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Effects of acid leaching on the Sr-Nd-Hf isotopic compositions of ocean island basalts. Nobre Silva, Ines G.; Weis, Dominique; Scoates, James S. 2010

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Article Volume 11, Number 9 25 September 2010 Q09011, doi:10.1029/2010GC003176 ISSN: 15252027 Effects of acid leaching on the SrNdHf isotopic compositions of ocean island basalts Inês G. Nobre Silva, Dominique Weis, and James S. Scoates Pacific Centre for Isotopic and Geochemical Research, Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4, Canada (inobre@eos.ubc.ca) [1] The ability to conduct multiisotopic analyses (e.g., SrNdHfPb) on the same sample is critical for studies that evaluate the mantle source components of oceanic basalts. The isotopic compositions of rela- tively immobile elements, such as Nd (and other REE) and Hf, are considered to be relatively resistant to alteration, however, accurate Sr and Pb isotopic analyses of oceanic basalts require thorough acid leaching prior to dissolution. A detailed study of the Sr, Nd and Hf isotopic systematics of acidleached oceanic basalts from Hawaii and Kerguelen was undertaken to assess how acid leaching affects their isotopic com- positions. Most of the Sr, Nd and Hf was removed in the first acid leaching steps. Hawaiian basalts lose up to 35% and 40% of their total Sr and Hf contents, respectively, whereas for Kerguelen basalts the corresponding losses are 63% and ∼70%. Acid leaching leads to significant loss of the original Nd content (up to 90%), which cannot be solely explained by the elimination of alteration phases and is likely related to preferential removal of the REE in the constituent silicate minerals (e.g., plagioclase, clinopyroxene). The leached residues yield Sr isotopic ratios significantly less radiogenic than their respective unleached powders and NdHf isotopic compositions that are within analytical uncertainty of the respective unleached powders. This study shows that multiisotopic analyses on the same acidleached sample aliquot can pro- duce reliable results for use in the discrimination of mantle source components of oceanic basalts. Components: 10,600 words, 7 figures, 5 tables. Keywords: ocean island basalts; acid leaching; SrNdHf isotopes; reproducibility; MCICPMS; TIMS. Index Terms: 1040 Geochemistry: Radiogenic isotope geochemistry; 1094 Geochemistry: Instruments and techniques. Received 14 April 2010; Revised 10 June 2010; Accepted 18 June 2010; Published 25 September 2010. Nobre Silva, I. G., D. Weis, and J. S. Scoates (2010), Effects of acid leaching on the SrNdHf isotopic compositions of ocean island basalts, Geochem. Geophys. Geosyst., 11, Q09011, doi:10.1029/2010GC003176. 1. Introduction [2] The combination of analyses of different iso- topic systems (e.g., SrNdHfPb) on samples of oceanic basalts constitutes a powerful tool for the determination and characterization of their mantle sources and components [e.g., Gast et al., 1964; Tatsumoto, 1966, 1978; White and Hofmann, 1982; Dupré and Allègre, 1983; Hart, 1984; White, 1985; Hamelin et al., 1986; Zindler and Hart, 1986; Weaver, 1991; Hofmann, 1997, 2003; Stracke et al., 2005]. Inherent to their emplacement in an oceanic environment, oceanic basalts are susceptible to seawater alteration, and the effects of this alter- ation on RbSr and UPb isotope systematics have long been recognized [e.g., Hart et al., 1974; Copyright 2010 by the American Geophysical Union 1 of 20Hawkesworth and Morrison, 1978; Cohen and O’Nions, 1982; Verma, 1992; Staudigel et al., 1995; KrolikowskaCiaglo et al., 2005]. Another well known cause of disturbance of the isotopic compositions of basalts, especially for Pb isotopes, is contamination during crushing and grinding [e.g., Woodhead and Hergt, 2000; McDonough and Chauvel, 1991; Weis et al., 2005]. To overcome the chemical disturbance caused by alteration, plus possible contamination during sample handling, careful acid leaching of samples prior to dissolution and analysis has proven to be an effective and essential step in sample processing for mantle geo- chemistry studies [e.g., Manhès et al., 1978; Dupré and Allègre, 1980; McDonough and Chauvel, 1991; Eisele et al., 2003; Stracke et al., 2003; Weis et al., 2005; Thompson et al., 2008; Hanano et al., 2009; Nobre Silva et al., 2009]. [3] Given the geochemical nature of Nd and Hf (rare earth element (REE) and high field strength element (HFSE), respectively), they are considered to be relatively immobile elements in aqueous solutions and their isotopic compositions to be rel- atively resistant to alteration [e.g., Cohen and O’Nions, 1982; Verma, 1992; Staudigel et al., 1995; Lassiter et al., 1996; Chauvel and Blichert Toft, 2001; Mattielli et al., 2002; Krolikowska Ciaglo et al., 2005]. Also, in contrast to Sr, which is relatively abundant in seawater, the concentra- tions of Nd and Hf in seawater are very low [e.g., Faure, 1986]. Hence, an acidleaching treatment prior to chemical separation for Nd and Hf isotopic analysis is not considered to be a requirement. Several studies, however, do show evidence for some mobility of Nd (and other REE) during hydrother- mal alteration [e.g., Ludden and Thompson, 1979; Cotten et al., 1995; Staudigel et al., 1995; Bau et al., 1996; Smith et al., 2000; Kempton et al., 2002]. Furthermore, the 143Nd/144Nd and 176Hf/177Hf ratios of basalts may be affected to some extent by sea- water alteration under some specific conditions [e.g., Kempton et al., 2002; Thompson et al., 2008]. [4] Depending on the sample collection method in studies of oceanic basalts (i.e., by hammering an outcrop, dredging along the slope of a seamount, ridge, or ocean floor, or by drilling), the sample sizes available for geochemical analysis can range from kilograms to less than a few grams. To properly characterize the sample and avoid any sample het- erogeneity effect, it is important that multiisotopic analyses are performed on the same sample aliquot (same sample powders, chips, and/or glasses). This means that sequenced chemical separations should be performed on a single dissolution of each leached sample. A wide variety of leaching protocols are used by different laboratories [e.g., Mahoney, 1987; McDonough and Chauvel, 1991; Weis and Frey, 1991; Stracke and Hegner, 1998; Abouchami et al., 2000; Thirlwall, 2000; Eisele et al., 2003; Stracke et al., 2003; Baker et al., 2004; Weis et al., 2005], but not all leaching techniques seem to provide results with the desired reproducibility for high precision isotopic studies and interlaboratory com- parison [Abouchami et al., 2000; Eisele et al., 2003; Stracke et al., 