Effects of acid leaching on the Sr-Nd-Hf isotopic compositions of ocean island basalts

Article Volume 11, Number 9 25 September 2010 Q09011, doi:10.1029/2010GC003176 ISSN: 1525‐2027 Effects of acid leaching on the Sr‐Nd‐Hf 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 multi‐isotopic analyses (e.g., Sr‐Nd‐Hf‐Pb) on the same sample is critical for studies that evaluate the mantle source components of oceanic basalts. The isotopic compositions of relatively immobile elements, such as Nd (and other REE) and Hf, are considered to be relatively resistant to alteration, however, accurate Sr and Pb isotopic analyses of oceanic basalts require thorough acid leaching prior to dissolution. A detailed study of the Sr, Nd and Hf isotopic systematics of acid‐leached oceanic basalts from Hawaii and Kerguelen was undertaken to assess how acid leaching affects their isotopic compositions. Most of the Sr, Nd and Hf was removed in the first acid leaching steps. Hawaiian basalts lose up to 35% and 40% of their total Sr and Hf contents, respectively, whereas for Kerguelen basalts the corresponding losses are 63% and ∼70%. Acid leaching leads to significant loss of the original Nd content (up to 90%), which cannot be solely explained by the elimination of alteration phases and is likely related to preferential removal of the REE in the constituent silicate minerals (e.g., plagioclase, clinopyroxene). The leached residues yield Sr isotopic ratios significantly less radiogenic than their respective unleached powders and Nd‐Hf isotopic compositions that are within analytical uncertainty of the respective unleached powders. This study shows that multi‐isotopic analyses on the same acid‐leached sample aliquot can produce reliable results for use in the discrimination of mantle source components of oceanic basalts. Components: 10,600 words, 7 figures, 5 tables. Keywords: ocean island basalts; acid leaching; Sr‐Nd‐Hf isotopes; reproducibility; MC‐ICP‐MS; 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 Sr‐Nd‐Hf isotopic compositions of ocean island basalts, Geochem. Geophys. Geosyst., 11, Q09011, doi:10.1029/2010GC003176. 1. Introduction [2] The combination of analyses of different isotopic systems (e.g., Sr‐Nd‐Hf‐Pb) on samples of oceanic basalts constitutes a powerful tool for the determination and characterization of their mantle sources and components [e.g., Gast et al., 1964; Tatsumoto, 1966, 1978; White and Hofmann, 1982; Copyright 2010 by the American Geophysical Union Dupré and Allègre, 1983; Hart, 1984; White, 1985; Hamelin et al., 1986; Zindler and Hart, 1986; Weaver, 1991; Hofmann, 1997, 2003; Stracke et al., 2005]. Inherent to their emplacement in an oceanic environment, oceanic basalts are susceptible to seawater alteration, and the effects of this alteration on Rb‐Sr and U‐Pb isotope systematics have long been recognized [e.g., Hart et al., 1974; 1 of 20 Geochemistry Geophysics Geosystems 3 G NOBRE SILVA ET AL.: Sr‐Nd‐Hf ISOTOPES OF ACID LEACHED OIB Hawkesworth and Morrison, 1978; Cohen and O’Nions, 1982; Verma, 1992; Staudigel et al., 1995; Krolikowska‐Ciaglo et al., 2005]. Another well known cause of disturbance of the isotopic compositions of basalts, especially for Pb isotopes, is contamination during crushing and grinding [e.g., Woodhead and Hergt, 2000; McDonough and Chauvel, 1991; Weis et al., 2005]. To overcome the chemical disturbance caused by alteration, plus possible contamination during sample handling, careful acid leaching of samples prior to dissolution and analysis has proven to be an effective and essential step in sample processing for mantle geochemistry studies [e.g., Manhès et al., 1978; Dupré and Allègre, 1980; McDonough and Chauvel, 1991; Eisele et al., 2003; Stracke et al., 2003; Weis et al., 2005; Thompson et al., 2008; Hanano et al., 2009; Nobre Silva et al., 2009]. [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 relatively resistant to alteration [e.g., Cohen and O’Nions, 1982; Verma, 1992; Staudigel et al., 1995; Lassiter et al., 1996; Chauvel and Blichert‐ Toft, 2001; Mattielli et al., 2002; Krolikowska‐ Ciaglo et al., 2005]. Also, in contrast to Sr, which is relatively abundant in seawater, the concentrations of Nd and Hf in seawater are very low [e.g., Faure, 1986]. Hence, an acid‐leaching treatment prior to chemical separation for Nd and Hf isotopic analysis is not considered to be a requirement. Several studies, however, do show evidence for some mobility of Nd (and other REE) during hydrothermal alteration [e.g., Ludden and Thompson, 1979; Cotten et al., 1995; Staudigel et al., 1995; Bau et al., 1996; Smith et al., 2000; Kempton et al., 2002]. Furthermore, the 143Nd/144Nd and 176Hf/177Hf ratios of basalts may be affected to some extent by seawater alteration under some specific conditions [e.g., Kempton et al., 2002; Thompson et al., 2008]. [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 heterogeneity effect, it is important that multi‐isotopic analyses are performed on the same sample aliquot (same sample