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Leaching systematics and matrix elimination for the determination of high-precision Pb isotope compositions.. Nobre Silva, Ines G.; Weis, Dominique; Barling, Jane; Scoates, James S. 2009

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Leaching systematics and matrix elimination for the determination of high-precision Pb isotope compositions of ocean island basalts Ineˆs Garcia Nobre Silva, Dominique Weis, Jane Barling, 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] Ocean island basalts from Hawaii and Kerguelen were analyzed for their Pb isotopic compositions to assess the effect of acid leaching and matrix elimination by Pb anion exchange columns on reproducibility and accuracy. Unleached samples consistently yield Pb isotopic ratios that reflect the incorporation of foreign material. Leaching removes up to 70–80% of the total Pb content of the samples with corresponding weight losses between 35 and 60%. The older and more altered Kerguelen basalts show better external reproducibility than the Hawaiian basalts, which appears to be due to the presence in the Hawaiian samples of more radiogenic contaminants (e.g., seawater Pb, drilling mud, and related alteration phases). All leached samples purified twice on anion exchange columns show more radiogenic Pb isotopic ratios than those processed once. The difference is larger for tholeiitic basalts (Hawaiian and Kerguelen Plateau) than for transitional to alkalic basalts (Kerguelen Archipelago). The small differences in measured ratios of total procedural triplicates reflect differential elimination of residual alteration via leaching and matrix effects. The effectiveness of matrix elimination depends on the specific basalt composition, and tholeiitic basalts (i.e., low Pb concentrations) require two passes on anion exchange columns. This study shows that all steps in sample processing are critical for achieving accurate high-precision Pb isotopic compositions of ocean island basalts. Components: 11,199 words, 8 figures, 6 tables. Keywords: acid leaching; Pb isotopes; reproducibility; MC-ICP-MS; ocean island basalts; matrix effects. Index Terms: 1040 Geochemistry: Radiogenic isotope geochemistry; 1094 Geochemistry: Instruments and techniques. Received 6 April 2009; Revised 22 June 2009; Accepted 29 June 2009; Published 15 August 2009. Nobre Silva, I. G., D. Weis, J. Barling, and J. S. Scoates (2009), Leaching systematics and matrix elimination for the determination of high-precision Pb isotope compositions of ocean island basalts, Geochem. Geophys. Geosyst., 10, Q08012, doi:10.1029/2009GC002537. 1. Introduction [2] The Pb isotopic compositions of oceanic basalts are extremely useful for evaluating and characterizing the mantle sources and components of both ocean island basalts (OIB) and mid-ocean ridge basalts (MORB) [e.g., Gast et al., 1964; Tatsumoto, 1966, 1978; Abouchami et al., 2000, 2005; Eisele et al., 2003; Blichert-Toft et al., 2003; Marske et al., 2007]. In order for the measured Pb isotopic compositions of basalts to be representa- tive of their mantle source, any foreign elemental contribution introduced by postmagmatic processes, including secondary phases produced during sea- G3GeochemistryGeophysicsGeosystems Published by AGU and the Geochemical Society AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Technical Brief Volume 10, Number 8 15 August 2009 Q08012, doi:10.1029/2009GC002537 ISSN: 1525-2027 Copyright 2009 by the American Geophysical Union 1 of 23water alteration and weathering or any surficial contamination from sample crushing, must be eliminated. Acid leaching of samples prior to dissolution and isotopic analysis has long been recognized as a means of removing such secondary material [e.g., Manhe`s et al., 1978; Dupre´ and Alle`gre, 1980; Hamelin et al., 1986; McDonough and Chauvel, 1991; Weis et al., 2005] and a wide variety of leaching protocols are used by different laboratories [e.g.,Mahoney, 1987;McDonough and Chauvel, 1991; Weis and Frey, 1991; Stracke and Hegner, 1998; Abouchami et al., 2000; Thirlwall, 2000; Eisele et al., 2003; Stracke et al., 2003; Baker et al., 2004; Weis et al., 2005]. [3] In the past, the reproducibility of leached samples was within the analytical precision of Pb isotope ratio measurements (per mil range). How- ever, with the increase in precision (e.g., <200 ppm range on 208Pb/204Pb) achieved by recent analytical developments, such as triple spike thermal ioniza- tion mass spectrometry (TS-TIMS) [e.g., Galer, 1999; Woodhead and Hergt, 2000] and double spike or Tl-corrected multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) [e.g., Longerich et al., 1987; Walder and Furuta, 1993; Belshaw et al., 1998; Rehka¨mper and Halliday, 1998; Thirlwall, 2000; White et al., 2000; Weis et al., 2005], it has become clear that not all leaching techniques provide reproducible results. Reproducibility problems have been reported for Pb isotopic compositions in several studies of oceanic basalts, such as those from the Hawaii Scientific Drilling Project (HSDP) [Abouchami et al., 2000; Eisele et al., 2003] and some Icelandic basalts [Stracke et al., 2003; Baker et al., 2004, 2005; Albare`de et al., 2005]. The poor reproduc- ibility in some of these studies was attributed to variable degrees of sample contamination, sample heterogeneity, and/or the inability of the leaching procedures to consistently eliminate various con- taminants. Recently, it has also been documented that the accuracy of radiogenic isotope ratio meas- urements determined by MC-ICP-MS can be af- fected by nonspectral interferences (matrix effects that affect the ionization and transmission of the analyte, as well as instrumental mass bias) due to residual sample matrix [e.g., Thirlwall, 2002; Woodhead, 2002; Barling and Weis, 2008]. [4] In this contribution, we report Pb isotopic analyses of basalts from twomajor hot spot systems, Hawaii (Pacific Ocean) and Kerguelen (Indian Ocean), by MC-ICP-MS to assess the efficiency of multistep acid leaching in obtaining reproducible Pb isotopic compositions of OIB. This is a companion paper to that of Hanano et al. [2009], which documented different alteration assemblages in Hawaiian and Kerguelen basalts and their behavior during acid leaching based on scanning electron microscopy (SEM) of thin sections and X-ray diffraction (XRD) characterization of sample pow- ders (unleached and leached). To investigate how Pb isotopic ratios are affected by acid-leaching process, two Hawaiian and two Kerguelen samples were chosen from the sample set, where both the Pb contents and isotopic compositions were measured in the acid solutions (leachates) of each leaching step and in the bulk leachates (all leaching step solutions combined), as well as for unleached and leached powders (residues). We then compared the isotopic compositions of unleached and leached powders for other samples from the same hot spot systems. To assess the importance of the residual elemental matrix in the Pb fraction after anion exchange chromatography, we also compared the isotopic compositions of two sets of full procedural triplicates of leached powder splits of samples that were passed once (1) and twice (2) through the purification process. 