2003; Baker et al., 2004, 2005; Albarède et al., 2005]. The recent improvements in precision and reproducibility offered by the current generations of multiple collector mass spectrometers (TIMS and MCICPMS) have enabled researchers to discriminate different isotopic trends within data sets that previously were within analytical uncer- tainty, especially for Pb and Hf. The question then arises, to what extent, if any, are the Nd and Hf isotopic compositions of OIB affected by acid leaching? [5] To complement an earlier study on the effect of acid leaching and matrix elimination on Pb isotopes of samples from ocean island basalts [Nobre Silva et al., 2009], we carried out a comprehensive study of the Sr, Nd and Hf isotopic systematics of two Hawaiian and two Kerguelen oceanic basalts (0–1.7 wt% LOI) that were subjected to multistep acid leaching (up to 14 steps). Here we report the Sr, Nd, and Hf elemental contents and isotope compositions for the unleached and respective leached powders (residues), as well as for the acid solutions (leachates) of each leaching step and the bulk leachates (all solutions combined). A full suite of trace element concentrations on both unleached and leached powders was also measured for one of the basalts. These data are used to assess the effects of our leaching procedure on basaltic rock compositions. 2. Samples [6] For this study, we selected two Hawaiian and two Kerguelen basalts among the 14 analyzed by Nobre Silva et al. [2009]. We focused on these samples as they are representative of basalts typi- cally analyzed for radiogenic isotopic compositions from these two islands and span a wide range of MgO (3.5–18.0 wt%) with relatively weak alter- ation (e.g., 0.85–1.65 wt% LOI). The samples are: J202023 from the Mile High Section of Mauna Loa volcano, SR0954–8.00 from the Mauna Kea volcano collected from the Hawaii Scientific Dril- ling Project (HSDP), OB93–165 from Mont Crozier Geochemistry Geophysics Geosystems G3 NOBRE SILVA ET AL.: SrNdHf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 2 of 20Tabl e 1. Summar y o fSampl e Geochemica lCharacteristics ,Hawai ian d Kerguele n Sampl e Eruptio n Environmen t Dept ha (mbsl/masl ) Roc k Typ e Si O 2 (wt% ) Mg O (wt% ) N a 2 O (wt% ) K 2O (wt% ) Pb (ppm ) Sr (ppm ) N d (ppm ) H f (ppm ) A.I .b LO Ic Ag e (Ma )d Reference s Hawaiia n Basalt s Maun a Lo a (Southwes tRif tZone ) J2  02 0 23 submarin e 48 9 tholeiiti c basal t 50.1 4 6.5 0 2.7 2 0.5 2 2.0 0 30 6 32.0 8 6.6 0 − 0.8 7 0.9 9  J. M .Rhode s (unpublishe d data ,2003 ) Maun a Ke a (HSD P 2e ) SR095 4– 8.0 0 submarin e 3009. 2 tholeiiti c basal t 47.6 7 18.0 3 1.6 5 0.2 3 0.6 6 23 9 15.4 3 2.9 5 − 1. 3 0.8 5 0.5 5 Rhode s an d Vollinge r [2004 ] Kerguele n Basalt s Mon tCrozie r OB9 3– 16 5 subaeria l 51 5 alkali c basal t 50.9 1 3.5 4 3.5 2 1.4 5 6.3 4 41 7 36.3 0 5.9 9 0.6 7  24. 5 D .Wei s (unpublishe d data ,2005 ) Norther n Kerguele n Platea u OD P Le g 183 ,1140 A  31 R 1, 57 – 61 submarin e 270.0 7 tholeiiti c basal t 49.6 4 5.5 4 2.6 4 0.4 9 1.5 2 26 2 20. 4 3.7 5 − 0.8 1 1.6 5 34. 3 Wei s an d Fre y [2002 ] a mbs l= meter s belo w se a level ,fo rsubmarin e basalts ;mas l= meter s abov e se a level ,fo rsubaeria lbasalts . b A.I .= alkalinit y inde x (A I= tota lalkali s − (Si O 2 × 0.3 7 − 14.43) )[ Rhode s, 1996] . c LO I= weigh tlos s o n  ignitio n afte r3 0 mi n at 1020°C . d Age sfro m Maun a Ke a samples :Shar p an d Renn e [2005] ;Mon tCrozie ran d Mon tBureau :Nicolayse n et al .[2000] ;Mon tde sRuche san d Mon tFontaine :Douce te tal .[2002] ;Sit e1140 :Dunca n [2002] . e HSD P 2 = Hawai iScientifi c Drillin g Project ,phas e 2. Geochemistry Geophysics Geosystems G3 NOBRE SILVA ET AL.: SrNdHf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 3 of 20Tabl e 2. Sr ,Nd ,an d H fElementa lContent s an d Isotopi c Composition s o fUnleache d Whol e Roc k Powders ,Leachat e Solutions ,an d Leache d Residue s fo rHawaiia n Basalt s Sampl e Sr N d H f Tota ln g in Eac h Fractio n % Sr in Eac h Fractio n 87 Sr /86 Sr 2 SE a Tota ln g in Eac h Fractio n % N d in Eac h Fractio n 14 3 Nd /14 4 N d 2 SE a Tota ln g in Eac h Fractio n % H fi n Eac h Fractio n 17 6 Hf /17 7 H f 2 SE a Sampl e J2  02 0 23 (Maun aLoa ) Unleache db 8546 6 10 0 0.70381 8 9 896 0 10 0 0.51294 3 7 184 3 10 0 0.28318 9 33 B.L.Homog .c 2976 8 34. 8 0.70393 0 10 475 4 53. 1 0.51293 4 9 64 7 35. 1   B.L.Aci dd 1841 5 21. 5 0.70420 3 9 439 5 49. 0 0.51293 1 8 50 3 27. 3 0.28310 9 10 1s t 1826 6 21. 4 0.70431 0 9 449 0 50. 1 0.51294 3 8 54 0 29. 3   2n d 303 3 3.5 5 0.70378 2 8 25 9 2.8 9 0.51293 8 7 61. 9 3.3 6   3r d 323 3 3.7 8 0.70367 1 9 17 6 1.9 7 0.51294 8 12 44. 5 2.4 1   4t h 201 2 2.3 5 12 1 1.3 5 21. 8 1.1 8   5t h 173 8 2.0 3 0.70368 7 8 10 1 1.1 3 0.51295 6 18 20. 4 1.1 1   6t h 66 9 0.78 3 0.70370 4 8 52. 8 0.58 9 0.51293 9 14 7.3 5 0.4 0   1s tH 2O  e            2n d H 2O 34 3 0.40 1   18. 8 0.20 9   0.45 0 0.02 4   Tota lf 2929 4 34. 3 521 8 58. 2 69 6 37. 8 Leach .Res .g 5617 2 65. 7 0.70368 3 10 374 2 41. 8 0.51294 8 7 114 7 62. 2 0.28308 4 6 Sampl e SR095 4– 8.0 0 (Maun aKea ) Unleache d 9428 6 10 0 0.70372 8 8 608 5 10 0 0.51300 9 8 116 4 10 0 0.28315 8 12 B.L.Homog . 1971 8 20. 9 0.70455 6 8 694 4 11 4 0.51299 7 9 407. 7 35. 0   B.L.Aci d 1957 9 20. 8 0.70463 4 9 702 0 11 5 0.51261 4 10 406. 5 34. 9   1s t 1559 7 16. 5 0.70451 1 9 490 2 80. 6 0.51299 5 7 261. 7 22. 5   2n d 195 1 2.0 7 0.70383 1 8 22 9 3.7 7 55.0 0 4.7 3   3r d 154 8 1.6 4 0.70366 1 17 76. 3 1.2 5 0.51302 7 42 39.9 1 3.4 3   4t h 117 5 1.2 5   46. 2 0.75 9 25.8 0 2.2 2   5t h 197 8 2.1 0 0.70357 0 7 48. 0 0.78 9 0.51287 2 11 2 28.1 0 2.4 1   6t h 113 0 1.2 0   32. 6 0.53 6   18.9 0 1.6 2   7t h 63 5 0.67 3 0.70359 2 9 16. 2 0.26 6   11.3 5 0.97 5   8t h 60 1 0.63 7   15. 7 0.25 8   10.7 5 0.92 4   1s tH 2O 24 5 0.26 0   6.8 0 0.11 2   0.80 0 0.06 9   2n d H 2O 22 2 0.23 6   7.8 0 0.12 8       Tota l 2508 3 26. 6 538 1 88. 4 45 2 38. 9 Leach .Res . 6920 3 73 0.70353 5 9 704. 4 11. 