.10 Equilibrium pressures and temperatures for primary and secondary garnet and clinopyroxene from Muskox and Jericho eclogites. The evolution of temperature shown is for single specimens that underwent partial melting at an assumed constant pressure (lines with arrows). The black line represents the Northern Slave geotherm (Kopylova et al. 1999a). Dotted lines represent possible pressures and temperatures for the recrystallized grains. These univariant lines are calculated using Nakamura 2009 geothermometer at all possible temperatures and pressures for the recrystallized grains. The bold line represents the dry eclogite solidus (Yasuda 1994). The dash-dot line represents the solidus curve for carbonated eclogite (Dasgupta et al. 2004). The red line represents the limit of the subsolidus stability field for phengite for mid-ocean ridge basalt bulk composition (Schmidt 1996). Equilibrium pressures and temperatures for all blue samples are from Jericho and are calculated based on chemical data from Kopylova et al. (2004). The following is a discussion of Figure 7.10 and an analysis of potential causes of melting. Initially, the Muskox eclogites consisted of mainly garnet and clinopyroxene. This implies that the melting behaviour of these eclogites can be approximated by a dry bulk composition of eclogite and crystallized below the dry eclogite solidus. Based on the abundance of secondary carbonate minerals (5 – 30 vol. %), an influx of carbonate material is postulated. This changed the bulk composition to a carbonated eclogite. The 115 figure also compares the estimated pressures and temperatures with eclogite solidi. Several of them (Yasuda et al. 1994; Hammouda 2003; Shirasaka and Takahashi 2003; Yaxley and Brey 2004) have been reported for differing initial compositions of eclogites. Dasgupta et al. (2004) were the only workers to use a natural eclogite xenolith (Salt Lake Crater, Hawaii) with a chemical composition containing both Na2O and K2O, which are abundant in Northern Slave eclogite xenoliths. Based on these criteria, only the Dasgupta et al. (2004) carbonated eclogite solidus is used. Pressures and temperatures of equilibrium of both the primary and secondary minerals also support the evolution of the bulk composition. The primary minerals plot below the dry solidus during initial crystallization. During secondary melting, the temperature of the recrystallized minerals increased to a point above the carbonated eclogite solidus (Figure 7.10). Therefore, the partial melting of these eclogites is caused by the change in the bulk composition of the mantle as well as the influx of a hot, CO2-rich material. This material could be a mantle fluid or a melt and could be related to a nearby carbonatite intrusion or the host kimberlite based on the abundance of secondary carbonate minerals. However, carbonatite intrusions are not known to occur at depths greater than 75 km (Wyllie et al. 1990). Other possible origins of the CO2-rich material may relate to mantle degassing, which releases trapped volatiles from the mantle (Zhang and Zindler 1993). Also, subduction of oceanic crust recycles CO2-rich material (carbonate sediments) back into the mantle, which subsequently escape due to an increase in pressure (Des Marais 1985). The infiltration of these rising volatiles may have contributed to the partial melting of these eclogites and the crystallization of secondary carbonates. The initial bulk composition of the Jericho eclogite is also dry as the rocks consist mainly of garnet and clinopyroxene. This implies that crystallization of these eclogites occurred below the dry eclogite solidus. Recrystallization changed the bulk composition. As the abundance of secondary phlogopites in zones of partial melting is low (< 15 vol. %), the melting behaviour of the eclogite can be predicted based on a solidus intermediate between dry and H2O-saturated. The wet eclogite solidus (Wyllie 1979) occurs at 700 oC and is an approximate vertical line (see Figure 7.11). Temperatures of equilibrium for secondary minerals indeed plot between wet and dry eclogite solidi (Figure 7.10). Secondary minerals should also record pressure and temperature conditions where phlogopite is stable. Unfortunately, pressure and temperature stability field for phlogopite in eclogite has not been experimentally determined. Phengite, 116 another type of mica, occurs at much lower temperatures and pressures than the recrystallized Jericho eclogites in bulk compositions of H2O-saturated mid-ocean ridge basalt (Figure 7.10). Pressure and temperatures of equilibrium of both the primary and secondary minerals also support the evolution of the bulk composition. The primary minerals plot below the dry solidus during initial crystallization. However, once an H2O- rich material infiltrated the eclogite, the temperature increased to a point above the H2O- bearing eclogite solidus. Therefore, the partial melting of these eclogites is caused by the change in bulk composition as well as the influx of a hot material rich in H2O. This material could be a mantle fluid or a melt and may be related to the host kimberlite based on the assemblage of the secondary minerals that resembles kimberlitic composition. Additionally, subducted oceanic crust injects seawater and hydrated minerals into the mantle, which become progressively unstable with increasing pressure (Des Marais 1985). These volatiles degas and may contribute to the partial melting of these eclogites and the subsequent crystallization of secondary phlogopite (Des Marais 1985). Recrystallization of garnet and clinopyroxene into secondary MgO-rich end- members (i.e. pyrope and diopside) is a very common occurrence in cratonic eclogites (Taylor and Neal 1989; Ireland et al. 1994; Barth et al. 2001; Aulbach et al 2007). This recrystallization may have occurred during entrainment in the kimberlite melt, which results in partial melting of the eclogite in conjunction with metasomatism of a K-rich fluid at low pressure (Misra et al. 2004). The metasomatism and partial melting of the eclogites occurred during several periods as established through isotopic studies (Heaman et al. 2006). The oldest metasomatic event occurred around the time of eclogite metamorphism (1785 Ma) and is responsible for the enrichment in HFSE as well as formation of Nb2O5-rich rutile (Heaman et al. 2006). The second metasomatic event occurred at 1.0 – 1.3 Ga when a carbonatitic fluid facilitated the enrichment of LREE in eclogites as well as the growth of carbonate and phlogopite in the eclogite xenoliths (Heaman et al. 2006). This metasomatism may also have been associated with diamond formation (Spetsius 1999; De Stefano et al. 2009). Since the metasomatism affecting these eclogite is complex, the influence of kimberlite is unlikely to be the only contributing factor. Eclogite xenoliths from the Udachnaya kimberlite pipe, Yakutia, Russia show secondary recrystallization of clinopyroxene, whereas the garnets remain unaltered 117 (Misra et al. 2004). Geothermometry of these samples show that analyses of primary grains yield higher temperatures (by ~150 – 250 oC) of equilibrium than analyses of secondary grains. This is the opposite trend to that of the Northern Slave. The secondary clinopyroxenes from Udachnaya have significantly greater (2 – 6 wt. %) amounts of MgO and CaO than the primary grains. This results in a lower ratio of Fetotal/(Fetotal + Mg2+ + Ca2+) in secondary clinopyroxenes compared to primary, and therefore lower the temperature. Recrystallized clinopyroxenes from Jericho are the opposite, showing higher Fetotal/(Fetotal + Mg2+ + Ca2+) ratios. The secondary alteration products of Udachnaya eclogites are phlogopite, low-Na2O clinopyroxene, alkali-rich amphibole, sulfides, and carbonates (Misra et al. 2004). These secondary minerals are produced by two metasomatic events. The first event occurs prior to incorporation into the kimberlite and produced the phlogopite by the infiltration of a K- and H2O-rich fluid (Misra et al. 2004). The second event contained a K-rich fluid with lesser amounts of H2O and CO2; this event occurred after incorporation into the kimberlite (Misra et al. 2004). This second event also produced the Na-depleted clinopyroxenes due to decreasing pressure associated with kimberlite ascent (Carswell 1975; Misra et al. 2004). The partial melting of the eclogite is attributed to a fluid influx, which resulted in a substantial depression in temperatures of the eclogite solidus in the presence of an H2O-rich fluid (Misra et al. 2004). Eclogite xenoliths from the Roberts Victor kimberlites, South Africa, the Daldyn- Alakit and Malo-Botuobia pipes, Yakutia, Russia, the Bellsbank kimberlites, South Africa, and the A154S pipe, Central Slave, Canada all exhibit secondary clinopyroxenes with very similar compositional trends of increasing FeO, MgO, and CaO and decreasing Na2O contents (Taylor and Neal 1989; Spetsius and Taylor 2002; Aulbach et al. 2007; Gréau et al. 2011). This type of secondary clinopyroxene was first analysed by Donaldson (1978) in peridotite nodules who suggested that the texture and chemistry is derived from incongruent melting of peridotite minerals. However, Taylor and Neal (1989) suggested that the breakdown of the clinopyroxene to a Na2O-poor compositon was due to the reaction of clinopyroxene with kimberlitic fluids. This metasomatism leaches Na2O and Al2O3 from the clinopyroxene into the fliud during the partial melting of the clinopyroxenes (Taylor and Neal 1989; Spetsius and Taylor 2002). This partial melting event began after the eclogite is incorporated into the kimberlite, between 900 – 1300 oC and 30 kbar, but was not complete until 10 – 15 kbar. The latter pressure 118 estimate was derived based on the presence of glass and feldspars that accompany the secondary clinopyroxene (Spetsius and Taylor 2002). Due to the lack of clinopyroxene and garnet compositional data in the literature, temperatures of equilibrium for the aforementioned eclogite suites cannot be calculated. However, considering that these eclogites were infiltrated by the hot, kimberlitic fluid, it is likely that the secondary clinopyroxene grains yield higher temperatures of equilibrium than the primary analyses. 