The Solar System primordial lead

Earth and Planetary Science Letters 300 (2010) 152–163

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Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l

The Solar System primordial lead Janne Blichert-Toft a,⁎, Brigitte Zanda b, Denton S. Ebel c, Francis Albarède a a b c

Laboratoire de Sciences de la Terre, CNRS UMR 5570, Ecole Normale Supérieure de Lyon, Université Claude Bernard Lyon 1, 46 Allée d'Italie, 69364 Lyon Cedex 07, France LMCM-CNRS UMR 7202, Museum National d'Histoire Naturelle, 75005 Paris, France American Museum of Natural History, New York, NY 10024, USA

a r t i c l e

i n f o

Article history: Received 8 June 2010 Received in revised form 30 September 2010 Accepted 1 October 2010 Available online 30 October 2010 Editor: T. Spohn Keywords: iron meteorites troilite primordial Pb Pb isotopes MC-ICP-MS TIMS double-spike Canyon Diablo Nantan solar nebula early Solar System

a b s t r a c t Knowledge of the primordial isotope composition of Pb in the Solar System is critical to the understanding of the early evolution of Earth and other planetary bodies. Here we present new Pb isotopic data on troilite (FeS) nodules from a number of different iron meteorites: Canyon Diablo, Mundrabilla, Nantan, Seeläsgen, Toluca (IAB– IIICD), Cape York (IIIA), Mt Edith (IIIB), and Seymchan (pallasite). Lead abundances and isotopic compositions typically vary from one troilite inclusion to another, even within the same meteorite. The most primitive Pb was found in three leach fractions of two exceptionally Pb-rich Nantan troilite nodules. Its 204Pb/206Pb is identical to that of Canyon Diablo troilite as measured by Tatsumoto et al. [M. Tatsumoto, R.J. Knight, C.J. Allègre, Time differences in the formation of meteorites as determined from the ratio of lead-207 to lead-206, Science 180 (1973) 1279–1283]. However, our measurements of 207Pb/206Pb and 208Pb/206Pb are significantly higher than theirs, as well as other older literature data obtained by TIMS, while consistent with the recent data of Connelly et al. [J.N. Connelly, M. Bizzarro, K. Thrane, J.A. Baker, The Pb–Pb age of Angrite SAH99555 revisited, Geochim. Cosmochim. Acta 72(2008) 4813–4824], a result we ascribe to instrumental mass fractionation having biased the older data. Our current best estimate of the Solar System primordial Pb is that of Nantan troilite, which has the following isotopic composition: 204Pb/206Pb = 0.107459(16), 207Pb/206Pb = 1.10759 (10), and 208Pb/206Pb = 3.17347(28). This is slightly less radiogenic than the intercept of the bundle of isotopic arrays formed in 207Pb/206Pb–204Pb/206Pb space by our measurements of Canyon Diablo, Nantan, Seeläsgen, Cape York, and Mundrabilla, as well as literature data, which, in spite of rather large uncertainties, suggests a common primordial Pb component for all of these meteorites. The radiogenic Pb present in most of these irons is dominantly asteroidal and indicates evolution in a high-U/Pb environment. The apparent age of the radiogenic Pb component is consistent with the 39Ar–40Ar ages of silicate inclusions found in the same meteorites. We propose that the radiogenic Pb was introduced more recently into troilite, from the surface rubble of the parent asteroid, possibly during the impacts that generated the IAB iron meteorites. The excellent correlation between 208Pb/206Pb and 204Pb/206Pb translates into a Th/U ratio of 3.876 ± 0.016 for the asteroid, which is the most precise estimate for the solar nebula to date. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Very few measurements of a natural property have withstood the test of time as well as Patterson's (1956) determination of the isotopic composition of primordial (or primitive) Pb in the Solar System as hosted by troilite (FeS) in the Canyon Diablo iron meteorite. Exact knowledge of the primordial Pb component is crucial to the chronology of meteorites. Because radioactive U and Th are refractory and produce radiogenic isotopes of the volatile element Pb, the understanding of the early evolution of the Earth and other planetary bodies hinges on the determination of the primordial Pb isotopic abundances. Contamination of meteorite samples is a ubiquitous and overwhelming problem, and repeated attempts over the years at measuring primordial Pb, therefore,

⁎ Corresponding author. Tel.: +33 4 72 72 84 88. E-mail address: [email protected] (J. Blichert-Toft). 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.10.001

have involved both increasingly sophisticated techniques of multiple and aggressive leaching steps and enormous efforts to reduce chemistry blanks to the extreme (Chen and Wasserburg, 1983; Connelly et al., 2008a; Göpel et al., 1985; Tatsumoto et al., 1973). Despite having thus succeeded rather spectacularly in improving the precision of the original estimate of the Solar System primordial Pb, all quests for the most primitive Pb have consistently failed to demonstrate that Pb significantly less radiogenic than that of Canyon Diablo troilite could actually be isolated from meteorites. At the time, almost forty years ago, when the isotopic ratios that are still in use today as the reference for primordial Pb (Tatsumoto et al., 1973) were acquired, the isotopic measurements were obtained by thermal-ionization mass spectrometry (TIMS). The ion beam intensity produced by TIMS depends on the conditions of the run, notably the efficiency of silica-based activators, and may vary substantially during a single measurement, rendering the Pb mass bias highly unsteady. A few decades ago, the main limitation on the accuracy of this technique was, therefore, the control of

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instrumental mass fractionation, which was assessed by comparing standard and sample runs under approximately similar conditions. For decades, the accuracy of Pb isotope compositions remained of the order of 1 × 10− 3 per unit of mass difference (Chen and Wasserburg, 1983; Göpel et al., 1985; Tatsumoto et al., 1973). In contrast to TIMS, multicollector inductively-coupled plasma mass spectrometry (MC-ICP-MS) produces steady ion beams, but the mass bias still needs calibration with respect to a reference material. A first solution involved the use of Pb double-spikes made of either natural (Dodson, 1963; Galer, 1999; Hamelin et al., 1985) or artificial (Amelin and Davis, 2006; Connelly et al., 2008a) isotopes. Lead double-spikes can be used with both thermalionization and ICP mass spectrometers. Alternatively, owing to the stability of the signal, instrumental mass fractionation effects with MCICP-MS can be controlled by adding a thallium spike to the Pb solutions, and the accuracy even further improved by bracketing samples with standards. For all these modern techniques, the potential precision of Pb isotopes currently lies well within the 0.01% range, which for purely radiogenic Pb corresponds to an age resolution of about 0.2 Ma (Albarède et al., 2004; Amelin, 2006; Bouvier et al., 2007; Hirata, 1996; White et al., 2000). It is now agreed upon that, for equivalent quantities of Pb, the respective performances of MC-ICP-MS and TIMS are essentially identical (Amelin et al., 2008). These quoted uncertainties represent the best achievable precision on Pb–Pb ages, but the actual error on ages depends on a number of other factors, notably the possibility of extracting very radiogenic Pb from the samples analyzed. In this study, we adopted the standard strategy of sequential and increasingly aggressive leaching to measure Pb isotope compositions in a number of troilite nodules from a variety of iron meteorites in yet another attempt to identify and further improve precision and accuracy on the isotopic composition of primordial Pb. It is the first time this has been tried using MC-ICP-MS. We present new high-precision MC-ICP-MS data showing that Pb in troilite from Canyon Diablo, Mundrabilla, Nantan, Seeläsgen, Toluca (IAB–IIICD), Cape York (IIIA), Mt Edith (IIIB), and Seymchan (pallasite) contains a very unradiogenic component. Although our sequential leachings reproduce, to a first order, the literature values of primordial Pb (Chen and Wasserburg, 1983; Connelly et al., 2008a; Göpel et al., 1985; Tatsumoto et al., 1973) for those of our samples having the highest Pb abundances, we take one step further and use an alternative data projection technique to demonstrate that this isotope composition is common to all the meteorites investigated, including those for which pure primordial Pb could not be isolated by acid leaching.