powders, chips, and/or glasses). This means that sequenced chemical separations should be performed on a single dissolution of each leached 10.1029/2010GC003176 sample. A wide variety of leaching protocols are used by different laboratories [e.g., Mahoney, 1987; McDonough and Chauvel, 1991; Weis and Frey, 1991; Stracke and Hegner, 1998; Abouchami et al., 2000; Thirlwall, 2000; Eisele et al., 2003; Stracke et al., 2003; Baker et al., 2004; Weis et al., 2005], but not all leaching techniques seem to provide results with the desired reproducibility for high‐ precision isotopic studies and interlaboratory comparison [Abouchami et al., 2000; Eisele et al., 2003; Stracke et al., 2003; Baker et al., 2004, 2005; Albarède et al., 2005]. The recent improvements in precision and reproducibility offered by the current generations of multiple collector mass spectrometers (TIMS and MC‐ICP‐MS) have enabled researchers to discriminate different isotopic trends within data sets that previously were within analytical uncertainty, especially for Pb and Hf. The question then arises, to what extent, if any, are the Nd and Hf isotopic compositions of OIB affected by acid leaching? [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 typically analyzed for radiogenic isotopic compositions from these two islands and span a wide range of MgO (3.5–18.0 wt%) with relatively weak alteration (e.g., 0.85–1.65 wt% LOI). The samples are: J2‐020‐23 from the Mile High Section of Mauna Loa volcano, SR0954–8.00 from the Mauna Kea volcano collected from the Hawaii Scientific Drilling Project (HSDP), OB93–165 from Mont Crozier 2 of 20 3 G Geochemistry Geophysics Geosystems Table 1. Summary of Sample Geochemical Characteristics, Hawaii and Kerguelen Eruption Deptha Environment (mbsl/masl) Rock Type Pb Sr Nd Hf Age SiO2 MgO Na2O K2O (wt%) (wt%) (wt%) (wt%) (ppm) (ppm) (ppm) (ppm) A.I.b LOIc (Ma)d References Hawaiian Basalts Mauna Loa (Southwest Rift Zone) J2‐020‐23 submarine 489 tholeiitic basalt 50.14 6.50 2.72 0.52 2.00 306 32.08 6.60 −0.87 0.99 ‐ Mauna Kea (HSDP‐2e) SR0954–8.00 submarine 3009.2 tholeiitic basalt 47.67 18.03 1.65 0.23 0.66 239 15.43 2.95 −1.3 0.85 0.55 Rhodes and Vollinger [2004] ‐ J. M. Rhodes (unpublished data, 2003) Kerguelen Basalts Mont Crozier OB93–165 subaerial 515 alkalic basalt 50.91 3.54 3.52 1.45 6.34 417 36.30 5.99 0.67 Northern Kerguelen Plateau ODP Leg 183, 1140A‐31R‐1, 57–61 submarine 270.07 tholeiitic basalt 49.64 5.54 2.64 0.49 1.52 262 20.4 a 24.5 D. Weis (unpublished data, 2005) 3.75 −0.81 1.65 34.3 Weis and Frey [2002] 3 of 20 10.1029/2010GC003176 mbsl = meters below sea level, for submarine basalts; masl = meters above sea level, for subaerial basalts. A.I. = alkalinity index (AI = total alkalis − (SiO2 × 0.37 − 14.43)) [Rhodes, 1996]. c LOI = weight loss‐on‐ignition after 30 min at 1020°C. d Ages from Mauna Kea samples: Sharp and Renne [2005]; Mont Crozier and Mont Bureau: Nicolaysen et al. [2000]; Mont des Ruches and Mont Fontaine: Doucet et al. [2002]; Site 1140: Duncan [2002]. e HSDP‐2 = Hawaii Scientific Drilling Project, phase 2. b NOBRE SILVA ET AL.: Sr‐Nd‐Hf ISOTOPES OF ACID LEACHED OIB Sample Table 2. Sr, Nd, and Hf Elemental Contents and Isotopic Compositions of Unleached Whole‐Rock Powders, Leachate Solutions, and Leached Residues for Hawaiian Basalts 0.703818 0.703930 0.704203 0.704310 0.703782 0.703671 9 10 9 9 8 9 0.703687 0.703704 ‐ ‐ 8 8 ‐ ‐ 0.703683 10 Unleached B.L.Homog. B.L.Acid 1st 2nd 3rd 4th 5th 6th 7th 8th 1st H2O 2nd H2O Total Leach. Res. 94286 19718 19579 15597 1951 1548 1175 1978 1130 635 601 245 222 25083 69203 100 20.9 20.8 16.5 2.07 1.64 1.25 2.10 1.20 0.673 0.637 0.260 0.236 26.6 73 0.703728 0.704556 0.704634 0.704511 0.703831 0.703661 ‐ 0.703570 ‐ 0.703592 ‐ ‐ ‐ 8 8 9 9 8 17 ‐ 7 ‐ 9 ‐ ‐ ‐ 0.703535 9 a 143 Nd/144Nd Sample J2‐020‐23 (Mauna Loa) 8960 100 0.512943 4754 53.1 0.512934 4395 49.0 0.512931 4490 50.1 0.512943 259 2.89 0.512938 176 1.97 0.512948 121 1.35 101 1.13 0.512956 52.8 0.589 0.512939 ‐ ‐ ‐ 18.8 0.209 ‐ 5218 58.2 3742 41.8 0.512948 Sample SR0954–8.00 (Mauna Kea) 6085 100 0.513009 6944 114 0.512997 7020 115 0.512614 4902 80.6 0.512995 229 3.77 76.3 1.25 0.513027 46.2 0.759 48.0 0.789 0.512872 32.6 0.536 ‐ 16.2 0.266 ‐ 15.7 0.258 ‐ 6.80 0.112 ‐ 7.80 0.128 ‐ 5381 88.4 704.4 11.6 0.513000 2 SEa 7 9 8 8 7 12 18 14 ‐ ‐ 7 8 9 10 7 42 112 ‐ ‐ ‐ ‐ ‐ 3 Hf % Hf in Each Fraction 1843 647 503 540 61.9 44.5 21.8 20.4 7.35 ‐ 0.450 696 1147 100 35.1 27.3 29.3 3.36 2.41 1.18 1.11 0.40 ‐ 0.024 37.8 62.2 0.283189 ‐ 0.283109 ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ 33 ‐ 10 ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ 0.283084 6 1164 407.7 406.5 261.7 55.00 39.91 25.80 28.10 18.90 11.35 10.75 0.800 ‐ 452 711 100 35.0 34.9 22.5 4.73 3.43 2.22 2.41 1.62 0.975 0.924 0.069 ‐ 38.9 61 0.283158 ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ 12 ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ 0.283120 5 176 Hf/177Hf 2 SEa 2 SE, indicates the uncertainty in the last digits of the measured isotopic ratios. Unleached, unleached