6 0.51300 0 3 71 1 61 0.28312 0 5 a 2 SE ,indicate s th e uncertaint y in th e las tdigit s o fth e measure d isotopi c ratios . b Unleached ,unleache d roc k powders . c B.L.Homog. ,bul k leachat e solutio n homogenize d (aci d+ fin e powde rparticles) . d B.L.Acid ,bul k leachat e solutio n withou tth e fin e powde rparticles . e Dashe s indicat e n o isotopi c analysi s performed . f Tota l= su m o fth e elementa lcontent s o fal lindividua lleachin g steps .Th e Sr ,Nd ,an d H fcontent s o fth e bul k leachat e solution sdoe sno tequa lth e su m o fth e individua lleachin g step sbecaus e the y wer e m ea su re d o n di ffe re n t al iq u o ts o f th e sa m e sa m pl e po w de rs . N ot e: th e el em en ta lc o n te n ts (ng )o f th e u n le ac he d po w de rs ar e ca lc u la te d fro m th e in iti al am o u n ts o f po w de r an d th e re sp ec tiv e sa m pl e concentration s (se eTabl e 1). Th e content s o fth e bul k an d individua lleachat e solution s ar e thos e measure d by H R  IC P M S o n a 10 % aliquo to feac h solution .Th e elementa lcontent s (ng )o fth e leache d residue s ar e calculate d by mas s balanc e betwee n th e tota ln g in th e startin g unleache d powder s an d th e su m o fth e elementa lcontent s in al lth e individ ua lleachat e solutions . g Leach .Res. ,leache d powde rresidues . Geochemistry Geophysics Geosystems G3 NOBRE SILVA ET AL.: SrNdHf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 4 of 20Tabl e 3. Sr ,Nd ,an d H fElementa lContent s an d Isotopi c Composition s o fUnleache d Whol e Roc k Powders ,Leachat e Solutions ,an d Leache d Residue s fo rKerguele n Basalt s Sampl e Sr N d H f Tota ln g in Eac h Fractio n % Sr in Eac h Fractio n 87 Sr /86 Sr 2 SE a Tota ln g in Eac h Fractio n % N d in Eac h Fractio n 14 3 Nd /14 4 N d 2 SE a Tota ln g in Eac h Fractio n % H fi n Eac h Fractio n 17 6 Hf /17 7 H f 2 SE a Sampl e OB9 3– 16 5 (Mon tCrozier ) Unleache db 12022 1 10 0 0.70534 0 8 1046 5 10 0 0.51262 1 6 172 7 10 0 0.28285 0 6 B.L.Homog .c 3701 6 30. 8 0.70543 0 10 908 8 86. 8 0.51261 7 7 48 8 28. 3 0.28284 9 9 B.L.Aci dd 3448 9 28. 7 0.70544 0 9 919 6 87. 9   33 4 19. 4   1s t 3977 4 33. 1 0.70541 4 7 877 4 83. 8 0.51262 6 8 67 7 39. 2 0.28276 1 15 2n d 1381 3 11. 5 0.70530 3 8 35 7 3.4 1 0.51263 2 10 16 8 9.7 2 0.28283 5 8 3r d 380 8 3.1 7 0.70529 7 9 83. 5 0.79 8 0.51263 3 10 49. 3 2.8 5   4t h 774 0 6.4 4  e  15 3 1.4 6   88. 5 5.1 2   5t h 328 9 2.7 4 0.70529 1 8 75. 7 0.72 3 0.51206 2 11 36. 7 2.1 3   6t h 360 7 3.0 0   79. 7 0.76 2   36. 1 2.0 9   7t h 129 7 1.0 8 0.70528 4 9 27. 8 0.26 5 0.51263 2 40 13. 8 0.79 6   8t h 166 1 1.3 8   35. 3 0.33 7   18. 2 1.0 5   1s tH 2O 31 3 0.26 1   9.6 0 0.09 2   1.3 5 0.07 8   2n d H 2O 19 4 0.16 1   4.1 0 0.03 9       Tota lf 7549 5 62. 8 959 9 91. 7 108 9 63. 1 Leach .Res .g 4472 6 37. 2 0.70529 8 10 86 6 8.2 8 0.51262 5 6 63 8 36. 9 0.28284 9 6 OD P Le g 183 ,1140 A 31 R 1, 57 – 61 (Norther nKerguele n Plateau ) Unleache d 10343 8 10 0 0.70442 2 8 803 7 10 0 0.51280 8 7 148 1 10 0 0.28302 0 5 B.L.Homog . 5476 2 52. 9 0.70454 5 8 628 9 78. 3 0.51280 9 7 88 5 59. 8 0.28302 3 5 B.L.Aci d 4640 4 44. 9 0.70466 8 8 614 3 76. 4 0.51280 6 7 77 9 52. 6 0.28296 2 10 1s t 3278 2 31. 7 0.70472 0 6 574 5 71. 5 0.51281 8 8 67 2 45. 4 0.28290 8 18 2n d 550 8 5.3 2 0.70461 5 8 30 7 3. 8 0.51221 3 19 12 7 8.5 5 0.28302 3 6 3r d 256 7 2.4 8 0.70462 9 9 87. 7 1. 1   37. 9 2.5 6   4t h 171 8 1.6 6   49. 5 0. 6   21. 6 1.4 6   5t h 198 1 1.9 2 0.70454 8 9 58. 6 0. 7   20. 7 1.4 0   6t h 265 2 2.5 6   73. 4 0. 9   23. 7 1.6 0   7t h 348 3 3.3 7 0.70430 0 8 11 3 1. 4   32. 5 2.1 9   8t h 226 3 2.1 9   76. 6 1. 0   16. 4 1.1 1   9t h 277 3 2.6 8 0.70422 6 8 10 9 1. 4 0.51280 6 20 24. 2 1.6 3   10t h 101 4 0.9 8   35. 0 0. 4   2.2 0 0.14 8   11 th 139 9 1.3 5 0.70422 7 8 53. 2 0. 7 0.51283 3 22 7.1 9 0.48 6   12 th 145 5 1.4 1 0.70422 8 10 54. 4 0. 7 0.51271 2 34 13. 3 0.89 8   13 th 168 8 1.6 3   64. 8 0. 8   16. 0 1.0 8   14t h 85 0 0.82 1   25. 8 0. 3   4.6 5 0.31 4   1s tH 2O 37 3 0.36 0   16. 7 0. 2   0.1 5 0.01 0   2n d H 2O 15 4 0.14 9   6.8 5 0. 1       Tota l 6266 0 60. 6 687 6 85. 6 101 9 68. 8 Leach .Res . 4077 8 39. 4 0.70422 2 23 116 1 14. 4 0.51283 9 6 46 2 31. 2 0.28302 8 5 Geochemistry Geophysics Geosystems G3 NOBRE SILVA ET AL.: SrNdHf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 5 of 20on the Courbet Peninsula of the Kerguelen Archi- pelago, and 1140A31R1, 57–61 cm recovered during ODP Leg 183 on the Northern Kerguelen Plateau (see Table 1 for additional sample charac- terization). To assess the effects of our multistep acid leaching procedure on the elemental con- centrations and Sr, Nd, and Hf isotopic composi- tions of these four samples, all column separations and analyses were performed on the same chemical digestions of all sample fractions (i.e., unleached and leached sample powders, leachate solutions collected at each leaching step, and respective accumulate bulk leachates) as the Pb isotopic anal- yses published by Nobre Silva et al. [2009]. 3. Analytical Techniques 3.1. Sample Preparation [7] All chemical digestion and separation were carried out in Class 1000 clean laboratories and the mass spectrometric analyses were performed in Class 10,000 laboratories at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia. Sample handling in all labs was carried out in Class 100 laminar flow hoods. All reagents used for leaching, disso- lution and separation were subboiled, all dilutions were made using ≥ 18.2 MW · cm deionized water, and all labware was acidwashed prior to use. The acid leaching procedure used follows that of Weis et al. [2005] and is detailed by Nobre Silva et al. [2009]. Briefly, the sample powders were acid leached using 10 mL of 6 M HCl in screwtop Savillex® beakers, in a warm (∼50°C) ultrasonic bath for 20 min. Immediately after, the supernatant (leachate solution) was decanted without centrifu- gation, to ensure the removal of the silt to finesize particle fraction, which is mostly dominated by secondary alteration phases. The whole process was repeated (6 to 14 times for the samples in this study) until the leachate was clear and free of fine size particles. For all sample specimens, Sr, Nd and Hf elemental concentrations and isotopic compo- sitions were determined on a single dissolution, using methods similar to those described by Weis et al. [2005, 2006, 2007]; the total Sr, Nd and Hf procedural blanks were of the order of 200, 60, and 30 pg, respectively. The results are reported in Tables 2 and 3. In Table 4, a summary of the percentages of powder and elemental losses is presented. 