2. Samples [5] Fifteen basalts from the Hawaiian and Kerguelen oceanic islands were selected for this leaching investigation (see Table 1 for brief sample charac- terization). These samples are representative of basalts typically analyzed for radiogenic isotopic compositions from these two islands and span a wide range of MgO (3.5–18.0 wt %) with weak to moderate alteration (e.g., 0.40–2.8 wt % LOI) (Table 1). Four samples from this study were examined in detail for alteration mineralogy by Hanano et al. [2009]. For the Hawaiian hot spot system, we chose seven tholeiitic samples from the Mauna Loa and Mauna Kea volcanoes; the Mauna Kea samples were all collected from the HSDP-2 drill core. All of the Hawaiian basalts have ages less than 500 ka [Sharp and Renne, 2005] and, with the exception of one sample from Mauna Loa (SW-70), all are submarine in origin. For the Kerguelen hot spot system, seven samples from the Kerguelen Archipelago were chosen, varying from transitional tholeiites to alkalic basalts with ages ranging from 29 to 24 Ma [Nicolaysen et al., 2000], and all were erupted in a subaerial environ- ment. In addition, to evaluate if compositional differences between samples would influence Pb isotopic compositions during leaching, a submarine Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 2 of 23Ta bl e 1. Su m m ar y o fS am pl e G eo ch em ic al Ch ar ac te ris tic s: H aw ai ia n d K er gu el en Sa m pl e Er up tio n En vi ro nm en t D ep th a (m bsl /m asl ) R oc k Ty pe Si O 2 (w t% ) M gO (w t% ) N a 2 O (w t% ) K 2O (w t% ) Pb (pp m) A .I. b LO Ic (w t% ) A ge d (M a) R ef er en ce s H aw ai ia n Ba sa lts M au na Lo a SW - 70 su ba er ial – th ol ei iti c ba sa lt 51 .5 2 9. 33 2. 35 0. 41 2. 50  1. 87 – 0. 14 0 J. M .R ho de s (un pu bli she dd ata ,2 00 3) J2 -0 20 -2 3 su bm ar in e 48 9 th ol ei iti c ba sa lt 50 .1 4 6. 50 2. 72 0. 52 2. 00  0. 87 0. 99  J. M .R ho de s (un pu bli she dd ata ,2 00 3) J2 -0 19 -0 4 su bm ar in e 19 86 th ol ei iti c ba sa lt 49 .8 6 14 .1 9 1. 62 0. 27 1. 00  2. 28 0. 42 0. 45 0 J. M .R ho de s (un pu bli she dd ata ,2 00 3) M au na K ea (H SD P-2 e ) SR 07 05 -0 .1 5 su bm ar in e 18 23 .2 hy al oc la sti te 49 .1 6 11 . 69 1. 81 0. 17 0. 73  1. 76 2. 32 0. 48 8 Rh od es a n d Vo lli ng er [20 04 ] SR 07 68 -1 1. 20 su bm ar in e 21 57 .4 hy al oc la sti te 50 .3 9 6. 76 2. 26 0. 37 1. 09  0. 95 0. 40 0. 50 6 Rh od es a n d Vo lli ng er [20 04 ] SR 09 54 -8 .0 0 su bm ar in e 30 09 .2 th ol ei iti c ba sa lt 47 .6 7 18 .0 3 1. 65 0. 23 0. 66  1. 31 0. 85 0. 55 0 Rh od es a n d Vo lli ng er [20 04 ] SR 09 56 -1 8. 35 su bm ar in e 30 19 .0 th ol ei iti c ba sa lt 49 .7 0 7. 01 2. 25 0. 35 1. 08  1. 34 0. 61 0. 55 0 Rh od es a n d Vo lli ng er [20 04 ] K er gu el en Ba sa lts M on tC ro zi er O B9 3- 16 5 su ba er ial 51 5 al ka lic ba sa lt 50 .9 1 3. 54 3. 52 1. 45 6. 34 0. 67 – 24 .5 D .W ei s (un pu bli she dd ata ,2 00 5) O B9 3- 17 7 su ba er ial 38 0 al ka lic ba sa lt 48 .7 5 3. 89 3. 54 1. 86 – 1. 90 – 24 .5 D .W ei s (un pu bli she dd ata ,2 00 5) O B9 3- 20 2 su ba er ial 78 al ka lic ba sa lt 48 .9 3 4. 05 3. 19 1. 65 5. 58 1. 28 – 24 .5 D .W ei s (un pu bli she dd ata ,2 00 5) M on td es R uc he s B Y 96 -2 7 su ba er ial 45 5 tra ns iti on al ba sa lt 46 .9 0 10 .6 5 2. 16 0. 72 0. 96  0. 04 2. 01 28 .0 D ou ce te t a l. [20 02 ] B Y 96 -3 1 su ba er ial 40 8 tra ns iti on al ba sa lt 49 .9 1 4. 31 2. 84 1. 21 1. 69 0. 01 1. 82 28 .0 D ou ce te t a l. [20 02 ] M on tF on ta in e B Y 96 -8 6 su ba er ial 30 0 tra ns iti on al ba sa lt 45 .8 7 11 .5 0 1. 94 0. 77 1. 71 0. 17 2. 80 28 .0 D ou ce te t a l. [20 02 ] M on tB ur ea u G M 92 -4 8 su ba er ial 25 0 tra ns iti on al ba sa lt 47 .0 9 8. 71 2. 50 0. 43 0. 27 0. 05 – 29 .5 Ya n g et a l. [19 98 ] N or th er n K er gu el en Pl at ea u O D P Le g 18 3, 11 40 A -3 1R -1 , 57 -6 1 su bm ar in e 27 0. 07 th ol ei iti c ba sa lt 49 .6 4 5. 54 2. 64 0. 49 1. 52  0. 81 1. 65 34 .3 W ei s a n d Fr ey [20 02 ] a Fo rs u bm ar in e ba sa lts ,m bs lm ea n s m et er s be lo w se a le ve l; fo rs u ba er ia lb as alt s, m as lm ea n s m et er s ab ov e se a le ve l. b A .I. ,a lk al in ity in de x (A I= to ta la lk al is  (Si O 2  0. 37  14 .4 3)) [R ho de s, 19 96 ]. c LO I, w ei gh tl os s o n ig ni tio n af te r 30 m in at 10 20  C. d A ge sf ro m M au na Lo a, B .S in ge r(u np ub lis he dd ata ,2 00 7); M au na K ea sa m pl es , Sh ar p a n d Re nn e [20 05 ];M on tC ro zi er an d M on tB ur ea u , Ni co la ys en et a l. [20 00 ];M on td es R uc he sa n d M on tF on ta in e, D o u ce t et a l. [20 02 ]; Si te 11 40 ,D un ca n [20 02 ]. e H SD P- 2, H aw ai iS ci en tif ic D ril lin g Pr o jec t,p ha se 2. Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 3 of 23tholeiitic basalt from the 34 Ma Northern Kergue- len Plateau (NKP) recovered during ODP Leg 183 [Weis and Frey, 2002] was also analyzed. All Hawaiian sample powders were prepared from chips that were repeatedly rinsed in deionized water and pulverized following the crushing and washing procedures described by Rhodes [1996] and Rhodes and Vollinger [2004]. For the Kergue- len samples, the powders were prepared following the method described by Doucet et al. [2002]. 3. Analytical Techniques [6] All leaching and chemical separation were carried out in Class 1000 clean labs and the mass spectrometric analyses were performed in Class 10,000 labs at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia. Sample handling in all labs was carried out in Class 100 laminar flow hoods. All reagents used for leaching, dissolution and separation were subboiled, all dilutions were made using 18.2 MWcm deionized water, and all labware was acid-washed prior to use. Whenever sample size permitted, the complete analytical procedure (separate leaching, dissolution and chemistry) was carried out in triplicate (i.e., three separate aliquots or splits of the same starting sample powder). 3.1. Leaching Procedure [7] The sequential acid-leaching procedure used in this study follows that of Weis et al. [2005], which was slightly modified after Mahoney [1987]. Ap- proximately 0.2–0.4 g of sample powder (grain size < 100 mm) were acid-leached with 10 mL of 6M HCl in a 15 mL screw top Savillex1 beaker in an ultrasonic bath for 20 min. This process brought the temperature up to 50C. The supernatant (leachate solution) was immediately decanted be- fore the fines had time to settle. This procedure was repeated until a transparent (i.e., free of fine-size particles), pale yellow to colorless solution was obtained. The same procedure was repeated two more times using 18.2 MWcm water to eliminate any trace of acid. The leached rock powders were then dried to completion on a hot plate at 120C and weighed after cooling. A minimum of six acid- leaching steps was required for the least altered samples and up to 14 steps for the most altered ones. In addition, for four samples (Hawaii: J2- 020-23 (Mauna Loa), SR0954-8.00 (Mauna Kea); Kerguelen: OB93-165 (Mont Crozier, Kerguelen Archipelago), ODP leg 183, 1140A-31-R1, 57-61 (Northern Kerguelen Plateau)), the leachate solu- tions were collected at each step, as well as accumulated for bulk leachate analysis, and were measured for both their Pb contents and Pb isotopic compositions. 3.2. Sample Digestion and Pb Separation [8] Unleached and leached rock powders were digested in a closed vessel using a 1:10 mixture of concentrated HNO3 and HF acids on a hotplate at 120C over 48 h. During this period, the sample solutions were ultrasonicated for 30 min to ensure complete digestion, after which they were dried, redigested in 10 mL of 6M HCl for 24 h and dried again. In preparation for Pb chemistry, the samples were redissolved in 2 mL of 0.5M HBr. As a final precaution, sample solutions were ultrasonicated for 20 min and centrifuged at 14,500 rpm for 6 min before being loaded onto a precleaned and conditioned 200 mL column of fresh AG1-X8 100-200 mesh resin (Bio-Rad Lab- oratories, USA). The matrix was washed out with 0.5M HBr after which Pb was eluted in 6M HCl [Weis et al., 2006]; this column chemistry results in close to 100% recovery of Pb (D. Weis, unpub- lished data, 2005). To assess the efficiency of the Pb purification and potential matrix effects, a second set of leached triplicates of 11 samples was subjected to two passes through the same anion exchange resin. After chemical purification, the eluted Pb fraction was dried, a small quantity of concentrated HNO3 was added to destroy any organic material eluted from the resin along with the Pb, and it was dried again. In preparation for isotopic analyses, the dried Pb fractions were redissolved in 1 mL of 0.05N subboiled HNO3 in an ultrasonic bath. Seven total procedural blanks, including leaching, were measured by ID-TIMS using a 205Pb spike; their Pb concentrations were between 50 to 160 pg (average 100 pg), which is negligible in comparison to the Pb content of the sample powders analyzed (52 to 428 ng). 