7.4.2 Processes forming mineral zonation Both garnet and clinopyroxene grains show zoning in eclogites from the Northern Slave. Zonation of garnet grains from Muskox eclogites exhibit a very similar increase in MgO content from the core to the rim as with primary to secondary garnets. The zoned grains are not formed in veins of partial melting or in close proximity to them. This zoning at Jericho is caused by a more recent partial melting event than the metasomatic events, which produced the secondary crystallization of garnet and clinopyroxene (De Stefano et al. 2009). This is concluded based on the estimate of time required to eliminate the zoning. The diffusion rate for Mg in garnet is 8x10-18 m2/s at 1300 oC and 4 GPa (Smith and Boyd 1992). The garnet zoning in Jericho eclogites therefore persisted for only hundreds to thousands of years as the rims are ~ 200 µm thick (De Stefano et al. 2009). The temperatures of equilibrium of zoned garnets with homogenous clinopyroxenes show an increase in temperature from the core to the rim (Table C83, C84, and C112). This increase in temperature is constrained by the increase in MgO content along the rim of the garnet grains. These temperature estimates are less accurate than those calculated for recrystallization because zoned garnet, in most samples, is assumed to be equilibrated with non-evolving clinopyroxene compositions. However, sample 26-6 contains zoned clinopyroxene and garnet grains. Figure 7.4 shows the evolution of equilibrium temperature from the core to the rim of garnet grains. The temperatures for the recrystallized eclogites were calculated at the same pressure as the fresh eclogites. During partial melting and metasomatism, the geotherm is perturbed and so the equilibrium temperature and pressure for the rim analyses of these eclogites cannot be determined based on the assumption that pressures and temperatures fall on the steady-state geotherm. The dotted lines represent the multitude of temperatures and pressures for which the secondary minerals may have crystallized. 119 Figure 7.11 Equilibrium pressures and temperatures for zoned garnet and clinopyroxene grains from Muskox and Jericho eclogite xenoliths. The evolution of temperature shown for single specimens is for an assumed constant pressure (solid lines with arrows). The black line represents the Northern Slave geotherm (Kopylova et al. 1999a). Dotted lines represent possible pressures and temperatures for the zoned eclogites. These univariant lines are calculated using Nakamura 2009 geothermometer at all possible temperatures and pressures for grain rims. The dash-dot line represents the solidus curve for carbonated eclogite (Dasgupta et al. 2004). The bold, dashed line represents the wet eclogite solidus (Wyllie 1979). Equilibrium pressures and temperatures for Jericho sample LGS8 Mx3 (Kopylova et al. 1999a) and 26-6 (Kopylova et al. 2004). The red line represents the phengite solidus for mid-ocean ridge basalt bulk composition (Maruyama and Okamoto 2007). Initial crystallization of only garnet and clinopyroxene suggests bulk composition of dry eclogite. All of the eclogites have temperatures of equilibrium below the dry eclogite solidus for the core. Analyses of garnet and clinopyroxene rims show an increase in temperature of equilibrium due to an increase in MgO content from the core to the rim. The zonation may be the result of kimberlite metasomatism as garnet is replaced by carbonate and phlogopite along grain boundaries and in fractures (Chapter 3.4; Appendix B). The eclogite was infiltrated by a K2O-, CO2-, and H2O-rich fluid likely from the host kimberlite. Temperatures of equilibrium for zoned rims indeed plot 120 between the wet and dry eclogite solidi (Figure 7.11). Zoned rims should also record pressure and temperature conditions where phlogopite is stable. Unfortunately, pressure and temperature stability field for phlogopite in eclogite has not been experimentally determined. The limit for phengite stability in mid-ocean ridge basalt occurs at lower temperatures and pressures than those for grain rims in the zoned eclogites (Figure 7.11). Eclogite xenoliths from the Mir kimberlite, Yakutia, Russia, show zoned garnets and homogeneous clinopyroxenes (except in one sample) (Beard et al. 1996). Geothermometry of these samples show that the rim yields higher temperatures of equilibrium (between 50 – 200 oC) than the core. This is the same pattern exhibited by Northern Slave eclogites. The zonation is chemically defined by an increase in Mg and a decrease in Ca from the core to the rim. The zonation of the garnets is the result of kimberlitic metasomatism (K2O-, CO2-, H2O-rich fluid), as the compositional variation in the garnets occurs along fractures infilled by phlogopite and amphibole (Beard et al. 1996). Eclogite xenoliths from the Udachnaya kimberlite, Yakutia, Russia, show slightly zoned garnets and homogeneous clinopyroxenes (Keller et al. 1999). The zonation is chemically defined by a slight decrease in MgO content and slight increases in CaO and Na2O contents from the core to the rim. These eclogites do not show a significant change in temperatures of equilibration from the core to the rim. 7.5 Depth distribution of Northern Slave eclogites The majority of diamondiferous eclogites occupy a narrow range of depths from 145 to 155 km. This narrow lens of diamondiferous, massive Group A eclogites was likely formed from a thin mantle layer of re-melted peridotite (Cookenboo 1998; Heaman et al. 2006). There are three Group B, diamondiferous eclogites, which formed at depths from 175 to 185 km. The foliation of eclogites in the Northern Slave is attributed to plastic deformation during a subduction event (Jin et al. 2001). The foliation is controlled mostly by omphacite because it is much weaker than garnet; omphacite exhibits greater strain at lower stress (Jin et al. 2001) (Figure 7.12). Group B eclogites tend to exhibit a bimodal distribution over depths (Figure 7.13). The first group clusters from 120 to 145 km depth and the second group clusters from 170 to 185 km. 121 Figure 7.12 Comparison of strengths of eclogite, omphacite, garnet, and harzburgite under constant strain rate (4.6 X 10-4 s-1), temperature (1500 K), and pressure (3 GPa). Modified after Jin et al. 2001. Figure 7.13 Depth distribution histogram of Group B and C eclogites from Northern Slave kimberlites. Outlined is the proposed single slab of subducted oceanic crust. 122 This bimodal distribution could be the result of two subduction events of oceanic crust, each of which may have entrained eclogite into the Slave lithosphere. Evidence that supports this theory includes the following. A. Models for amalgamation of the Slave craton suggest a minimum of three subduction events prior to kimberlite emplacement. The first proposed event is the subduction of oceanic crust associated with the Anton micro-continent from 2.67 to 2.6 Ga (Fyson and Helmstaedt 1988; Kusky 1989). The second oceanic plate subduction events occurred at 1.9 Ga and resulted in the production of the Hottah arc. The third subduction event at 1.84 Ga was related to the Great Bear arc development (Bowring and Grotzinger 1992). B. Geophysical studies of the Northern Slave craton show three high-velocity zones beneath the Slave craton (at 75, 135, and 195 km depths), which have been interpreted as subducted slabs as they are colder than the surrounding rocks (Bostock 1997; Cook et al. 1999). The lower cluster of Group B eclogites occurs at the depth of the 135 km high-velocity zone and the deeper cluster of Group B eclogites matches with the 195 km high-velocity zone. The evidence against multiple subduction events includes U-Pb zircon dates from zircon-bearing Jericho eclogites. The ages indicate eclogite metamorphism at approximately 1785 Ma (Heaman et al. 2006). Thus, these eclogites represent the subducted oceanic crust from the time of building the Great Bear magmatic arc (1.88 – 1.84 Ga) (Heaman et al. 2006). Any older dates of eclogite metamorphism are absent. Another explanation for the bimodal distribution of Group B eclogites is their non-uniform distribution in a single, thick slab. Since both Group B and C eclogites are considered to be derived from subducted oceanic crust and Group C eclogites are more abundant in the gap (150 – 165 km) where Group B eclogites are deficient, this indicates a single subducted slab. Group B eclogites are more depleted than those from Group C. The lower portion of the slab containing more Group B eclogites may have experienced more partial melting than the remaining slab. Substantial partial melting of subducted oceanic crust may occur when high shear stresses (> 100 MPa) are maintained by rocks at or above their melting temperatures or if the subducted slab is very young (< 5 Ma) (Peacock et al. 123 1994). The majority of Group B eclogites at the lower part of the proposed slab exhibit a massive texture indicating little to no shear stresses and suggesting that more extensive partial melting of the basal part of the slab was not related to high shear stress. The total thickness of this proposed slab is 75 km. Oceanic crust is known to be only approximately 15 km thick (Winter 2001) and therefore, this very thick layer of Group B and C eclogites may represent stacked subducted slabs of oceanic crust. Sharp compositional and thermal boundaries between individual slabs have likely been destroyed and thus multiple slabs are not detected through geophysical methods. 7.6 Comparison of Northern and Central Slave eclogites For comparison with Northern Slave eclogites, eclogites from the Lac de Gras area are examined in the following discussion. The overall consensus on the origin of eclogite xenoliths from each of these studies indicates a protolith from subducted oceanic crust (Pearson et al. 2004; Aulbach et al. 2007; Schmidberger et al. 2007; Aulbach et al. 2011). The majority of all eclogites analysed in these studies correspond to Groups B and C. The following characteristics are common to both Northern and Central Slave eclogites. A. Group A eclogites from A154 exhibit Mg-numbers within the mantle range and enriched Mg and Cr, which occupy similar fields to Group A eclogites from the Northern Slave (Figure 7.14). B. Major element analyses of Group B and C eclogites generally exhibit low Mg- number and depletion in Mg and Cr, which occupy similar fields to Group B and C eclogites from the Northern Slave (Figure 7.15). C. Group B and C eclogites show similar REE and trace element patterns to Group B and C eclogites from the Northern Slave – positive Eu and Sr anomalies, flat HREE, depleted LREE (Figure 7.16). D. Group B eclogites from both the Central and Northern Slave exhibit a bimodal distribution, which may indicate multiple subducted slabs (Figures 7.13 and 7.17). E. Diamondiferous eclogites from the Northern and Central Slave both occur at narrow ranges of depths (Figures 6.8, 6.19, 6.30). 