3.2. Mass Spectrometry 3.2.1. Sr, Nd, and Hf Abundances [8] To determine the amount of Sr, Nd, and Hf present at each step of the acidleaching procedure, and consequently how much is lost, the respective Notes to Table 3: a2 SE indicates the uncertainty in the last digits of the measured isotopic ratios. bUnleached, unleached rock powders. cB.L.Homog., bulk leachate solution homogenized (acid + fine powder particles). dB.L.Acid, bulk leachate solution without the fine powder particles. eDashes indicate no isotopic analysis performed. fTotal = sum of the elemental contents of all individual leaching steps. The Sr, Nd, and Hf contents of the bulk leachate solutions does not equal the sum of the individual leaching steps because they were measured on different aliquots of the same sample powders. Note: the elemental contents (ng) of the unleached powders are calculated from the initial amounts of powder and the respective sample concentrations (see Table 1). The contents of the bulk and individual leachate solutions are those measured by HRICPMS on a 10% aliquot of each solution. The elemental contents (ng) of the leached residues are calculated by mass balance between the total ng in the starting unleached powders and the sum of the elemental contents in all the individual leachate solutions. gLeach. Res., leached powder residues. Table 4. Summary of the Weight and Relevant Element % Losses During the Multistep Acid Leaching Procedure Each Leaching Step Sample Initial Weight (g) # Acid Leaching Steps Final Weight (g) Weight Loss (%) Pb Loss (%) Sr Loss (%) Nd Loss (%) Hf Loss (%) Mauna Loa J202020 0.2793 6 0.1828 34.6 68.7 34.3 58.2 37.8 Mauna Kea SR0954–8.00 0.3945 8 0.259 34.3 70.8 26.6 88.4 38.9 Mont Crozier OB93–165 0.2883 8 0.106 63.2 37.3 62.8 91.7 63.1 Northern Kerguelen Plateau ODP Leg 183, 1140A31R1, 57–61 0.3948 14 0.1598 59.5 76.1 60.6 85.6 68.8 Geochemistry Geophysics Geosystems G3 NOBRE SILVA ET AL.: SrNdHf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 6 of 20concentrations were measured on a 10% aliquot of the unleached sample powders, individual leachates and bulk leachate solutions. The analyses were performed on an ELEMENT2 highresolution (HR)ICPMS (Thermo Finnigan, Germany) and were quantified using external calibration curves and indium (In) as an internal standard. Multiele- ment (Sr, Nd, and Hf) standard solutions were prepared from 1000 ppm Specpure® (Alfa Aesar®, Johnson Matthey Company, USA) single element standard solutions. Samples and standards were diluted accordingly ([Sr] and [Hf] = 1–500 ppb; [Nd] 0.1–25 ppb) and run in 0.15M HNO3. All analyses were normalized to the internal standard and blank subtracted. 3.2.2. Sr, Nd, and Hf Isotopic Compositions [9] All Sr and Nd isotopic ratios were measured on a TRITON (Thermo Finnigan) thermal ionization mass spectrometer (TIMS) in static mode with relay matrix rotation on single Ta (Sr) or double ReTa (Nd) filaments. Sr and Nd isotopic compo- sitions were corrected for mass fractionation with the exponential law using 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. The data were then normalized using the average of the corresponding standard (NIST SRM 987 for Sr and La Jolla for Nd) ran in the barrel, relative to the values of NIST SRM 987 87Sr/86Sr = 0.710248 and La Jolla 143Nd/144Nd = 0.511858 [Weis et al., 2006]. During the course of the analyses presented in this study, the average value of the SRM 987 Sr standard was 0.710244 ± 0.000027 (n = 11) and the La Jolla Nd standard was 0.511854 ± 0.000014 (n = 9). [10] Hf isotopic compositions were determined on a Nu Plasma (Nu021; Nu Instruments Ltd, UK) multiple collector inductively coupled plasma mass spectrometer (MCICPMS), under dry plasma con- ditions using a membrane desolvator (Nu DSN100) for sample introduction, following the analytical procedures detailed by Weis et al. [2007]. All anal- yses were obtained in static multicollection mode, with interference corrections for 176Lu and 176Yb on 176Hf, and 174Yb on 174Hf, respectively. All sample Figure 1. Variation of Pb, Sr, Nd, and Hf concentrations throughout the leaching procedure for basalts from (a) Mauna Loa, (b) Mauna Kea, (c) Mont Crozier, and (d) the Northern Kerguelen Plateau. Elemental concentrations (ppb) refer to the abundances in each leachate solution measured by HRICPMS. The Sr abundances are divided by 100 to allow for plotting on the same graph as the other elements. Geochemistry Geophysics Geosystems G3 NOBRE SILVA ET AL.: SrNdHf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 7 of 20and standard solutions were prepared fresh for each analytical session and sample analysis followed a modified samplestandard bracketing protocol in which the JMC 475 standard solution was run after every two samples to monitor for any systematic daily drift of the standard value. Hf isotopic ratios were corrected for instrumental mass fractionation by exponentially normalizing to 179Hf/177Hf = 0.7325. The sample results were then normalized to the 176Hf/177Hf value of 0.282160 [Vervoort andBlichert Toft, 1999] using the daily average of the JMC475Hf standard. The JMC 475 Hf standard analyzed over the period of the analyses gave an average value of 176Hf/177Hf = 0.282181 ± 0.000008 (n = 32). 4. Results [11] Throughout the multistep acidleaching pro- cedure, the Hawaiian basalts lose ∼35% of their initial weight (up to 8 acid steps; Tables 2 and 4) and Kerguelen basalts lose up to 60% (up to 14 acid steps; Tables 3 and 4). For all samples, the largest amount of Sr, Nd and Hf is leached out in the first 1–2 acid leaching steps (Figure 1), as is the Pb. Overall, leaching removes up to 35% of the total Sr content and 40% of the Hf content for Hawaiian basalts, and up to 63% and ∼70% of the Sr and Hf contents, respectively, for the Kerguelen basalts (Tables 2–4). With the exception of one Hawaiian basalt, all samples lose ∼90% of their initial Nd content during leaching. Similar pro- portions of Sr and Nd losses after acid leaching have been reported for basalts from the first drilling phase of the HSDP [Hauri et al., 1996]. [12] The Hawaiian and Kerguelen basalts present a similar pattern of Sr isotopic variation throughout the leaching process (Figure 2). The magnitudes of the isotopic variations at each leaching step are Figure 2. 87Sr/86Sr variations throughout the acid leaching procedure for basalts from (a) Mauna Loa, (b) Mauna Kea, (c) Mont Crozier, and (d) the Northern Kerguelen Plateau. The numbers refer to the respective acid leaching step (see Tables 1 and 3). The size of the spheres is proportional to the amount of Sr in each sample fraction. The analytical uncertainty of each analysis is smaller than the symbol sizes. In the top left corner of the plots, the average 2SE of the Sr isotopic analysis of each experiment is shown. Geochemistry Geophysics Geosystems G3 NOBRE SILVA ET AL.: SrNdHf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 8 of 20Figure 3. 143Nd/144Nd variations throughout the acid leaching procedure for basalts from (a) Mauna Loa, (b) Mauna Kea, (c) Mont Crozier, and (d) Northern Kerguelen Plateau. Symbol colors and sizes are as described in the caption of Figure 2. The analytical uncertainty is smaller than the symbol sizes, except where represented. The average 2SE of the Nd isotopic analysis of each experiment is shown in the upper corners of the plots. Figure 4. 