3.3. Mass Spectrometry 3.3.1. Pb Concentrations [9] The Pb concentrations of leached and unleached sample powders, as well as of each acid-leaching step solution and bulk leachates were measured to determine the amount of Pb present at each step of the acid-leaching procedure. The analyses were performed on an ELEMENT2 high-resolution (HR)-ICP-MS (Thermo Finnigan, Germany) and were quantified using external cal- Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 4 of 23ibration curves and indium (In) as an internal standard. Standard solutions were prepared from a 1000 ppm Specpure1 (Alfa Aesar1, Johnson Matthey Company, USA) Pb standard solution. Samples and standards were diluted and run in 0.15M HNO3. All analyses were normalized to the internal standard and blank subtracted. 3.3.2. Pb Isotopic Compositions [10] The Pb isotopic analyses were performed on a Nu Plasma MC-ICP-MS (Nu Instruments Ltd, UK) under dry plasma conditions using a membrane desolvator (Nu DSN100) for sample introduction. Analyses were made by static multicollection with masses 202 to 208 measured in collectors L2 to H4, respectively. Instrumental mass fractionation was monitored and corrected online using a Spec- pure1 Tl standard solution with a 205Tl/203Tl = 2.3885. This in-house value provides SRM-981 Pb standard (NIST, USA) ratios within error of the triple spike isotopic values [Galer and Abouchami, 1998] and has remained constant since instrument installation (fall 2002). The potential 204Hg isobar- ic interference on the 204Pb ion beam was moni- tored at mass 202 and, when necessary, was corrected for assuming natural abundances (202Hg/204Hg = 4.35) adjusted for instrumental mass fractionation. As is standard practice for all MC-ICP-MS analyses at the PCIGR, all samples and standards were prepared fresh for each analyt- ical session, which can be particularly important for Pb-Tl solutions [Kamenov et al., 2004]. Stan- dard solutions were prepared by combining the SRM-981 Pb standard and the Tl standard solu- tions to give a [Pb]/[Tl] of 4 and diluted with 0.05M HNO3 to obtain an optimal 208Pb ion beam of 8V (and no less than 2V). Samples were also run in 0.05M HNO3 with Tl added to match the standard [Pb]/[Tl] of 4. Matching of [Pb]/[Tl] is done to ensure that sample and standard solutions are matrix-matched as much as possible. To ac- complish this, the Pb content of each solution was determined by HR-ICP-MS analysis of a small aliquot prior to isotopic analysis of the sample by MC-ICP-MS. [11] Sample analysis followed a modified sample- standard bracketing protocol in which the SRM- 981 Pb standard was run after every second sam- ple. The results were then normalized off-line to the triple spike values (206Pb/204Pb = 16.9405, 207Pb/204Pb = 15.4963, and 208Pb/204Pb = 36.7219) [Galer and Abouchami, 1998], using the ln-ln method as described by Albare`de et al. [2004]. For sessions where a systematic drift in the SRM-981 isotopic ratio reference values was ob- served (i.e., drift >2 SD on the average of the analyses for the day), the sample-standard brack- eting normalization was used instead. In all cases, the agreement between the two normalization methods was excellent (better than 2  103, or <50 ppm) for all Pb isotopic ratios. [12] During the course of this study, 176 analyses of the SRM-981 Pb standard yielded mean values of 206Pb/204Pb = 16.9421 ± 0.0030, 207Pb/204Pb = 15.4985 ± 0.0025, and 208Pb/204Pb = 36.7190 ± 0.0078, which are within error of the triple spike values [Galer and Abouchami, 1998]. Reproduc- ibility of the SRM-981 Pb standard on a per session basis was significantly better (±30, ±42 and ±60 ppm/amu for 208Pb/204Pb, 207Pb/204Pb and 206Pb/204Pb, respectively). 4. Results [13] All the analyses acquired during the course of this study are presented in Tables 2–6 and the results are illustrated in Figures 1–8. For the four samples indicated in section 3.1 that were studied in detail for characterization of the acid-leaching procedure, we report the Pb isotopic compositions for selected individual leachate solutions, which were chosen to cover, as best as possible, the entire leaching profile. For the last leaching steps, the Pb isotopic compositions are not reported as their Pb contents were insufficient (7 ng) for analysis. In addition, we subjected six splits each of powder from five Hawaiian and six Kerguelen samples to acid leaching following the procedure described in section 3.1. To assess the influence of residual sample matrix (e.g., Ca, Al, Fe, Mg) still present in the Pb fraction after anion exchange chromatog- raphy on the accuracy and reproducibility of the Pb isotopic compositions, one half of these same powder splits were passed 1 on columns and the other half was passed 2, as described in section 3.2. 4.1. Sequential Leaching [14] Throughout the multistep acid-leaching proce- dure, the Hawaiian basalts lost 35% of their initial weight and the Kerguelen basalts lost 60%, which is consistent with the higher degree of alteration presented by the older Kerguelen basalts [Hanano et al., 2009]. For both Hawaiian and Kerguelen samples, the amount of Pb (as well as Sr, Nd and Hf) that is leached out decreases significantly in the first 1–2 steps and then slowly Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 5 of 23decreases with each progressive step (Tables 2 and 3 and Figures 1–3). Leaching removes up to 70–80% of the original Pb content of each sample. Unleached sample powders are isotopically more radiogenic than their respective leached sample residues, with the exception of the submarine Kerguelen basalt (ODP leg 183, sample 1140A- 31R-1, 57-61), and fall along a mixing line formed by the leached sample residues and the respective bulk leachate solutions. Most of the foreign Pb (40 to 50% for the Hawaiian and submarine Kerguelen tholeiites and 25% for the subaerial Kerguelen Archipelago basalts) is removed during the first leaching step (Figures 1–3), as demon- strated by the similarity between the Pb isotopic compositions of the first leachate solutions and the bulk leachate solutions. As the number of acid- leaching steps increases, the relative amount of Pb Table 2. Pb Content and Pb Isotopic Composition of Each Leaching Step Solution for Two Hawaiian Basalts Fraction ng Pb in Each Fraction % Pb in Each Fraction Column Passes 206Pb/204Pb 2 SE 207Pb/204Pb 2 SE 208Pb/204Pb 2 SE Sample J2-020-23 (Mauna Loa) Unl.a 572.6 100 1 18.3052 0.0067 15.5121 0.0021 38.1726 0.0016 B.L.Homog.b 313.7 54.8 1 18.3463 0.0006 15.5306 0.0006 38.2486 0.0016 B.L.Homog. 313.7 54.8 2 18.3473 0.0007 15.5319 0.0006 38.2538 0.0017 B.L.Acidc 269.1 47.0 1 18.4045 0.0008 15.5575 0.0007 38.3594 0.0019 B.L.Acid 269.1 47.0 2 18.4026 0.0005 15.5553 0.0005 38.3528 0.0014 1st 340.5 59.5 1 18.3978 0.0013 15.5552 0.0012 38.3484 0.0031 2nd 18.6 3.2 1 18.2983 0.0014 15.5096 0.0012 38.1544 0.0036 3rd 12.1 2.1 1 18.2192 0.0021 15.4716 0.0019 37.9844 0.0046 4th 9.2 1.6 1 18.2400 0.0025 15.4816 0.0020 38.0073 0.0056 5th 6.7 1.2 –d – – – – – 6th 3.8 0.7 – – – – – – 1st H2O – – – – – – – – 2nd H2O 2.4 0.4 – – – – – – Totale 393.2 68.7 Leach. Res.f 179.4 31.33 1 18.1799 0.0007 15.4577 0.0006 37.9441 0.0015 Sample SR0954-8.00 (Mauna Kea) Unl. 258.8 100 1 18.6083 0.0025 15.4969 0.0029 38.1771 0.0028 B.L.Homog. 138.6 1 18.6633 0.0008 15.4987 0.0008 38.2166 0.0020 B.L.Homog. repg 138.6 1 18.6735 0.0036 15.5084 0.0030 38.2398 0.0060 B.L.Acid 139.7 1 18.6610 0.0008 15.5037 0.0007 38.2191 0.0017 1st 105.5 40.8 1 18.6687 0.0013 15.5020 0.0013 38.2270 0.0041 2nd 14.5 5.6 1 18.6288 0.0019 15.4962 0.0017 38.1966 0.0040 3rd 16.3 6.3 1 18.6219 0.0012 15.4942 0.0011 38.1924 0.0033 4th 11.3 4.3 – – – – – – 5th 11.1 4.3 – – – – – – 6th 9.2 3.5 1 18.6317 0.0024 15.4984 0.0020 38.1959 0.0058 7th 4.7 1.8 1 18.6477 0.0036 15.5139 0.0031 38.2144 0.0082 8th 5.5 2.1 – – – – – – 1st H2O 2.6 1.0 – – – – – – 2nd H2O 2.6 1.0 – – – – – – Total 183.2 70.8 Leach. Res. 75.6 29.2 1 18.6091 0.0011 15.4874 0.0010 38.1790 0.0029 aUnl., unleached rock powders. bB.L.Homog., bulk leachate solution homogenized (acid + fine powder particles). cB.L.Acid, bulk leachate solution without the fine powder particles. dDashes indicate no isotopic analysis performed. eThe Pb content of the bulk leachate solutions does not equal the sum of the individual leaching steps because they were measured on different aliquots of the same powders. fLeach. Res., leached powder residues. gReplicate analyses of the same sample solution. Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 6 of 23Table 3. Pb Content and Pb Isotopic Composition of Each Leaching Step Solution for Two Kerguelen Basalts Fraction ng Pb in Each Fraction % Pb in Each Fraction Column Passes 206Pb/204Pb 2 SE 207Pb/204Pb 2 SE 208Pb/204Pb 2 SE Sample OB93-165 (Mont Crozier) Unl.a 1827.8 100 1 18.6045 0.0017 15.5817 0.0018 39.3219 0.0036 B.L.Homog.b 443.3 1 18.6066 0.0009 15.5790 0.0008 39.3221 0.0024 B.L.Homog. 443.3 2 18.6118 0.0006 15.5846 0.0005 39.3402 0.0015 B.L.Acidc 419.8 1 18.6045 0.0016 15.5882 0.0018 39.3534 0.0048 B.L.Acid 419.8 2 18.6025 0.0006 15.5872 0.0007 39.3482 0.0019 B.L.Acid repd 419.8 2 18.5987 0.0007 15.5830 0.0006 39.3352 0.0018 1st 445.8 24.4 1 18.6128 0.0011 15.5872 0.0009 39.3470 0.0026 2nd 96.8 5.3 1 18.5939 0.0007 15.5787 0.0006 39.2950 0.0017 3rd 28.5 1.6 1 18.6016 0.0015 15.5811 0.0012 39.2880 0.0034 4th 45.3 2.5 –e – – – – – 5th 21.9 1.2 – – – – – – 6th 22.2 1.2 1 18.6014 0.0017 15.5786 0.0017 39.2741 0.0049 7th 8.6 0.5 1 18.6286 0.0042 15.5839 0.0030 39.2643 0.0086 8th 8.3 0.5 – – – – – – 1st H2O 3.2 0.2 – – – – – – 2nd H2O 1.8 0.1 – – – – – – Totalf 682.3 37.3 Leach. Res.g 1145.6 62.7 1 18.5959 0.0010 15.5769 0.0008 39.2961 0.0020 ODP Leg 183, 1140A-31R-1, 57-61 (Northern Kegguelen Plateau) Unl. 572.5 100.00 1 18.5451 0.0008 15.5619 0.0009 38.9056 0.0029 B.L.Homog. 332.7 1 18.5409 0.0011 15.5624 0.0013 38.9003 0.0042 B.L.Homog. 332.7 2 18.5420 0.0025 15.5622 0.0031 38.9019 0.0105 B.L.Acid 314.2 1 18.5428 0.0010 15.5625 0.0010 38.9025 0.0027 B.L.Acid 314.2 2 18.5431 0.0026 15.5623 0.0030 38.9039 0.0099 1st 262.6 45.9 1 18.5487 0.0011 15.5685 0.0011 38.9182 0.0030 2nd 59.9 10.5 1 18.5484 0.0008 15.5646 0.0007 38.9090 0.0019 3rd 27.3 4.8 1 18.5461 0.0012 15.5635 0.0012 38.8882 0.0028 4th 19.9 3.5 1 18.5436 0.0022 15.5627 0.0023 38.8781 0.0064 5th 15.6 2.7 – – – – – – 6th 14.9 2.6 1 18.5521 0.0012 15.5648 0.0011 38.8814 0.0031 7th 11.9 2.1 1 18.5635 0.0018 15.5659 0.0017 38.8893 0.0056 8th 7.5 1.3 1 18.5741 0.0027 15.5648 0.0023 38.8627 0.0064 9th 8.8 1.5 1 18.5856 0.0018 15.5723 0.0016 38.9003 0.0040 10th 5.1 0.9 1 18.6111 0.0032 15.5922 0.0029 38.8502 0.0067 11th 5.6 1.0 – – – – – – 12th 4.8 0.8 – – – – – – 13th 5.1 0.9 – – – – – – 14th 3.1 0.5 – – – – – – 1st H2O 2.6 0.4 – – – – – – 2nd H2O 2.0 0.3 – – – – – – Total 456.8 79.8 Leach. Res. 115.7 20.2 1 18.5575 0.0006 15.5648 0.0007 38.9405 0.0020 aUnl., unleached rock powders. bB.L.Homog., bulk leachate solution homogenized (acid + fine powder particles). cB.L.Acid, bulk leachate solution without the fine powder particles. dReplicate analyses of the same sample solution. eDashes indicate no isotopic analysis performed. fThe Pb content of the bulk leachate solutions does not equal the sum of the individual leaching steps because they were measured on different aliquots of the same powders. gLeach. Res., leached powder residues. Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 7 of 23Table 4. Pb Isotopic Compositions of Tholeiitic Basalts From Mauna Loa and Mauna Kea Sample Acid Leaching Steps % Weight Loss Column Passes 206Pb/204Pb 2 SE 207Pb/204Pb 2 SE 208Pb/204Pb 2 SE Mauna Loa SW-70-1a – – 1 18.1092 0.0014 15.4842 0.0016 37.9295 0.0045 SW-70-2 – – 1 18.1059 0.0020 15.4824 0.0023 37.9235 0.0079 SW-70-3 – – 1 18.0993 0.0014 15.4759 0.0016 37.9069 0.0051 Ext. Reprod.b (ppm) 558 566 619 SW-70-Ic 6 39.15 1 18.0721 0.0007 15.4841 0.0008 37.9454 0.0023 SW-70-II 6 34.50 1 18.0562 0.0009 15.4590 0.0011 37.8660 0.0033 SW-70-III 6 35.50 1 18.0551 0.0008 15.4568 0.0009 37.8588 0.0029 Ext. Reprod. (ppm) 1053 1960 2536 J2-020-23-1 – – 1 18.3052 0.0016 15.5121 0.0021 38.1726 0.0067 J2-020-23-I 7 37.28 1 18.1801 0.0007 15.4571 0.0006 37.9415 0.0018 J2-020-23-II 7 26.76 1 18.1772 0.0005 15.4539 0.0006 37.9323 0.0014 J2-020-23-III 7 38.45 1 18.1797 0.0012 15.4568 0.0013 37.9401 0.0041 J2-020-23-III repd 7 38.45 1 18.1800 0.0009 15.4571 0.0008 37.9414 0.0022 Ext. Reprod. (ppm) 153 198 230 J2-020-23-ae 7 37.39 2 18.1799 0.0008 15.4568 0.0007 37.9401 0.0020 J2-020-23-b 7 36.21 2 18.1805 0.0008 15.4573 0.0009 37.9418 0.0026 J2-020-23-c 7 30.74 2 18.1809 0.0013 15.4576 0.0013 37.9437 0.0042 J2-020-23-c rep 7 30.74 2 18.1779 0.0013 15.4544 0.0013 37.9355 0.0036 Ext. Reprod. (ppm) 146 187 185 J2-019-04-1 – – 1 18.4951 0.0012 15.5703 0.0014 38.4676 0.0042 J2-019-04-I 7 34.78 1 18.1796 0.0008 15.4583 0.0008 37.9751 0.0022 J2-019-04-II 7 34.83 1 18.1774 0.0007 15.4558 0.0006 37.9677 0.0018 Ext. Reprod. (ppm) 170 227 278 J2-019-04-a 7 28.89 2 18.1806 0.0009 15.4601 0.0009 37.9795 0.0024 Mauna Kea (HSDP-2) SR0705-0.15-1 – – 1 18.5130 0.0009 15.5042 0.0009 38.1661 0.0028 SR0705-0.15-2 – – 1 18.6619 0.0013 15.5289 0.0011 38.2650 0.0037 SR0705-0.15-3 – – 1 18.7945 0.0020 15.5518 0.0023 38.3567 0.0073 Ext. Reprod. (ppm) 15096 3065 4983 SR0705-0.15-I 6 40.05 1 18.3372 0.0017 15.4662 0.0020 38.0123 0.0067 SR0705-0.15-II 6 38.63 1 18.3201 0.0010 15.4768 0.0011 38.0054 0.0029 SR0705-0.15-III 6 37.02 1 18.3475 0.0017 15.4686 0.0020 38.0264 0.0063 Ext. Reprod. (ppm) 1514 719 564 SR0768-11.20-1 – – 1 18.5598 0.0015 15.4854 0.0019 38.1574 0.0064 SR0768-11.20-I 11 30.63 1 18.5380 0.0011 15.4819 0.0011 38.1435 0.0031 SR0768-11.20-II 11 27.31 1 18.5338 0.0006 15.4776 0.0006 38.1313 0.0019 SR0768-11.20-III 10 33.72 1 18.5373 0.0010 15.4817 0.0009 38.1422 0.0027 Ext. Reprod. (ppm) 243 309 351 SR0768-11.20-a 9 31.42 2 18.5374 0.0014 15.4801 0.0016 38.1426 0.0049 SR0768-11.20-b 9 32.67 2 18.5357 0.0017 15.4817 0.0020 38.1379 0.0062 SR0768-11.20-c 9 29.72 2 18.5376 0.0011 15.4813 0.0013 38.1422 0.0039 Ext. Reprod. (ppm) 115 106 137 SR0954-8.00-1 – – 1 18.6336 0.0014 15.4921 0.0012 38.1946 0.0034 SR0954-8.00-2 – – 1 18.6346 0.0012 15.4921 0.0011 38.1943 0.0033 SR0954-8.00-3 – – 1 18.6323 0.0010 15.4908 0.0011 38.1898 0.0034 Ext. Reprod. (ppm) 122 98 142 SR0954-8.00-I 6 27.60 1 18.5986 0.0012 15.4811 0.0016 38.1574 0.0043 SR0954-8.00-II 6 19.09 1 18.6031 0.0013 15.4832 0.0014 38.1641 0.0050 Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 8 of 23that is removed becomes increasingly smaller (<1.5%), after 3–4 steps for the Mauna Loa and Mont Crozier basalts and after 6–7 steps for the Mauna Kea and Northern Kerguelen Plateau basalts (Tables 2 and 3). [15] For the Hawaiian basalts (Figures 1, 3a, and 3b), the Pb isotopic compositions of the individual leachate solutions generally plot along a mixing trend between a more radiogenic end-member and the leached sample residue. In contrast, for the Kerguelen basalts (Figures 2, 3c, and 3d), the Pb isotopic compositions of the individual leachate solutions after leaching step 5 deviate from the mixing line formed by the residual leached sample powder and bulk leachate solutions. The leachate solutions corresponding to higher leaching steps are insignificant in terms of their Pb content (Figures 1–3). Nevertheless, these leachate solutions yield more radiogenic Pb isotope ratios, which indi- cates the presence of an additional minor component. [16] With the exception of three Hawaiian samples, the unleached sample powders show better repro- ducibility (e.g., 122 and 93 ppm on 206Pb/204Pb for samples SR954-8.00 and OB93-165, respectively) than their respective leached sample residues (e.g., 254 and 230 ppm on 206Pb/204Pb for the same two samples) (Tables 4–6). In a 208Pb/204Pb versus 206Pb/204Pb diagram (Figure 8a), the individual leaching trends have steeper slopes than the ‘‘Kea-mid8’’ and ‘‘Kea-hi8’’ groups of HSDP-2 basalts defined by Eisele et al. [2003]. The isotopic compositions of the unleached powders for three Hawaiian basalts (J2-019-04, J2-020-23, SR0705- 0.15) yield much higher dispersion and plot dis- tinctly outside the