rock powders. B.L.Homog., bulk leachate solution homogenized (acid + fine powder particles). d B.L.Acid, bulk leachate solution without the fine powder particles. e Dashes indicate no isotopic analysis performed. f Total = sum of the elemental contents of all individual leaching steps. The Sr, Nd, and Hf contents of the bulk leachate solutions does not equal the sum of the individual leaching steps because they were measured on different aliquots of the same sample powders. Note: the elemental contents (ng) of the unleached powders are calculated from the initial amounts of powder and the respective sample concentrations (see Table 1). The contents of the bulk and individual leachate solutions are those measured by HR‐ICP‐MS on a 10% aliquot of each solution. The elemental contents (ng) of the leached residues are calculated by mass balance between the total ng in the starting unleached powders and the sum of the elemental contents in all the individual leachate solutions. g Leach. Res., leached powder residues. b c 4 of 20 10.1029/2010GC003176 100 34.8 21.5 21.4 3.55 3.78 2.35 2.03 0.783 ‐ 0.401 34.3 65.7 2 SEa Total ng in Each Fraction NOBRE SILVA ET AL.: Sr‐Nd‐Hf ISOTOPES OF ACID LEACHED OIB 85466 29768 18415 18266 3033 3233 2012 1738 669 ‐e 343 29294 56172 Sr/86Sr Nd % Nd in Each Fraction 3 Unleachedb B.L.Homog.c B.L.Acidd 1st 2nd 3rd 4th 5th 6th 1st H2O 2nd H2O Totalf Leach. Res.g 87 Total ng in Each Fraction G Sr % Sr in Each Fraction Geochemistry Geophysics Geosystems Sample Total ng in Each Fraction Table 3. Sr, Nd, and Hf Elemental Contents and Isotopic Compositions of Unleached Whole‐Rock Powders, Leachate Solutions, and Leached Residues for Kerguelen Basalts Unleached B.L.Homog. B.L.Acid 1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th 13th 14th 1st H2O 2nd H2O Total Leach. Res. 103438 54762 46404 32782 5508 2567 1718 1981 2652 3483 2263 2773 1014 1399 1455 1688 850 373 154 62660 40778 100 52.9 44.9 31.7 5.32 2.48 1.66 1.92 2.56 3.37 2.19 2.68 0.98 1.35 1.41 1.63 0.821 0.360 0.149 60.6 39.4 0.705340 0.705430 0.705440 0.705414 0.705303 0.705297 ‐e 0.705291 ‐ 0.705284 ‐ ‐ ‐ 8 10 9 7 8 9 ‐ 8 ‐ 9 ‐ ‐ ‐ 0.705298 10 ODP 0.704422 0.704545 0.704668 0.704720 0.704615 0.704629 ‐ 0.704548 ‐ 0.704300 ‐ 0.704226 ‐ 0.704227 0.704228 ‐ ‐ ‐ ‐ 0.704222 143 Nd/144Nd Sample OB93–165 (Mont Crozier) 10465 100 0.512621 9088 86.8 0.512617 9196 87.9 ‐ 8774 83.8 0.512626 357 3.41 0.512632 83.5 0.798 0.512633 153 1.46 ‐ 75.7 0.723 0.512062 79.7 0.762 ‐ 27.8 0.265 0.512632 35.3 0.337 ‐ 9.60 0.092 ‐ 4.10 0.039 ‐ 9599 91.7 866 8.28 0.512625 Leg 183, 1140A‐31R‐1, 57–61 (Northern 8 8037 100 8 6289 78.3 8 6143 76.4 6 5745 71.5 8 307 3.8 9 87.7 1.1 ‐ 49.5 0.6 9 58.6 0.7 ‐ 73.4 0.9 8 113 1.4 ‐ 76.6 1.0 8 109 1.4 ‐ 35.0 0.4 8 53.2 0.7 10 54.4 0.7 ‐ 64.8 0.8 ‐ 25.8 0.3 ‐ 16.7 0.2 ‐ 6.85 0.1 6876 85.6 23 1161 14.4 2 SEa 6 7 ‐ 8 10 10 ‐ 11 ‐ 40 ‐ ‐ ‐ 6 Kerguelen Plateau) 0.512808 7 0.512809 7 0.512806 7 0.512818 8 0.512213 19 ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ 0.512806 20 ‐ ‐ 0.512833 22 0.512712 34 ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ 0.512839 6 Hf % Hf in Each Fraction 1727 488 334 677 168 49.3 88.5 36.7 36.1 13.8 18.2 1.35 ‐ 1089 638 100 28.3 19.4 39.2 9.72 2.85 5.12 2.13 2.09 0.796 1.05 0.078 ‐ 63.1 36.9 0.282850 0.282849 ‐ 0.282761 0.282835 ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ 6 9 ‐ 15 8 ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ 0.282849 6 1481 885 779 672 127 37.9 21.6 20.7 23.7 32.5 16.4 24.2 2.20 7.19 13.3 16.0 4.65 0.15 ‐ 1019 462 100 59.8 52.6 45.4 8.55 2.56 1.46 1.40 1.60 2.19 1.11 1.63 0.148 0.486 0.898 1.08 0.314 0.010 ‐ 68.8 31.2 0.283020 0.283023 0.282962 0.282908 0.283023 ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ 5 5 10 18 6 ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ 0.283028 5 176 Hf/177Hf 2 SEa 10.1029/2010GC003176 100 30.8 28.7 33.1 11.5 3.17 6.44 2.74 3.00 1.08 1.38 0.261 0.161 62.8 37.2 2 SEa Total ng in Each Fraction NOBRE SILVA ET AL.: Sr‐Nd‐Hf ISOTOPES OF ACID LEACHED OIB 120221 37016 34489 39774 13813 3808 7740 3289 3607 1297 1661 313 194 75495 44726 Sr/86Sr Nd % Nd in Each Fraction 3 Unleachedb B.L.Homog.c B.L.Acidd 1st 2nd 3rd 4th 5th 6th 7th 8th 1st H2O 2nd H2O Totalf Leach. Res.g 87 Total ng in Each Fraction G Sr % Sr in Each Fraction Geochemistry Geophysics Geosystems 5 of 20 Sample Total ng in Each Fraction Geochemistry Geophysics Geosystems 3 G Table 4. NOBRE SILVA ET AL.: Sr‐Nd‐Hf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 Summary of the Weight and Relevant Element % Losses During the Multistep Acid Leaching Procedure Each Leaching Step Sample Mauna Loa J2‐020‐20 Mauna Kea SR0954–8.00 Mont Crozier OB93–165 Northern Kerguelen Plateau ODP Leg 183, 1140A‐31R‐1, 57–61 Initial Weight (g) # Acid Leaching Steps Final Weight (g) Weight Loss (%) Pb Loss (%) Sr Loss (%) Nd Loss (%) Hf Loss (%) 0.2793 6 0.1828 34.6 68.7 34.3 58.2 37.8 0.3945 8 0.259 34.3 70.8 26.6 88.4 38.9 0.2883 8 0.106 63.2 37.3 62.8 91.7 63.1 0.3948 14 0.1598 59.5 76.1 60.6 85.6 68.8 on the Courbet Peninsula of the Kerguelen Archipelago, and 1140A‐31R‐1, 57–61 cm recovered during ODP Leg 183 on the Northern Kerguelen Plateau (see Table 1 for additional sample characterization). To assess the effects of our multistep acid leaching procedure on the elemental concentrations and Sr, Nd, and Hf isotopic compositions of these four samples, all column separations and analyses were performed on the same chemical digestions of all sample fractions (i.e., unleached and leached sample powders, leachate solutions collected at each leaching step, and respective accumulate bulk leachates) as the Pb isotopic analyses published by Nobre Silva et al. [2009]. 