124 Figure 7.14 Whole-rock Mg-number for Central Slave eclogite xenoliths. The purple field represents crustal values (Barth et al. (2001); red field represents mantle values (Barth et al. 2002). The green ellipse represents Group A eclogites from the Central Slave, the bold black ellipse represents Group A eclogites from the Northern Slave; yellow ellipse repre- sents Group B and C eclogites from the Central Slave, the black ellipse represents Group B and C eclogites from the Northern Slave. The Central Slave eclogites on this figure are estimates of bulk composition made as described in Chapter 6.5 for A154 eclogites from Schmidberger et al. (2007). There exists an overlap of between the diamondiferous eclog- ite data and the Group A, B, and C eclogite data as the two are not mutually exclusive. Figure 7.15 Whole-rock Cr2O3 vs. MgO (wt. %) for Central Slave eclogites. The purple field represents crustal values and the red field represents mantle values (Barth et al. 2002). The green field represents Group A eclogites from the Central Slave, the bold black ellipse represents Group A eclogites from the Northern Slave; yellow field represents Group B and C eclogites from the Central Slave, the black ellipse represents Group B and C eclogites from the Northern Slave. The Central Slave eclogites on this figure are estimates of bulk composition made as described in Chapter 6.5 for A154 eclogites from Schmidberger et al. (2007). There exists an overlap of between the diamondiferous eclogite data and the Group A, B, and C eclogite data as the two are not mutually exclusive. 125 Figure 7.16 N-MORB normalized trace element pattern for whole-rock analysis of eclog- ite xenoliths from the Central Slave. A) Modified after Schmidberger et al. (2007) – the dashed line represents mafic cumulates from ophiolites and the solid line represents other eclogite xenoliths from Diavik, Groups A, B, and C are all represented; B) modified after Aulbach et al. (2011), samples correspond to Groups B and C – grey (high-Mg) and green (high-Mg) fields represent Diavik eclogites are from Aulbach et al. (2007); C) modified after Aulbach et al. (2007), samples (high-Mg) correspond to Groups A and B – light grey field represents gabbros from the Oman ophiolite and the dark grey field represents gabbros from the Southeast Indian ridge, given for comparison. 126 Figure 7.17 Depth distribution histogram of Group B and C eclogites from Central Slave kimberlites. Outlined are the proposed slabs of subducted oceanic crust. 127 There are several differences between eclogites from the Central and Northern Slave. A. In contrast to the mantle origin of Group A eclogites from the Northern Slave, Group A eclogites from Schmidberger et al. (2007) give contradictory chemical evidence with respect to their origin. The major element evidence supports a mantle origin for these eclogites, whereas the trace element and REE signatures point toward an origin from subducted oceanic crust. Group A eclogites from Aulbach et al. (2007) only show evidence that support a subducted oceanic crustal origin through trace element analyses, because bulk major element analyses for these eclogites were not reported. B. REE patterns for Central Slave eclogites show either a negative Nb anomaly (Figure 7.16a, c), which indicates a subducted oceanic crustal origin, or no Nb anomaly (Figure 7.16b). This is in contrast to Northern Slave eclogites that show a positive Nb anomaly (Figure 7.11). The removal of rutile or rutile present in the source melt would cause a negative anomaly. A positive Nb anomaly Northern Slave eclogites is due to accumulation of rutile in the magma, which matches well to greater amounts of modal rutile (3 vol. %) than that in Central Slave eclogites (< 1 vol. %) (Heaman et al. 2006; Aulbach et al. 2011). C. Diamondiferous eclogites from the Central Slave correspond only to Groups B and C (Figures 7.14, 7.15, and 7.16b), whilst diamondiferous eclogites from the Northern Slave are predominately Group A. The crustal origin for the Central Slave diamondiferous eclogites is supported by δ13C data. δ13C values of diamonds from the Central Slave craton range from -10.5 ‰ to 0.7 ‰ (Donnelly et al. 2007), which is well beyond the mantle value of -5 ‰. D. Both Group B and C eclogites from the Central Slave craton exhibit a bimodal distribution at similar depths (Figure 7.17). However, only Group B eclogites from the Northern Slave craton exhibit a bimodal distribution (Figure 7.13). E. Eclogite xenoliths from the Central Slave yield older formation ages (Lu-Hf isotope composition of the A154 eclogites from Schmidberger et al. 2007), approximately 2.1 Ga, than those from the Northern Slave. Re-Os dating of sulphides from eclogitic diamonds yield an age of 1.85 Ga (Aulbach et al. 2011). Both of these ages of formation roughly correspond to the subduction event from 1.95 to 1.91 Ga, which generated the Hottah magmatic arc (Schmidberger et al. 128 2007). Eclogites from the Northern Slave show much younger ages (1785 Ma), which are related to a later subduction event that generated the Great Bear magmatic arc (1.88 – 1.84 Ga) (Heaman et al. 2006). F. The majority of diamondiferous eclogites from the Central Slave occur at greater depths (190 – 220 km) than those from the Northern Slave craton (145 – 155 km). G. Different diamond-forming processes are responsible for the formation of diamonds in the Central and Northern Slave. Diamonds from the Central Slave formed along the fluid conduits created through deserpentinization of the subducted oceanic crust (Aulbach et al. 2011) from carbon with δ13C -17 – -2 ‰ (Davies et al. 2004) (Figures 2.10 and 7.18). The bulk of deserpentinization occurs at pressures permissive of diamond formation (60 – 70 kbar) (Schmidt and Poli 1998). A different source of carbon is suggested for Northern Slave eclogites by values of δ13C -23 – -41 ‰. The origin of diamonds in the Northern Slave craton has been attributed to multiple metasomatic events and partial melting in the mantle (De Stefano et al. 2009). Another theory for the origin of diamond in the Northern Slave proposes metasomatism of eclogite that has been emplaced into peridotite in the cratonic lithospheric mantle (Smart et al. 2009) (Figure 7.19). H. Different processes were responsible for the occurrence of diamondiferous eclogites in a tightly restricted depth below both the Central and Northern Slave. Since diamonds formed in a subducted oceanic crust, the thickness of the crustal part of the subducted slab (15 – 20 km, Winter 2001) inherently restricts the interval of depth at which diamondiferous eclogites can occur in the Central Slave. The majority of diamondiferous eclogites from the Northern Slave craton show mineral chemistry corresponding to a mantle cumulate origin and are likely derived from a thin mantle layer of re-melted peridotite (Cookenboo 1998; Heaman et al. 2006). 7.7 The Central – Northern Slave mantle cross-section Based on geochronology of eclogite xenoliths and eclogitic diamonds from the Northern and Central Slave craton, a formation model for multiple subducted slabs of oceanic crust is proposed. Models for the formation of the entire Slave craton suggest three subduction events: the first occurred from 2.67 to 2.6 Ga, the second from 1.95 to 129 Figure 7.18 Schematic diagram of a subduction zone setting of the Central Slave craton depicting eclogite and eclogitic diamond formation. Thick blue arrows represent veins in which fluids move (wavy arrows), yellow zones represent a selvage area, and dotted areas represent metasomatic zones. Modified after Aulbach et al. (2011). Figure 7.19 Schematic diagram of petrogenesis of Jericho diamondiferous eclogite. Eclogite lenses were emplaced into peridotite in the Slave cratonic lithospheric mantle. Metsomatism (black arrows and dashes) of eclogite and peridotite facilitated diamond growth. Modified after Smart et al. 2009) 130 1.91 Ga, and the third from 1.88 to 1.84 Ga (Fyson and Helmstaedt 1988; Kusky 1989; Bowring and Grotzinger 1992; see Chapter 7.2 for further explanation). The bimodality of the depth distribution for Group B and C eclogites from the Central and Northern Slave match well with the two ages (1.9 and 1.8 Ga) determined from the eclogites and eclogitic diamonds (Figure 7.20). Below is a summary of the facts and observations that were the basis for the cross- section of Figure 7.20 showing spatial and age relationships between eclogite segments in the Slave craton. Figure 7.20 depicts the deepest subducted slab at 190 to 210 km below the Central Slave extending to shallower depths of 170 – 185 km below the Northern Slave. The apparent angle of this slab on the cross-section (Figure 7.20) is not the true dip of the subducting slab, as the orientation of the subduction zone is almost parallel to the line of the cross-section. The apparent dip of the subducted slab may result from slightly different dips of subduction below North and Central Slave. Based on the preferential occurrence of diamondiferous eclogites at 190 – 210 km, which are derived from subducted oceanic crust, and the age of these eclogitic diamonds of 1.85 Ga, the age of this slab is inferred to be 1.84 – 1.88 Ga. It is thus proposed that there exists a laterally continuous slab of subducted oceanic crust that spans from the Central to the Northern Slave because: 1) diamondiferous Group B eclogites occur at similar depths (175 – 210 km) below the Northern and Central Slave; 2) the ages of eclogite formation from the Central and Northern Slave are similar; and 3) geophysical studies of the Slave craton show a continuous, laterally extensive high-velocity zone, which has been interpreted as subducted oceanic crust, at a depth of 195 km (Bostock 1997). This slab with an age of 1.88 – 1.84 Ga has a thickness of 25 km, which is also an acceptable thickness for oceanic crust. This slab brought with it crustal organic carbon and is associated with the formation of the Great Bear magmatic arc, which is the third, and last, subduction event reported for the Slave craton (Heaman et al. 2006). The second slab is drawn below the Central Slave at depths that correspond to another mode of crustal eclogites (140 – 160 km) and another high-velocity zone at 135 km. The interval of these depths also corresponds to known thickness of oceanic crust. It is presumed that the age of the slab is 1.95 – 1.91 Ga because this is the only other age reported for subduction events that allegedly were the source of the Central Slave eclogites. This slab may be the ocean floor of the Hottah magmatic arc, which is the 131 second subduction event in the history of the Slave craton (Bowring and Grotzinger 1992). With respect to continuation of this second slab to the Northern Slave, there are several possible solutions. The first of these is to extend the slab to Northern Slave and assume that the slab gets thicker toward the north. It would satisfy the fact that many crustal eclogite lenses occur at depths from 130 to 170 km in the Northern Slave. The thickening of the slab can be explained by imbrication, which is common below continents (Kusky 1993). The presence of a continuous, high-velocity zone at 135 km would support the single extended and thickened slab at these depths. However, this is not supported by the geophysical studies completed for the Slave, which demonstrates the absence of a continuous reflection at this depth of 135 km (Bostock 1997). Another model proposes the abrupt termination of the slab