2. Sample descriptions Most of the following iron meteorite descriptions are taken from Buchwald's (1975) monograph and are complemented by data from Choi et al. (1995) and Wasson and Kallemeyn (2002). The origin (whether from museum or private collections, or through meteorite dealers) and identification numbers of each of the samples are listed in Table 1. Canyon Diablo is an octahedrite from the IAB main group with a standard (6.9–8.2 wt.%) Ni content and an exposure age of 640 ± 60 Ma (Voshage and Feldmann, 1979). The metal is mostly kamacite with 1–4 wt.% taenite and plessite, and cm-long crystals of schreibersite (Fe–Ni phosphide). Troilite occurs as 2–50 mm nodules, commonly elongated and with different proportions of graphite. Daubreelite (Fe–Cr sulfide), chromite, and sphalerite also have been described in these nodules (El Goresy, 1965). Silicate inclusions are present in Canyon Diablo, but are less abundant than in other meteorites from the same group, such as Odessa. Three samples were obtained for this study: a single nodule from George R. Tilton's private collection, courtesy of Richard W. Carlson from the Carnegie Institution of Washington, here labeled “Tilton”, and two samples (AMNH #2619 and #9) from the American Museum of Natural History in the

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form of small solid pieces of troilite extracted from two different samples using a stainless steel needle and chisel. Nantan (IAB) does not seem to have been described in much detail in the accessible literature. Its metal contains 7.0 wt.% Ni and its exposure age is 30–60 Ma (Nishiizumi et al., 1995). Three nodules (here labeled Nantan K, N1, and N2 for their laboratory run codes) were obtained from Alain Carion (a meteorite dealer in Paris). Mundrabilla is an unusual ungrouped IAB octahedrite with a moderate (7.5 wt.%) Ni content and an exposure age of 350 ± 90 Ma (Maruoka and Matsuda, 2001). It is composed of a very large number of precursor taenite crystals, each 2–5 cm across with 15–20 wt.% taenite and plessite. Schreibersite is common. Troilite occurs mostly as veins between grain boundaries with graphite, daubreelite, and sphalerite (Ramdohr and El Goresy, 1971). The occurrence of silicates is rare. Two samples (#63692 and #77992) in the form of thick slabs with abundant, thickly veined troilite, relatively easily pried out with a stainless steel chisel, were generously donated for this study by Jutta Zipfel, curator of the Max-Planck collection in Mainz. Seeläsgen is a coarse octahedrite from the IAB main group with a low (6.6 wt.%) Ni content, which makes it similar to Magura. The metal is mostly kamacite with b1 wt.% taenite and plessite and scattered mm-sized schreibersite crystals. Troilite occurs as elongated nodules up to 7 cm long with screibersite rims and up to 50 wt.% graphite. Daubreelite is not observed, but carlsbergite (Cr nitride) and chromite are present. This meteorite is free of silicate inclusions. Part of a large nodule situated towards the edge of the meteorite slab was obtained from the collection of the Muséum National d'Histoire Naturelle in Paris (MNHN n° 3056) using first a diamond saw to isolate the nodule from the main iron mass and then a stainless steel chisel to extricate pieces of the troilite. Toluca is a coarse octahedrite from the IAB main group with 8.0 wt.% Ni and an exposure age of 600 ± 40 Ma. The metal is kamacite with 15 wt.% taenite and plessite. Cohenite (Fe–Ni carbide) and skeletal cmsized schreibersite crystals are present. The mineral composition of nodules (b5 cm) ranges from troilite to graphite and they are commonly rimmed by schreibersite and cohenite. El Goresy (1965) identified clinoenstatite and olivine in myrmekitic intergrowths with troilite, and also found sphalerite and minor sulfides. Silicate fragments up to several cm occur as aggregates of olivine, diopside, and subordinate plagioclase. One sample (AMNH #4937) was acquired from the American Museum of Natural History, again, as for the Canyon Diablo samples, in the form of small solid troilite pieces extracted with a stainless steel needle and chisel. Cape York is a coarse octahedrite from the IIIA group with 7.6–7.8 wt.% Ni and an exposure age of 93 ± 16 Ma (Murty and Marti, 1987). Taenite and plessite represent 25–35 wt.% of the metal. Small crystals of schreibersite and cohenite are common. Troilite is abundant as Reichenbach lamellae overgrowing chromite, but even more abundant as elongated nodules of up to 10 cm in size. Daubreelite and chromite occur as minor phases in the nodules. There has been no report of silicate inclusions. Half of a large elongated nodule within a thick slab of metal was obtained from the Muséum National d'Histoire Naturelle in Paris (MNHN n° 2820). The inclusion was first freed from most of the surrounding iron using a diamond saw. Massive troilite chunks were then recovered from the inclusion using a press. Mount Edith is a shock-hardened medium octahedrite from the IIIB group with 9.4 wt.% Ni and an exposure age of 715 ± 65 Ma (Voshage and Feldmann, 1979). Taenite and plessite represent 40 wt.% of the metal and cm-sized schreibersite crystals are abundant. Troilite nodules up to 4 cm in size are common. Sarcopside (Fe–Mn phosphate) and other phosphates have been described in these nodules by Olsen and Steele (1994). No silicate inclusions have been reported. One sample (AMNH #242) was obtained from the American Museum of Natural History, which, like for Toluca and Canyon Diablo, was in the form of small solid troilite pieces removed from the main nodule with a stainless steel needle and chisel.

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Table 1 Blank-corrected Pb isotope compositions of troilite inclusions in iron meteorites. Samples

Type

Collection

Sample weight (g)

Sample ID

ng Pb

204

Pb/206Pb

Canyon Diablo #2619

IAB−IIICD

AMNH

1.1948

Canyon Diablo #9

IAB–IIICD

AMNH

0.6080

Canyon Diablo (Tilton)

IAB–IIICD

CIW

0.6657

Mundrabilla #63692

IAB–IIICD

Mainz

3.4578

Mundrabilla #77992

IAB–IIICD

Mainz

3.7739

Nantan K

IAB–IIICD

Alain Carion

4.6931

Nantan N1

IAB–IIICD

Alain Carion

2.2208

Nantan N2

IAB–IIICD

Alain Carion

3.0695

Seeläsgen n° 3056

IAB–IIICD

MNHN

3.0248

Toluca #4937

IAB–IIICD

AMNH

0.6178

Cape York n° 2820

IIIA

MNHN

2.3343

Mt Edith #242

IIIB

AMNH

0.8035

Seymchan

Pallasite

Luc Labenne

1.6777

CD-AMNH2619-L1-N1 CD-AMNH2619-L2-N1 CD-AMNH2619-L3-N1 CD-AMNH2619-L4-N1 CD-AMNH2619-L5-N1 CD-AMNH2619-L6-N1 CD-AMNH2619-L7-CHF CD-AMNH2619-L8-CHC CD-AMNH9-B-L1-N1 CD-AMNH9-B-L2-N1 CD-AMNH9-B-R-CN CD-Til-A-L1-N1 CD-Til-A-L1-N1-rr CD-Til-A-L2-N1 CD-Til-A-L2-N1-rr CD-Til-A-R-CN CD-Til-A-R-CN-rr Mun63-G-L1-N1 Mun63-G-L2-N1 Mun63-G-L3-N1 Mun63-G-R-CN Mun77-F-L1-N1 Mun77-F-L2-N1 Mun77-F-L3-N1 Mun77-F-R-CN Nan-K-L1-N1 Nan-K-L2-N1 Nan-K-L3-N1 Nan-K-R-CN N1-L1-N1 N1-L2-N1 N1-L3-N1 N1-L4-N1 N1-L5-N1 N1-L6-N1 N1-L7-CN N1-L8-CN N1-L9-CHB N2-L1-N1 N2-L2-N1 N2-L3-N1 N2-L4-N1 N2-L5-N1 N2-L6-N1 N2-L7-CN N2-L8-CN N2-L9-CHB N2-R-CN See-L1-N1 See-L2-N1 See-L3-N2.5 See-L4-N7 See-L5-N7 See-L6-N7 See-L7-R See-RR Tol-C-L1-N1 Tol-C-L2-N1 Tol-C-rR-CN CY-L1-N1 CY-L2-N1 CY-L3-N2.5 CY-L4-N7 CY-L5-N7 CY-L6-N7 CY-L7-R CY-RR Mt Edith-D-L1-N1 Mt Edith-D-L2-N1 Mt Edith-D-R-CN Seym-E-L1-N1 Seym-E-L2-N1 Seym-E-R-CN

N100 N100 N100 N100 N100 98 5 3 N100 N100 N100 N100 N100 N100 N100 N100 N100 25 21 8 7 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 N100 38 35 6 73 N100 37 N100 11 58 23 38 2 N100 N100 N100 N100 46 N100