3. 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, dissolution and separation were sub‐boiled, all dilutions were made using ≥ 18.2 MW · cm de‐ionized water, and all labware was acid‐washed prior to use. The acid leaching procedure used follows that of Weis et al. [2005] and is detailed by Nobre Silva et al. [2009]. Briefly, the sample powders were acid‐ leached using 10 mL of 6 M HCl in screw‐top Savillex® beakers, in a warm (∼50°C) ultrasonic bath for 20 min. Immediately after, the supernatant (leachate solution) was decanted without centrifugation, to ensure the removal of the silt to fine‐size particle fraction, which is mostly dominated by secondary alteration phases. The whole process was repeated (6 to 14 times for the samples in this study) until the leachate was clear and free of fine‐ size particles. For all sample specimens, Sr, Nd and Hf elemental concentrations and isotopic compositions were determined on a single dissolution, using methods similar to those described 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 acid‐leaching procedure, and consequently how much is lost, the respective Notes to Table 3: a 2 SE indicates the uncertainty in the last digits of the measured isotopic ratios. Unleached, unleached rock powders. B.L.Homog., bulk leachate solution homogenized (acid + fine powder particles). d B.L.Acid, bulk leachate solution without the fine powder particles. e Dashes indicate no isotopic analysis performed. f Total = sum of the elemental contents of all individual leaching steps. The Sr, Nd, and Hf contents of the bulk leachate solutions does not equal the sum of the individual leaching steps because they were measured on different aliquots of the same sample powders. Note: the elemental contents (ng) of the unleached powders are calculated from the initial amounts of powder and the respective sample concentrations (see Table 1). The contents of the bulk and individual leachate solutions are those measured by HR‐ICP‐MS on a 10% aliquot of each solution. The elemental contents (ng) of the leached residues are calculated by mass balance between the total ng in the starting unleached powders and the sum of the elemental contents in all the individual leachate solutions. g Leach. Res., leached powder residues. b c 6 of 20 Geochemistry Geophysics Geosystems 3 G NOBRE SILVA ET AL.: Sr‐Nd‐Hf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 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 HR‐ICP‐MS. The Sr abundances are divided by 100 to allow for plotting on the same graph as the other elements. concentrations were measured on a 10% aliquot of the unleached sample powders, individual leachates and bulk leachate solutions. The analyses were performed on an ELEMENT2 high‐resolution (HR)‐ICP‐MS (Thermo Finnigan, Germany) and were quantified using external calibration curves and indium (In) as an internal standard. Multielement (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 Re‐Ta (Nd) filaments. Sr and Nd isotopic compo- sitions were corrected for mass fractionation with the exponential law using 86Sr/88Sr = 0.1194 and 146 Nd/144Nd = 0.7219, respectively. The data were then normalized using the average of the corresponding standard (NIST SRM 987 for Sr and La Jolla for Nd) ran in the barrel, relative to the values of NIST SRM 987 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 (MC‐ICP‐MS), under dry plasma conditions using a membrane desolvator (Nu DSN100) for sample introduction, following the analytical procedures detailed by Weis et al. [2007]. All analyses were obtained in static multicollection mode, with interference corrections for 176Lu and 176Yb on 176 Hf, and 174Yb on 174Hf, respectively. All sample 7 of 20 Geochemistry Geophysics Geosystems 3 G NOBRE SILVA ET AL.: Sr‐Nd‐Hf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 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. and standard solutions were prepared fresh for each analytical session and sample analysis followed a modified sample‐standard 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 176 Hf/177Hf value of 0.282160 [Vervoort and Blichert‐ Toft, 1999] using the daily average of the JMC 475 Hf standard. The JMC 475 Hf standard analyzed over the period of the analyses gave an average value of 176 Hf/177Hf = 0.282181 ± 0.000008 (n = 32). 