The thallium isotope composition of carbonaceous chondrites — New evidence for live 205Pb in the early solar system

Earth and Planetary Science Letters 291 (2010) 39–47

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The thallium isotope composition of carbonaceous chondrites — New evidence for live 205Pb in the early solar system R.G.A. Baker a,b,⁎, M. Schönbächler a,c, M. Rehkämper a,b,⁎, H.M. Williams d, A.N. Halliday d a

Department of Earth Science and Engineering, Imperial College, London SW7 2AZ, UK The Natural History Museum, Cromwell Road, London SW7 5BD, UK Department of Earth Sciences, University of Manchester, Manchester M13 9PL, UK d Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, UK b c

a r t i c l e

i n f o

Article history: Received 17 June 2009 Received in revised form 14 December 2009 Accepted 23 December 2009 Available online 25 January 2010 Editor: R.W. Carlson Keywords: chronology isochron extinct radionuclides lead thallium carbonaceous chondrites chondrites meteorites solar nebula solar system volatile elements nucleosynthesis

a b s t r a c t The extinct radionuclide 205Pb, which decays to 205Tl with a half-life of 15 Ma, is of considerable cosmochemical interest, as it is the only short-lived isotope that is produced exclusively by s-process nucleosynthesis. Evidence for the existence of 205Pb in the early solar system has only recently been obtained from analyses of IAB iron meteorites, but significant uncertainties remain about the initial 205Pb abundance and Tl isotope composition of the solar system. In an attempt to better constrain these values, a comprehensive 205Pb–205Tl isochron study was carried out on ten carbonaceous chondrites of groups CI, CM, CV, CO and CR. The Pb and Cd isotope compositions of the meteorites were also determined, to correct for terrestrial Pb contamination and eliminate samples that exhibit fractionated Tl isotope compositions from thermal processing. The analyses revealed only limited variation in ε205Tl, with values of between − 4.0 and + 1.2, but nonetheless the Tl isotope compositions correlate with Pb/Tl ratios. This correlation is unlikely to be due to stable isotope fractionation from terrestrial weathering or early solar system processes, and is most readily explained by in situ decay of 205Pb to 205Tl. Previous 53Mn–53Cr and 107Pd–107Ag studies of bulk carbonaceous chondrites provide evidence that the Pb–Tl isochron records volatile fractionation in the solar nebula at close to 4567 Ma. The isochron thus yields the initial 205Pb abundance and Tl isotope composition of the solar system, with values of 205Pb/204PbSS,0 = (1.0 ± 0.4) × 10− 3 and ε205TlSS,0 = − 7.6 ± 2.1, respectively. These results confirm the previous Pb–Tl data for IAB iron meteorites, which provided the first clear evidence for the existence of live 205Pb in the early solar system. The initial 205PbSS,0 abundance inferred from carbonaceous chondrites demonstrates that the 205Pb–205Tl decay system is well suited for chronological studies of early solar system processes that produce fractionations in Pb/Tl ratios, including core crystallization and the mobilization of volatiles during thermal processing. The 205PbSS,0 abundance is close to the upper limit of nucleosynthetic production estimates for AGB stars and thus in accord with contributions of such stars to the early solar system budget of freshly synthesized radioisotopes. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Extinct radionuclide systems are powerful cosmochemical tools that provide quantitative information on the presolar production sites of elements as well as the timescales of early solar system processes. The nuclide 205Pb, which decays to 205Tl with a half-life of 15.1 Ma (Pengra et al., 1978), is of particular astrophysical importance, as it is the only short-lived nuclide produced solely by s-process nucleosynthesis. Aside from being a unique nucleosynthetic tracer, the 205Pb– ⁎ Corresponding authors. Department of Earth Science and Engineering, Imperial College, London SW7 2AZ, UK. E-mail addresses: [email protected] (R.G.A. Baker), [email protected] (M. Rehkämper). 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.12.044