176Hf/177Hf variations throughout the acid leaching procedure for basalts from (a) Mont Crozier and (b) the Northern Kerguelen Plateau basalts. Symbol colors and sizes are as described in the caption of Figure 2. The average 2SE of the Hf isotopic analysis of each experiment is shown in the upper right corner of the plots. Note the nonoverlapping range of Hf isotopic ratios between the two plots. Geochemistry Geophysics Geosystems G3 NOBRE SILVA ET AL.: SrNdHf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 9 of 20different between the four samples as they are dependent on the amount of fine particles (mostly alteration phases and contaminants) that are removed when decanting the individual leachate solutions. The leached powder residues of all four samples yield less radiogenic Sr isotopic compositions than the respective unleached powders. The Sr isotopic compositions of the individual leachate solutions and unleached powders, of both Hawaiian and Kerguelen basalts, generally plot along a mixing trend between a more radiogenic endmember (represented by the bulk leachate solutions and 1st leachate solution) and the leached sample residue (Figure 2). [13] For both Hawaiian and Kerguelen basalts, no significant difference (i.e., outside individual ana- lytical error) is observed between the Nd isotopic compositions of the unleached rock powders, the bulk leachate solutions, the individual leachate solutions, and the leached residues (Figure 3). For each individual sample, the results for nearly all sample fractions plot within error of each other. The few exceptions with lower 143Nd/144Nd repre- sent a very minor relative proportion of the whole sample Nd content (e.g., leachate #2 for sample 1140A31R1, 57–61; leachate #5 for sample OB93–165; and a bulk leachate solution for sample SR0954–8.00). After acid leaching, the leached powder residues of both Hawaiian and Kerguelen basalts yield similar Nd isotopic com- positions to those of their respective unleached sample powders. [14] Statistically acceptable Hf runs (i.e., >1.5 V on the 177Hf beam) could only be obtained for the Kerguelen samples. This was because during the course of analysis of the leachate solutions, we encountered tungsten (W) interference problems on some of the Hawaiian samples processed in a tungsten carbide (WC) crusher and mill. In addi- tion, the Hf contents of some fractions were not sufficient to provide good runs, hence no data is reported. For the two Kerguelen samples, the Hf isotope compositions of the unleached sample powders, bulk leachate solutions, and leached sam- ple residues are within error of each other (Figure 4). In both cases, the first leachate solution yields lower 176Hf/177Hf values. After acid leaching, the leached powder residues of both Hawaiian and Kerguelen basalts yield identical Hf isotopic com- positions to those of their respective unleached sample powders (Tables 2 and 3). 5. Discussion 5.1. Effects of Acid Leaching of OIB in SrPb Isotopic Space [15] The bulk leachate solutions consist of acid, a combination of secondary mineral phases, and any contaminant removed from the sample by the acid leaching process. The isotopic compositions of the bulk leachate solutions are thus a proxy for the total alteration and contaminant removed with the acid leaching procedure, whereas those of the leached residues can be considered to represent the original isotopic composition of the samples. In multi isotopic space, the bulk leachate solutions and leached residues are hence expected to lie along mixing trends that encompass the isotopic compo- sitions of the initial unleached rock powders and that provide insight about the nature of the contaminant. In Figures 5 and 6, we combine the Sr isotopic results of our leaching experiments with their respective Pb isotopic compositions from Nobre Silva et al. [2009], and discuss the Figure 5. 87Sr/86Sr versus 206Pb/204Pb diagrams showing the relationships between the unleached sample powders, leached residues, and leachate solutions for the two Hawaiian basalts compared to the isotopic compositions of pos- sible external contaminants. Pb isotopic compositions and concentrations are from Nobre Silva et al. [2009]. In both diagrams, the size of the symbols is proportional to the Sr (lighter color) and Pb (darker color) contents in each sample fraction. The analytical uncertainty of each analysis is represented by the 2SE, which is much smaller than the symbol sizes. (a) The results for the Mauna Loa sample (J202023) and several possible contaminants: seawater (elemental abundances (EA): Faure [1986]; isotopic compositions (IC): Abouchami and Galer [1998] and Eisele et al. [2003]); authigenic carbonates (EA: Faure [1986] and Chester [2003]; IC: same as seawater); FeMn oxides (EA and IC: Ling et al. [1997]); deepsea interbedded claybearing carbonate sediments such as those now subducting at the IzuBonin trench (EA and IC: Hauff et al. [2003] and Plank et al. [2007]); Chinese loess (EA: Nakai et al. [1993] and Gallet et al. [1996]; IC: Kennedy et al. [1998] and Jones et al. [2000]). The thick red line represents the mixing trend between the leached sample residue and the bulk leachate solutions (circles indicate 10% increments). (b) The results for the Mauna Kea sample (SR0954–8.00) and three external contaminants: seawater, carbonates, HSDP2 bulk mud [Abouchami et al., 2000], and a mixture of claybearing carbonates plus drilling mud. The mixing trend between each of these contaminants and the leached sample residue are shown. All unlabelled tick marks on the mixing curves indicate 10% increments. Geochemistry Geophysics Geosystems G3 NOBRE SILVA ET AL.: SrNdHf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 10 of 20Figure 5 Geochemistry Geophysics Geosystems G3 NOBRE SILVA ET AL.: SrNdHf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 11 of 20plausibility of several natural contaminants to explain the mixing trends defined by the bulk lea- chates and the leached sample residues. [16] The differences between the Sr and Pb isotopic compositions of the unleached and leached rock powders of the submarine tholeiites from Mauna Loa (sample J202023) and Mauna Kea (sample SR0954–8.00) reflect exchange with a more radio- genic contaminant (Figures 2a, 2b, and 5). Given their high Sr contents and radiogenic Sr isotopic compositions, possible natural contaminants to consider for these two Hawaiian basalts are sea- water, local deepsea sediments and/or FeMn crusts, or a mixture of each. A distal contaminant that must also be considered, and that has been identified in soil horizons of Hawaiian lavas, is windblown Chinese loess into the central north Pacific [e.g., Kennedy et al., 1998; Jones et al., 2000; Abouchami et al., 2000]. [17] In PbSr isotopic