0.065395 0.067745 0.068301 0.072979 0.092636 0.094402 0.083923 0.079740 0.055852 0.055733 0.058351 0.086440 0.086394 0.106115 0.106114 0.106375 0.106486 0.064134 0.064382 0.062892 0.064542 0.087598 0.097608 0.096623 0.103638 0.099562 0.105414 0.106794 0.107401 0.091516 0.105476 0.106859 0.107380 0.107271 0.107315 0.107343 0.107458 0.107468 0.105929 0.101905 0.107137 0.107202 0.107062 0.106846 0.107452 0.107356 0.107353 0.107352 0.077758 0.094872 0.106231 0.107010 0.107213 0.107092 0.107102 0.106817 0.060739 0.068780 0.072935 0.071502 0.074500 0.097998 0.102090 0.105540 0.105061 0.105926 0.070666 0.052557 0.052603 0.052492 0.055844 0.055934 0.055949

2s%

207

Pb/206Pb

0.0018 0.0028 0.0026 0.0027 0.0071 0.0085 0.1259 0.2102 0.0007 0.0004 0.0006 0.0075 0.0080 0.0105 0.0147 0.0108 0.0120 0.0071 0.0091 0.0211 0.0308 0.0088 0.0070 0.0115 0.0107 0.0080 0.0127 0.0113 0.0118 0.0063 0.0098 0.0114 0.0123 0.0116 0.0114 0.0094 0.0103 0.0112 0.0116 0.0113 0.0129 0.0131 0.0115 0.0115 0.0117 0.0105 0.0139 0.0273 0.0038 0.0080 0.0112 0.0145 0.0138 0.0110 0.0148 0.0292 0.0033 0.0466 0.0050 0.0041 0.0113 0.0097 0.1091 0.0201 0.0518 0.0300 0.2512 0.0005 0.0003 0.0005 0.0005 0.0011 0.0004

0.89354 0.90568 0.90887 0.93284 1.03289 1.04088 0.98935 0.96881 0.85178 0.85091 0.86348 0.98860 0.98850 1.10053 1.10045 1.10214 1.10246 0.88964 0.90517 0.89786 0.90426 1.01609 1.06188 1.09939 1.08707 1.06803 1.09694 1.10374 1.10734 1.02718 1.09754 1.10472 1.10709 1.10665 1.10685 1.10695 1.10753 1.10762 1.10003 1.07989 1.10664 1.10672 1.10616 1.10522 1.10762 1.10733 1.10798 1.10735 0.96638 1.04766 1.10164 1.10502 1.10601 1.10549 1.10557 1.10437 0.87700 0.91935 0.93845 0.94715 0.96214 1.06653 1.08408 1.09906 1.09700 1.10041 0.93753 0.82495 0.82508 0.82438 0.86975 0.86859 0.86635

2s%

208

Pb/206Pb

2s%

0.006 0.011 0.010 0.011 0.033 0.039 0.575 0.939 0.001 0.001 0.001 0.031 0.034 0.050 0.070 0.051 0.057 0.025 0.045 0.105 0.147 0.043 0.034 0.066 0.050 0.038 0.060 0.054 0.056 0.029 0.047 0.054 0.059 0.055 0.054 0.045 0.049 0.053 0.056 0.054 0.061 0.063 0.055 0.055 0.056 0.050 0.066 0.129 0.018 0.038 0.054 0.069 0.066 0.053 0.070 0.140 0.011 0.209 0.023 0.022 0.060 0.047 0.527 0.096 0.249 0.144 1.303 0.004 0.003 0.005 0.002 0.005 0.002

2.30130 2.35039 2.36166 2.45991 2.86867 2.89963 2.68718 2.60381 2.11033 2.10942 2.16468 2.72665 2.72647 3.14426 3.14516 3.14915 3.15320 2.23897 2.28589 2.25551 2.28711 2.74969 2.96076 3.08028 3.07960 3.00767 3.12858 3.15681 3.17226 2.83840 3.13140 3.16140 3.17114 3.16956 3.16996 3.17029 3.17333 3.17346 3.14217 3.05799 3.17016 3.17048 3.16758 3.16378 3.17361 3.17292 3.17606 3.17286 2.56643 2.91443 3.14528 3.15897 3.16434 3.16211 3.16171 3.15458 2.20819 2.37675 2.46222 2.43103 2.49156 2.97784 3.06222 3.13363 3.12367 3.14148 2.43515 2.03683 2.03771 2.03570 2.11172 2.11313 2.11032

0.034 0.055 0.051 0.053 0.147 0.173 2.569 4.258 0.006 0.003 0.010 0.151 0.162 0.216 0.302 0.219 0.246 0.111 0.180 0.416 0.601 0.178 0.142 0.270 0.216 0.163 0.259 0.232 0.241 0.128 0.203 0.234 0.252 0.237 0.234 0.193 0.211 0.230 0.239 0.232 0.265 0.271 0.235 0.236 0.241 0.217 0.286 0.558 0.078 0.165 0.230 0.297 0.282 0.226 0.301 0.598 0.063 0.944 0.102 0.082 0.228 0.198 2.244 0.413 1.066 0.618 5.483 0.007 0.005 0.009 0.004 0.011 0.003

AMNH = American Museum of Natural History (New York); CIW = The Carnegie Institution of Washington; Mainz = The Max-Planck Institut für Chemie; MNHN = Muséum National d'Histoire Naturelle (Paris); Alain Carion and Luc Labenne are meteorite dealers based in Paris. R and RR = Residue; L = Leachate (L1–L9); N1, N2.5, and N7 = 1M, 2.5 M, and 7 M HNO3, respectively; CN, CHF, CHC, and CHB = concentrated HNO3, HF, HCl, and HBr, respectively. Capital letters A, B, C, D, E, F, G, and K for some sample names are laboratory run codes that have been carried over; rr = rerun; 2s = 2 sigma.

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Seymchan is a coarse pallasite with 9.15 wt.% Ni, which was initially classified as a IIE octahedrite by Scott and Wasson (1976). No detailed description is available in the open literature. Our sample contains long, coarse kamacite lamellae with troilite distinctly lining grain boundaries. We acquired a sample from Luc Labenne, a French meteorite dealer, and delicately pried out the troilite, which was present in thin veins dispersed within the iron, using a stainless steel needle and chisel. 3. Analytical techniques Except for the Canyon Diablo troilite sample donated by George R. Tilton and the Nantan troilite samples, all of which were in the form of individual whole nodules directly ready for processing in the chemistry laboratory, the troilite from the other samples, as described earlier, first had to be extracted. In some cases, the troilite was freed from most of the host iron by a diamond saw and then, in all cases, either extricated from the remaining host iron with the help of a stainless steel needle and chisel or, when the troilite was too hard and massive, popped out from its position in the surrounding iron using a hydraulic press. The various troilite samples weighed between 0.6 and 4.7 grams (Table 1). Each troilite sample was sequentially leached in nitric acid (1 M, 2.5 M, and 7 M HNO3) on a hot plate at 120–130 °C for durations that varied from several days up to two weeks and the resulting residue dissolved in concentrated HNO3. For a few samples, concentrated HF, HCl, and HBr were also experimented with as leaching agents towards the end of the step leaching procedure. HBr, in particular, seems to have been efficient at getting at the remaining primordial Pb (fractions N1-L9-CHB and N2-L9-CHB; Table 1). Prior to the Pb column chemistry, Fe was removed from each of the leachate and residue fractions using the classic solvent extraction technique of Dodson et al. (1936) and Myers and Metzler (1950) (see also BlichertToft et al., 2010). Eliminating the Fe from the sample matrix greatly simplifies the subsequent Pb separation chemistry and ensures full Pb yields on the column. Isopropylether was used as the ether phase and 8 M HCl as the complementary water phase, in which the leachates and residues readily dissolved. A 1:1 ratio of 8 M HCl and isopropylether was found to be optimum for efficient extraction of Fe. Prior to use, the fresh isopropylether was first purified for any trace amounts of contaminant Pb by several back-extractions into dilute (~ 0.5 M) HCl. Although, in principle, the Fe extracted into the isopropylether from the sample can subsequently be removed again with dilute HCl, the isopropylether was never re-used in the present study for fear of residual Pb blanks. Two Fe-extraction steps with fresh isopropylether were undertaken for each sample in order to achieve complete removal of Fe (~ 98% of Fe is extracted during the first step, the rest during the second step). Lead for all the leachates and residues was then separated on 150 μl teflon columns filled with anion-exchange resin (Bio-Rad AG1X8, 200–400 mesh) using 1 M HBr and 6 M HCl. All samples were run through the Pb micro-columns twice to ensure the cleanest possible Pb for isotopic analysis. Double-distilled reagents were used throughout for all sample leaching, dissolution, Fe extraction, and column elution procedures. The isotopic compositions of the purified Pb were analyzed on the Nu Plasma 500 HR MC-ICP-MS at the Ecole Normale Supérieure in Lyon using standard bracketing (the SRM-981 Pb standard was run systematically every two samples) and added normal Tl to monitor instrumental mass bias and correct for mass fractionation. All the runs were done using Faraday cups fitted with 1011 Ω resistors. An average yield of ~ 1.0 ion per 100 atoms nebulized was estimated from the beam intensities on standard solutions of known concentrations. We used the triple-spike values of Abouchami et al. (2005) for normalization to the SRM-981 Pb standard (see Albarède et al., 2004, for details). Typical internal uncertainties on the Pb isotopic ratios were about 50–100 ppm. The amounts of Pb analyzed for each