205

Tl decay system also has potential uses in chronometry. The Pb/Tl ratios of early solar system materials will be affected by a range of processes, such as partial evaporation/condensation, metal-silicate and metal-sulfide fractionation, as well as fractional crystallization of solid metal from a metallic liquid. As a result, the 205Pb–205Tl decay system may be useful for dating a number of early solar system processes, including volatilization during thermal processing, planetary differentiation and the crystallization of asteroidal cores. Lead-205 is predicted by astrophysical models to have been sufficiently abundant in the early solar system to produce meteorite reservoirs with distinct Tl isotope compositions (Wasserburg et al., 1994) and this has prompted a number of 205Pb–205Tl decay system studies in the past (Anders and Stevens, 1960; Ostic et al., 1969; Huey and Kohman, 1972; Arden, 1983; Chen and Wasserburg, 1994).

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Definitive evidence for the former presence of live 205Pb in the solar system remained elusive, however, until the recent investigation of Nielsen et al. (2006a). The latter study found a strong positive correlation between ε205Tl and 204Pb/203Tl ratios for metal samples of IAB iron meteorites, and this was interpreted as an isochron that reflects in situ decay of formerly live 205Pb, following metal segregation and crystallization. This isochron demonstrates that 205 Pb was alive in the early solar system but it fails to link the 205 Pb–205Tl decay system to an absolute timescale. Such a link is desirable but requires Pb–Tl isotope data for meteorites that were precisely dated with an independent chronometer. The present study was conducted to investigate whether bulk rock samples of carbonaceous chondrites display Tl isotope variations from the decay of 205Pb that can be exploited for chronological studies. Additional Pb and Cd isotope analyses were carried out, to identify samples that were affected by terrestrial Pb contamination and which exhibit fractionated Cd and Tl stable isotope compositions from mobilization of these labile elements during thermal processing of the parent body. 2. Samples A total of ten carbonaceous chondrites were analyzed in this study. The majority of the samples were provided by the Natural History Museum in London. Exceptions are the Allende sample from the Smithsonian Institution and the CR2 chondrites EET 92042 and GRA 95229, which are from the NASA-Johnson Space Center. Except for Allende Smithsonian, all samples were received as small rock chips that were ground to a fine powder prior to digestion, using an alumina mortar and pestle. 3. Analytical methods A comprehensive description of the laboratory methods employed at the Imperial College MAGIC Laboratories is given in Baker et al. (2009). Only a summary and details that are specific to this study are provided here and in Table 1. 3.1. Sample dissolution The samples were first weighed into Savillex Teflon beakers, refluxed in 8 ml 28 M HF and 4 ml 15 M HNO3 on a hotplate for 72 h, dried and taken up in 4.5 ml 15 M HNO3. All but four samples (Allende NHM digestions 1 to 4; Table 2) were then placed in Parr bombs for 72 h at 170 °C, whilst the others were further digested on a hotplate. Following this step, the samples were dried again, taken up in 10 ml 6 M HCl and returned to the hotplate for a further 72 h. Nearly perfect solutions were obtained for all samples at this stage and these were centrifuged before a 10% aliquot was removed from all but three samples (see below) for Cd isotope analysis. The meteorites EET 92042, GRA 95229 and Allende Smithsonian were analyzed at a later stage and they were spiked with a Cd double spike directly after weighing, such that the removal of a separate Cd aliquot was not necessary. The acid molarity of the main aliquot was then reduced from 6 M to 1 M HCl, by several cycles of evaporation and addition of H2O before saturated bromine water (∼1% of solution volume) was added and the solutions left to stand overnight. A further 10% solution aliquot was then removed for the determination of the Pb and Tl concentrations with the isotope dilution (ID) technique, whilst the remainder was prepared for the Tl and Pb isotope composition (IC) measurements. 3.2. Chemical separation procedures 3.2.1. Separations for isotope composition measurements The anion-exchange separation methods are similar to those described by Baker et al. (2009). The main difference is that the

Table 1 Anion-exchange chemistry for the separation and purification of Pb, Cd and Tl from meteorite samples. First stage: Separation of Pb, Cd, Tl from bulk samples 1 ml resin in quartz-glass columns 18 ml 0.1 M HCl-SO2 11 ml 0.1 M HCl 4 × 1 ml 0.1 M HCl-1% Br2 Up to 30 ml sample sol. in 0.1 M HCl-1% Br2 10 ml 0.03 M HBr–0.5 M HNO3-1% Br2 40 ml 0.03 M HBr–2 M HNO3-1% Br2 10 ml 0.1 M HCl–1% Br2 16 ml 0.1 M HCl-SO2