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 proportions of Sr and Nd losses after acid leaching have been reported for basalts from the first drilling phase of the HSDP [Hauri et al., 1996]. 4. Results [12] The Hawaiian and Kerguelen basalts present a [11] Throughout the multistep acid‐leaching pro- cedure, the Hawaiian basalts lose ∼35% of their similar pattern of Sr isotopic variation throughout the leaching process (Figure 2). The magnitudes of the isotopic variations at each leaching step are 8 of 20 Geochemistry Geophysics Geosystems 3 G NOBRE SILVA ET AL.: Sr‐Nd‐Hf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 Figure 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 non‐overlapping range of Hf isotopic ratios between the two plots. 9 of 20 Geochemistry Geophysics Geosystems 3 G NOBRE SILVA ET AL.: Sr‐Nd‐Hf ISOTOPES OF ACID LEACHED OIB different between the four samples as they are dependent on the amount of fine particles (mostly alteration phases and contaminants) that are removed when decanting the individual leachate solutions. The leached powder residues of all four samples yield less radiogenic Sr isotopic compositions than the respective unleached powders. The Sr isotopic compositions of the individual leachate solutions and unleached powders, of both Hawaiian and Kerguelen basalts, generally plot along a mixing trend between a more radiogenic end‐member (represented by the bulk leachate solutions and 1st leachate solution) and the leached sample residue (Figure 2). [13] For both Hawaiian and Kerguelen basalts, no significant difference (i.e., outside individual analytical error) is observed between the Nd isotopic compositions of the unleached rock powders, the bulk leachate solutions, the individual leachate solutions, and the leached residues (Figure 3). For each individual sample, the results for nearly all sample fractions plot within error of each other. The few exceptions with lower 143Nd/144Nd represent a very minor relative proportion of the whole sample Nd content (e.g., leachate #2 for sample 1140A‐31R‐1, 57–61; leachate #5 for sample OB93–165; and a bulk leachate solution for sample SR0954–8.00). After acid leaching, the leached powder residues of both Hawaiian and Kerguelen basalts yield similar Nd isotopic compositions to those of their respective unleached sample powders. [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 10.1029/2010GC003176 some of the Hawaiian samples processed in a tungsten carbide (WC) crusher and mill. In addition, the Hf contents of some fractions were not sufficient to provide good runs, hence no data is reported. For the two Kerguelen samples, the Hf isotope compositions of the unleached sample powders, bulk leachate solutions, and leached sample residues are within error of each other (Figure 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 compositions to those of their respective unleached sample powders (Tables 2 and 3). 5. Discussion 5.1. Effects of Acid Leaching of OIB in Sr‐Pb 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 compositions 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 possible external contaminants. Pb isotopic compositions and concentrations are from Nobre Silva et al. [2009]. In both diagrams, the size of the symbols is proportional to the Sr (lighter 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 (J2‐020‐23) and several possible contaminants: seawater (elemental abundances (EA): Faure [1986]; isotopic compositions (IC): Abouchami and Galer [1998] and Eisele et al. [2003]); authigenic carbonates (EA: Faure [1986] and Chester [2003]; IC: same as seawater); Fe‐Mn oxides (EA and IC: Ling et al. [1997]); deep‐sea interbedded clay‐bearing carbonate sediments such as those now subducting at the Izu‐Bonin trench (EA and IC: Hauff et al. [2003] and Plank et al. [2007]); Chinese loess (EA: Nakai et al. [1993] and Gallet et al. [1996]; IC: Kennedy et al. [1998] and Jones et al. [2000]). The thick red line represents the mixing trend between the leached sample residue and the bulk leachate solutions (circles indicate 10% increments). (b) The results for the Mauna Kea sample (SR0954–8.00) and three external contaminants: seawater, carbonates, HSDP‐2 bulk mud [Abouchami et al., 2000], and a mixture of clay‐bearing carbonates plus drilling mud. The mixing trend between each of these contaminants and the leached sample residue are shown. All unlabelled tick marks on the mixing curves indicate 10% increments. 