space, the leaching experi- ments for the Mauna Loa basalt (sample J202023) produce a shallow trend defined by the leached sample residue, unleached sample powder, bulk lea- chates, and individual leachate solutions (Figure 5a). This trend reflects a larger difference in the Pb isotopic values of the leached fractions relative to their Sr isotopic compositions. Mixing between the fresh basaltic rock sample and a contaminant with a high Pb content is thus needed to explain this “leaching” trend. Given their very low Pb contents, Pacific Ocean seawater ([Sr] = ∼ 8 ppm [after Faure, 1986]; 87Sr/86Sr = 0.70918 [Thomas et al., 1996]; [Pb]1000–2000 m = ∼5 × 10−5 ppm [after Chester, 2003]) and authigenic carbonate deposits can be ruled out as viable contaminants (see mixing curves in Figure 5a). FeMn deposits, which are efficient scavengers of trace metals, have very elevated Pb contents (e.g., Central Pacific FeMn deposits: 997–1054 ppm [after Ling et al., 1997]). Even a contribution of 1% of FeMn oxides would be too high to explain the observed trend between the sample and leachate fractions (Figure 5a). Chinese eolian loess has elemental and isotopic compositions ([Sr] = ∼200 ppm [after Nakai et al., 1993]; 87Sr/86Sr = 0.717 [after Kennedy et al., 1998]; [Pb] = ∼20 ppm [after Gallet et al., 1996]; 206Pb/204Pb = 18.775 [after Jones et al., 2000]) suitable to explain the trend produced by leaching. However, incorporation of as much as 15% of loess into the fresh lava would be needed (Figure 5a). This is a very large amount of sediment to be incorporated into a basalt erupted in a submarine environment and thus unlikely to be the sole explanation for the observed trend. A preferred explanation is incorpo- ration of a composite mixture of deepsea inter- bedded claybearing carbonate sediments, similar in composition to those now subducting at the IzuBonin trench [Hauff et al., 2003; Plank et al., 2007], and some minor FeMn oxides, deposited on the surface of the basalt (Figure 5a). [18] For the HSDP2 Mauna Kea submarine tho- leiite (sample SR0954–8.00), the isotopic trend produced by the acid leaching reflects interaction of the basalt with a highly radiogenic contaminant, different than that discussed above for the subma- rine Mauna Loa basalt (Figures 2b and 5b). Several previous studies have suggested possible contami- nation of the HSDP samples by drilling mud [e.g., Hauri et al., 1996; Abouchami et al., 2000; Eisele et al., 2003; Nobre Silva et al., 2009]. Hanano et al. [2009] also identified distinct patches of barite/ celestite within a thin section of this sample. This is unexpected given the careful sample washing Figure 6. 87Sr/86Sr versus 206Pb/204Pb diagrams showing the isotopic compositions of each sample fraction obtained throughout the leaching procedure for the Kerguelen basalts compared to the isotopic compositions of possible exter- nal contaminants. Pb isotopic compositions and concentrations are from Nobre Silva et al. [2009]. In both diagrams, the size of the symbols is proportional to the Sr (lighter color) and Pb (darker color) contents in each sample fraction. The analytical uncertainty of each individual analysis is represented by the black error bars centered on the respective symbol. (a) The relationship between the Sr and Pb isotopic compositions of the leached and unleached powders, individual leaching steps, and bulk leachate solutions of the Northern Kerguelen Plateau basalt (ODP Leg 183, 1140A31R15761). The thick red line with 10% increments represents the mixing trend between the leached sample residue and the bulk leachate solutions. Mixing lines are shown between the leached sample residue and Indian Ocean seawater (dashed line) (EA: Faure [1986]; IC: Vlastélic et al. [2001]), FeMn oxides, drilling muds, and volcanic components weathered from older parts of the Kerguelen Plateau (gray arrow) (e.g., CKP: Frey et al. [2002] and Neal et al. [2002]). (b) The relationship between the Sr and Pb isotopic compositions of the leached and unleached powders, individual leaching steps, and bulk leachate solutions of the Mont Crozier basalt (OB93–165). The thick red line with 10% increments shows the mixing trend between the leached sample residue (proxy for the fresh sample) and the bulk leachate solutions (proxy for the total alteration removed). The black dashed line passing though the unleached sample (altered basalt) represents the trend that would be expected between a simple binary mixing between the “fresh sample” and the “alteration removed.” Geochemistry Geophysics Geosystems G3 NOBRE SILVA ET AL.: SrNdHf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 12 of 20Figure 6 Geochemistry Geophysics Geosystems G3 NOBRE SILVA ET AL.: SrNdHf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 13 of 20procedures employed for the HSDP sample suite prior to crushing [Rhodes, 1996; Rhodes and Vollinger, 2004]. The bulk mud used during HSDP2 has distinctly more radiogenic Sr and Pb isotopic compositions [Abouchami et al., 2000; Eisele et al., 2003], however its Pb isotopic composition is too radiogenic to explain the trend defined by the lea- ched sample residue, unleached sample powder, bulk leachates and individual leachate solutions of the Mauna Kea basalt (SR0954–8.00) (Figure 5b). Interaction of loess particles with this sample could potentially account for the observed trend produced by leaching. Based on its Pb isotopic composition, the carbonate fraction of Asian loess has been con- sidered a possible source of the radiogenic Pb con- tamination of Hawaiian basalts recovered during the HSDP [Abouchami et al., 2000; Eisele et al., 2003]. However, as much as 25% of the carbonate fraction of the loess alone would be needed (Figure 5b). This represents an excessively large amount of sediment to be incorporated into the sample, and requires a process that would facilitate interaction with the carbonate fraction of the loess, but not with the sil- icate fraction. Another alternative is the interaction of the fresh basalt with 20% of drilling mud and deep sea carbonates mixed in a proportion of 30:70 (black thick mixing curve in Figure 5b). However, 20% is also a large amount of a contaminant to be incor- porated into the basalt. A more plausible explanation for the observed trend is interaction of the basaltic rock sample with a combination of distinct end members, including the drilling muds used during the HSDP2. [19] Acid leaching of the submarine tholeiite from the Northern Kerguelen Plateau (sample 1140A 31R1, 57–61) lowers its Sr isotopic composition (Figures 2 and 6b). In contrast, its Pb isotopic Figure 7. Extended primitive mantlenormalized trace element diagram showing