Resin cleaning Resin cleaning Resin equilibration Collect Pb Matrix elution Collect Cd Matrix elution Collect Tl

Second stage: Tl purification 0.1 ml resin in small Teflon columns 2.5 ml 0.1 M HCl-SO2 2.5 ml 0.1 M HCl 4 × 0.1 ml 0.1 M HCl-10% Br2 Up to 2 ml sample sol. in 0.1 M HCl-10% Br2 1.5 ml 0.03 M HBr–0.5 M HNO3-1% Br2 1.1 ml 0.03 M HBr–2 M HNO3-1% Br2 1.1 ml 0.1 M HCl-1% Br2 1.6 ml 0.1 M HCl-SO2

Resin cleaning Resin cleaning Resin equilibration Matrix elution Matrix elution Matrix elution Matrix elution Collect Tl

Second stage: Pb purification 1 ml resin in quartz-glass columns 17 ml 0.1 M HCl 17 ml 0.1 M HNO3 4 × 1 ml 0.2 M HBr-1% Br2 Up to 30 ml sample sol. in 0.2 M HBr-1% Br2 7 ml 0.2 M HBr–0.5 M HNO3-1% Br2 3 ml 0.03 M HBr–0.5 M HNO3-1% Br2 8 ml 0.03 M HBr–0.5 M HNO3-1% Br2

Resin cleaning Resin cleaning Resin Equilibration Matrix elution Matrix elution Matrix elution Collect Pb

All separations utilized Biorad AG 1-X8 200–400 mesh anion-exchange resin. A fresh resin bed was prepared for each sample.

elution procedure for the primary anion-exchange column was modified to permit the collection of separate Pb, Cd and Tl fractions for isotopic analysis (Table 1). Each fraction was then further purified by anion-exchange chromatography to obtain elemental separates that were sufficiently clean for precise and accurate isotopic analyses. The total procedural blank was ∼15 pg for Tl, ∼ 300 pg for Pb, and less than ∼20 pg for Cd. In all cases the blank was negligible, constituting less than 0.1% (Tl, Pb) or 0.8% (Cd) of the indigenous element budget. The elution technique for the purification of Tl (Table 1) produced a final fraction that was essentially Pb-free but which contained N99% of the Tl present in samples. The Pb fractions from the first stage columns (Table 1) were dried down, redissolved in concentrated HBr, dried again, and taken up in 0.2 M HBr. Saturated bromine water was then added to a concentration of about 1% and the resultant samples were left to stand for at least 6 h prior to the anion-exchange purification of Pb (Table 1). Cadmium fractions were collected from the primary separation chemistry (Table 1) for all samples. This was followed by doublespiking, except for EET 92042, GRA 95229 and Allende Smithsonian, which had been double-spiked for Cd isotope analysis directly after weighing. The procedure that was applied for the separation and purification of Cd from these fractions or the 10% aliquots of sample solutions was essentially identical to the methodology of Wombacher et al. (2003). This involved (i) an initial anion-exchange column for the isolation of Cd from the bulk matrix and (ii) further purification of Cd from Sn using 200 µl TRU-Spec resin columns. 3.2.2. Separations for isotope dilution concentration measurements The signal intensities measured in the IC analyses were evaluated, to estimate the Tl and Pb contents of the samples. The spiking of the respective ID aliquots with 203Tl and 208Pb tracer solutions was then optimized, based on these results. After spiking, the solutions were dried down, redissolved in 1 ml 0.2 M HBr–1% Br2 and then processed

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Table 2 Analytical results obtained in this study for carbonaceous chondrites. Sample

Group

Weight (g)