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leachate and residue fraction are provided in Table 1 and correspond to the sample signal intensities calibrated with respect to the intensity of the Pb ion beam signal obtained for the 35 ppb SRM-981 Pb standard solution run in alternation with the samples. For most of the samples (as split into multiple leachate and residue fractions), the Pb abundances in each fraction were so large that the sample had to be subdivided multiple times by repeated dilutions into fractions small enough (~100 ng Pb diluted in 3 ml 0.05 M HNO3, giving a sample solution of ~35 ppb similar to the bracketing standard solution) so as not to saturate the 208Pb Faraday collector during isotopic measurement. Hence, all the samples could be analyzed with large ion beam intensities and for a long enough duration that a sufficient number of ratios could be collected (90–120 ratios of 10-second integration time each) ensuring excellent precision on all the isotopic measurements. The Pb abundances in each fraction are not precisely known (except to be well above the microgram level) because of the multiple dilutions that turned out to be necessary for most of the samples and had to be carried out quickly and efficiently as the samples were analyzed on the mass spectrometer in order to keep sample and standard measurements in close succession. This particular constraint, i.e., keeping pace with the standards, had priority over keeping track of the dilutions, hence rendering it difficult to simultaneously record the exact dilution factors applied. The high Pb concentrations were unexpected as, in order to preserve precious Pb samples, thought to be very small, for mass spectrometric analysis, they had not been checked for their Pb concentrations by quadrupole ICP-MS prior to isotopic analysis. It is clear, however, that some of the meteorites (e.g., Nantan, Seeläsgen, and Canyon Diablo) had released tens of microgram-quantities of Pb during each leaching step, whereas others had overall lower Pb contents (e.g., Mundrabilla #63692, Toluca, Seymchan, and Cape York), but still ample quantities for precise isotopic measurement. Total procedural Pb blanks were estimated in two different ways: first by running a complete chemistry without any sample added (15 pg), and, second, by assuming that the smallest sample analyzed during the period when this work was carried out (see Blichert-Toft et al., 2010) only contains radiogenic and contaminant Pb. This assumption leads to a maximum estimate of the Pb blank of ~ 40 pg. Given the micrograms of Pb contained by the present troilite samples, even the worst-case-scenario Pb blank of ~40 pg is negligible. Mass fractionation correction of Pb using Tl assumes that Pb and Tl fractionate identically, which generally is a reasonable approximation (see later discussion). Additional standard bracketing relieves this constraint by estimating the difference between Tl and Pb fractionation factors, which amounts to a normalization of the Pb isotopic ratios to SRM-981 values, and interpolating this difference between standards. The reader is referred to Albarède et al. (2004) for a detailed discussion of this method. To assess how effective the Tl correction technique prior to sample-standard bracketing is, we here report the unweighted average and 2-sigma dispersion of Tl-corrected values for the isotope compositions of 84 runs of the SRM-981 Pb standard measured during the course of this study, which was a 12-month period (from February 2009 to February 2010) as 204Pb/206Pb = 0.059041 ± 0.000017, 207 Pb/ 206 Pb = 0.91468 ± 0.00014, and 208 Pb/ 206 Pb = 2.16683 ± 0.00045. For 204Pb/206Pb and 207Pb/206Pb, our numbers are indistinguishable within the given uncertainties from the original values of Catanzaro et al. (1968) and from the triple-spike values reported by Abouchami et al. (2005) (updated from Eisele et al., 2003) of, respectively, 0.059029 ± 0.000005 and 0.91475± 0.00003, which we used for normalization of our final Tl-corrected data to a unique reference. The small bias between our 208Pb/206Pb ratio with respect to the value of 2.16770 ± 0.00005 reported by Abouchami et al. (2005) does not affect the accuracy on either the Pb isotope ratios or the Pb–Pb ages. The sample-standard bracketing step applied after the Tl-correction further improves the reproducibility on the samples by a factor of 3–5 over the simple Tl normalization reported here for the standards.

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indistinguishable from that of Pb in Nantan metal measured by Chen and Wasserburg (1983), while our 207Pb/206Pb is 2‰ higher. Given the remarkable agreement between their and our 204Pb/206Pb values and the similar 207Pb/206Pb ratios of our bracketing SRM-981 Pb standard (0.915) and the Nantan and Canyon Diablo troilite samples (~1.106), it is unlikely that the differences observed for 207Pb/206Pb are due to analytical issues with the MC-ICP-MS technique, such as memory effects or non-linearity on the mass spectrometer. The position of Tatsumoto et al.'s (1973) values to the right of individual isochrons (or mixing lines) in the 207Pb/206Pb vs 204Pb/206Pb plot (Fig. 3) was already observed previously for Cape York by Connelly et al. (2008a) and, hence, now seems to call for further systematic investigation. As an important caveat, we note that Connelly et al.'s (2008a) double-spike TIMS data plot right on the array defined by our MC-ICP-MS data (Fig. 3). Our observations further are consistent with those of Chen and Wasserburg (1983), who found that Pb in their samples of Nantan and Canyon Diablo was equally unradiogenic, but still more radiogenic than the Canyon Diablo troilite Pb analyzed by Tatsumoto et al. (1973). Mundrabilla #63692, Seymchan, Mt Edith, and Toluca all contain radiogenic Pb that could not be completely removed by the present acid leaching procedures. This indicates that they either contain some U, as was also found for Muonionalusta IVA troilite (Blichert-Toft et al., 2010), or contain recent planetary, yet not necessarily terrestrial (Chen and Wasserburg, 1983), Pb. This, in turn, is consistent with primitive Pb appearing in Toluca and Canyon Diablo leachates only after N99.7% of total Pb had been removed in Göpel et al.'s (1985) experiments. On the contrary, Connelly et al. (2008a) observed the least radiogenic Pb in the leachates of Cape York troilite at much earlier stages of leaching (N34 and N60% of Pb removal). The 207 Pb/206Pb of Mundrabilla #77992 (fraction 77-F-L3-N1; Table 1) is almost as high as the Canyon Diablo value (Fig. 1), which requires that some minerals in this particular sample were perturbed well after the formation of the meteorite.

Measurement errors for Pb isotope compositions are either the 2sigma relative in-run errors or the standard reproducibility, whichever is greater. Data uncertainties were calculated by Monte Carlo error propagation assuming a Pb isotope composition of the blank of 204 Pb/206Pb = 0.0545, 207Pb/206Pb = 0.8549, and 208Pb/206Pb = 2.085, with errors of 2, 0.4, and 0.4%, respectively, and a blank of 15 pg, which we estimate to be known to within a factor of two. The details of these calculations can be found in Bouvier et al. (2007). 4. Results The blank-corrected Pb isotope data for troilite from Canyon Diablo (“Tilton” and AMNH #2619 and #9), Mundrabilla (#63692 and #77992), Nantan (K, N1, and N2), Seeläsgen, Toluca, Cape York, Mt Edith, and Seymchan, together with the sample weights and the amounts of Pb analyzed for each leachate and residue fraction, are listed in Table 1 and shown in Figures 1 and 2. 4.1. Uranogenic Pb For an individual meteorite, the Pb concentrations and isotope compositions commonly vary from one troilite inclusion to another. This is especially clear for Canyon Diablo, for which the three samples analyzed here gave widely different results, with the most primitive being the troilite nodule donated by George R. Tilton (Table 1). This is consistent with the heterogeneous mineralogy of the Canyon Diablo troilite nodules as described by El Goresy (1965). The differences between the present data and literature data for Canyon Diablo (Chen and Wasserburg, 1983; Göpel et al., 1985; Tatsumoto et al., 1973), Cape York (Chen and Wasserburg, 1983; Connelly et al., 2008a), Mundrabilla (Chen and Wasserburg, 1983), and Toluca (Chen and Wasserburg, 1983; Göpel et al., 1985) are substantial. Although they may be due, in part, to different leaching protocols, it is also possible that each troilite inclusion simply has a distinctive U/Pb ratio. In the present work, the least radiogenic Pb was measured on the last leach fractions of the three samples of Nantan (Table 1 and Figs. 1 and 2), although Seeläsgen, Cape York, and the Canyon Diablo troilite nodule from George R. Tilton all came close. Our 204Pb/206Pb in Nantan troilite is