the results for three unleached powder splits (diamonds) and three leached powders splits (circles) of sample OB93–165 from the Kerguelen Archi- pelago. The inset diagram shows the chondritenormalized rare earth element patterns for the same powder splits. Normalizing values are from McDonough and Sun [1995]. Geochemistry Geophysics Geosystems G3 NOBRE SILVA ET AL.: SrNdHf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 14 of 20composition increases after being acid leached [Nobre Silva et al., 2009]. This implies that the contaminant being leached out has more radiogenic 87Sr/86Sr values, but less radiogenic Pb isotopic compositions, than those of the basalt. Given the low Pb content and radiogenic Sr and Pb isotopic compositions of seawater, alteration of this basalt by Indian Ocean seawater or contamination with carbonated sediments (Figure 6a) cannot account for this opposite behavior during leaching. Assuming that the isotopic compositions of the drilling muds used during the ODP Leg 183 are comparable to those used by the HSDP (i.e., highly radiogenic in Pb), contamination by the drilling muds cannot be the cause for the observed differences in the isotopic compositions of the unleached and leached pow- ders of sample 1140A31R1, 57–61 (Figure 6a). Volcanic components released during weathering of older parts of the Kerguelen Plateau (e.g., the ca. 100 Ma Central Kerguelen Plateau), could also potentially be a viable source of contamination by deposition after eruption and emplacement of the Northern Kerguelen Plateau. However, the high Sr and relatively low Pb contents of the Central Kerguelen Plateau basalts, as well as their much less radiogenic Pb isotopic compositions [Frey et al. 2002; Neal et al., 2002], indicates that they alone are not viable contaminants (Figure 6a). The combined information given by the Sr and Pb isotopic compositions of the leaching experiment with sample ODP Leg 183, 1140A31R1, 57–61, does not allow the identification the exact nature of its unradiogenic Pb contaminant. As with the other basalts in this study, the most probable scenario is Table 5. ICPMS Trace Element Abundances of Unleached and Leached Splits of the Mont Crozier, Kerguelen Archipelago, Basalta Element Unleached OB93–165 Leached OB93–165 Unleach./Leach.Split 1 Split 2 Split 3 Average SD Split A Split B Split C Average SD Li 4.2 4.9 5.2 4.8 0.53 4.0 3.9 3.6 3.8 0.19 1.2 Sc 22 23 24 23 1.0 28 27 26 27 1.13 0.85 V 292 298 339 310 26 146 142 137 141 4.8 2.2 Co 34 35 37 35 1.8 21 21 21 21 0.19 1.7 Ni 17 19 19 18 1.1 9.3 9.3 9.3 9.3 0.01 2.0 Cu 61 71 72 68 6.4 0.9 0.9 0.9 0.9 0.01 76 Zn 92 104 110 102 9.2 51 51 50 51 0.81 2.0 Ga 25 27 28 27 1.7 23 23 24 23 0.42 1.1 Rb 25 30 31 29 2.9 28 28 27 27 0.59 1.0 Sr 459 483 507 483 24 441 444 441 442 1.76 1.1 Y 31 32 34 32 1.5 16 16 15 16 0.21 2.1 Zr 252 288 283 275 20 249 241 256 249 7.74 1.1 Nb 30 31 36 32 3.0 45 43 42 43 1.94 0.75 Mo 1.9 2.1 2.4 2.1 0.28 1.1 1.0 1.0 1.0 0.02 2.1 Cd 0.19 0.21 0.22 0.21 0.02 0.12 0.12 0.11 0.12 0.00 1.8 Sn 1.7 1.7 2.0 1.8 0.17 0.83 0.86 0.89 0.86 0.03 2.1 Cs 0.07 0.08 0.09 0.08 0.01 0.03 0.03 0.02 0.03 0.00 3.1 Ba 320 351 355 342 19 342 340 358 347 9.99 0.99 La 34 37 38 37 2.1 6.9 7.1 7.1 7.0 0.09 5.2 Ce 75 78 83 79 4.0 15 15 15 15 0.22 5.1 Pr 8.8 9.1 9.7 9.2 0.45 2.0 2.0 2.0 2.0 0.04 4.6 Nd 35 37 39 37 1.8 9.4 9.1 9.1 9.2 0.18 4.0 Sm 7.5 7.8 8.2 7.8 0.38 2.7 2.6 2.6 2.6 0.04 3.0 Eu 2.4 2.5 2.7 2.5 0.14 2.2 2.2 2.3 2.2 0.02 1.1 Gd 7.2 7.6 7.9 7.6 0.37 3.1 3.0 3.0 3.0 0.08 2.5 Tb 1.1 1.2 1.2 1.2 0.06 0.53 0.52 0.52 0.53 0.01 2.2 Dy 6.5 6.7 7.1 6.8 0.33 3.4 3.3 3.3 3.3 0.08 2.0 Ho 1.3 1.3 1.4 1.3 0.07 0.71 0.67 0.68 0.69 0.02 1.9 Er 3.4 3.5 3.7 3.6 0.14 2.0 1.9 1.9 1.9 0.04 1.8 Tm 0.46 0.48 0.52 0.49 0.03 0.29 0.28 0.29 0.29 0.00 1.7 Yb 2.9 3.0 3.2 3.0 0.16 2.0 1.9 1.9 1.9 0.04 1.6 Lu 0.43 0.44 0.46 0.44 0.02 0.30 0.29 0.29 0.29 0.01 1.5 Hf 6.3 7.2 7.1 6.9 0.48 6.2 5.9 6.0 6.0 0.14 1.1 Ta 2.0 2.2 2.6 2.3 0.30 3.5 3.4 3.4 3.4 0.04 0.65 Pb 2.7 2.8 2.9 2.8 0.12 1.9 2.0 2.0 1.9 0.03 1.4 Th 4.4 4.7 4.8 4.6 0.22 2.4 2.2 2.3 2.3 0.11 2.0 U 1.1 1.1 1.2 1.1 0.05 0.71 0.71 0.73 0.72 0.01 1.6 aUnit is ppm. Geochemistry Geophysics Geosystems G3 NOBRE SILVA ET AL.: SrNdHf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 15 of 20that the source of contamination of this basalt is a composite one. [20] In contrast to the submarine basalts in this study, the unleached rock powder of the subaerial Kerguelen Archipelago basalt from Mont Crozier (sample OB93–165) does not plot along the mixing trend defined by the leached sample residue and the bulk and first leachate solutions (Figure 6b). The scatter observed in the Pb isotope values of the individual leachate solutions compared with their Sr isotopic compositions (Figure 6a and Nobre Silva et al. [2009, Figure 2a]) suggests an additional source of variability. This is consistent with the progressive removal at each step of the leaching procedure of a variety of secondary alteration mi- nerals (carbonates, oxides, sulfides, clays, epidote, zeolites [e.g., Nougier et al., 1982; Verdier, 1989; Hanano et al., 2009]). The different Pb isotopic compositions of each leaching step relative to their Sr isotopic compositions suggest that the alteration minerals removed during the acidleaching proce- dure may have been formed by more than one alteration process, including surficial chemical and physical weathering, possible interaction with sea- water transported by hydrothermal fluids, or inter- action with sea spray. 5.2. Effects of Acid Leaching on Trace Element Abundances and NdHf Isotopic Compositions of OIB [21] Both Hawaiian and Kerguelen basalts lose a large relative amount (∼90%) of Nd during the acid leaching procedure (Tables 2 and 3 and Figure 1). Comparing the trace element abundances of unleached and leached powder splits of the Mont Crozier basalt (sample OB93–165; Table 5 and Figure 7), the elements that are the most affected (i.e., showing the highest proportion of removal) by the leaching process are Cs, Rb, Th, U, Pb, and the rare earth elements (REE), espe- cially the light REE. The LREE abundances in the leached powder splits are ∼5 times less than in the original unleached powder splits, whereas the heavy REE are only a factor of ∼1.5 less abundant. This extraction and fractionation of the REE by acid leaching has been previously noted by other authors [e.g., Verma, 1992; Thompson et al., 2008]. [22] A surprising effect of the leaching process is the higher concentrations of Nb and Ta in the leached powder splits than in the unleached splits (Figure 7). The elemental concentrations in the leached powder splits are measured on a residue weight of about ∼0.2 g (versus ∼0.4 g for the unleached splits) that represents what is left after differential mineral removal by acid leaching. Nb and Ta are high field strength elements and are predominantly concentrated in primary FeTi oxide minerals, such as titanomagnetite or ilmenite. The resistance of these minerals to the acidleaching process will promote the concentration of Nb and Ta relative to other elements in the final powder residue, hence the leached residues will have higher concentrations of these elements than the unleached powders. [23] The trace element abundances measured on the leached powder splits are ∼2–5 times less variable than the abundances measured on the unleached powder splits (e.g., %RSDunl/leach = 4.45 for La; = 1.94 for Lu). This demonstrates the reproducibility of the leaching process and that mineral phases are not randomly removed from the altered crystalline basalt. The acid leached residue of whole rock powders of oceanic basalts is composed mainly of plagioclase and clinopyroxene ± olivine and FeTi oxides [e.g., Mahoney, 1987; Regelous et al., 2003: Hanano et al., 2009]. The chondritenormalized REE patterns of the leached powder splits of sample OB93–165 (Figure 7) are consistent with removal of olivine, pyroxene, and some plagio- clase, and with plagioclase being the main mineral left after acid leaching (e.g., positive Eu anomaly). The irregular trace element pattern of the leached powder residues reflects the selective removal of some elements relative to others. This indicates that the elemental abundances measured on the leached residue powders cannot be considered representa- tive of the primary magma elemental concentra- tions. The REE, given their ionic radii (La: 1.16 Å; Lu: 0.97 Å) and valence (+3), do not readily sub- stitute for other elements in the lattices of early crystallizing minerals (olivine and pyroxene) of a mafic melt [e.g., Krauskopf and Bird, 1995]. Instead, the REE are mostly located in nonstructural sites and defects, where the bonds are most easily bro- ken. Hence, the observation that the REE, espe- cially the LREE, are more easily leached out likely reflects their relatively weak bonding in the min- eral structures. As a result, the REE are easily remobilized and preferentially concentrated into the leachable secondary mineral phases during alte- ration [Verma, 1992], which subsequently allows for their differential removal by the acid leaching process. [24] Previous studies have suggested that Nd and Hf isotope ratios of oceanic basaltic rocks are to some extent modified by alteration, generally becoming slightly lower in the altered rock [e.g., Geochemistry Geophysics Geosystems G3 NOBRE SILVA ET AL.: SrNdHf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 16 of 20Kempton et al., 2002; Thompson et al., 2008]. Despite the large relative amounts of Nd (and other REE) lost during the acid leaching procedure, there is very little effect on the Nd isotopic compositions (Figure 3). Seawater has Nd isotopic composition that varies among and within the ocean basins (mean 143Nd/144NdPacific Ocean = ∼0.5125 [Piepgras and Wasserburg, 1980]; mean 143Nd/144NdIndian Ocean = ∼0.5123 [Albarède et al., 1997]) and that is overall significantly lower than the Nd isotopic composi- tion of oceanic basalts. However, given the very low Nd concentrations in seawater (∼2.6 × 10−6 ppm [Faure, 1986]), seawaterbasalt interaction will not produce a significant change in the REE content and Nd isotopic compositions of the basalts unless the water/rock ratios are greater than 105 [Ludden and Thompson, 1979]. For each of the four samples in this study, the unleached and leached sample powders, bulk and individual leachate solutions mostly plot within analytical uncertainty of each other, except for a few leachate solutions that present lower isotopic compositions. The cause of the lower isotopic compositions of these odd frac- tions is unknown, but may be due to some het- erogeneity in the secondary alteration phases. [25] Reliable Hf isotopic data could only be obtained for the Kerguelen basalts (Figure 4). The Hf iso- topic compositions of both the unleached and leached sample powders are the same within ana- lytical uncertainty. The Hf isotopic composition of seawater (mean 176Hf/177HfIndian Ocean = ∼0.28288 [David et al., 2001]) is significantly different from that of ocean island basalts, being more radiogenic than the Kerguelen Archipelago basalt and less radiogenic than the Northern Kerguelen Plateau basalt. However, given the geochemical nature of Hf (high field strength element) and its low con- centration in seawater (∼7 × 10–6 ppm [Faure, 1986; Chester, 2003]), no change in the Hf isoto- pic compositions of the basalts is expected. 6. Conclusions [26] The results of this study on the effects of acid leaching of wholerock powders on radiogenic isotopes (PbSrNdHf) commonly used to discern and characterize the mantle sources of oceanic basalts demonstrate that accurate multiisotopic analysis can be acquired on the same sample aliquots after an acid leaching treatment. The Sr isotopic ratios of all leached sample residues (submarine or subaerial) are significantly less radiogenic than their respective unleached powders. The higher 87Sr/86Sr ratios of the unleached powders are generally related to seawater interaction/alteration and (in the case of cored samples) drilling mud contamination. Sea- water alone cannot explain the Sr and Pb isotopic compositional differences between unleached and leached powders of the submarine tholeiites, and interaction with other external contaminants such as FeMn oxides, deepsea sediments, and loess is also likely. For all the samples, the Nd and Hf isotopic compositions of leached and unleached powders are within analytical uncertainty of each other, despite the significant losses (∼90%) of Nd (and other REE) and to a lesser extent Hf. Given these relatively significant elemental losses, care should be taken to ensure that enough material is available to measure isotopic compositions with the required precision after acid leaching of the samples. Due to the trace element losses and REE fractionation during acid leaching, the elemental abundances measured on leached sample powders are not reliable magmatic elemental signatures. Hence, for calculation of ini- tial isotopic ratios at the time of crystallization, parent/daughter ratios should be determined from elemental concentration ratiosmeasured on unleached sample powders. Acknowledgments [27] We thank Bruno Kieffer, Jane Barling, and Bert Mueller for their help in operating the TIMS, MCICPMS, and HR ICPMS instruments, respectively. 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