and, thus, its absence below the Northern Slave (Figure 7.20). This model would be based on the absence of a single, tight cluster of Group B and C eclogites from the Northern Slave at similar depths. The subducted slab dips up toward the Northern Slave because the cross section is drawn at an angle where the Northern Slave is closer to the subduction zones than the Central Slave. The termination of the slab in between Central and Northern Slave may result from the limited extent of the slab and the localization of the Hottah subduction zone closer to the Central Slave (Figure 7.20). Alternatively, the slab there may have disappeared due to extensive partial melting (Peacock et al. 1994), or it broke off due to increased density and slow subduction velocity (Davies and von Blanckenburg 1995). The third, shallowest slab is drawn below Northern Slave at depths 120 – 130 km to account for the concentration of crustal eclogites at this depth. It is possible that the high-velocity zone at a depth of 135 km relates to this subducted slab. The age of this slab may be 2.67 – 2.6 Ga, which corresponds to the earliest subduction event that built the Slave craton (Fyson and Helmstaedt 1988; Kusky 1989). This slab would be related in origin to the accretion of the Anton Terrane and therefore, would dip downward toward the Central Slave (Figure 7.20). Although no analogous ages are reported from the Northern Slave eclogites, there are two reasons why an Archean age for this slab is appropriate. Firstly, the age is consistent with the pattern of progressively older eclogites at shallower depths, which is observed below the Central Slave (Figure 7.20). Secondly, the shallow position of Archean slabs is expected based on the model of hot and shallow 132 subduction in the Archean (Condie 1986). The slab is unlikely to extend to the Central Slave because there is no evidence for existence of crustal eclogites at 120 – 130 km in the Central Slave. The termination of the slab corresponds well to the position of the Anton subduction zone closer to the Northern Slave than the Central Slave. The relic slabs of subducted oceanic crust below the Slave craton are interspersed with zones of mantle partial melting, which are seen as crystallized magma chambers filled with cumulates (Group A eclogites). They exist only below the Northern Slave, where they are most common from 135 to 150, but also occur at 175 km and from 190 to 205 km (Figure 7.20). Interestingly, the magma chambers are most abundant immediately beneath or at similar depths to where subducted slabs are present. This, however, does not conform to the current model of mantle melting. During dehydration of subducted oceanic crust, fluids are released above the slab; this would result in partial melting of the mantle producing magma chambers above the subducted slab (Winter 2001; Figure 7.18). 133 134 Figure 7.20 A) Map of the Slave craton (modified after Aulbach et al. 2011; location of Anton Terrane from Kusky 1989). Inset shows location of kimberlite pipes and location of the cross-section (modified after Aulbach et al. 2007; position of Muskox kimberlite pipe from Heaman et al. 2006). B) Schematic lithological mantle cross-section from Central to Northern Slave. Eclogites are depicted as lenses because eclogite is very rare in the mantle (3 – 15 %, Schulze 1989) and is interbedded with surrounding peridotite (Schulze 1989). Gray areas represent potential subducted slabs based on bimodal distribution of Group B and C eclogites from the Central Slave and Group B eclogites from the Northern Slave. The boundaries between spinel peridotite, spinel-garnet peridotie, garnet peridotite, lithosphere, and asthenosphere for the Central Slave are determined from Menzies et al. (2004) and Pearson et al. (1999). Boundaries between spinel peridotite, spinel-garnet peridotite, garnet peridotite, lithosphere, and asthenosphere for the Northern Slave are determined from Kopylova and Caro (2004) and Kopylova et al. (1999). The boundary between graphite and diamond is based on the intersection of the graphite-diamond constraint with the Central and Northern Slave geotherms. High- velocity areas are from Bostock 1997. Occurrences of diamondiferous eclogites are denoted with diamonds. Question marks represent uncertainty of lateral extent of the subducted oceanic crust. See Chapter 7.5 for sources of slab ages. 135 7.4 Conclusions 1. Northern Slave eclogites found as xenoliths in the Jericho and Muskox kimberlite pipes have foliated and massive textures. These eclogites are dominantly comprised of primary low-Cr Pyrope(21-72)-Almandine(15-64)-Grossular(10-35) and omphacite (Diopside45-91Jadeite9-55) with rutile and forsterite (Fo86-93). The eclogites, based on garnet and clinopyroxene compositions, correspond to eclogites of Groups A, B, and C defined by Coleman et al. (1965). 2. Jericho eclogites have a mafic to ultramafic bulk composition similar to picrites and contain olivine. Muskox eclogites have a more mafic bulk composition, which explains the absence of olivine. 3. There are two populations of eclogite xenoliths from the Northern Slave, Group A eclogites with a mantle origin, and Group B and C eclogites with a protolith from subducted oceanic crust. The eclogite xenoliths from the Jericho kimberlite formed at 730 to 1300 °C and 30 – 70 kbar. The eclogite xenoliths from the Muskox kimberlite equilibrated at 670 to 1290 °C and 25 – 70 kbar. 4. Diamondiferous eclogites from the Northern Slave are mostly Group A massive eclogites. The common criteria for distinguishing diamondiferous eclogites based on high Na2O content in garnet and high K2O content in clinopyroxenes can be applied only to the Muskox eclogites, but the Na2O and K2O thresholds are higher than in other worldwide eclogite suites. The high Mg content in both garnet and clinopyroxene best distinguishes the diamondiferous eclogites from Jericho, in which the common criteria do not work. 5. The majority of diamondiferous eclogites from the Northern Slave occur at shallower depths (145 – 155 km) than those from the Central Slave craton (190 – 220 km). Different diamond-forming processes are responsible for the formation of diamonds in the Central and Northern Slave. 6. Textures of partial melting and recrystallization of garnet and clinopyroxene is common (10 – 90 vol. %) in eclogites from the Northern Slave. Partial melting and recrystallization produced highly magnesian secondary garnet and clinopyroxene as well as phlogopite, ferrotschermakite, taramite, calcite, siderite, magnesite, dolomite, magnetite, spinel, and ulvöspinel. The recrystallization was caused by an influx of carbonatitic and hydrous hot fluid. Zoning, present in 136 garnet and clinopyroxene, is attributed to a recent heating event immediately predating kimberlite eruption. 7. A model with multiple subducted slabs of oceanic crust below the Slave craton is proposed. The deepest subducted slab with an age of 1.88 – 1.84 Ga at 190 to 210 km below the Central Slave extends to shallower depths of 170 – 185 km below the Northern Slave. Another slab dated at 1.95 – 1.91 Ga at 140 – 160 km below the Central Slave may extend to the Northern Slave where it gets progressively thicker from imbrication. The third, shallowest slab dated at 2.67 – 2.6 Ga occurs below Northern Slave at depths 120 – 130 km. 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Nd and Sr isotopic study of a mafic layer from Ronda ultramafic complex, Nature 304, 226-230. 146 Appendix A: Macro-specimen descriptions 147 Jericho DJ99001 256.30 Texture: undetermined Garnet Orange, coarse, altered CPX Green-yellow, altered Phlogopite Secondary DJ99001 150.75 Texture: massive Garnet Purple/pink, coarse, altered CPX Green-yellow, coarse, altered Phlogopite Secondary DJ99001 197.10 Texture: massive Garnet Orange/purple, coarse CPX Green-yellow, coarse DJ99001 142.22 Texture: foliated Garnet Orange, coarse CPX Green-yellow, elongated DJ99001 201.00 Texture: undetermined Garnet Orange/purple, coarse CPX Green, altered Olivine Green-yellow, coarse JD40 Mx103 Texture: massive Garnet Orange/dark orange, coarse CPX Green, coarse Spinel Very little Phlogopite Secondary JD35 Mx3 Texture: foliated Garnet Orange, coarse CPX Light green, elongated JDO14 Mx88 Texture: massive Garnet Light to dark orange, coarse CPX Light to medium green, coarse Phlogopite Secondary 148 JDO14 Mx128 Texture: massive Garnet Light orange, coarse CPX Green, coarse Phlogopite Secondary JD35 Mx9 Texture: massive Garnet Orange, coarse CPX Dark green, coarse Phlogopite Secondary, rare JDO14 Mx80 Texture: massive Garnet Dark orange, coarse CPX Dark green, coarse Phlogopite Secondary JDO14 Mx71 Texture: foliated Garnet Light to dark orange, coarse CPX Green, elongated Phlogopite secondary JDO33 Mx2 Texture: massive Garnet Orange, coarse CPX Dark green, coarse Phlogopite Secondary JD35 Mx27 Texture: foliated Garnet Orange, coarse CPX Light green, elongated Phlogopite Secondary, rare LGS08 Mx13 Texture: massive Garnet Light orange to pink, coarse CPX Light green, coarse Phlogopite Secondary LGS09 Mx18 Texture: foliated Garnet Orange, coarse CPX Green, elongated 149 LGS1 Mx1 Texture: massive Garnet Orange, coarse, idioblastic CPX Green, coarse, idioblastic Phlogopite Secondary, rare LGS08 Mx12 Texture: foliated Garnet Dark orange, coarse CPX Dark green, elongated Phlogopite Secondary LGS047 Mx1 Texture: massive Garnet Orange, coarse CPX Green, coarse Olivine Yellow Phlogopite Secondary, rare LGS044 Mx3 Texture: foliated Garnet Orange, coarse CPX Green, coarse, some altered Phlogopite Secondary, rare Spinel Secondary, rare LGS044 Mx15 Texture: foliated Garnet Light orange to pink, coarse CPX Light green, coarse Phlogopite Secondary, rare Spinel Secondary, abundant LGS044 Mx9 Texture: massive Garnet Dark to light orange, coarse CPX Green, coarse Olivine Yellow, abundant Phlogopite Secondary, rare LGS022 Mx9 Texture: foliated Garnet Orange, coarse CPX Green, mostly altered, some fresh, elongated Phlogopite Secondary 150 LGS025 Mx11 Texture: massive Garnet Orange, coarse CPX Light green, elongated, mostly altered Phlogopite Secondary, rare LGS027 Mx2 Texture: massive Garnet Dark orange, coarse CPX Dark green, coarse Phlogopite Secondary, rare LGS021 Mx5 Texture: foliated Garnet Light orange, coarse CPX Light green, elongated Phlogopite Secondary, rare LGS026 Mx6 Texture: foliated Garnet Orange, coarse CPX Green, fine grained Phlogopite Secondary LGS053 Mx5 Texture: undetermined Garnet Orange, coarse CPX Dark green Phlogopite Secondary, abundant LGS035 Mx2 Texture: massive Garnet Light to dark orange, coarse CPX Green, coarse Phlogopite Secondary, rare LGS035 Mx1 Texture: massive Garnet Light orange, coarse CPX Green, coarse Phlogopite Secondary, rare 151 LGS033 Mx1 Texture: foliated Garnet Orange, coarse CPX Light green, elongated Phlogopite Secondary, rare Carbonate Secondary, abundant LGS055 Mx7 