4.2. Thorogenic Pb The alignment of all the samples is much better in the 208Pb/206Pb vs Pb/206Pb plot (Fig. 2) than in the 207Pb/206Pb vs 204Pb/206Pb plot 204

1.15

1.10

1.05

so

lin

e

0.95

on en

vir

0.90

me

nt

ga

207

Pb/ 206Pb

1.00

Canyon Diablo (Tatsumoto et al., 1973) Canyon Diablo AMNH 9 Canyon Diablo AMNH 2619 Canyon Diablo "Tilton" Mundrabilla #63692 Mundrabilla #77992

0.85

0.80

0.75 0.05

0.06

0.07

0.08 204

Pb/

207

206

204

206

0.09

Nantan N1 Nantan N2 Nantan K Seeläsgen Toluca Cape York Mt Edith Seymchan

0.10

0.11

206

Pb

Fig. 1. Pb/ Pb vs Pb/ Pb for all the troilite samples analyzed in this work. Most error ellipses are smaller than the symbol size and, therefore, have been omitted. The variability is too large for a unique 207Pb*/206Pb* intercept, and, hence, a single apparent age, to be calculated. The field of environmental and gasoline Pb (England and France) is from Monna et al. (1997). In order to satisfy the 207Pb/206Pb data, nearly 100% of terrestrial gasoline-free Pb is required to account for the Pb in some of the samples, which is an extreme, highly unlikely hypothesis. Connelly et al. (2008) in the figure legend refers to Connelly et al. (2008a) in the reference list.

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157

3.35

3.15

2.95

16

/U

=

7 3.8

Th 2.55

208

Pb/ 206Pb

0.0

6±

2.75

2.35

gasoline

2.15

Canyon Diablo (Tatsumoto et al., 1973) Canyon Diablo AMNH 9 Canyon Diablo AMNH 2619 Canyon Diablo "Tilton" Mundrabilla #63692 Mundrabilla #77992

environment

1.95

1.75 0.05

Nantan N1 Nantan N2 Nantan K Seeläsgen Toluca Cape York Mt Edith Seymchan

0.06

0.07

0.08

0.09

0.10

0.11

204

Pb/ 206Pb

Fig. 2. 208Pb/206Pb vs 204Pb/206Pb for all the troilite samples analyzed in this work. The Th/U ratio of 3.876 ± 0.016 of the radiogenic component is calculated from the 208Pb*/206Pb* intercept. The field of environmental and gasoline Pb (England and France) is from Monna et al. (1997).

defined by our data (Fig. 4). The least radiogenic value observed for Pb/206Pb in Nantan is 3‰ higher than Tatsumoto et al.'s (1973) value. As was the case for 207Pb/206Pb, 208Pb/206Pb of Mundrabilla #77992 (fraction 77-F-L3-N1; Table 1) also is almost as high as the Canyon Diablo value (Fig. 2).

(Fig. 1). This is largely due to the slow decay, nearly linear in time, of 232 Th and 238U with respect to the far more rapid decay of 235U. The initial 208Pb*/206Pb* value of 0.9572 ± 0.0038 defines a Th/U ratio of the non-primordial component of 3.876 ± 0.016 (where * stands for the radiogenic component). This value is in agreement with previously reported Th/U ratios for carbonaceous chondrites of 3.9 ± 0.2 (Rocholl and Jochum, 1993) and indicates that radiogenic Pb is produced by Th and U in chondritic proportions. Again, Tatsumoto et al.'s (1973) data for the Canyon Diablo troilite and Chen and Wasserburg's (1983) data for Nantan plot to the right of the arrays obtained in the present work (Fig. 4). In contrast, as for 207Pb/206Pb vs 204 Pb/206Pb (Fig. 3), Connelly et al.'s (2008a) data plot on the array

208

5. Discussion Compared to the existing literature data, the primordial Pb values deduced from the new data of this study (see later discussion) essentially reside within this large body of precise measurements on different samples from the same meteorites. Although the isotopic

1.108 1.107 1.106

Canyon Diablo "Tilton" Nantan N1 Nantan N2 Nantan K Seeläsgen Cape York (DS-TIMS, Connelly et al., 2008)

Toluca, troilite (Göpel et al., 1985) Canyon Diablo, troilite (Tatsumoto et al., 1973)

207

Pb/ 206Pb

1.105 Nantan, metal Canyon Diablo, troilite (Chen and Wasserburg, 1983) (Göpel et al., 1985)

1.104 1.103

n

tio

ec

ers

int

y rra

a

Canyon Diablo, metal (Chen and Wasserburg, 1983)

1.102

ma

ss dir dis ec crim tio n ina

tio

n

1.101 1.100 1.099

0.1055

0.1060

0.1065

0.1070 204

0.1075

0.1080

Pb/ 206Pb

Fig. 3. Close-up of Fig. 1 for selected samples. Error ellipses are calculated using Monte Carlo loops. The strong correlation attests to the effect of the Pb blank correction. Error ellipses for the literature values (full circles and crosses) are not shown because the error correlation coefficients are not known. The samples of this study are shown as squares and diamonds. The blue error ellipse represents the confidence interval for the intersection of all the sample arrays (see caption to Fig. 7). DS-TIMS in the legend stands for double-spike TIMS data. Connelly et al. (2008) in the figure legend refers to Connelly et al. (2008a) in the reference list.

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3.18

208

Pb/ 206Pb

3.17

Canyon Diablo "Tilton" Nantan N1 Nantan N2 Nantan K Seeläsgen Cape York (DS-TIMS, Connelly et al., 2008)

Canyon Diablo, troilite (Tatsumoto et al., 1973) Canyon Diablo, troilite (Göpel et al., 1985)

3.16

Nantan, metal (Chen and Wasserburg, 1983) Canyon Diablo, metal (Chen and Wasserburg, 1983)

3.15

ma

ss d dir iscri ec mi tio na n ti

3.14

on

3.13

0.1055

0.1060

0.1065

0.1070 204

0.1075

0.1080

Pb/ 206Pb

Fig. 4. Close-up of Fig. 2 for selected samples (refer to the caption of Fig. 3 for details). No array intersection ellipse is shown (see caption to Fig. 7). Connelly et al. (2008) in the figure legend refers to Connelly et al. (2008a) in the reference list.

composition of Pb in Canyon Diablo by Tatsumoto et al. (1973) has remained the absolute reference for primitive Pb for almost four decades, enough new data have now been collected by different laboratories using different techniques (both leaching protocols and mass spectrometry), that a reassessment has become both possible and timely. 5.1. Primordial lead The strongest constraint comes from the most primitive Pb extracted by sequential leaching. Only some of our Canyon Diablo samples (the leach fraction CD-Til-A-L2-N1 and residue CD-Til-A-R-CN of the “Tilton” sample) provided unradiogenic Pb with 204Pb/206PbN 0.106 (Table 1 and Figs. 1–4), which is not nearly as unradiogenic as Tatsumoto et al.'s (1973) measurement. Since the primordial Pb component is likely to be hosted in sphalerite, which is rather abundant in Canyon Diablo and other similar samples (El Goresy, 1965), we speculate that perhaps none of the inclusions of the Canyon Diablo sample analyzed here had the optimum mineralogy for encountering the ultimate primordial Pb. The amounts of Pb analyzed for Nantan troilite, in contrast, were by far the largest among the samples investigated here, making this meteorite the prime target for the primordial Pb component, as well as rendering it essentially immune to contamination and potential blank issues. This high Pb concentration may be related to the observation by Weber et al. (1986) and Nishiizumi et al. (1995) that Nantan has the lowest contents of cosmogenic nuclides ever measured in any iron meteorite. This suggests that this meteorite was buried deeply within the asteroid, well below the regolith, and, therefore, protected from both aqueous surface alteration and evaporation due to micrometeorite bombardment. Unlike Canyon Diablo troilite, the three Nantan troilite samples from this study seem to be equally rich in Pb and to yield similar isotopic compositions for the most primitive Pb. Table 2 lists (in bold) our best estimate of the most primitive Pb identified in the present study, which was calculated as the unweighted mean of the three fractions of Nantan, L8-CN and L9-CHB from the N1 nodule and L7-CN from the N2 nodule, with the highest 204 Pb/206Pb (Table 1 and Fig. 5). The isotope compositions of Pb in these three fractions are essentially indistinguishable and only marginally different from the values obtained for fraction R-CN from the third Nantan nodule labeled K (Table 1). As mentioned before, our least radiogenic Nantan results for troilite agree with Chen and Wasserburg's (1983) metal data for 204Pb/206Pb, but are 1.4‰ higher for 207Pb/206Pb (Fig. 3) and 2.0‰ higher for 208Pb/206Pb (Fig. 4). In contrast, although Connelly et