ε205Tld

204

Pb/203Tle,f

Allende NHM (1)a,b Allende NHM (2)a,b Allende NHM (3)a,b Allende NHM (4)a,b Allende NHM (5)a Allende NHM (6)a Allende NHM (7)a Allende NHM (8)a Allende NHM averagec Orgueil Cold Bokkeveld Murchison Allende Smithsonian Leoville Colony Kainsaz EET 92042 GRA 95229 NWA 801

CV3 CV3 CV3 CV3 CV3 CV3 CV3 CV3

1.401 1.507 ∼1.2 1.342 ∼0.9 0.936 1.257 1.158

CI1 CM2 CM2 CV3 CV3 CO3 CO3 CR2 CR2 CR2

0.157 0.453 0.431 0.677 0.561 0.604 0.587 0.460 0.578 0.617

− 3.3 − 3.3 − 3.0 − 3.3 − 2.8 − 3.2 − 2.7 − 3.3 − 3.1 ± 0.5 − 1.7 − 1.5 − 1.6 − 3.6 − 1.2 + 1.2 − 0.7 − 2.1 − 0.9 − 4.0

1.24 ± 0.01 1.22 ± 0.01 1.21 ± 0.01 1.22 ± 0.01 1.18 ± 0.01 1.20 ± 0.01 1.18 ± 0.01 1.17 ± 0.01 1.20 ± 0.05 1.42 ± 0.02 1.41 ± 0.12 1.40 ± 0.02 1.38 ± 0.21 1.37 ± 0.02 2.20 ± 0.44 1.47 ± 0.01 1.39 ± 0.05 1.48 ± 0.05 0.790 ± 0.071

Mol fraction primitive Pbg

Pb (ng/g)e,f

Tl (ng/g)e

1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

– – – –

– – – – 52.82±0.03 58.28 ± 0.03 50.99 ± 0.02 59.35 ± 0.02 55.4 ± 8.4 100.4 ± 0.1 52.4 ± 0.1 58.5 ± 0.1 51.1 ± 0.1 29.0 ± 0.04 31.70 ± 0.04 33.84 ± 0.04 21.4 ± 0.6 19.2 ± 0.6 23.92 ± 0.03

1.00 0.40 ± 0.03 1.00 0.87 ± 0.13 1.00 0.23 ± 0.04 1.00 1.00 1.00 0.91 ± 0.09

982±2 1099 ± 2 948 ± 1 1097 ± 2 1031 ± 156 2199 ± 23 1137 ± 97 1260 ± 13 1161 ± 215 631 ± 7 1112 ± 224 793 ± 7 479 ± 7 452 ± 8 337 ± 5

ε114Cd/110Cd

4.8 ± 2.4 5.5 ± 1.8 3.7 ± 1.2 2.4 ± 1.2 32.9 ± 3.5 3.0 ± 1.3 15.8 ± 4.2 1.3 ± 0.5 – 2.9 ± 1.7

206

Pb/204Pb

10.29 10.28 10.28 10.29 10.29 10.29 10.29 10.29 10.29 ± 0.01 9.89 13.89 9.82 11.81 10.51 16.66 10.52 10.73 10.57 11.70

207

Pb/204Pb

10.91 10.90 10.91 10.91 10.92 10.91 10.91 10.91 10.91±0.01 10.66 13.13 10.63 11.84 11.05 14.27 11.04 11.19 11.09 11.75

208

Pb/204Pb

30.51 30.49 30.49 30.49 30.51 30.50 30.49 30.49 30.50 ± 0.02 30.10 33.97 30.03 31.96 30.61 36.35 30.67 30.98 30.76 31.83

a

The eight Allende NHM samples are separate dissolutions of a single ∼ 30 g sized Allende powder aliquot. Samples dissolved by hotplate dissolution; all other meteorites were dissolved using Parr bombs. c The uncertainties of the Allende NHM average are reported at the 2 sd level, based on the results for 8 separate dissolutions. d The ε205Tl data are assigned a 2 sd uncertainty of ±0.5 ε, based on the results obtained for multiple dissolutions of Allende NHM. e The 2 sd uncertainties of the trace element data were obtained by propagating the uncertainties of relevant individual sources of error (see text). f Corrected for terrestrial contamination, incl. laboratory blank. g The Pb contamination was calculated by assuming that the meteorites had the following primitive Pb isotope compositions: Cold Bokkeveld–Murchison; Colony–Kainsaz; Allende Smithsonian/NWA 801 — any value between the measured Pb isotope composition and the composition of Allende NHM/EET 92042 was considered acceptable, within uncertainty. The correction also accounts for any contamination by the laboratory Pb blank. See Supplementary data for a detailed description of the correction procedure. b