fields of their respective volcano (Figure 8), which reflects the presence of a much higher proportion of contamination and secondary components (Figure 6 and see section 5.1 below). The Hawaiian basalts, despite significantly youn- ger ages and less alteration than the Kerguelen basalts, show much larger differences between the Pb isotopic compositions of unleached powders, bulk leachate solutions and their residue. [17] The number of acid-leaching steps affects the final Pb isotopic reproducibility of a sample. In Figure 4, we compare results for an alkalic basalt (OB93-165) from the Kerguelen Archipelago, for which powder splits were subjected either to six acid-leaching steps (3 aliquots) or eight acid-leaching steps (7 aliquots). The Pb isotopic compositions of the powder splits that were subjected to six acid- leaching steps have a reproducibility of 230 ppm on 206Pb/204Pb, whereas those subjected to eight steps have a reproducibility of 136 ppm (Table 5). Comparable improvements in reproducibility were observed for 207Pb/204Pb and for 208Pb/204Pb. Al- though just within error, the average isotopic composition of the powder splits that were sub- jected to eight acid-leaching steps is slightly less radiogenic (i.e., further from the composition of the unleached powder and leachate solutions) than the average of those that were leached six times. Table 4. (continued) Sample Acid Leaching Steps % Weight Loss Column Passes 206Pb/204Pb 2 SE 207Pb/204Pb 2 SE 208Pb/204Pb 2 SE SR0954-8.00-III 6 49.92 1 18.6020 0.0014 15.4804 0.0013 38.1561 0.0041 Ext. Reprod. (ppm) 254 186 225 SR0954-8.00-a 6 28.64 2 18.6100 0.0006 15.4874 0.0006 38.1787 0.0018 SR0954-8.00-b 6 28.75 2 18.6089 0.0008 15.4864 0.0008 38.1761 0.0019 SR0954-8.00-c 6 31.82 2 18.6075 0.0010 15.4869 0.0009 38.1764 0.0027 Ext. Reprod. (ppm) 135 62 74 SR0956-18.35-1 – – 1 18.5075 0.0009 15.4786 0.0009 38.1409 0.0025 SR0956-18.35-I 9 34.93 1 18.4892 0.0006 15.4748 0.0005 38.1281 0.0014 SR0956-18.35-II 8 34.62 1 18.4864 0.0009 15.4717 0.0009 38.1182 0.0026 Ext. Reprod. (ppm) 210 283 370 SR0956-18.35-a 8 35.45 2 18.4857 0.0010 15.4742 0.0011 38.1250 0.0033 aNumbers 1, 2, and 3 refer to complete procedural duplicates of unleached samples. bExt. Reprod., external reproducibility of the set of the same type of analyses (i.e., 1, 2, and 3; I, II, and III; and a, b, and c), expressed in ppm (2 SD on the mean of individual analyses of each set of triplicates of the same sample: 2 SD/mean  106). cRoman numerals I, II, and III refer to complete procedural duplicates of leached samples that were passed once on anion exchange columns. dReplicate analyses of the same sample solutions by MC-ICP-MS are indicated by rep. eLetters a, b, and c refer to complete procedural duplicates of leached samples that were passed twice on anion exchange columns. Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 9 of 23Table 5. Pb Isotopic Compositions of Subaerial Alkalic Basalts From Mont Crozier on the Kerguelen Archipelago Sample Acid Leaching Steps % Weight Loss Column Passes 206Pb/204Pb 2 SE 207Pb/204Pb 2 SE 208Pb/204Pb 2 SE OB93-165-1a – – 1 18.6054 0.0015 15.5830 0.0012 39.3245 0.0037 OB93-165-2 – – 1 18.6038 0.0015 15.5812 0.0017 39.3212 0.0034 OB93-165-3 – – 1 18.6037 0.0011 15.5811 0.0010 39.3207 0.0028 OB93-165-3 repb – – – 18.6050 0.0009 15.5815 0.0013 39.3212 0.0043 Ext. Reprod.c (ppm) 93 114 90 OB93-165-Id 6 36.20 1 18.5969 0.0010 15.5788 0.0010 39.3010 0.0030 OB93-165-II 6 35.49 1 18.5930 0.0010 15.5769 0.0012 39.2908 0.0036 OB93-165-III 6 34.67 1 18.5936 0.0009 15.5751 0.0010 39.2894 0.0033 Ext. Reprod. (ppm) 230 239 324 OB93-165-A2e 8 50.44 1 18.5892 0.0007 15.5765 0.0007 39.2840 0.0023 OB93-165-A3 8 50.44 1 18.5895 0.0009 15.5763 0.0010 39.2853 0.0032 OB93-165-B2 8 45.31 1 18.5895 0.0014 15.5778 0.0018 39.2870 0.0055 OB93-165-B3 8 45.31 1 18.5868 0.0021 15.5750 0.0026 39.2786 0.0091 OB93-165-C2 8 41.31 1 18.5868 0.0015 15.5749 0.0017 39.2803 0.0059 OB93-165-C3 8 41.31 1 18.5871 0.0011 15.5746 0.0010 39.2778 0.0028 OB93-165-C3 rep 8 41.31 1 18.5881 0.0018 15.5744 0.0019 39.2773 0.0064 Ext. Reprod. (ppm) 136 158 199 OB93-165-af 7 62.00 2 18.5890 0.0008 15.5758 0.0009 39.2823 0.0025 OB93-165-b 7 52.49 2 18.5924 0.0008 15.5756 0.0011 39.2878 0.0031 OB93-165-b rep 7 52.49 2 18.5920 0.0009 15.5775 0.0011 39.2890 0.0026 OB93-165-c 7 62.65 2 18.5920 0.0009 15.5770 0.0010 39.2898 0.0031 Ext. Reprod. (ppm) 172 120 172 OB93-165-A1 8 50.44 2 18.5898 0.0015 15.5773 0.0019 39.2851 0.0065 OB93-165-B1 8 45.31 2 18.5868 0.0014 15.5749 0.0015 39.2808 0.0049 OB93-165-C1 8 41.31 2 18.5892 0.0028 15.5765 0.0035 39.2833 0.0120 Ext. Reprod. (ppm) 175 161 110 OB93-177-1 – – 1 18.4802 0.0013 15.5682 0.0015 39.1292 0.0044 OB93-177-I 7 55.99 1 18.4459 0.0009 15.5606 0.0009 39.0594 0.0022 OB93-177-II 6 36.86 1 18.4493 0.0010 15.5609 0.0010 39.0603 0.0026 OB93-177-III 4 80.64 1 18.4534 0.0010 15.5615 0.0009 39.0610 0.0025 Ext. Reprod. (ppm) 410 57 43 OB93-177-a 6 61.99 2 18.4528 0.0014 15.5624 0.0014 39.0639 0.0035 OB93-177-b 5 54.36 2 18.4534 0.0008 15.5630 0.0008 39.0654 0.0020 OB93-177-c 5 45.91 2 18.4480 0.0008 15.5598 0.0011 39.0583 0.0021 Ext. Reprod. (ppm) 322 220 191 OB93-202-1 – – 1 18.4917 0.0009 15.5734 0.0010 39.1247 0.0029 OB93-202-I 7 62.62 1 18.4957 0.0008 15.5724 0.0007 39.1123 0.0015 OB93-202-II 5 52.21 1 18.4934 0.0007 15.5728 0.0007 39.1090 0.0017 Ext. Reprod. (ppm) 176 35 118 OB93-202-a 5 58.25 2 18.4976 0.0009 15.5740 0.0009 39.1166 0.0025 aNumbers 1, 2, and 3 refer to complete procedural duplicates of unleached samples. bReplicate analyses of the same sample solutions by MC-ICP-MS are indicated by rep. cExt. Reprod., external reproducibility of the set of the same type of analyses (i.e., 1, 2, and 3; I, II, and III; and a, b, and c), expressed in ppm (2 SD on the mean of individual analyses of each set of triplicates of the same sample: 2 SD/mean  106). dRoman numerals I, II, and III refer to complete procedural duplicates of leached samples that were passed once on anion exchange columns. eCapital letters A2, A3, B2, B3, C2, and C3 refer to portions of three leached duplicates that were passed once on anion exchange columns, and capital letters A1, B1, and C1 refer to fractions of three leached duplicates that were passed twice on Pb anion exchange. fLetters a, b, and c refer to complete procedural duplicates of leached samples that were passed twice on anion exchange columns. Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 10 of 23Table 6. Pb Isotopic Compositions of Subaerial Tholeiitic Transitional Basalts From Mont des Ruches, Mont Fontaine, and Mont Bureau on the Kerguelen Archipelago and of a Submarine Tholeiitic Basalt From ODP Leg 183, Site 1140, on the Northern Kerguelen Plateau Sample Acid Leaching Steps % Weight Loss Column Passes 206Pb/204Pb 2 SE 207Pb/204Pb 2 SE 208Pb/204Pb 2 SE Mont Des Ruches BY96-27-1a – – 1 18.2551 0.0006 15.5272 0.0005 38.8273 0.0014 BY96-27-2 – – 1 18.2543 0.0011 15.5273 0.0010 38.8260 0.0029 BY96-27-3 – – 1 18.2534 0.0011 15.5258 0.0011 38.8210 0.0035 Ext. Reprod.b (ppm) 95 110 171 BY96-27-Ic 6 29.46 1 18.2539 0.0014 15.5201 0.0016 38.7853 0.0055 BY96-27-II 6 30.03 1 18.2542 0.0010 15.5211 0.0010 38.7885 0.0031 BY96-27-III 6 31.34 1 18.2495 0.0006 15.5167 0.0006 38.7737 0.0021 Ext. Reprod. (ppm) 288 301 402 BY96-31-1 – – 1 18.3490 0.0008 15.5516 0.0010 39.0494 0.0042 BY96-31-I 11 44.32 1 18.3222 0.0014 15.5514 0.0017 39.0013 0.0055 BY96-31-II 10 41.67 1 18.3225 0.0019 15.5497 0.0022 38.9980 0.0075 Ext. Reprod. (ppm) 28 155 118 BY96-31-ad 12 58.54 2 18.3258 0.0015 15.5528 0.0014 39.0097 0.0037 BY96-31-b 12 54.62 2 18.3200 0.0012 15.5479 0.0010 38.9925 0.0029 BY96-31-c 11 41.74 2 18.3228 0.0018 15.5510 0.0007 39.0034 0.0007 Ext. Reprod. (ppm) 318 320 445 Mont Fontaine BY96-86-1 – – 1 18.3610 0.0008 15.5436 0.0007 39.0256 0.0022 BY96-86-1 repe – – 1 18.3624 0.0014 15.5448 0.0017 39.0217 0.0037 Ext. Reprod. (ppm) 106 110 138 BY96-86-I 13 58.93 1 18.3803 0.0008 15.5397 0.0007 39.0083 0.0019 BY96-86-II 10 59.20 1 18.3776 0.0012 15.5387 0.0009 39.0034 0.0027 Ext. Reprod. (ppm) 212 85 178 BY96-86-a 9 55.45 2 