al.'s (2008a) double-spike TIMS data for Cape York are less primitive than our MC-ICP-MS data for Nantan, they plot on exactly the same trend as our data above the reference Canyon Diablo troilite data of Tatsumoto et al. (1973) (Figs. 3 and 4). The most likely interpretation is that the Canyon Diablo troilite data of Tatsumoto et al. (1973) and the Nantan metal data of Chen and Wasserburg (1983) are biased towards lower 207Pb/206Pb and 208Pb/206Pb values because of a different correction of the TIMS mass bias based on the value assumed at the time for the 208Pb/206Pb ratio in the SRM-981 Pb standard (J.H. Chen, pers. comm., 2010). To a slightly lesser extent, this is also the case for Göpel et al.'s (1985) Pb data on Toluca and Canyon Diablo troilite (Figs. 3 and 4). The anomalous behavior of 207Pb during TIMS analysis observed by Doucelance and Manhès (2001) and Amelin et al. (2005), which probably reveals an even-odd isotopic effect, does not seem to affect the MC-ICP-MS data. The case can therefore be made that the present MCICP-MS data on Pb in a number of different troilite samples and Connelly et al.'s (2008a) data on Cape York obtained by double-spike TIMS supersede the more ancient values of Tatsumoto et al. (1973), Chen and Wasserburg (1983), and Göpel et al. (1985). The present MC-ICP-MS data are normalized to the SRM-981 values obtained by the true triplespike ‘intersection’ technique of Galer (1999), which entails no assumption on any reference value: the ratios derived in this way are absolute ratios. This is not the case for the unspiked TIMS data of Tatsumoto et al. (1973), Chen and Wasserburg (1983), and Göpel et al. Table 2 Estimates of isotope compositions of primordial Pb. The new values for Nantan (in bold) are our current best estimate of the Solar System primordial Pb. 204 a

Canyon Diablo Canyon Diablob Cape Yorkc Nantand Nantane Intersectionf a b c d e

f

Pb/206Pb

0.106(1) 0.10745(3) 0.10712(6) 0.10743(4) 0.107459(16) 0.10675(56)

207

Pb/206Pb

1.09(1) 1.1060(5) 1.1063(5) 1.1047(3) 1.10759(10) 1.1041(27)

208

Pb/206Pb

3.11(3) 3.1671(14) 3.1684(7) 3.1596(9) 3.17347(28) 3.159(3)

Patterson (1956). Tatsumoto et al. (1973). Connelly et al. (2008a), unweighted average of the three most primitive fractions. Chen and Wasserburg (1983). This work, unweighted average of the three most primitive fractions N2-L7-CN, N1L8-CN, and N1-L9-CHB. This work.

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159

1.109

0.1076

0.1075

1.108

0.1074

1.106 0.1072

Pb/ 206Pb

204

0.1073

207

Pb/ 206Pb

1.107

1.105 0.1071

1.104

0.1070

1.103

0.1069 See- N2-L5- See- See- N2-L3- N2-L4- See- N1-L5- N1-L6- N1-L7- N2-R- N2-L9- N2-L8- N1-L4- Nan- N2-L7- N1-L8- N1-L9L4-N7 N1 L6-N7 L7-R N1 N1 L5-N7 N1 N1 CN CN CHB CN N1 K-R- CN CN CHB CN

Fig. 5. 204Pb/206Pb (blue open symbols, continuous blue line) and 207Pb/206Pb (red bull's eye symbols, dotted red line) of the most primitive fractions measured in the present study arranged by increasing 204Pb/206Pb. The primitive Pb isotope values reported in Table 2 (in bold) are the unweighted average of the three fractions (all from Nantan) with the highest 204 Pb/206Pb.

(1985), which depend on calibrating the mass bias with respect to a reference ratio, most commonly 208Pb/206Pb in a specific reference material, typically SRM-981 or SRM-982 (Doucelance and Manhes, 2001; J.H. Chen, pers. comm., 2010). Likewise, since the 202Pb-205Pb double-spike technique of Amelin and Davis (2006) lacks isotopes common to the sample (natural 204, 206, 207, and 208) and the spike (artificial 202 and 205), there is, contrary to proper double- or triplespike techniques (e.g., Dodson, 1963; Galer, 1999; Hamelin et al., 1985; Rudge et al., 2009), no unique, ‘assumption-free’ solution to the set of mass-balance equations. In other words, there is no 3-dimensional space in which all the isotopic ratios have finite coordinates. Again, a calibration of the relative abundances of 202Pb and 205Pb in the spike with respect to 208Pb/206Pb in SRM-981 is needed and, hence, the method loses its absolute character. Fortunately, the value of 208Pb/ 206 Pb = 2.1677 for SRM-981 adopted by Amelin and Davis (2006) for normalization, and, by extension, Connelly et al. (2008a), is identical to that obtained by Abouchami et al. (2005) by a genuine triple-spike technique. Only negligible discrepancy with the present work is, therefore, to be expected and the mutual agreement between our standard-bracketed, Tl-doped MC-ICP-MS data and Connelly et al.'s (2008a) double-spike TIMS data validates the accuracy of both datasets and assigns the differences with respect to older work to mass bias issues. The isotope composition of primordial Pb derived in this work (Nantan) should not, in most cases, significantly change the “Canyon Diablo” model ages of radiogenic Pb found in planetary materials, such as CAIs and chondrules. Using Eq. (1), we can estimate that, with the new values (Table 2), changes in the model 207Pb*/206Pb* are approximately 207

Δ

206 Pb
= Pb
e 204

×

204 206

Pb=

h

207

Pb=206 Pb   Pb− 204 Pb=206 Pb old 206

Pb=

 Pb

old

ð1Þ

  207 206 − Pb= Pb

i

new

with “old” and “new” referring to, respectively, the old (Tatsumoto et al., 1973; Table 2) and the new (this study; Table 2) values for primordial Pb. In contrast, for very unradiogenic samples, for which

the fraction is of the order of unity or higher, the new primordial Pb values may to some extent affect the “Canyon Diablo” Pb–Pb ages of meteorites. Figure 6 summarizes the shifts of “Canyon Diablo” ages induced by adopting the new isotopic values of primordial Pb for different Pb isotope compositions. All shifts are towards younger ages. Dramatic changes (in excess of several million years) are predicted in two cases: (i) old unradiogenic samples, such as most of the fractions analyzed here, as a result of the leverage effect, and (ii) samples younger than 1 Ga, for which 207Pb*/206Pb* only very slowly deviates from λ235U/λ238U /137.88, in which λ stands for the decay constant. In contrast, the effect of the new Nantan primordial Pb values of this study on the “Canyon Diablo” ages of chondrules (Amelin and Krot, 2007; Amelin et al., 2002; Connelly et al., 2008b) and angrites (Amelin, 2008a,b; Connelly et al., 2008a), which contain fairly radiogenic Pb, can be safely disregarded. It will now be shown that primordial Pb in all the troilite samples analyzed so far, both in the present work and in the literature, is isotopically uniform within the given errors of the measurements. This is an important observation because more than one primordial Pb component could have been present or radiogenic Pb ingrowth in some of the meteorites could have gone through a multistage history. To aid with the solution, we will use a familiar approach inspired from Stacey and Kramers (1975): if a unique primordial Pb component is present in all troilite material, all the linear arrays, whether mixing lines or isochrons, defined by individual troilite inclusions should converge towards the primordial value. Figure 7 shows that, with a few exceptions, this is indeed the case and that they define a bundle of straight-lines converging towards one point in 207Pb/206Pb–204Pb/206Pb space (see inset in Fig. 7). A strong positive indication that this approach is valid is that literature TIMS data for Canyon Diablo and Toluca (Göpel et al., 1985) and for Cape York (Connelly et al., 2008a) are all part of this bundle (open circles in Fig. 7 as opposed to bull's-eye symbols for the samples of this study). An elementary property, related to Legendre transforms, of linear bundles of (x, y) straight lines going through the point (x0, y0) is that they all obey the equation: y = y0 + sðx−x0 Þ = i + sx0