Texture: foliated Garnet Light orange, coarse CPX Light green, elongated Kyanite Blue, rare Phlogopite Secondary LGS054 Mx5 Texture: foliated Garnet Orange, coarse CPX Light green, mostly altered Phlogopite Secondary LGS047 Mx6 Texture: undetermined Garnet Light orange to pink, coarse CPX Light green Carbonate Secondary Phlogopite Secondary LGS053 Mx11 Texture: foliated Garnet Orange, coarse CPX Light green, elongated Phlogopite Secondary, rare Carbonate Secondary, abundant LGS052 Mx1 Texture: foliated Garnet Orange, coarse CPX Green, foliated Phlogopite Secondary LGS052 Mx8 Texture: foliated Garnet Orange, coarse CPX Dark green, elongated Phlogopite Secondary, abundant Carbonate Secondary, abundant 152 LGS10 Mx17 Texture: massive Garnet Orange, coarse CPX Light green, coarse Phlogopite Secondary, abundant Carbonate Secondary, abundant LGS09 Mx1 Texture: foliated Garnet Orange, coarse CPX Dark green, elongate, ~ 20% altered Phlogopite Secondary, very rare LGS07 Mx5 Texture: massive Garnet Orange, coarse CPX Light green, coarse Phlogopite Secondary, rare LGS06 Mx23 Texture: foliated Garnet Orange, coarse CPX Green, elongated Phlogopite Secondary LGS06 Mx3 Texture: foliated Garnet Orange, coarse CPX Dark green, elongated Olivine Yellow Phlogopite Secondary, rare LGS053 Mx14 Texture: foliated Garnet Dark orange, coarse CPX Dark green, mostly altered Phlogopite Secondary, abundant LGS051 Mx4 Texture: foliated Garnet Orange, coarse CPX Light green Phlogopite Secondary, abundant 153 LGS053 Mx3 Texture: foliated Garnet Orange, coarse CPX Green, ~20% altered Phlogopite Secondary, abundant LGS052 Mx2 Texture: foliated Garnet Orange, coarse CPX Green, elongated Phlogopite Secondary Carbonate Secondary, rare LGS054 Mx8 Texture: foliated Garnet Orange, coarse, idioblastic CPX Green, elongated Phlogopite Secondary LGS054 Mx4 Texture: massive Garnet Light orange, coarse CPX Green, coarse Kyanite Associated with phlogopite, blue Phlogopite Secondary, abundant LGS051 Mx11 Texture: foliated Garnet Light orange, coarse CPX Light green, very fine grained Carbonate Secondary, rare Phlogopite Secondary LGS052 Mx9 Texture: foliated Garnet Light orange, coarse CPX Light green, ~90 % altered Phlogopite Secondary LGS47 Mx15 Texture: foliated Garnet Light orange, coarse CPX Light green, ~80 % altered, elongated Phlogopite Secondary 154 LGS53 Mx6 Texture: foliated Garnet Light orange, coarse CPX Light green, ~40 % altered, elongated Phlogopite Secondary, minor LGS52 Mx3 Texture: foliated Garnet Orange, coarse CPX Light green, elongated Phlogopite Secondary, abundant JDF6N #4 Texture: massive Garnet Dark orange, coarse CPX Light green, coarse Olivine Yellow Phlogopite Secondary JD67 Mx2 Texture: massive Garnet Light orange, coarse CPX Light green, coarse Phlogopite Secondary JD41 Mx15 Texture: foliated Garnet Orange, coarse CPX Green, elongated Phlogopite Secondary, abundant JDF6N #2 Texture: foliated Garnet Orange, coarse CPX Green, elongated Phlogopite Secondary, minor LGS033 Mx6 Texture: foliated Garnet Light orange to pink, coarse CPX Light green, ~ 90 % altered, elongated Phlogopite Secondary, abundant 155 LGS035 Mx4 Texture: massive Garnet Orange, coarse CPX Dark green, coarse Olivine Yellow, coarse Phlogopite Secondary, abundant Carbonate Secondary LGS037 Mx8 Texture: foliated Garnet Orange, coarse CPX Dark green, elongated Olivine Yellow Phlogopite Secondary, rare LGS039 Mx17 Texture: foliated Garnet Orange, coarse CPX Green, elongated Phlogopite Secondary, rare LGS039 Mx22 Texture: massive Garnet Orange, coarse CPX Green, coarse Phlogopite Secondary, rare LGS039 Mx12 Texture: foliated Garnet Dark orange, coarse CPX Dark green, elongated Phlogopite Secondary LGS011 Mx16 Texture: massive Garnet Light orange to pink, coarse CPX Light green, ~80 % altered, coarse Phlogopite Secondary, abundant Carbonate Secondary, rare LGS016 Mx8 Texture: foliated Garnet Orange, coarse, idioblastic CPX Green, elongated Phlogopite Secondary, very rare 156 LGS019 Mx3 Texture: foliated Garnet Light orange, coarse CPX Light green, coarse Phlogopite Secondary, rare Carbonate Secondary, abundant LGS020 Mx8 Texture: foliated Garnet Orange, coarse CPX Light green, ~80 % altered Phlogopite Secondary, rare LGS013 Mx3 Texture: foliated Garnet Orange, coarse CPX Dark green, elongated Phlogopite Secondary, rare Carbonate Secondary, very rare LGS019 Mx1 Texture: foliated Garnet Light orange, coarse CPX Green, elongated Phlogopite Secondary, rare LGS017 Mx16 Texture: foliated Garnet Orange, coarse CPX Green, elongated Phlogopite Secondary, minor LGS016 Mx12 Texture: massive Garnet Light orange, coarse CPX Light to dark green, ~40 % altered, coarse Phlogopite Secondary, rare LGS012 Mx3 Texture: foliated Garnet Orange, coarse CPX Green, elongated, ~80 % altered Phlogopite Secondary, abundant 157 LGS017 Mx7 Texture: foliated Garnet Orange, coarse CPX Light green, elongated Phlogopite Secondary, abundant LGS020 Mx4 Texture: foliated Garnet Orange, coarse CPX Light green, elongated Phlogopite Secondary LGS011 Mx25 Texture: foliated Garnet Orange, coarse CPX Dark green, elongated Phlogopite Secondary, abundant LGS011 Mx17 Texture: foliated Garnet Orange, coarse CPX Light green, elongated Phlogopite Secondary, minor Carbonate Secondary, abundant Muskox 10204 Type: 1 Texture: hypidioblastic Garnet 70% Light orange, fractured, carbonate and biotite alteration around edges and in fractures, hypidioblastic CPX 10% Green, 70-90% altered by carbonate, hypidioblastic Biotite 2% Alters garnet around edges, hypidioblastic Diamond <1% Inclusion in CPX, 3mm, grey Carbonate 18% Mainly alters CPX, some garnet alteration 158.8 Type: 1 Texture: hypidioblastic Garnet 50% Light orange/orange-grey, idioblastic, some carbonate alteration CPX 30% Light green, hypidioblastic, carbonate altered Biotite 5% Idio-hypidoblastic laths, as alteration around garnet grains Carbonate 15% Alters CPX and garnet in fractures and along grain boundaries 158 MOX3 69.2A Type: 1 Texture: idioblastic Garnet 60% Orange, idioblastic CPX 35% Light green, hypidioblastic Carbonate 5% Vein running through, alters garnet boundaries and in CPX fractures 10215 Type: 1 Texture: xenoblastic Garnet 60% Hypidioblastic porphyroblasts, some carbonate alteration CPX 0-35% Completely altered by carbonate, xenoblastic, angular in spaces between garnet grains Sulphide 5% Silver/gold/black, xenoblastic, rounded-angular Carbonate 35% Mainly alters CPX, some alteration of garnet in fractures 10288 Type: 1 Texture: hypidioblastic CPX 45% Up to 80% altered by carbonate, hypidioblastic, rounded, green Garnet 40% Large porphyroblasts, orange, hypidioblastic, rounded Carbonate 14% Alter CPX in fractures and along grain boundaries, alters garnet in fractures Sulphide 1% Gold/black, rounded, hypidioblastic 10285 Type: 1 Texture: hypidioblastic Garnet 50% Red/orange, rounded, hypidioblastic, carbonate alteration in fractures and boundaries CPX 0-42% 100% carbonate altered, no fresh grains Sulphide 5% Silver, xenoblastic, rounded Biotite 3% Brown, some alteration veins, alter garnet grain boundaries Carbonate 42% Mainly alter CPX, garnet grain boundaries, some veins associated with biotite 10340 Type: 1 Texture: hypidioblastic CPX 50% Up to 90% altered by carbonate, elongated, hypidioblastic Garnet 30% Orange, hypidioblastic, some elongated, some carbonate altered in fractures Sulphide 3% Silvery, hypidioblastic, rounded Biotite 2% Alter garnet and CPX, associated with colourless carbonate pockets, brown, alteration veins through garnet and pink carbonate Carbonate 15% Alter CPX and garnet, veins, some occur as colourless/white pockets, some pinkish in large area altering CPX, fibrous 159 141.87 Type: 2 Texture: hypidioblastic, poikiloblastic garnet, foliated CPX 40-60% Green, elongated, 90% replaced/altered to carbonate, hypidio- xenoblastic, inclusions in garnet Garnet 40% Orange/red, elongated porphyroblasts, hypidioblastic, poikiloblastic (inclusions of CPX) Biotite 10% Associated with garnet, hypidioblastic laths Sulphides 1% Brown/gold, xenoblastic Carbonate 20% Alters CPX and some garnet fractures 161.90 Type: 1 Texture: hypidioblastic, poikiloblastic garnet, fresh, foliated, coarse grained Garnet 50% Orange, porphyroblasts, rounded, idioblastic, poikiloblastic (inclusions of CPX and sulphides) CPX 35% Dark green, elongated, hypidioblastic, 10% altered to carbonate Phlogopite 5% Brown, platey, idioblastic, associated with garnet Sulphides 3% Black/gold, xenoblastic Carbonate 10% Mainly alters CPX, some alteration of garnet in fractures 162.37 Type: 2 Texture: hypidioblastic, poikiloblastic garnet, medium grained, massive CPX 60% Dark green, altered 80% to carbonate, hypidio-xenoblastic, elongated Garnet 35% Orange, hypidioblastic, elongated, poikiloblastic (inclusion of CPX) Phlogopite 5% Brown, elongated laths, associated with garnet Sulphide 3% Black/gold, xenoblastic Carbonate 10% Alters CPX, some alteration in garnet fractures 197.70 Type: 1 Texture: hypidioblastic, massive, fine grained CPX 50% Green, altered to carbonate Garnet 35% Orange, hypidioblastic, carbonate alteration in fractures Phlogopite 5% Idioblastic laths, brown Sulphide 5% Gold/brown, 1-3mm, hypidio-xenoblastic, secondary alteration in grain boundaries Carbonate 5% Alters CPX, some colourless/white aggregates 202.43 Type: 2 Texture: extremely altered, unconsolidated CPX 0-90% Completely altered by carbonate, dull green Garnet 5% porphyroblasts, orange, hypidioblastic Biotite 5% Brown, idioblastic Carbonate 90% Completely altered CPX 160 224.36 Type: 2 Texture: xenoblastic, foliated, fine grained Garnet 50% Orange, hypidio-xenoblastic CPX 35% Green, xenoblastic, altered by carbonate Phlogopite 10% Brown, veinlets run through sample Carbonate 5% Alters fractures and edges of CPX, edges of garnet 10223 Type: 2 Texture: hypidioblastic Garnet 50% Orange, elongated, hypidioblastic, carbonate alteration along edges CPX 30% Light green, 60-80% altered by carbonate, elongate, hypidioblastic Sulphide 5% Silver/black, rounded, hypidioblastic Carbonate 15% Alters CPX and garnet 10214 Type: 2 Texture: hypidioblastic CPX 10-50% Up to 40% altered by carbonate, elongated, dark green Biotite 30% Altered?, brown, soft Garnet 20% Orange, rounded, hypidioblastic Carbonate 40% Some fresh, some altering garnet and CPX 10283 Type: 2 Texture: hypidioblastic CPX 50% Dark green, 30-80% carbonate altered, elongated, hypidioblastic Garnet 40% Orange/brown, elongated, hypidioblastic, carbonate alteration around edges Sulphide 2% Silver/black, gold, rounded, hypidioblastic Carbonate 8% White, veins, rounded grains 10337 Type: 2 Texture: hypidioblastic Garnet 40% Light orange, smaller grains, hypidioblastic, rounded CPX 40% Light green, smaller grains, hypidioblastic, rounded Biotite 5% Brown, very tiny grains, hypidioblastic Sulphide 1% Gold/black, rounded, hypidioblastic Carbonate 15% Alteration around grain boundaries of garnet and CPX 