10 of 20 Geochemistry Geophysics Geosystems 3 G NOBRE SILVA ET AL.: Sr‐Nd‐Hf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 Figure 5 11 of 20 Geochemistry Geophysics Geosystems 3 G NOBRE SILVA ET AL.: Sr‐Nd‐Hf ISOTOPES OF ACID LEACHED OIB plausibility of several natural contaminants to explain the mixing trends defined by the bulk leachates 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 J2‐020‐23) and Mauna Kea (sample SR0954–8.00) reflect exchange with a more radiogenic 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 seawater, local deep‐sea sediments and/or Fe‐Mn crusts, or a mixture of each. A distal contaminant that must also be considered, and that has been identified in soil horizons of Hawaiian lavas, is wind‐blown Chinese loess into the central north Pacific [e.g., Kennedy et al., 1998; Jones et al., 2000; Abouchami et al., 2000]. [17] In Pb‐Sr isotopic space, the leaching experiments for the Mauna Loa basalt (sample J2‐020‐23) produce a shallow trend defined by the leached sample residue, unleached sample powder, bulk leachates, and individual leachate solutions (Figure 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). Fe‐Mn deposits, which are 10.1029/2010GC003176 efficient scavengers of trace metals, have very elevated Pb contents (e.g., Central Pacific Fe‐Mn deposits: 997–1054 ppm [after Ling et al., 1997]). Even a contribution of 1% of Fe‐Mn oxides would be too high to explain the observed trend between the sample and leachate fractions (Figure 5a). Chinese eolian loess has elemental and isotopic compositions ([Sr] = ∼200 ppm [after Nakai et al., 1993]; 87 Sr/ 86 Sr = 0.717 [after Kennedy et al., 1998]; [Pb] = ∼20 ppm [after Gallet et al., 1996]; 206 Pb/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 incorporation of a composite mixture of deep‐sea interbedded clay‐bearing carbonate sediments, similar in composition to those now subducting at the Izu‐Bonin trench [Hauff et al., 2003; Plank et al., 2007], and some minor Fe‐Mn oxides, deposited on the surface of the basalt (Figure 5a). [18] For the HSDP‐2 Mauna Kea submarine tholeiite (sample SR0954–8.00), the isotopic trend produced by the acid leaching reflects interaction of the basalt with a highly radiogenic contaminant, different than that discussed above for the submarine Mauna Loa basalt (Figures 2b and 5b). Several previous studies have suggested possible contamination of the HSDP samples by drilling mud [e.g., Hauri et al., 1996; Abouchami et al., 2000; Eisele et al., 2003; Nobre Silva et al., 2009]. Hanano et al. [2009] also identified distinct patches of 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 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 individual analysis is represented by the black error bars centered on the respective symbol. (a) The relationship between the Sr and Pb isotopic compositions of the leached and unleached powders, individual leaching steps, and bulk leachate solutions of the Northern Kerguelen Plateau basalt (ODP Leg 183, 1140A‐31‐R1‐57‐61). The thick red line with 10% increments represents the mixing trend between the leached sample residue and the bulk leachate solutions. Mixing lines are shown between the leached sample residue and Indian Ocean seawater (dashed line) (EA: Faure [1986]; IC: Vlastélic et al. [2001]), Fe‐Mn oxides, drilling muds, and volcanic components weathered from older parts of the Kerguelen Plateau (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.” 12 of 20 Geochemistry Geophysics Geosystems 3 G NOBRE SILVA ET AL.: Sr‐Nd‐Hf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 Figure 6 13 of 20 Geochemistry Geophysics Geosystems 3 G NOBRE SILVA ET AL.: Sr‐Nd‐Hf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 Figure 7. Extended primitive mantle‐normalized trace element diagram showing the results for three unleached powder splits (diamonds) and three leached powders splits (circles) of sample OB93–165 from the Kerguelen Archipelago. The inset diagram shows the chondrite‐normalized rare earth element patterns for the same powder splits. Normalizing values are from McDonough and Sun [1995]. procedures employed for the HSDP sample suite prior to crushing [Rhodes, 1996; Rhodes and Vollinger, 2004]. The bulk mud used during HSDP‐2 has distinctly more radiogenic Sr and Pb isotopic compositions [Abouchami et al., 2000; Eisele et al., 2003], however its Pb isotopic composition is too radiogenic to explain the trend defined by the leached sample residue, unleached sample powder, bulk leachates and individual leachate solutions of the Mauna Kea basalt (SR0954–8.00) (Figure 5b). Interaction of loess particles with this sample could potentially account for the observed trend produced by leaching. Based on its Pb isotopic composition, the carbonate fraction of Asian loess has been considered a possible source of the radiogenic Pb contamination of Hawaiian basalts recovered during the HSDP [Abouchami et al., 2000; Eisele et al., 2003]. However, as much as 25% of the carbonate fraction of the loess alone would be needed (Figure 5b). This represents an excessively large amount of sediment to be incorporated into the sample, and requires a process that would facilitate interaction with the carbonate fraction of the loess, but not with the silicate fraction. Another alternative is the interaction of the fresh basalt with 20% of drilling mud and deep sea carbonates mixed in a proportion of 30:70 (black thick mixing curve in Figure 5b). However, 20% is also a large amount of a contaminant to be incorporated into the basalt. A more plausible explanation for the observed trend is interaction of