ð2Þ

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CD

1.0

hron

0.6

-2

-1

Geoc

-5

-1

-20

0

207

Pb/ 206Pb

0.8

0.4

0.2

0.02

0.04

0.06

0.08

0.10

204

Pb/ 206Pb

Fig. 6. Shifts incurred in Pb–Pb ages (in millions of years) when the primordial Pb isotope composition of Nantan troilite (this work) is used instead of Pb from the Canyon Diablo troilite (Tatsumoto et al., 1973). CD equals Pb from Canyon Diablo troilite (Tatsumoto et al., 1973). See text for explanations.

where s is the slope and i the intercept i = y0 −s x0

ð3Þ

A simple way to identify such a bundle is to plot the intercepts versus the slopes for all the different alignments: the slope and the intercept of the new alignment give (−x0, y0) and define in the leastsquare sense the point common to all the lines. The present Pb isotope data on Canyon Diablo (“Tilton” and AMNH #2619), Nantan (K, N1, and N2), Cape York, Seeläsgen, and Mundrabilla #77992 (Table 1) tightly define a common isotopic com-

position (Fig. 7 and Table 2). Toluca (shown with a red bull's-eye symbol in Fig. 7), Mt Edith, Seymchan, and Mundrabilla #63692 (not shown) all have too uniform Pb isotope compositions among their leachate and residue fractions (Table 1) to define useful alignments with statistically significant slopes and intercepts. Again, literature TIMS data for Canyon Diablo and Toluca (Göpel et al., 1985), as well as for Cape York (Connelly et al., 2008a), are consistent with the present results and fall on the same alignment as our MC-ICP-MS data (Fig. 7). The values of the common point of intersection (Table 2) are slightly more radiogenic than those of the most primitive leachates of Nantan and Canyon Diablo by 1–2.5‰, but the errors on the position of the common intercept are fairly large. If

0.70 1.15 Canyon Diablo (Tatsumoto et al., 1973)

Pb/ 206Pb

Age Ga 4.65

0.65 Mun77

4.55

intercept

0.60

Cape York Cape York Connelly CD Göpel

207

4.6

1.05

Toluca Göpel Seeläsgen

4.5

0.09 Nantan K Toluca Nantan 2 Nantan N1

4.45

0.55

1.10

4.4

204

0.10

Pb/

206

0.11

Pb

CD AMNH 4.35 4.3

0.50

least-squares solution Tatsumoto et al. (1973)

4.25

CD Tilton

4.2

0.45 4.2

4.4

4.6

4.8

5.0

5.2

5.4

5.6

5.8

slope Fig. 7. Correlation diagram between the slope and the intercept of the least-square straight-lines calculated for each sample array in 207Pb/206Pb–204Pb/206Pb space. Toluca (red bull's-eye) is not included in the regression calculations. The negative correlation shows that these arrays form a bundle and intercept at one common point, which represents the common primordial component (see inset). The slope and intercept of this correlation give the coordinates of the common point. Note that the slope y0 = 1.1060 and intercept −x0 = −0.10745 (see Eq. (4)) of the dashed red line, calculated from the 207Pb/206Pb and 204Pb/206Pb values of the Canyon Diablo troilite of Tatsumoto et al. (1973), are statistically distinguishable from those of the solid black line, which were calculated from a regression of all slopes and intercepts. This indicates that the arrays do not intersect at the composition of the Canyon Diablo troilite (see blue ellipse in Fig. 3). The bundle of the sample arrays in the 208Pb/206Pb vs 204Pb/206Pb diagram is less well defined.

J. Blichert-Toft et al. / Earth and Planetary Science Letters 300 (2010) 152–163

real, such a discrepancy cannot be the effect of mass fractionation during leaching (Bouvier and Wadhwa, 2007) or ion-exchange separation (Baker et al., 2004; Blichert-Toft et al., 2003) because the slopes of the mass fractionation trends in 207Pb/206Pb–204Pb/206Pb and 208Pb/206Pb– 204 Pb/206Pb space are negative (Figs. 3 and 4). The difference rather reflects a somewhat protracted history of the parent asteroid of the IAB irons, which involves, according to 182Hf–182W chronometry, silicate-metal differentiation at 2.5+ 2.3/− 2.0 Ma, silicate melting at 4.6 + 0.7 / − 0.6 Ma, and metamorphic events taking place until 10.8 + 2.4 / − 2.0 Ma after the formation of calcium–aluminum-rich refractory inclusions (CAIs) (Schulz et al., 2009). 5.2. Radiogenic lead If all the Pb that is not primordial is considered to be radiogenic, the age at which the U–Pb system closed can be calculated. Very few reliable ages exist for iron meteorites and when they exist they are not necessarily for the same samples as those analyzed in the present work. When Widmanstätten textures are present, a first isotopeindependent chronometer of iron meteorites is given by the thickness of kamacite lamellae and Ni diffusion profiles (Goldstein and Short, 1967). For slowly cooled IAB irons, the lack of Widmanstätten textures is an issue (Herpfer et al., 1994; Rasmussen, 1989), but the correlation between cooling rates and Ni contents in the bulk metal observed at higher Ni contents can be used to infer that the low-Ni IABs crystallized very slowly. Based on their Ni contents, all the IAB– IIICD samples analyzed in the present work are very similar to samples for which 39Ar–40Ar ages are available (Landes, Mundrabilla, Copiapo, Toluca) and which mostly range between 4.40 and 4.52 Ga (Bogard et al., 1967; Herpfer et al., 1994; Niemeyer, 1979; Takeda et al., 2000; Vogel and Renne, 2008). The fact that this age span overlaps the 207Pb*/206Pb* ages obtained here for Nantan, Toluca, and Seeläsgen (see vertical y-axis in Fig. 7) is probably not coincidental and indicates that the non-primordial Pb in troilite was not acquired on Earth, but results from radiogenic ingrowth, most likely on the meteorite parent body (see Chen and Wasserburg, 1983). These young ages are difficult to ascribe to a terrestrial Pb contaminant because some samples would then have to consist of ≈ 100% contamination Pb, yet, at the same time, be essentially devoid of a gasoline component (Fig. 1). In addition, in the absence of contamination, any natural system (hence all the samples considered here) is a binary mixture between primordial (i.e., initial or unradiogenic) and radiogenic Pb. Contamination would make Pb from the samples ternary mixtures of primordial, radiogenic, and terrestrial Pb, whereas the alignment in the 208Pb/206Pb vs 204Pb/206Pb plot (Fig. 2) is excellent and considerably strengthens the case for a binary mixture. The presence of common asteroidal Pb further is to be expected for the following reason: if sulfur, with its 50% condensation temperature T50 of 664 K (Lodders, 2003), is ubiquitous in the asteroid (which is the case as testified to by the very existence of abundant troilite in the present meteorite samples), substantial amounts of common Pb, which has a higher condensation temperature (T50 = 727 K) than sulfur, also must have been retained. The young ages do not fit a scenario of ancient resetting events either and a two-stage history of the troilite secondary isochrons would not intersect anywhere near the primordial isotope composition. Silicate-rich inclusions are found in many IAB meteorites with approximately chondritic bulk compositions and FeO-poor silicates (Benedix et al., 2000; McCoy et al., 1993). Wasson and Kallemeyn (2002) pointed out that the silicate composition is reminiscent of ordinary chondrites, and we suggest that it must, therefore, be slightly depleted in volatile elements. A first explanation for the radiogenic component is that a U–Th rich phase, presumably hosted in silicates, with a 238U/204Pb ratio b 8, but with chondritic Th/U, was entrained with the molten metal. Zircons, such as those described in Toluca by Marvin and Klein (1964), and phosphates (McCoy et al., 1993), may have played a role