161 10292 Type: 2 Texture: hypidioblastic CPX 52% Green, 70-100% altered by carbonate, hypidioblastic Garnet 40% Orange/brown, elongated, hypidioblastic Sulphide 3% Gold, silver, xenoblastic, angular Carbonate 5% Some white veins and patches, alters CPX and garnet 246.1 Type: 1 Texture: altered CPX 70% Completely altered to pink/yellow mineral Garnet 30% Large grains, idioblastic, orange 161.5 Type: 1 Texture: medium grained, hypidioblastic, massive CPX 60% Up to 70% altered by carbonate, light green, hypidioblastic Garnet 30% Orange, rounded, hypidioblastic Carbonate 10% alters CPX and grain boundaries of garnet 200.5 Type: 1 Texture: hypidioblastic CPX 70% Green, carbonate alters up to 80%, hypidioblastic, elongated Garnet 20% Orange, elongated, hypidioblastic Carbonate 10% Alters CPX only 120.6 Type: 1 Texture: hypidioblastic CPX 60% Green , elongated, altered by carbonate 70%, hypidioblastic Garnet 30% Orange, rounded, hypidioblastic, carbonate alteration around edges Carbonate 10% Alters CPX and garnet 10216 Type: 1 Texture: hypidioblastic Garnet 75% Orange, rounded, hypidioblastic, carbonate alteration in fractures and edges CPX 10% Bright green (Chr di), angular, xenoblastic, 20% altered to carbonate Biotite 1% Altered, brown, soft, platey Carbonate 15% Alters CPX and garnet, some white pockets 162 10403 Type: 2 Texture: hypidioblastic, foliated Garnet 35% Rounded to elongate, orange, clay and carbonate altered in edges CPX 54% Light green, 5% altered to carbonate in fractures and edges, xenoblastic, elongate Biotite 5% Brown/black, platey, associated with clay alteration around garnet Sulphide 1% Gold, inclusion in garnet and in clay alteration, rounded, hypidioblastic Carbonate 5% Alters garnet and CPX, some orange/yellow veins 10207 Type: 2 Texture: hypidioblastic, foliated CPX 45% Altered by carbonate 80-100%, elongated, hypidioblastic Garnet 40% Red/orange, hypidioblastic, elongated, rounded Carbonate 5% Altered CPX and garnet, some white pockets 10402 Type: 1 Texture: hypidioblastic CPX 60% 70% altered by carbonate, green, rounded, hypidioblastic Garnet 20% Orange, rounded, hypidioblastic, carbonate alteration around edges Carbonate 20% Alters CPX and garnet 10289 Type: 1 Texture: hypidioblastic, massive Garnet 50% Orange, rounded, carbonate alters edges CPX 45% Carbonate altered 90-100%, interstitial, rounded, hypidioblastic Clay 3% Alters edges of garnet Carbonate 2% Alters CPX and garnet, some white veins through garnet 10298 Type: 1 Texture: hypidioblastic, foliated Garnet 50% Red/orange, rounded, carbonate altered edges, hypidioblastic CPX 35% Green, rounded, elongated, 30% carbonate altered in fractures and edges Sulphide 5% Silvery, hypidioblastic, elongated Carbonate 10% Alters CPX and garnet, pink veins 163 10168 Type: 1 Texture: altered, hypidioblastic, massive Garnet 40% Orange, rounded, carbonate alteration on edges, carbonate veins run though, biotite inclusions CPX Completely altered by carbonate Biotite Associated with garnet, in alteration veins Sulphide Silver, elongate, rounded, inclusion in CPX, hypidioblastic Carbonate Alters CPX and garnet, colourless/white pockets 10334 Type: 2 Texture: hypidioblastic, foliated CPX 53% Elongate, rounded, hypidioblastic, 40% altered by carbonate Garnet 30% Orange, rounded, elongate, hypidioblastic, carbonate alteration Sulphide 2% Silver, elongate, rounded, hypidioblastic Carbonate 15% Alters CPX and garnet 10339 Type: 1 Texture: hypidioblastic, altered Garnet 50% Rounded, hypidioblastic, orange, carbonate alteration along edges CPX 43% 80-100% carbonate altered in fractures and edges, green, elongated, hypidioblastic, rounded Biotite 2% Associated with garnet in edges, hypidioblastic, very fine grained Carbonate 15% Alteration of CPX and garnet, some white pockets 10164 Type: 1 Texture: hypidioblastic, massive, fresh Garnet 50% Orange, rounded, hypidioblastic CPX 50% Green, hypidioblastic Carbonate 2% Very little alteration of CPX 20000 Type: 2 Texture: hypidioblastic, altered CPX 60% Orange, fine grained, rounded, hypidioblastic Garnet 40% No fresh grains – not carbonate 10209 Type: 2 Texture: altered, foliatged CPX 68% No fresh grains, completely altered to carbonate, interstitial Garnet 30% Orange, rounded, some carbonate altered edges, elongated Carbonate 2% Few thin veins, white 164 10217 Type: 2 Texture: hypidioblastic, foliated Garnet 50% Orange, elongated, carbonate altered edges CPX 50% 95-100% carbonate altered (some white centres), elongated, hypidioblastic Carbonate 2% Alters CPX and garnet – white centres of CPX 165 Appendix B: Petrographic descriptions 166 Jericho LGS08 Mx13 Name: Eclogite Texture: massive, hypidioblastic Mineral Modal % Description Garnet 50-55% PPL – colourless; fractured; very high relief; hypidioblastic; inclusions of opaques and CPX; alteration of serpentine and opaques in fractures and boundaries; kelyphitic rims occurs around grains in contact with carbonate veins – thickest rims are closest to the veins; as inclusion in CPX; 3.4 mm average XPL – black; isotropic CPX 45-48% PPL – colourless; high relief; cleavage traces visible in one direction; in 95% of grains have clay alteration along cleavage planes; alteration of opaques and serpentine infill fractures; clay alteration infill thinner fractures; few grains cut by late carbonate veining; inclusions of garnet, opaques, and CPX; hypidioblastic; 1.8 mm average XPL – maximum 2nd order yellow/orange birefringence; inclined extinction; all grains exhibit undulatory extinction Opaques 1% PPL – black; hypidioblastic; 0.25 mm average; as inclusion in garnet and CPX; some xenoblastic grains infill fractures; serpentine alteration around boundaries XPL – remains black Secondary Carbonate 3% PPL – colourless; largest vein has a width of 0.32 mm; average grain size is 0.12 mm; serpentine alteration occurs along outer edges of the veins along with some very fine crystals of biotite; hypidioblastic grains XPL – from low 3rd order blue to high order pearly white birefringence Serpentine 5% PPL – grey/green/brown; very fine crystals; occur in fractures and boundaries of all grains in thin section XPL – low 2nd order pink maximum birefringence; parallel extinction Biotite <1% PPL – pleochroic colourless to orange/brown, 1 hypidioblastic lath, 1.2 mm; moderate relief; one perfect cleavage trace visible XPL – low 3rd order pink/blue; bird’s eye extinction; undulatory extinction – almost bent; serpentine alteration around boundary Comments: Some deformation is present – undulatory extinction in CPX and biotite Garnet has more alteration by serpentine than CPX Late carbonate veining crosscuts garnet and CPX Kelyphitic rims on garnet is thickest closest to carbonate veins 167 JD35 Mx9 Name: Eclogite/megacryst? Texture: very coarse grained, altered, massive, hypidioblastic Mineral Modal % Description Garnet 42-44% PPL – colourless; average 5.2 mm; maximum 1 cm; very high relief; hypidioblastic; alteration pockets (CPX, biotite, rutile, carbonate, serpentine, and spinel) within grains; alteration suite of serpentine, biotite, spinel, and CPX in fractures, veins, and along grain boundaries; fractured XPL – isotropic; black CPX 15-55% PPL – colourless; high relief; highly fractures; average 7.6 mm; highly altered (50 – 95 %) to serpentine; fractures in-filled with serpentine; some grains contain alteration to serpentine; inclusions of garnet and biotite; hypidioblastic XPL – maximum 1st order yellow; inclined extinction; some grains have undulatory extinction; few faint cleavage traces visible Biotite 1% PPL – colourless to light tan pleochroic; moderate relief; mostly plates, few laths; as inclusion in CPX; 0.32 mm average; hypidioblastic XPL – maximum 2nd order green/yellow; bird’s eye extinction; few grains with undulatory extinction; one grain has other biotite blebs/exsolution/twinning Spinel 1 grain PPL – 0.72 mm; black rim; core pleochroic foxy brown- green/brown; hypidioblastic XPL – undulatory extinction in core; no change in birefringence Secondary Serpentine 75% PPL – colourless/yellow; some green; low relief; mainly alters CPX grains; present in garnet alteration pockets, along grain boundaries, in fractures, and veins; very fine crystals; usually needles, some laths, some fibrous; hypidioblastic-idioblastic XPL – colourless/yellow grains show low 1st order grey birefringence; green grains have high 1st to low 2nd order yellow/orange; parallel extinction; veins running through garnet have green serpentine on outer edges with colourless/yellow in the centre Spinel/ Opaque 2% PPL – black; some with green centres; 0.3 mm average; occur in alteration pockets or in alteration veins; fractures; hypidioblastic; high relief XPL – remains black; isotropic, black Carbonate 2% PPL – colourless; low relief; occur only in alteration pockets XPL – low 3rd order blue; 0.10 mm average; hypidioblastic plates Biotite 1% PPL – colourless to light tan pleochroism; some idioblastic laths; some hypidioblastic plates; 0.21 mm average; plates occur in alteration pockets mainly; laths occur in alteration veins or along grain boundaries of garnet grains XPL – maximum 2nd order blue; bird’s eye extinction 168 Comments: Section is cut thinner than normal – lower interference colours Some deformation is present, evidenced by undulatory extinction Serpentine alteration is mainly constrained to CPX Garnet contains only alteration pockets, no primary mineral inclusions LGS035 Mx4 Name: Eclogite Texture: highly altered CPX, massive, hypidoblastic Mineral Modal % Description Garnet 53% PPL – colourless; very high relief; hypidioblastic; fractured; 2.8 mm average; inclusions of apatite and opaques; alteration in fractures and boundaries: serpentine, carbonate in centre, some opaques, some amphibole, little biotite; alteration pockets: green serpentine along edges, colourless serpentine in centre, amphibole, opaques, carbonate, and biotite; rare kelyphitic rims XPL – isotropic, black CPX 10-40% PPL – high relief; colourless; highly fractured; hypidioblastic; 3 mm average, heavily altered; serpentine altered in fractures completely separates pieces of grains; 30-95% altered; rare cleavage traces visible XPL – low 1st order grey/white to yellow; inclined extinction; some grains show slight undulatory extinction; most grains are constrained to one small area of slide Apatite 5% PPL – moderate relief; most colourless; some have pleochroism colourless to pink; idioblastic hexagonal crystals; some fractures; 0.4 mm average; as inclusions in garnet; distributed evenly throughout slide XPL – maximum 2nd order yellow; parallel extinction; some have inclusions of opaques and serpentine pockets; uniaxial Opaques 2% PPL – black; inclusion in garnet; hypidioblastic; 0.16 mm average XPL – remains black Secondary Serpentine 30% PPL – colourless, green/brown; low relief; very fine crystals; fibrous in some areas; mainly alter garnet in veins, fractures, and