using the procedures outlined in Baker et al. (2009). The total Tl and Pb blanks of the ID procedures were 5.0 ± 0.5 pg and 250 ± 100 pg, respectively, which amounted to blank corrections of b0.5% for all samples. 3.3. Mass spectrometry 3.3.1. Thallium isotope measurements The Tl isotope measurements were performed with a Micromass IsoProbe multiple collector inductively coupled plasma-mass spectrometer (MC-ICPMS), using previously described procedures that utilized both external normalization to NIST SRM 981 Pb and standard-sample bracketing for mass bias correction (Baker et al., 2009). All results are reported in the ε notation, whereby: 205

205

ε

Tl =

ð

Tl =

203

TlÞsample

ð205 Tl = 203 TlÞstd

! −1

× 10; 000

ð1Þ

The NIST SRM 997 Tl isotope reference material, which is characterized by 205Tl/203Tl = 2.38710 (Dunstan et al., 1980), was used as ε205Tl = 0 standard throughout. Thallium isotope analyses that were carried out for 8 separate dissolutions of a large (∼30 g) homogenized powder sample of Allende, yielded an external reproducibility (2 sd) of ±0.5 ε205Tl (Table 2). This value provides a conservative estimate of the analytical uncertainty, because it may be biased by Tl isotope variations between the different powder splits. The accuracy of the method was verified through analyses of doped natural samples. To this end, the Tl-free matrix fractions of the meteorites Cold Bokkeveld, Leoville, Murchison and NWA 801 were collected from the first stage anion-exchange chemistry (Table 1) and doped with NIST 997 Tl. The doped matrix fractions were then treated in the same manner as initial dissolutions and the Tl was separated using the standard methodology (Table 1) prior to isotopic analyses. These samples all displayed Tl isotope compositions that were identical to the true result (ε205Tl = 0), to within ±0.5 ε.

3.3.2. Lead and cadmium isotope measurements The Pb IC measurements were also conducted with the IsoProbe, using external normalization relative to admixed NIST SRM 997 Tl for mass bias correction. The analytical methods were similar to those outlined by Rehkämper and Mezger (2000) and all data are reported relative to the NIST SRM 981 Pb data of Galer and Abouchami (1998). Repeated analyses of NIST SRM 981 Pb solutions that were carried out interspersed with the sample measurements typically yielded external precisions (2 sd) of about ±200 ppm for 206Pb/204Pb, ±300 ppm for 208 Pb/204Pb, and ±400 ppm for 207Pb/204Pb (207Pb was measured with a 1010 Ω resistor, whilst all other collectors featured 1011 Ω resistors). Duplicate analyses of sample solutions displayed similar or slightly worse reproducibilities. The Pb IC data was acquired mainly to estimate the extent of terrestrial Pb contamination for the samples. No additional measures were therefore applied to improve the precision and accuracy of the results, and conservative uncertainties (2 sd) of about ±800 ppm (206Pb/204Pb) to ±1500 ppm (207Pb/204Pb, 208Pb/204Pb) were adopted for most Pb IC data. The Cd stable isotope analyses were carried out with a Nu Plasma HR MC-ICPMS instrument at the University of Oxford. A 111Cd–113Cd double spike was employed for the correction of instrumental mass fractionation, using techniques that were similar to those of Ripperger and Rehkämper (2007). The zoom lens system of the Nu Plasma instrument was adjusted to allow simultaneous collection of 111 Cd, 112Cd, 113Cd, 114Cd, 115In (for correction of interference from 113 In), and 117Sn (for the correction of 112,114,115Sn). Each analysis involved an electronic baseline measurement of 20 s, followed by 30 data acquisition cycles of 10 s each. The double spike data reduction was carried out off-line, using a spreadsheet that implements the iterative procedures of mass bias correction developed by Siebert et al. (2001). The Cd isotope results are reported in ε-notation (Eq. (1)), as ε114/110Cd values relative to the isotope composition of the Alfa Cd Zürich standard (Ripperger and Rehkämper, 2007). This standard is offset by +0.5 ε114/110Cd (Z. Xue and M. Rehkämper, unpublished results) from the isotope composition of the JMC Cd Münster solution utilized by Wombacher et al. (2003, 2008). Multiple analyses of mixed solutions with 20 ng/ml Alfa Cd Zürich and 20 ng/ml Cd double