the basaltic rock sample with a combination of distinct end‐ members, including the drilling muds used during the HSDP‐2. [19] Acid leaching of the submarine tholeiite from the Northern Kerguelen Plateau (sample 1140A‐ 31R‐1, 57–61) lowers its Sr isotopic composition (Figures 2 and 6b). In contrast, its Pb isotopic 14 of 20 Geochemistry Geophysics Geosystems 3 G Table 5. Basalta NOBRE SILVA ET AL.: Sr‐Nd‐Hf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 ICP‐MS Trace Element Abundances of Unleached and Leached Splits of the Mont Crozier, Kerguelen Archipelago, Unleached OB93–165 Leached OB93–165 Element Split 1 Split 2 Split 3 Average SD Split A Split B Split C Average SD Unleach./Leach. Li Sc V Co Ni Cu Zn Ga Rb Sr Y Zr Nb Mo Cd Sn Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U 4.2 22 292 34 17 61 92 25 25 459 31 252 30 1.9 0.19 1.7 0.07 320 34 75 8.8 35 7.5 2.4 7.2 1.1 6.5 1.3 3.4 0.46 2.9 0.43 6.3 2.0 2.7 4.4 1.1 4.9 23 298 35 19 71 104 27 30 483 32 288 31 2.1 0.21 1.7 0.08 351 37 78 9.1 37 7.8 2.5 7.6 1.2 6.7 1.3 3.5 0.48 3.0 0.44 7.2 2.2 2.8 4.7 1.1 5.2 24 339 37 19 72 110 28 31 507 34 283 36 2.4 0.22 2.0 0.09 355 38 83 9.7 39 8.2 2.7 7.9 1.2 7.1 1.4 3.7 0.52 3.2 0.46 7.1 2.6 2.9 4.8 1.2 4.8 23 310 35 18 68 102 27 29 483 32 275 32 2.1 0.21 1.8 0.08 342 37 79 9.2 37 7.8 2.5 7.6 1.2 6.8 1.3 3.6 0.49 3.0 0.44 6.9 2.3 2.8 4.6 1.1 0.53 1.0 26 1.8 1.1 6.4 9.2 1.7 2.9 24 1.5 20 3.0 0.28 0.02 0.17 0.01 19 2.1 4.0 0.45 1.8 0.38 0.14 0.37 0.06 0.33 0.07 0.14 0.03 0.16 0.02 0.48 0.30 0.12 0.22 0.05 4.0 28 146 21 9.3 0.9 51 23 28 441 16 249 45 1.1 0.12 0.83 0.03 342 6.9 15 2.0 9.4 2.7 2.2 3.1 0.53 3.4 0.71 2.0 0.29 2.0 0.30 6.2 3.5 1.9 2.4 0.71 3.9 27 142 21 9.3 0.9 51 23 28 444 16 241 43 1.0 0.12 0.86 0.03 340 7.1 15 2.0 9.1 2.6 2.2 3.0 0.52 3.3 0.67 1.9 0.28 1.9 0.29 5.9 3.4 2.0 2.2 0.71 3.6 26 137 21 9.3 0.9 50 24 27 441 15 256 42 1.0 0.11 0.89 0.02 358 7.1 15 2.0 9.1 2.6 2.3 3.0 0.52 3.3 0.68 1.9 0.29 1.9 0.29 6.0 3.4 2.0 2.3 0.73 3.8 27 141 21 9.3 0.9 51 23 27 442 16 249 43 1.0 0.12 0.86 0.03 347 7.0 15 2.0 9.2 2.6 2.2 3.0 0.53 3.3 0.69 1.9 0.29 1.9 0.29 6.0 3.4 1.9 2.3 0.72 0.19 1.13 4.8 0.19 0.01 0.01 0.81 0.42 0.59 1.76 0.21 7.74 1.94 0.02 0.00 0.03 0.00 9.99 0.09 0.22 0.04 0.18 0.04 0.02 0.08 0.01 0.08 0.02 0.04 0.00 0.04 0.01 0.14 0.04 0.03 0.11 0.01 1.2 0.85 2.2 1.7 2.0 76 2.0 1.1 1.0 1.1 2.1 1.1 0.75 2.1 1.8 2.1 3.1 0.99 5.2 5.1 4.6 4.0 3.0 1.1 2.5 2.2 2.0 1.9 1.8 1.7 1.6 1.5 1.1 0.65 1.4 2.0 1.6 a Unit is ppm. composition increases after being acid leached [Nobre Silva et al., 2009]. This implies that the contaminant being leached out has more radiogenic 87 Sr/86Sr values, but less radiogenic Pb isotopic compositions, than those of the basalt. Given the low Pb content and radiogenic Sr and Pb isotopic compositions of seawater, alteration of this basalt by Indian Ocean seawater or contamination with carbonated sediments (Figure 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 powders of sample 1140A‐31R‐1, 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, 1140A‐31R‐1, 57–61, does not allow the identification the exact nature of its unradiogenic Pb contaminant. As with the other basalts in this study, the most probable scenario is 15 of 20 Geochemistry Geophysics Geosystems 3 G NOBRE SILVA ET AL.: Sr‐Nd‐Hf ISOTOPES OF ACID LEACHED OIB that 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 minerals (carbonates, oxides, sulfides, clays, epidote, zeolites [e.g., Nougier et al., 1982; Verdier, 1989; Hanano et al., 2009]). The different Pb isotopic compositions of each leaching step relative to their Sr isotopic compositions suggest that the alteration minerals removed during the acid‐leaching procedure may have been formed by more than one alteration process, including surficial chemical and physical weathering, possible interaction with seawater transported by hydrothermal fluids, or interaction with sea spray. 5.2. Effects of Acid Leaching on Trace Element Abundances and Nd‐Hf 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), especially the light REE. The LREE abundances in the leached powder splits are ∼5 times less than in the original unleached powder splits, whereas the heavy REE are only a factor of ∼1.5 less abundant. This extraction and fractionation of the REE by acid leaching has been previously noted by other authors [e.g., Verma, 1992; Thompson et al., 2008]. [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 10.1029/2010GC003176 unleached splits) that represents what is left after differential mineral removal by acid leaching. Nb and Ta are high field strength elements and are predominantly concentrated in primary Fe‐Ti oxide minerals, such as titanomagnetite or ilmenite. The resistance of these minerals to the acid‐leaching process will promote the concentration of Nb and Ta relative to other elements in the final powder residue, hence the leached residues will have higher concentrations of these elements than the unleached powders. [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 Fe‐Ti oxides [e.g., Mahoney, 