pockets; heavily alters CPX in fractures and edges XPL – colourless grains show 1st order grey; green/brown grains show upper 1st order yellow birefringence; parallel extinction Amphibole 3% PPL – pleochroic light brown/yellow to green/brown; some grains show 56/124 cleavage traces; hypidioblastic; 0.48 mm average in alteration pockets; more lath grains form in alteration veins; occurs in garnet alteration only; inclusions of opaques, biotite, and serpentine; moderate relief XPL – maximum 1st order yellow; inclined extinction; undulatory extinction 169 Mineral Modal % Description Biotite 1% PPL – light tan to brown pleochroism; moderate relief; 0.24 mm average; plates associated with amphibole; serpentine and garnet inclusions; occur in alteration patches and veining XPL – 2nd order yellow; bird’s eye extinction Opaques 1% PPL – black; idioblastic; overprints amphibole and biotite alteration XPL – remains black Carbonate <1% PPL – colourless; low relief; 0.1 mm average; occurs only in alteration pockets in garnet XPL – upper 4th-5th order pink; parallel extinction Comments: Highly altered CPX grains Some deformation present in CPX and amphibole LGS1 Mx1 Name: Eclogite Texture: massive, hypidoblastic Mineral Modal % Description Garnet 45-50% PPL – very light pink; very high relief; hypidioblastic; serpentine alteration in fractures; alteration suite of biotite, serpentine, carbonate, and opaque along grain boundaries; 2.2 mm average; fractured XPL – black, isotropic CPX 45-50% PPL – colourless; high relief; 1.84 mm average; mainly hypidioblastic, some angular xenoblastic grains; cleavage traces rarely visible; fractured; in fracture (near partial melting) alters to carbonate and rare biotite XPL – maximum 2nd order purple, mostly 1st order orange/yellow; inclined extinction; some undulatory extinction; lamellae are rare; partial melting along grain boundaries – recrystallization of idioblastic CPX grains in between grains of CPX; partial melting most prevalent along touching grains of CPX; inclusions of garnet Secondary Serpentine 2% PPL – colourless; green; low relief; very fine crystals; occur in fractures and along grain boundaries of garnet XPL – colourless grains have low 1st order grey birefringence; green grains show 1st order yellow birefringence; colourless grains are fibrous and occur in centre of fractures; green grains occur on outer edges of fractures Biotite 1% PPL – colourless to light brown pleochroism; moderate relief; xenoblastic; infilling fractures and along grain boundaries; 0.12 mm average XPL – maximum 2nd order yellow/green; bird’s eye extinction; some undulatory extinction 170 Mineral Modal % Description Carbonate 1% PPL – colourless; low relief; in fractures and boundaries of CPX grains; 0.16 mm average; xenoblastic CPL – upper 3rd order green; parallel extinction; occurs in centres of serpentine alteration veins in garnet; late stage veining cuts through CPX and garnet grains Comments: Clinopyroxene grains exhibit partial melting Some deformation indicated by undulatory extinction in biotite Late stage carbonate veining cuts through CPX and garnet LGS025 Mx11 Name: Eclogite Texture: massive, hypidioblastic, Mineral Modal % Description CPX 60% PPL – colourless; high relief; 4.8 mm; fractured; cleavage traces visible; some grains show lamellae; inclusions of garnet, rutile, and opaques; alteration of fractures with carbonate and serpentine; alteration along grain boundaries composed of serpentine, carbonate, biotite, opaques; alteration patches composed of biotite, serpentine, opaques, and chlorite XPL – maximum 2nd order orange/yellow; inclined extinction Garnet 25% PPL – very light pink; very high relief; fractured; larger grains elongated, hypidioblastic, 2.8 mm average; as inclusions (blebs) in CPX – rounded, hypidioblastic, 0.8 mm average; inclusions of CPX and opaques; alteration in fractures and boundaries composed of serpentine, chlorite, carbonate in centre, opaques, zircon, and biotite (in grain boundaries only); kelyphitic rims occur in areas where garnet crystals are in contact with carbonate alteration; alteration patches composed of biotite, carbonate, serpentine, and opaques XPL – isotropic, black Opaques 1% PPL – black; rounded; as inclusions in garnet and CPX; hypidioblastic; 0.4 mm average XPL – remains black Rutile <1% PPL – one large elongate grain; 1.52 mm; very high relief; yellow/brown; serpentine alteration in centre; biotite and serpentine along boundary; half of grain is surrounded by a garnet grain; cluster of needles showing colour, but mainly opaque XPL – no change in colour; needles show parallel extinction Zircon <1% PPL – colourless; very high relief; in alteration in garnet fracture XPL – maximum 3rd order pink/blue birefringence; parallel extinction 171 Secondary Mineral Modal % Description Serpentine 7% PPL – light yellow/green; very finely crystalline; elongate, radiating laths/needles; occurs in alteration veins and patches throughout slide in garnet, CPX, and rutile XPL – low 1st order grey; parallel extinction Carbonate 2% PPL – colourless; low relief; slightly fractured; xenoblastic; 0.09 mm average; occurs in alteration patches in garnet and CPX; alteration in fractures in CPX; large alteration zones associated with biotite, serpentine, and chlorite XPL – maximum 5th order pink/pearly white; parallel extinction Biotite 2% PPL – pleochroic colourless to light brown; moderate relief; xenoblastic – infill fractures and grain boundaries; in alteration patches in garnet and CPX; cleavage rarely visible; 0.24 mm average; lath-like hypidioblastic XPL – maximum 2nd order green; bird’s eye extinction; undulatory extinction Opaques 1% PPL – black; very fine crystals; occur with serpentine in garnet fractures; some larger clusters in grain boundaries between garnet and CPX; also in large alteration zones; 0.1 mm average XPL – remains black Chlorite <1% PPL – light green; very fine grained; mainly in larger alteration patches or with serpentine in alteration of fractures in garnet XPL – low 1st order grey; radiating needles Comments: Unusual texture with garnet blebs within CPX Some deformation indicated by undulatory extinction in biotite LGS016 Mx12 Name: Eclogite Texture: massive, hypidioblastic Mineral Modal % Description CPX 90% PPL – colourless; fractured; cleavage traces visible; high relief; hypidioblastic; 4.72 mm average; alteration of biotite and serpentine in fractures; alteration of serpentine and biotite in grain boundaries; late carbonate vein cuts through some grains XPL – maximum 2nd order blue; inclined extinction; lamellae present in a few grains; few grains with undulatory extinction; partial melting prevalent in fractures and along grain boundaries Garnet 3% PPL – colourless; very high relief; 1.36 mm average; fractured; alteration of serpentine in fractures; biotite, spinel, and amphibole alteration in grain boundaries; hypidioblastic; rounded XPL – isotropic, black 172 Secondary Mineral Modal % Description Serpentine 2% PPL – colourless to light yellow; very fine crystals; alters garnet and CPX in fractures and grain boundaries; fibrous and lath forms; hypidioblastic; low relief XPL – 1st order grey to yellow birefringence; parallel extinction; fibrous crystals form radiating arrays Biotite 2% PPL – moderate relief; pleochroic colourless to light brown; cleavage visible in larger grains; average 0.16 mm; occurs as alteration of garnet and CPX in fractures, but mainly in grain boundaries; hypidioblastic laths; xenoblastic when infilling fractures XPL – 2nd order yellow; bird’s eye extinction; some undulatory extinction Amphibole 1% PPL – pleochroic light green to green; fractured; no cleavage visible; occurs as alteration along grain boundaries of garnet; 0.16 mm average; hypidioblastic; platey; spinel inclusions; moderate relief XPL – low 1st order yellow; inclined extinction; faint undulatory extinction Carbonate 1% PPL – colourless; hypidioblastic; low relief; very thin late stage veins cut through CPX; faint cleavage visible; very finely crystalline XPL – 4th-5th order green/pink; symmetrical extinction; 0.09 mm average Spinel <1% PPL – black/green; 0.05 mm average; in alteration of garnet grain boundaries; rounded; hypidioblastic; high relief XPL – black, isotropic Comments: Piece of kimberlite enclosed by CPX Some deformation indicated by undulatory extinction in CPX, biotite, and amphibole Partial melting is prevalent in CPX JD40 Mx103 Name: Eclogite Texture: massive, hypidioblastic Mineral Modal % Description Garnet 45% PPL – colourless; fractured; inclusions of CPX and biotite laths; hypidioblastic; 6 mm average; alteration of biotite, serpentine, chlorite, carbonate, opaque and spinel in fractures; alteration of biotite, chlorite, serpentine, carbonate and opaques in grain boundaries; very high relief XPL – isotropic, black 173 Mineral Modal % Description CPX 35% PPL – colourless; no visible cleavage traces; 3.6 mm average; hypidioblastic; inclusions of biotite laths; alteration of carbonate, biotite, serpentine, chlorite, and opaques in fractures and grain boundaries; high relief XPL – maximum 2nd order orange birefringence; inclined extinction; lamellae present in some grains; partial melting is prevalent in all grains in fractures and grain boundaries Rutile 1% PPL – high relief; mostly black, some yellow/brown colour visible in areas of partial melting; hypidioblastic; 3.2 mm; fractured XPL – parallel extinction; remains same black and yellow/brown; fractures in-filled with carbonate; mainly surrounded by carbonate with some serpentine Secondary Biotite 15% PPL – colourless to light brown; moderate relief; 1 perfect cleavage visible; 0.8 mm average with maximum of 7.6 mm; idioblastic laths; hypidioblastic plates infill fractures and grain boundaries of garnet and CPX XPL – 2nd order red; bird’s eye extinction; most show undulatory extinction; some bent laths near edge of xenolith in contact with kimberlite Serpentine 2% PPL – colourless; low relief; very finely crystalline; alters garnet and CPX in fractures and grain boundaries XPL – maximum 1st order yellow; mostly fibrous, radiating crystals Carbonate 1% PPL – colourless; low relief; alters garnet, CPX, and rutile in grain boundaries and fractures; 0.4 mm average; hypidioblastic plates XPL – upper 5th order pink; symmetrical extinction Chlorite 1% PPL – green; low relief; hypidioblastic plates; alters garnet and CP in fractures and grain boundaries; 0.24 mm average XPL – low 1st order grey; inclined extinction; no cleavage visible Spinel <1% PPL – green/black; occurs in fractures of garnet alteration; rounded; hypidioblastic; high relief; 0.08 mm average XPL – isotropic, black Opaques <1% PPL – black; high relief; 0.04 mm average; alteration of garnet and CPX in fractures and grain boundaries XPL – remains black Comments: Partial melting of CPX and rutile is prevalent Some deformation indicated by undulatory extinction