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spike typically yielded external reproducibilities (2 sd) of ±0.8 to ±1.5 ε114/110Cd. 3.3.3. Pb and Tl ID concentration measurements The ID concentration measurements were carried out using procedures similar to those employed for the IC analyses (Baker et al., 2009). The uncertainties quoted for the Pb, Tl concentrations and 204 Pb/203Tl ratios (Table 2), were obtained by propagating individual sources of error, which are the correction for terrestrial Pb contamination, the reproducibility of the mass spectrometric analyses (better than ±0.2‰ and ±0.4‰ for Pb and Tl, respectively, based on 2 se within-run errors) and the uncertainty of the procedural blank corrections. With this, the Tl concentration data have errors of about 0.1 to 3% (Table 2). The uncertainties for the Pb contents and 204Pb/ 203 Tl ratios are about 0.5 to 20%, whereby the biggest errors reflect large corrections for terrestrial Pb contamination (Table 2; see Supplementary data for details). 4. Results and discussion 4.1. Lead isotope compositions and corrections for terrestrial contamination of the meteorites A number of studies have shown that the Pb isotope compositions of carbonaceous chondrites correlate with the degree of volatile depletion, such that the volatile-rich CI and CM chondrites with low primordial U/Pb ratios have less radiogenic Pb isotope ratios than CV chondrites, which have higher U/Pb (Huey and Kohman, 1973; Tatsumoto et al., 1976). Significant Pb isotope variations have also been reported for different sub-samples of the same meteorite (e.g., Allende; Tatsumoto et al., 1976), and this probably reflects the inhomogeneous distribution of chondrite constituents with different 238 U/204Pb ratios. The contamination of meteorite samples with terrestrial Pb was shown to be a problem in a number of previous investigations (e.g., Göpel et al., 1985; Nielsen et al., 2006a). Lead contamination is also an issue for the present study, as accurate isochron calculations for the extinct 205Pb–205Tl decay system require the indigenous (uncontaminated) 204Pb contents of the meteorites. It was therefore necessary to adopt appropriate correction procedures. A common approach of many previous studies is the removal of terrestrial Pb from contaminated meteorites by repeated leaching with mineral acids (Göpel et al., 1985). Such an approach was not used in the present study, to ensure that the Tl isotope compositions were not fractionated during laboratory processing. The alternative approach that was adopted here, applied the Pb isotope compositions of the meteorites (Table 2) to screen and correct the samples for terrestrial Pb contamination. The effect of such contamination can be readily seen in a plot of 206Pb/204Pb vs. Tl concentration (Fig. 1). The majority of the samples define a trend, which links more radiogenic Pb isotope compositions with lower Tl contents. Such a correlation is expected, as lower Tl concentrations reflect a larger extent of volatile depletion, and this will be associated with higher U/Pb ratios. Four of the meteorites, Cold Bokkeveld, Allende Smithsonian, Colony and NWA 801, however, clearly plot above the correlation defined by the other samples (Fig. 1) and these unusually radiogenic Pb isotope compositions are likely to reflect terrestrial Pb contamination. This conclusion is supported by the observation that the complete dataset of Table 2 yields a 207Pb–206Pb isochron age of 4362 ± 73 Ma (as determined with the Isoplot program of Ken Ludwig, Berkeley Geochronology Center). This age is too young in comparison to previous results, which demonstrate that carbonaceous chondrites and their constituents formed within the first few million years of solar system history (Tatsumoto et al., 1976; Amelin et al., 2002). If the four samples with radiogenic Pb are excluded, the remaining