1987; Regelous et al., 2003: Hanano et al., 2009]. The chondrite‐normalized REE patterns of the leached powder splits of sample OB93–165 (Figure 7) are consistent with removal of olivine, pyroxene, and some plagioclase, and with plagioclase being the main mineral left after acid leaching (e.g., positive Eu anomaly). The irregular trace element pattern of the leached powder residues reflects the selective removal of some elements relative to others. This indicates that the elemental abundances measured on the leached residue powders cannot be considered representative of the primary magma elemental concentrations. The REE, given their ionic radii (La: 1.16 Å; Lu: 0.97 Å) and valence (+3), do not readily substitute for other elements in the lattices of early crystallizing minerals (olivine and pyroxene) of a mafic melt [e.g., Krauskopf and Bird, 1995]. Instead, the REE are mostly located in non‐structural sites and defects, where the bonds are most easily broken. Hence, the observation that the REE, especially the LREE, are more easily leached out likely reflects their relatively weak bonding in the mineral structures. As a result, the REE are easily remobilized and preferentially concentrated into the leachable secondary mineral phases during alteration [Verma, 1992], which subsequently allows for their differential removal by the acid leaching process. [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., 16 of 20 Geochemistry Geophysics Geosystems 3 G NOBRE SILVA ET AL.: Sr‐Nd‐Hf ISOTOPES OF ACID LEACHED OIB 10.1029/2010GC003176 Kempton et al., 2002; Thompson et al., 2008]. Despite the large relative amounts of Nd (and other REE) lost during the acid leaching procedure, there is very little effect on the Nd isotopic compositions (Figure 3). Seawater has Nd isotopic composition that varies among and within the ocean basins (mean 143 Nd/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 composition of oceanic basalts. However, given the very low Nd concentrations in seawater (∼2.6 × 10−6 ppm [Faure, 1986]), seawater‐basalt interaction will not produce a significant change in the REE content and Nd isotopic compositions of the basalts unless the water/rock ratios are greater than 105 [Ludden and Thompson, 1979]. For each of the four samples in this study, the unleached and leached sample powders, bulk and individual leachate solutions mostly plot within analytical uncertainty of each other, except for a few leachate solutions that present lower isotopic compositions. The cause of the lower isotopic compositions of these odd fractions is unknown, but may be due to some heterogeneity in the secondary alteration phases. to seawater interaction/alteration and (in the case of cored samples) drilling mud contamination. Seawater alone cannot explain the Sr and Pb isotopic compositional differences between unleached and leached powders of the submarine tholeiites, and interaction with other external contaminants such as Fe‐Mn oxides, deep‐sea sediments, and loess is also likely. For all the samples, the Nd and Hf isotopic compositions of leached and unleached powders are within analytical uncertainty of each other, despite the significant losses (∼90%) of Nd (and other REE) and to a lesser extent Hf. Given these relatively significant elemental losses, care should be taken to ensure that enough material is available to measure isotopic compositions with the required precision after acid leaching of the samples. Due to the trace element losses and REE fractionation during acid leaching, the elemental abundances measured on leached sample powders are not reliable magmatic elemental signatures. Hence, for calculation of initial isotopic ratios at the time of crystallization, parent/daughter ratios should be determined from elemental concentration ratios measured on unleached sample powders. [25] Reliable Hf isotopic data could only be obtained Acknowledgments for the Kerguelen basalts (Figure 4). The Hf isotopic compositions of both the unleached and leached sample powders are the same within analytical uncertainty. The Hf isotopic composition of seawater (mean 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 concentration in seawater (∼7 × 10 –6 ppm [Faure, 1986; Chester, 2003]), no change in the Hf isotopic compositions of the basalts is expected. [27] We thank Bruno Kieffer, Jane Barling, and Bert Mueller for their help in operating the TIMS, MC‐ICP‐MS, and HR‐ ICP‐MS instruments, respectively. We are grateful to Bruno Kieffer, Jane Barling and Mike Garcia for stimulating discussions and their input on earlier versions of the manuscript. Catherine Chauvel, an anonymous reviewer, and the editor Joel Baker, are thanked for their constructive comments that led to an improved manuscript. I. G. Nobre Silva was supported by the POCTI program of the Fundação para a Ciência e Tecnologia (Portugal). Funding for this research was provided by NSERC Discovery Grants (Canada) to D. Weis and J. S. Scoates. References 6. 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