and bent biotite 174 Muskox MOX24 206.9 Name: Eclogite Texture: massive, coarse, diamond-type, xenoblastic CPX with idioblastic garnet Mineral Modal % Description Garnet 50% PPL – light pink; idioblastic; 6.2 mm average, maximum 1cm; highly fractured; 10 – 80 % partially melted; alteration of opaques in fractures; one large fracture (0.32 mm thick) filled with phlogopite, opaques, carbonate, and chlorite; mottled alteration on all garnet grains composed of opaques, chlorite, carbonate, and phlogopite; phlogopite most often surrounds garnet grain boundaries XPL – inclusions of CPX, opaques, and phlogopite; isotropic; alteration and partial melting follows fractures and grain boundaries CPX 45% PPL – colourless; xenoblastic; average 0.24 mm, maximum 1cm; partial melting 30 – 90 % is ubiquitous in fractures and along grain boundaries; inclusions of opaques, and garnet XPL – birefringence typically 2nd order blue, largest grain 1st order grey; inclined extinction; few grains show cleavage; very thin lamellae on grain Rutile 1% PPL – green/brown in centre surrounded by opaque alteration; altered by spinel XPL – parallel extinction; birefringence same as colour in PPL Secondary Carbonate 30% PPL – grey; alters garnet, CPX, and phlogopite in fractures and along cleavage; few large veins run through samples – some contain only carbonate, some with phlogopite, chlorite, and opaques XPL – high 5th order birefringence Opaque 3% PPL – black, often associated with carbonate alteration; inclusions in garnet and CPX; very fine grained; average of 0.04 mm; hypidioblastic to idioblastic; sulphides occur in alteration as cubes; larger grains are rounded XPL – remains black Phlogopite 3% PPL – pleochroic colourless to light brown; xenoblastic to hypidioblastic; cleavage visible in larger grains; appears with carbonate alteration in garnet and CPX; along grain boundaries of garnet grains; mostly plates XPL – birefringence 2nd order green; 0.24 mm average; when associated with carbonate alteration also altered by carbonate; one twinned grain Chlorite 1% PPL – light green; some pleochroic green; plate to needle shaped; hypidioblastic XPL – low 1st order yellow birefringence, some show anomalous 2nd order blue Comments: Coarser grained and appears less metamorphosed than other samples Partial melting ubiquitous throughout slide 175 MOX28 308.4 Name: eclogite Texture: coarse, massive, hypidioblastic Mineral Modal % Description Garnet 60% PPL – pink/brown; hypidioblastic; average 3 mm, maximum 6 mm; fractured; inclusions of opaques and CPX; altered to carbonate, chlorite, opaques, and phlogopite mainly along grain boundaries and in fractures XPL - isotropic CPX 35% PPL – light green; hypidioblastic; highly fractured; some show cleavage; carbonate alteration in fractures and along cleavage; alteration patches of carbonate; opaque and carbonate inclusions; phlogopite alters along edges of some grains; 4 mm average; some clay alteration along cleavage XPL – inclined; extinction; 2nd order orange birefringence with some 1st order grey/yellow; 20 – 30 % altered Opaques 1% PPL – larger grains as inclusions in garnet and CPX; hypidioblastic; rounded; average of XPL – remains black Secondary Carbonate 2% PPL – colourless/grey; changes relief on rotation; patches rimmed by opaques and chlorite and rarely phlogopite; xenoblastic to hypidioblastic plates XPL – extreme birefringence; some grains show twin lamellae; patches average of 1.6 mm; grain size average of 0.56 mm Rutile 1% PPL – dark brown to black in areas; shows 90o cleavage; rounded; hypidioblastic; some angular grains; 0.8 mm average XPL – same dark brown colour; inclined extinction; one grain shows dulled 2nd order birefringence Opaques 1% PPL – black; mostly around edges of carbonate patches; fine grained average 0.16 mm XPL – remains black Phlogopite 1% PPL – colourless to orange/brown pleochroic; cleavage visible in some; plate to lath shaped; associated with carbonate patches XPL – 2nd order green birefringence; bird’s eye extinction Chlorite 0.5% PPL – light green; some pleochroic green; plate to needle shaped; hypidioblastic; associated with phlogopite XPL – low 1st order yellow birefringence; alters phlogopite and garnet grains Comments: No partial melting Infiltrated by kimberlite 176 MOX7 53.9 Name: foliated eclogite Texture: foliated, hypidioblastic, magmatic Mineral Modal % Description CPX 75% PPL – light green; elongated; fractured; hypidioblastic; inclusions of apatite and opaques; cleavage traces visible in most grains; fractures in-filled with opaques, carbonate, and rare phlogopite; altered 20 – 90 %; average 2.3 mm, maximum 1 cm XPL – maximum 2nd order orange; some show 1st order yellow; some grains exhibit undulatory extinction; secondary CPX along grain boundaries; some partial melting in fractures; occur in few magma patches with garnet Garnet 15% PPL – light pink; elongated; fractured; hypidioblastic; inclusions of apatite, CPX, and opaques; fractures in-filled with opaques, carbonate, phlogopite, and chlorite; boundaries of grains surrounded by opaques, phlogopite, chlorite, and rare CPX; as inclusions in phlogopite; 5 % altered XPL – isotropic; average 2 mm, maximum 5 mm; occur in few magma patches with CPX Rutile 7% PPL – black; as inclusions in garnet and CPX; mostly elongated and xenoblastic; larger grains occur between grains of CPX and/or garnet along foliation; average 2 mm XPL – remains black Secondary Phlogopite 3% PPL – colourless to orange/brown pleochroic; larger grains show cleavage traces; undulatory extinction common; elongated; hypidioblastic; some surround garnet and opaque grains; fractures of some garnets are in-filled with phlogopite and opaques on outer edges; in larger fractures forms on edges with carbonate centre XPL – mostly 2nd order yellow birefringence; average of 0.56 mm to maximum of 1.7 mm; bird’s eye extinction Carbonate 2% PPL – colourless/grey; occurs in fractures with phlogopite and opaques through garnet and CPX; hypidioblastic; elongated in fractures and veins XPL – extreme white birefringence; average of 0.2 mm Opaques 1% Amphibole <1% PPL – pleochroic green; rare; associated with garnet; xenoblastic plates XPL – 2nd order blue birefringence; Comments: Secondary melting exhibited by magmatic texture in some areas Deformation revealed by undulatory extinction Little partial melting; only in CPX 177 MOX25 207 Name: eclogite Texture: diamond-type, hypidioblastic with some idioblastic garnet Mineral Modal % Description Garnet 75 PPL – colourless; idioblastic to hypidioblastic; partial melting throughout; alteration in fractures by carbonate, opaques and phlogopite; alteration along grain boundaries by carbonate and phlogopite; average of 6.8 mm; inclusions of CPX, some 100 % altered to carbonate; fractured XPL – isotropic; ~80 % altered and/or partially melted CPX 20 PPL – colourless; hypidioblastic; fractured; partial melting along fractures – idioblastic secondary CPX along grain boundaries; alteration of opaques, phlogopite, and carbonate along grain boundaries; cleavage visible in some grains XPL – 1st order orange birefringence; inclined extinction; rare lamellae; one grain shows undulatory extinction; average 3.8 mm Secondary Carbonate 5% PPL – grey; very fine grained; mottled; alters CPX in fractures and grain boundaries up to 25%; alters garnet in fractures up to 1% XPL – 3rd order blue birefringence; associated with opaque in fractures of garnet grains Phlogopite 1% PPL – pleochroic colourless to orange/brown; average of 0.48 mm; hypidioblastic laths; cleavage visible on most grains; associated with opaques and carbonates in fractures and along grain boundaries of garnet and CPX grains XPL – 2nd order orange birefringence; rare 1st order grey; undulatory extinction common; bird’s eye extinction common Opaques 1% PPL – black; average 0.01 mm; in fractures and grain boundaries of CPX and garnet; often associated with carbonate and phlogopite; rounded; xenoblastic XPL – remains black Comments: Diamondiferous in hand sample Partial melting throughout slides ~80 % Deformation evidenced by undulatory extinction 178 Appendix C: Major element chemistry of minerals and equilibrium temperatures 179 Table Sample Table Sample Table Sample C1 minimum detection limits C43 LGS012 Mx3 C85 10164 C2 JDO14 Mx88 C44 LGS016 Mx12 C86 10403 C3 JD35 Mx9 C45 LGS039 Mx12 C87 10339 C4 JD40 Mx103 C46 LGS020 Mx4 C88 10334 C5 LGS8 Mx12 C47 DJ99001 142.22 C89 10289 C6 LGS1 Mx1 C48 LGS047 Mx1 C90 10298 C7 LGS035 Mx1 C49 LGS047 Mx6 C91 10292 C8 LGS025 Mx11 C50 LGS020 Mx8 C92 161.9 C9 LGS011 Mx17 C51 LGS011 Mx25 C93 10340 C10 LGS019 Mx1 C52 LGS044 Mx15 C94 10402 C11 LGS039 Mx22 C53 JD35 Mx3 C95 10288 C12 JDO-14 Mx71 C54 LGS052 Mx9 C96 10337 C13 LGS06 Mx23 C55 LGS47 Mx15 C97 10214 C14 LGS035 Mx2 C56 LGS016 Mx8 C98 120.6 C15 LGS06 Mx3 C57 LGS054 Mx4 C99 10283 C16 DJ99001 197.10 C58 DJ99001 150.75 C100 141.87 C17 JDO14 Mx128 C59 LGS08 Mx13 C101 10215 C18 LGS044 Mx9 C60 LGS053 Mx11 C102 158.8 C19 JDO33 Mx2 C61 LGS055 Mx7 C103 224.36 C20 LGS07 Mx5 C62 LGS054 Mx8 C104 197.7 C21 LGS044 Mx3 C63 LGS033 Mx1 C105 200.5 C22 JD35 Mx27 C64 LGS053 Mx3 C106 MOX3 69.2 C23 LGS09 Mx18 C65 LGS051 Mx4 C107 10223 C24 JDO14 Mx80 C66 LGS011 Mx16 C108 10217 C25 DJ99001 201.00 C67 LGS017 Mx7 C109 MOX7 53.9 C26 LGS10 Mx17 C68 LGS051 Mx11 C110 MOX24 206.9 C27 LGS09 Mx1 C69 LGS022 Mx9 C111 MOX25 207 C28 LGS026 Mx6 C70 LGS035 Mx4 C112 MOX28 308.4 C29 LGS027 Mx2 C71 LGS053 Mx14 C30 LGS021 Mx5 C72 LGS052 Mx3 C31 JDF6N #4 C73 LGS052 Mx8 C32 LGS033 Mx6 C74 LGS037 Mx8 C33 LGS052 Mx2 C75 LGS05 Mx5 C34 LGS3 Mx6 C76 LGS054 Mx5 C35 JD67 Mx2 C77 LGS017 Mx16 C36 JD41 Mx15 C78 47-8 C37 JDF6N#2 C79 52-5 C38 LGS052 Mx1 C80 55-4 C39 DJ99001 256.30 C81 47-2 C40 LGS039 Mx17 C82 JDF6N C41 LGS013 Mx3 C83 26-6 C42 LGS019 Mx3 C84 LGS8 Mx3 180 Source Jericho Muskox Jericho Muskox Jericho Muskox Muskox Muskox Muskox Mineral Garnet Garnet Clinopyroxene Clinopyroxene Olivine Mica Amphibole Spinel Carbonate SiO2 0.02 0.02 0.03 0.03 0.03 0.03 0.03 0.03 - TiO2 0.03 0.03 0.03 0.03 - 0.04 0.03 0.04 - Al2O3 0.02 0.02 0.02 0.02 0.02 0.03 0.02 0.03 - Cr2O3 0.05 0.05 0.05 0.05 0.03 0.07 0.05 0.05 - FeO 0.06 0.06 0.06 0.06 0.06 0.08 0.06 0.06 0.09 MnO 0.05 0.05 0.05 0.05 0.05 0.07 0.05 0.06 0.08 MgO 0.02 0.02 0.02 0.02 0.02 0.03 0.02 0.03 0.05 CaO 0.03 0.03 0.03 0.03 0.03 0.04 0.03 0.03 0.04 Na2O 0.01 0.03 0.05 0.05 - 0.04 0.05 - - K2O - - 0.02 0.02 - 0.04 0.03 - - NiO - - - - 0.07 - - 0.08 - Nb2O5 - - - - - - - 0.12 - C1. Minimum detection limits of minerals from Jericho and Muskox eclogites 181 Sample ID mount1-1 mount1-2 mount1-3 mount1-4 mount1-7 mount1-8 Mineral garnet garnet garnet garnet clinopyroxene clinopyroxene garnet* clinopyroxene* Used for average Y Y Y Y Y Y Number of analyses averaged 4 2 SiO2 39.81 39.91 39.91 39.82 55.20 55.28 39.86 55.24 TiO2 0.05 0.06 0.09 0.06 0.14 0.11 0.07 0.13 Al2O3 23.28 23.16 23.14 23.06 11.15 11.24 23.16 11.20 Cr2O3