161

in this component, but these minerals are expected to fractionate Th from U, which does not fit the remarkably constant Th/U ratio revealed by the 208Pb/206Pb vs 204Pb/206Pb plot (Fig. 2). In addition, because U and Th solubility in iron and sulfide is particularly low, introduction of the radiogenic component almost certainly post-dates ingrowth of radiogenic Pb. Contamination by radiogenic Pb from the shallow silicate-rich layers of the asteroid during its breakup, a series of recent events attested to by the 5 to 600 Ma range of exposure ages of IAB irons (Lavielle et al., 1999; Takeda et al., 2000; Voshage and Feldmann, 1979) is, therefore, a much more plausible explanation. Ordinary, and to a lesser extent, carbonaceous chondrites, such as COs and CVs, are indeed depleted in volatile elements. Lead much more radiogenic than that of CI chondrites, and even than terrestrial common Pb, is found in bulk chondrites (e.g., Bouvier et al., 2007; Göpel et al., 1994; Tatsumoto et al., 1976; Tilton, 1973) and is presumably hosted by chondrules (Amelin and Krot, 2007; Amelin et al., 2002; Connelly et al., 2008b) and phosphates (Göpel et al., 1994). Both the 39Ar–40Ar and the U–Pb ages of IAB iron meteorites record the closure of this non-primordial component some 50–150 Ma after the formation of the asteroid. The recent introduction of an old radiogenic asteroidal Pb component in troilite makes it unlikely that the IAB irons represent an asteroidal core. The presence of angular silicate inclusions (Benedix et al., 2000; McCoy et al., 1993) suggests that they are metal-rich fragments of an asteroidal regolith. Whether these fragments formed by early catastrophic breakup and re-accretion of a partially molten and metamorphic asteroid, as envisaged by McCoy et al. (1993) and Benedix et al. (2000), or as impact-related pockets of metal percolating into a porous chondritic matrix, as suggested by Choi et al. (1995) and Wasson and Kallemeyn (2002), is unclear. In some sense, these two mechanisms may be envisaged as grading into each other. The major limitation is that silicates could not have been heated to the point where the 39Ar–40Ar chronometer became reset on a large scale (Vogel and Renne, 2008), a first-order constraint that both models can be configured to conform to. The 39Ar– 40 Ar and U–Pb age agreement does not hold as well for Mundrabilla, for which the 207Pb*/206Pb* age is very old (Herpfer et al., 1994; Niemeyer, 1979). Both the occurrence of troilite as veins and the scarcity of silicates in this meteorite reveal that its history is unique. The age comparison is difficult to assess for Canyon Diablo, for which each sample seems to provide a different alignment, in keeping with the variability of nodule mineralogy in this meteorite (El Goresy, 1965). The Th/U ratio of 3.876 ± 0.016 inferred for the parent bodies from the excellent correlation between 208Pb/206Pb and 204Pb/206Pb (Fig. 2) is the most precise estimate for a planetary object to date, an order of magnitude improvement over previous estimates (Rocholl and Jochum, 1993). Contrary to the parent body of IVA iron meteorites (Blichert-Toft et al., 2010), there is no indication that segregation of IAB–IIICD irons led to fractionation of Th from U. Since the data for Mount Edith (IIIB) and Seymchan (pallasite) plot on the 208Pb/ 206 Pb–204Pb/206Pb isochron of the IAB–IIICD irons, this indicates that the Th/U ratio of 3.876 ± 0.016 derived from the radiogenic component likely represents the value of the solar nebula. 6. Conclusions Finding primitive Pb in a particular sample of troilite mostly depends on the Pb concentration of the troilite. As in Chen and Wasserburg (1983), the most primitive Pb found in the present study comes from Nantan, the sample with by far the highest Pb abundances in troilite, but is only marginally more primitive than our most primitive Canyon Diablo Pb. The 204Pb/206Pb of this primitive Pb is indistinguishable from that of Tatsumoto et al. (1973), while the 207Pb/206Pb and 208Pb/206Pb are significantly higher, which we ascribe to instrumental mass fractionation biasing the older data. The most primitive Pb found in the

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present study has the following composition: 204Pb/206Pb = 0.107459 (16), 207Pb/206Pb = 1.10759(10), and 208Pb/206Pb= 3.17347(28). Canyon Diablo, Nantan, Seeläsgen, Cape York, and Mundrabilla form, together with literature data, a bundle of secondary isochrons that intersect at a point only slightly more radiogenic than the most primitive Pb recorded in Nantan. This observation demonstrates that all these meteorites contain the same primordial Pb component. The radiogenic component present in most of the meteorites analyzed here represents Pb from the silicate-rich part of the parent body and the inferred 238U/204Pb suggests a chondritic source. The apparent age of this radiogenic Pb component is consistent with 39Ar– 40 Ar ages of silicate inclusions found in the same samples. The radiogenic Pb was added more recently to the troilite, probably during the impacts that formed the IAB irons, transferred from the surface chondritic rubble of the parent body. The remarkable correlation between 208Pb/206Pb and 204Pb/206Pb for all the meteorites analyzed here corresponds to a Th/U ratio of 3.876 ± 0.016, which is the most precise estimate to date of this ratio for the solar nebula. Acknowledgements In admiration of his life’s work, we dedicate this study to George Tilton, who had an immense impact on the concept of primordial Pb in the Solar System. JBT and FA acknowledge the financial support from the French Programme National de Planétologie (PNP) of the Institut National des Sciences de l'Univers (INSU) and Centre National d'Etudes Spatiales (CNES), and from the French Agence Nationale de la Recherche (ANR grant T-TauriChem). DSE acknowledges the support from the US NASA Cosmochemistry program (#NNX09AE84G). We are grateful to George Tilton, Rick Carlson, Jutta Zipfel, Joe Boesenberg, Luc Labenne, and Alain Carion for providing the samples analyzed in this study. We further thank Jim Connelly and Mary Horan for constructive reviews that helped us improve the manuscript. JBT also thanks Philippe Telouk for assistance with the Nu Plasma MC-ICP-MS. Finally, JBT and FA are indebted to Alan Levander for providing an inspiring and peaceful work environment at Rice University that allowed us to ponder the present data during our stay as Wiess Visiting Professors. References Abouchami, W., Hofmann, A.W., Galer, S.J.G., Frey, F.A., Eisele, J., Feigenson, M., 2005. Lead isotopes reveal bilateral asymmetry and vertical continuity in the Hawaiian mantle plume. Nature 434, 851–856. Albarède, F., Télouk, P., Blichert-Toft, J., Boyet, M., Agranier, A., Nelson, B.K., 2004. Precise and accurate isotopic measurements using multiple-collector ICPMS. Geochim. Cosmochim. Acta 68, 2725–2744. Amelin, Y., 2006. The prospect of high-precision Pb isotopic dating of meteorites. Meteoritics Planet. Sci. 41, 7–17. Amelin, Y., 2008a. U–Pb ages of angrites. Geochim. Cosmochim. Acta 72, 221–232. Amelin, Y., 2008b. The U–Pb systematics of angrite Sahara 99555. Geochim. Cosmochim. Acta 72, 4874–4885. Amelin, Y., Davis, W.J., 2006. Isotopic analysis of lead in sub-nanogram quantities by TIMS using a 202Pb–205Pb spike. J. Anal. At. Spectrom. 21, 1053–1061. Amelin, Y., Krot, A., 2007. Pb isotopic age of the Allende chondrules. Meteoritics Planet. Sci. 42, 1321–1335. Amelin, Y., Krot, A.N., Hutcheon, I.D., Ulyanov, A.A., 2002. Lead isotopic ages of chondrules and calcium–aluminum-rich inclusions. Science 297, 1678–1683. Amelin, Y., Davis, D.W., Davis, W.J., 2005. Decoupled fractionation of even- and oddmass isotopes of Pb in TIMS, Goldschmidt Conference, Idaho, p. A215. Amelin, Y., Janney, P., Chakrabarti, R., Wadhwa, M., Jacobsen, S.B., 2008. Isotopic analysis of small Pb samples using MC-ICPMS: The limits of precision and comparison to TIMS. EOS Trans. AGU, 89, Fall Meet. Suppl. Abstract V13A-2088. Baker, J., Peate, D.W., Waight, T., Meyzen, C., 2004. Pb isotopic analysis of standards and samples using a 207Pb–204Pb double spike and thallium to correct for mass bias with a doublefocusing MC-ICP-MS. Chem. Geol. 211, 275–303. Benedix, G.K., McCoy, T.J., Keil, K., Love, S.G., 2000. A petrologic study of the lAB iron meteorites: constraints on the formation of the IAB-Winonaite parent body. Meteoritics Planet. Sci. 35, 1127–1141. Blichert-Toft, J., Weis, D., Maerschalk, C., Agranier, A., Albarède, F., 2003. Hawaiian hot spot dynamics as inferred from the Hf and Pb isotope evolution of Mauna Kea volcano. Geochem. Geophys. Geosyst. 4. doi:10.1029/2002GC000340

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