Fig. 1. Plot of 206Pb/204Pb ratios vs. Tl contents for the carbonaceous chondrites. The majority of the samples fall on a broad correlation (marked by the bold line), whilst four meteorites plot above this trend, presumably due to terrestrial Pb contamination. A correlation coefficient of r2 = 0.75 is obtained for the dataset, when the four outlier are disregarded. The uncertainties are smaller than the symbols except for the mean Tl concentration of Allende NHM (Table 2).

carbonaceous chondrites provide a more reasonable Pb–Pb isochron age of 4529 ± 80 Ma. Based on these observations, the Pb concentrations of the four contaminated meteorites were corrected for terrestrial contamination. These calculations assumed that the measured Pb isotope compositions were produced by the addition of a terrestrial Pb contaminant (Göpel et al., 1985) to an appropriate primitive Pb isotope composition (see caption of Table 2). Simple mass balance calculations were then applied to estimate the mol fraction of terrestrial Pb that contributes to the total Pb budget of each sample (the Supplementary data of the article provides a detailed description of the correction procedure). The calculations suggest that the molar fraction of primitive Pb present in the contaminated samples varies between about 23 ± 4% for Colony to 87 ± 13% for Allende Smithsonian. The correction procedure utilizes a simplified approach but several lines of evidence suggest that it is sufficiently robust for the purposes of this study. First, it is possible that further samples also exhibit minor terrestrial Pb contamination. Inspection of Fig. 1 suggests, however, that such contributions are unlikely to exceed 5 to 10% for other meteorites. The failure to account for such contributions will not generate any large systematic errors. Second, Fig. 2a demonstrates that the corrected Pb concentrations of all samples are well correlated with the Tl contents. Such a correlation is not expected for meteorites that feature a significant component of terrestrial Pb or that were subjected to an unreasonable contamination correction. 4.2. Lead and thallium abundances and

204

Pb/203Tl concentration ratios

Both Tl and Pb are volatile elements, with 50% condensation temperatures TC50% of 727 K for Pb and 532 K for Tl (Lodders, 2003). The new data are used in the following, to evaluate whether this difference in condensation temperatures is mirrored in Pb/Tl ratios that increase with increasing extent of volatile depletion. Overall, the Pb and Tl concentrations of the samples vary by more than a factor of 5, between about 340 to 2200 ng/g for Pb and about 19 to 100 ng/g for Tl (Fig. 2a, Table 2). The concentrations of both elements furthermore decrease approximately in the order CI N CM ≥ CV ≥ CO N CR (Fig. 2a). The observed trend thus deviates slightly from the expected order CI N CM N CV ≈ CO, which is generally attributed to nebular fractionation processes, such as incomplete

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with previous studies reporting results of 140 ± 10 ng/g (Reed et al., 1960), 142 ng/g (mean CI value of Wasson and Kallemeyn, 1988), and 166.7 ± 2.4 ng/g (Arden and Cressey, 1984). In contrast, the present study obtained a much lower Tl abundance of 100.4 ± 0.1 ng/g for Orgueil (Table 2). The new results of this investigation reveal an excellent Pb–Tl covariation (Fig. 2a), and this is in accord with the observation that ratios of more highly volatile elements with TC50% similar to or lower than FeS (664 K; Lodders, 2003), such as Pb, Zn, Cd, In, and Tl, show only limited variability amongst the main groups of carbonaceous chondrites (Palme et al., 1988; Wombacher et al., 2008). Possibly, this may reflect quantitative condensation of the last nebular gas that remained with the most volatile constituents, after partial condensation of less volatile species had been completed (Wulf et al., 1995). The 204Pb/203Tl ratios that were determined for carbonaceous chondrites vary by only about a factor of 2.5, with most samples having values of 204Pb/203Tl ≈ 1.4 (Table 2). Furthermore, there is no systematic correlation of 204Pb/203Tl with the degree of volatile depletion, as denoted by differences in Pb or Tl concentrations (Fig. 2b). Consequently, it is reasonable to exploit the trend of Fig. 2a (and a similar plot of 204Pb vs. 203Tl; not shown), to derive average carbonaceous chondrite and, by inference, solar system Pb/Tl and 204 Pb/203Tl ratios of 22 ± 2 and 1.43 ± 0.14 respectively (uncertainties are 2sd). 4.3. Thallium isotope compositions