18.3788 0.0009 15.5406 0.0009 39.0084 0.0021 Mont Bureau GM92-48-1 – – 1 18.4760 0.0013 15.5660 0.0012 38.5495 0.0036 GM92-48-1 rep – – 1 18.4748 0.0008 15.5639 0.0008 38.5457 0.0024 Ext. Reprod. (ppm) 90 196 142 GM92-48-I 10 59.85 1 18.4475 0.0014 15.5199 0.0014 38.6163 0.0043 GM92-48-II 9 60.08 1 18.4472 0.0015 15.5179 0.0014 38.6113 0.0040 GM92-48-III 11 61.24 1 18.4454 0.0014 15.5170 0.0013 38.6106 0.0036 Ext. Reprod. (ppm) 123 194 160 Northern Kerguelen Plateau: ODP Leg 183, Site 1140 1140A-31R-1, 57-61-1 – – 1 18.5451 0.0008 15.5619 0.0009 38.9056 0.0029 1140A-31R-1, 57-61-I 13 64.01 1 18.5583 0.0012 15.5637 0.0010 38.9384 0.0027 1140A-31R-1, 57-61-II 13 54.72 1 18.5519 0.0015 15.5570 0.0015 38.9214 0.0046 1140A-31R-1, 57-61-III 10 50.99 1 18.5567 0.0009 15.5631 0.0008 38.9349 0.0023 Ext. Reprod. (ppm) 359 475 461 1140A-31R-1, 57-61-a 10 49.73 2 18.5571 0.0008 15.5636 0.0007 38.9383 0.0020 1140A-31R-1, 57-61-b 11 54.89 2 18.5622 0.0009 15.5642 0.0008 38.9369 0.0021 1140A-31R-1, 57-61-c 14 59.99 2 18.5594 0.0013 15.5646 0.0012 38.9420 0.0032 Ext. Reprod. (ppm) 274 68 134 aNumbers 1, 2, and 3 refer to complete procedural duplicates of unleached samples. bExt. Reprod., external reproducibility of the set of the same type of analyses (i.e., 1, 2, and 3; I, II, and III; and a, b, and c), expressed in ppm (2 SD on the mean of individual analyses of each set of triplicates of the same sample: 2 SD/mean  106). cRoman numerals I, II, and III refer to complete procedural duplicates of leached samples that were passed once on anion exchange columns. dLetters a, b, and c refer to complete procedural duplicates of leached samples that were passed twice on anion exchange columns. eReplicate analyses of the same sample solutions by MC-ICP-MS are indicated by rep. Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 11 of 23Figure 1. Diagrams of 208Pb/204Pb versus 206Pb/204Pb showing the results of the leaching experiments for two tholeiitic Hawaiian basalts: (a) sample J2-020-23 (Mauna Loa) and (b) sample SR0954-8.00 (Mauna Kea). Each circle represents the Pb isotopic composition for each individual leaching fraction, and the size of the circle reflects the relative amount of Pb present in the fraction. The open circles indicate the analyses of the unleached sample powders and correspond to 100% of the Pb content in the sample. The filled circles without outlines (labeled as ‘‘Leached Sample’’) indicate analyses of the leached sample residues, and the symbol size corresponds to the amount of Pb that remained after the entire leaching procedure. The smaller filled circles indicate the analyses of leachates, and the number labeled adjacent to each of these circles refers to the leaching step (see inset diagrams). The dark and light gray outlined circles represent the bulk leachate solution recovered throughout the leaching procedure; the size of these two circles is the same, as the Pb content is equivalent at the scale of Figure 1. The lighter gray circle refers to the isotopic composition of the acid solution plus silt size particles that were removed during leaching, whereas the darker gray circle refers to the isotopic composition of the acid solution alone, after separation of the particles by centrifugation. In the bottom right corner of each diagram, the average 2 SE of the Pb isotopic analysis for each experiment is indicated. The inset diagrams show the percentage of Pb that was eliminated at each leaching step (histogram) and the amount of Pb (ppm) present in each leachate (line graph) as well as the amounts of Sr, Nd, and Hf (also in ppm) for comparison (I. G. Nobre Silva, unpublished data, 2009). Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 12 of 234.2. Pb Purification [18] The matrix elimination experiments show dif- ferences between full procedural sample triplicates that were processed once and twice by anion ex- change chromatography (Tables 4–6 and Figure 5). Sample triplicates subjected to the same number of acid-leaching steps (six for the Hawaiian sample and eight for the Kerguelen sample) that were purified twice yield Pb isotopic compositions that are in general more reproducible (e.g., 135 and 74 ppm difference in 206Pb/204Pb and 208Pb/204Pb for Hawaii and 175 and 110 ppm for Kerguelen, respectively) than the triplicates that were purified only once (e.g., 254 and 225 ppm difference in 206Pb/204Pb and 208Pb/204Pb for Hawaii and 136 and 199 ppm for Kerguelen, respectively). The differences in reproducibility are more significant Figure 2. Diagrams of 208Pb/204Pb versus 206Pb/204Pb showing the results of the leaching experiments for two Kerguelen basalts: (a) sample OB93-165 (Mont Crozier, Kerguelen Archipelago) and (b) sample 1140A-31-R1, 57- 61 (ODP Leg 183, Northern Kerguelen Archipelago). Color coding, symbol sizing, labeling, and insets are as described in the caption of Figure 1. KA, Kerguelen Archipelago; NPK, Northern Kerguelen Plateau. Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 13 of 23for the Hawaiian sample and may reflect greater matrix effects (see section 5.2 below). [19] The number of acid-leaching steps that the powder splits are subjected to affects the relative differences in the measured isotopic ratios of the samples processed once versus twice on columns. For theMont Crozier sample OB93-165 (Figure 7a), the difference in the Pb isotopic ratios of powder splits that were purified once or twice decreases with an increasing number of acid-leaching steps. For the Mauna Loa samples J2-019-04 and J2-020- 23 (Figure 6a), the powder splits were subjected to seven acid-leaching steps and show negligible difference in their isotopic compositions after one and two passes on the Pb anion exchange columns, whereas for the Mauna Kea sample SR0954-8.00 (Figures 6a and 6b), where the powder splits were subjected to six leaching steps, the Pb isotopic signatures are clearly distinct. [20] Finally, all triplicates, from Hawaiian and Kerguelen samples, that were processed twice on columns show more radiogenic Pb isotopic com- positions (Tables 4–6). The results for the tripli- cates of Mauna Kea sample SR0954-8.00 that were subjected to one pass versus two passes on columns are not within error of each other (Figures 5a and 6b). For the Kerguelen Archipelago basalts, the differences in Pb isotopic compositions for samples with one versus two passes on columns are within error (Figures 5b and 7a). For the submarine basalt on the Northern Kerguelen Plateau (Figure 7b), even though the powder splits were subjected to higher numbers of acid-leaching steps (10–14), the relationship between one versus two passes on columns is preserved. 5. Discussion 5.1. Implications of Sequential Acid Leaching [21] Acid leaching of whole rock powders of oceanic basalts leaves a residue of plagioclase Figure 3. Diagrams of 207Pb/204Pb versus 206Pb/204Pb showing the results of the leaching experiments: (a) Hawaiian basalt sample J2-020-23, (b) Hawaiian basalt sample SR0954-8.00, (c) Kerguelen basalt sample OB93-165, and (d) Northern Kerguelen Plateau basalt sample 1140A-31-R1, 57-61. Color coding, symbol sizing, and labeling are as described in the caption of Figure 1; abbreviations are as indicated in the caption of Figure 2. Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 14 of 23and clinopyroxene ± olivine and oxides that pro- vides the closest estimate of the magmatic isotopic compositions [e.g., Mahoney, 1987; Regelous et al., 2003; Hanano et al., 2009]. For both the Hawaiian and Kerguelen basalts examined in this study, the relationship between full procedural triplicates of unleached and leached samples defines trends that are oblique to calculated mass fractionation lines (Figures 6 and 7). These trends result from the removal by acid leaching of foreign Pb hosted in low-temperature alteration minerals or in any potential contaminant (i.e., drilling mud, secondary minerals related to seawater alteration) [Hanano et al., 2009]. [22] For the tholeiitic Hawaiian basalts, the differ- ence in the Pb isotopic compositions between full procedural triplicates of unleached and leached sample splits reflects the incorporation of a local contaminant, such as seawater Pb as represented by Pacific Fe-Mn nodules [Abouchami and Galer, 1998; Eisele et al., 2003] (Figure 6a). For the two submarine tholeiites from Mauna Loa (J2- 019-04 and J2-020-23), the compositions of the unleached and leached sample splits trend toward the field of the Pacific Fe-Mn nodules (Figure 6a), which is consistent with the observation by Hanano et al. [2009] of finely banded Mn oxides filling void spaces in sample J2-019-04. [23] For the Mauna Kea samples collected from the HSDP-2 drill core (Figures 6 and 8), the trends of the leaching results do not intercept the field for Pacific Fe-Mn nodules indicating that seawater Pb is not a major contaminant. The trends point toward a much more radiogenic contaminant, with potentially variable Pb isotopic compositions, such as the drilling mud used during HSDP-2 (borehole mud and mud from the right slag pond [Abouchami et al., 2000; Eisele et al., 2003]). Distinct patches of barite/celestite have been identified within sam- ple SR0954-8.00 [Hanano et al., 2009], thus despite the careful sample washing procedures employed for the HSDP sample suite prior to crushing [Rhodes and Vollinger, 2004], unleached HSDP-2 samples may be contaminated by interac- tion with drilling mud. The distinctly larger differ- ences in Pb isotopic compositions between unleached and leached powder splits observed for the Hawaiian basalts compared to the Kerguelen Figure 4. Diagram of 208Pb/204Pb versus 206Pb/204Pb showing the reproducibility of the Pb isotopic compositions of powder splits from the same alkalic basalt (OB93-165) from the Kerguelen Archipelago that were subjected to six acid-leaching steps (three aliquots) and eight acid-leaching steps (seven aliquots). All these samples were purified once through Pb anion exchange columns prior to analysis by MC-ICP-MS. The error bars on each individual symbol represent the 2 SE of the individual run. The average (mean) Pb isotopic composition and 2 SD for each set of aliquots subjected to different numbers of leaching steps are represented by the larger gray symbols with thicker error bars. The number of acid-leaching steps is indicated as well as the external reproducibility (2 SDm in parts per million (ppm)) of 206Pb/204Pb and 208Pb/204Pb. Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 15 of 23Figure 5. Diagrams of 208Pb/204Pb versus 206Pb/204Pb for powder aliquots of (a) a tholeiitic Hawaiian basalt (sample SR0954-8.00 (Mauna Kea)) and (b) an alkalic Kerguelen basalt (sample OB93-165 (Mont Crozier)) showing the effect of purifying samples by anion exchange chromatography. For each sample, the powder aliquots were subjected to the same number of acid-leaching steps (six steps for the Hawaiian sample and eight steps for the Kerguelen sample) and processed once or twice through the Pb anion exchange columns prior to analysis by MC- ICP-MS. The error bars on the individual symbols represent the 2 SE of the individual runs. The average Pb isotopic compositions and 2 SD for the different groups of analyses are represented by the larger gray symbols and thicker error bars. Also identified are the number of acid-leaching steps that each group of powders was subjected to as well as their reproducibility (2 SDm expressed in ppm) for 206Pb/204Pb and 208Pb/204Pb. Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 16 of 23Figure 6. Diagrams of 208Pb/204Pb versus 206Pb/204Pb showing the isotopic compositions obtained from the acid- leaching and matrix elimination experiments for tholeiitic Hawaiian basalts (samples J2-019-04, J2-020-23, and SR0954-8.00). (a) Larger scale. Potential contaminants are also reported, including seawater Pb, represented by the field for Pacific Fe-Mn nodules [Abouchami and Galer, 1998], and HSDP-2 borehole mud and mud from the right slag pond [Abouchami et al., 2000; Eisele et al., 2003], represented as triangles. Also shown are the mass fractionation lines (red solid lines) calculated for these samples. (b) Enlarged portion of Figure 6a, focusing on the isotopic results for leaching and matrix elimination experiments of sample SR0954-8.00 from Mauna Kea. The calculated mass fractionation line is shown as the red solid line, and its associated 2 SD is shown as subparallel red dashed lines. The gray arrows on both diagrams show the direction of possible contamination that could explain the relationship between the isotopic results for the leached (colored symbols) and unleached (open symbols) samples. Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 17 of 23Figure 7. Diagrams of 208Pb/204Pb versus 206Pb/204Pb showing the isotopic compositions obtained with the acid- leaching and matrix elimination experiments for Kerguelen basalts. (a) The results for the subaerial alkalic sample OB93-165 from Mont Crozier on the Kerguelen Archipelago and (b) the results for the submarine tholeiitic sample 1140A-31R-1, 57-61, recovered during ODP Leg 183 (Site 1140) from the Northern Kerguelen Plateau (NKP). Also reported are the mass fractionation lines (red solid lines) calculated for these samples and their associated 2 SD (red dashed lines); the pale gray areas represent the range of uncertainty of the calculated mass fractionation. The inset diagram in the top left of Figure 7b shows the results for sample 1140A-31R-1, 57-61, and the Pb isotopic composition of Indian Ocean seawater, which is represented by the field for Fe-Mn deposits from the Kerguelen Plateau, Crozet Basin, and Australian-Antarctic Basin [Vlaste´lic et al., 2001]. The relationship between the results for the unleached and leached duplicates in these experiments cannot be explained by interaction with seawater. Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 18 of 23basalts (Figures 8a and 8b) is likely accounted for by contamination of the HSDP-2 samples with drilling mud and by interaction with seawater for the Mauna Loa samples. These larger differences were surprising as the Hawaiian basalts are much younger (500 ka versus 24–34 Ma) and signif- icantly less altered than the Kerguelen basalts. [24] For the Kerguelen Archipelago basalts, the unleached powder splits plot within 2s error of each other and typically have more radiogenic Pb isotopic compositions than the leached splits (Figures 7a and 8b). The results for the leached powder splits for sample OB93-165 (Figure 7a) trend toward less radiogenic isotopic compositions. This trend is systematic and correlates with an increasing number of leaching steps, which is consistent with progressive removal of secondary alteration minerals by sequential acid leaching. For this subaerial alkalic basalt, the trend corresponds to a mixing line with an unidentified component associated with alteration. All the other Kerguelen samples show comparable alteration systematics (Figure 8b), except the Mount Bureau sample GM92-48. Identification of the contaminant in the subaerial basalts from the Kerguelen Archipel- ago is more difficult to trace than that for the submarine basalts and may be related to the wide variety of secondary minerals (carbonates, oxides, Figure 8. Diagrams of 207Pb/204Pb versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb for leached and unleached samples from this study compared to fields for reported compositions of Hawaiian and Kerguelen basalts. (a) Hawaii. Fields for high-precision Pb isotopic compositions by TS-TIMS [Abouchami et al., 2000; Eisele et al., 2003] and MC-ICP-MS [Blichert-Toft et al., 2003; Wanless et al., 2006; D. Weis, unpublished data, 2005] are reported for basalts from Mauna Loa and Mauna Kea volcanoes. Lines for Kea-lo8, Kea-mid8, and Kea-hi8 are those defined by Eisele et al. [2003] on HSDP-2 basalts. Results for Hawaiian basalts analyzed in this study are shown as individual symbols (the analytical uncertainty for individual samples is smaller than the symbol sizes). Open symbols correspond to unleached samples, light-colored symbols (with dark outlines) correspond to leached samples purified once on Pb anion exchange columns, and dark-colored symbols indicate leached samples passed twice on Pb anion exchange columns. For reference, dashed lines connect results for unleached and leached samples (color-coded to match the corresponding symbols); they have been extrapolated to more radiogenic values for the HSDP-2 samples to clearly show the difference in slope compared to the Pb-Pb arrays of Eisele et al. [2003]. (b) Kerguelen. Fields for Pb isotopic compositions by TIMS [Yang et al., 1998; Doucet et al., 2002] and MC-ICP-MS [Doucet et al., 2005; D. Weis, unpublished data, 2005] are reported for basalts from the Kerguelen Archipelago and Northern Kerguelen Plateau [Weis and Frey, 2002]. Results for the Kerguelen basalts analyzed in this study are shown as individual symbols. Analytical uncertainty, color coding, and legend are as noted above. In both diagrams, note the significantly reduced dispersion of high-precision Pb isotopic ratios for leached samples compared to results from unleached samples and TIMS analyses. Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 19 of 23sulfides, quartz, clays, epidotes, zeolites) produced during subsequent hydrothermal alteration of the subaerial basalts [Nougier et al., 1982; Verdier, 1989; Hanano et al., 2009]. [25] In contrast to all the other studied Hawaiian and Kerguelen basalts, the submarine tholeiite (ODP leg 183, 1140A-31R-1, 57-61) from the Northern Kerguelen Plateau (Figures 2, 3, and 7b) shows more radiogenic isotopic compositions for the leached sample splits than for the unleached sample. The reason for this reverse leaching be- havior is enigmatic, especially as the field for Indian seawater Pb is distinctly more radiogenic (Figure 7b). Thus, interaction with seawater Pb, as represented by the Fe-Mn deposits from the Kerguelen Plateau, Crozet Basin and Australian- Antarctic Basin [Vlaste´lic et al., 2001], cannot account for this observation. 5.2. Effect of Matrix Elimination [26] The accuracy of radiogenic isotope ratio meas- urements by MC-ICP-MS can be affected by non- spectral interferences (matrix effects that affect the ionization and transmission of the analyte and instrumental mass bias) due to residual sample matrix [e.g., Thirlwall, 2002; Woodhead, 2002; Barling and Weis, 2008]. Samples for isotope ratio analysis by MC-ICP-MS thus need to be as pure as possible to obtain accurate high-precision results. Another possible source of inaccuracy could be mass fractionation during Pb anion exchange chro- matography [Blichert-Toft et al., 2003], as under certain circumstances, incomplete elution of the Pb fraction leads to lighter (less radiogenic) Pb isoto- pic compositions. In this case, if a significant difference was to be observed in the measured Pb isotopic ratios for a sample that was purified once and twice on columns (with close to 100% recov- ery in the Pb chemistry), then the twice-purified Pb fraction would be isotopically lighter than the once-purified Pb fraction. [27] In this study, all samples processed twice on columns have heavier isotopic ratios (i.e., more radiogenic) than those processed once, thus we observe the opposite relationship of what would be expected from column fractionation experi- ments [Blichert-Toft et al., 2003] (Figures 6b and 7b). These observed differences between the two types of triplicates (purified 1 and 2 on col- umns), which are more pronounced in the tholeiitic basalts (Hawaiian and submarine Northern Ker- guelen Plateau basalts), appear to fall along the calculated mass fractionation lines (within their 2s deviations). However, these differences are best explained by a nonspectral matrix effect as Barling and Weis [2008] show that the presence of mag- nesium and calcium in the matrix leads to lighter Pb isotopic compositions. [28] The lack of a mass fractionation effect pro- duced during chromatography is also supported by the Pb isotopic results for the transitional and alkalic basalts from the Kerguelen Archipelago, which have comparable CaO, but lower MgO and higher NaO and SiO2 contents than the Ha- waiian basalts. For these Kerguelen basalts, the Pb isotopic compositions for full procedural triplicates of leached powder splits that were purified once and twice on Pb anion exchange columns are within error of each other (Tables 5 and 6). In addition, their 2s mean deviations (160 ppm for 206Pb/204Pb and 208Pb/204Pb for the alkalic basalt OB93-165 (Figure 7a)) are within instrumental external reproducibility (200 ppm). [29] One possible explanation for the occurrence of a nonspectral matrix effect due to residual sample matrix during processing of the tholeiitic basalts may lie in their relatively low Pb concentrations (sub-ppm to ppm range) compared to the alkalic basalts (Table 1) and thus the amount of sample powder (mg) that is purified. For the tholeiitic samples, more sample powder is needed to ensure enough Pb for MC-ICP-MS analysis after leaching and chromatography. All of the sample splits for which the leached residue exceeded 170 mg (e.g., 250–300 mg for Mauna Kea (HSDP-2) samples and 130–190 mg for the NKP sample) show differences in Pb isotopic compositions when passed once and twice on columns. Full procedural sample triplicates where less sample was loaded (e.g., 150–170 mg for the Mauna Loa samples, and 76–130 mg for the Kerguelen Archipelago sam- ples) show smaller to negligible differences (i.e., within errors) in Pb isotopic compositions for one and two passes on columns. Larger sample sizes, still well below column saturation, mean that more sample matrix is loaded into the columns and this may not be effectively removed with just a single pass through columns [Barling and Weis, 2008]. Thus, for oceanic tholeiitic basalts, a second pass of chemical separation may be needed to reduce the sample matrix to negligible levels. 6. Conclusions [30] Acid-leaching and matrix elimination experi- ments on the Pb isotopic compositions of ocean Geochemistry Geophysics Geosystems G3 nobre silva et al.: technical brief 10.1029/2009GC002537 20 of 23island basalts from Hawaii and Kerguelen yield the following conclusions: [31] 1. Acid leaching removes the effects of con- tamination and alteration that disturb the magmatic Pb isotopic composition of OIB, which is essential for evaluating the isotopic composition of the mantle source of these basalts. [32] 2. In most cases, acid leaching leads to dis- tinctly less radiogenic (>500 ppm) Pb isotopic ratios of the samples compared to unleached sam- ples and is associated with a slight decrease in reproducibility. [33] 3. Independent of basalt composition (i.e., tholeiitic versus alkalic), leaching results in 35– 60% weight loss throughout the entire process and 70–80% of the Pb is lost in the first three acid- leaching steps. [34] 4. The reproducibility of Pb isotopic compo- sitions of OIB improves with the number of acid- leaching steps that the samples are subjected to, even if most of the Pb is leached out in the first steps. [35] 5. Another factor controlling the reproducibil- ity of Pb isotopic compositions of leached oceanic basalts is matrix elimination through column chro- matography. Samples purified twice on columns show more radiogenic Pb isotopic compositions. The effect is stronger for tholeiitic (Hawaiian and Kerguelen Plateau basalts) than for alkalic (Kerguelen Archipelago) compositions. [36] 6. The difference in the Pb isotopic composi- tions of leached samples that are passed once versus twice through chromatography is not due to fractionation on the anion exchange columns, but instead depends on the specific basalt compo- sition (i.e., the Pb concentration of the sample). Whenever a relatively large amount of material (>170 mg) needs to be dissolved for analysis because of low Pb concentrations (sub-ppm to ppm), a second chemical purification step is rec- ommended to avoid matrix effects. [37] 7. All steps are crucial during sample process- ing for obtaining accurate, high-precision Pb iso- topic compositions, to discern the mantle sources and components of ocean island basalts. 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