Fig. 2. Plot of (a) primitive Pb concentrations and (b) 204Pb/203Tl ratios vs. Tl abundances for the carbonaceous chondrites. Small open circles in panel (b) represent individual dissolutions of Allende (digestions 5–8; Table 2). The excellent correlation (r2 = 0.91) of the data in panel (a) is suggestive of a bulk solar system Pb/Tl ratio of 22± 2.

condensation of volatiles from an initially hot solar nebula (Palme et al., 1988). This deviation, however, most likely reflects primarily heterogeneities in the distribution of Pb- and Tl-rich phases in the samples that were analyzed in this study. Support for this conclusion is provided by the results for the eight separate dissolutions of Allende NHM, which were all prepared from a large mass of homogenized powder (Table 2). The Pb and Tl concentrations of these samples show sizeable deviations of about ±5 to 10% from the average. In contrast, the individual 204Pb/203Tl ratios scatter by less than ±3.3% from the mean value. These observations are best explained by either the variable distribution of volatile-depleted refractory components or small-scale heterogeneities in one or several chondrite phases that concentrate both Pb and Tl. This conclusion is in accord with (i) nebular condensation models, which suggest that both Pb and Tl are likely to be concentrated in sulfide phases (Lodders, 2003) and (ii) a detailed analytical study of Allende, which showed that Pb and Tl are primarily present in the fine-grained matrix, where they may be associated with sulfides and/or carbonaceous matter (Arden and Cressey, 1984). Based on the Allende NHM data (Table 2), it is therefore reasonable to conclude that differences between the Tl and Pb abundances reported in this study and the literature for various meteorites are, at least in part, plausibly the result of sample heterogeneities. For example, the Tl concentrations determined for Orgueil vary widely,

The Tl isotopic compositions of the carbonaceous chondrites vary by about 5 ε-units, from ε205Tl = − 4.0 for NWA 801 to ε205Tl = +1.2 for Colony. Between these extremes, the majority of the samples cluster at intermediate values of ε205Tl ≈ −1.5 and 204Pb/203Tl ≈ 1.4 (Fig. 3). The carbonaceous chondrites thus exhibit a much more limited range of Tl isotope compositions than the previously analyzed iron meteorites, which had ε205Tl values between −19 for a Canyon Diablo sulfide and + 30 for metal from Mundrabilla (Nielsen et al., 2006a). Despite the narrow range of observed ε205Tl, the carbonaceous chondrites exhibit Tl isotope variations that are significantly greater than the external precision of the measurements. The positive correlation that exists between ε205Tl and 204Pb/203Tl (Fig. 3) is furthermore consistent with the decay of 205Pb. The trend of Fig. 3 also exhibits significant scatter, however, and Tl possesses only two stable isotopes, which hinders the identification of isotopic variations that

Fig. 3. Pb–Tl isochron diagram for the carbonaceous chondrites. Only the average result is shown for Allende NHM (Table 2). The excellent correlation of the data appears to reflect primarily in situ decay of 205Pb to 205Tl.

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are unrelated to radiogenic ingrowth of 205Tl. It is therefore prudent to consider alternative processes, which may be responsible for either producing the positive correlation of ε205Tl with 204Pb/203Tl or that may have altered the slope of or generated scatter in this trend. 4.4. Potential alternative sources of Tl isotope variations It is conceivable that disturbances in the Pb–Tl systematics of carbonaceous chondrites are produced by heterogeneities in the distribution of distinct nucleosynthetic materials, with different initial 205 Pb abundances or Tl isotope compositions. This is considered to be an unlikely scenario, however, because 203Tl and 205Tl are both predominantly produced by s-process nucleosynthesis (Arlandini, 1999) and Tl is hence not expected to exhibit large nucleosynthetic isotope anomalies. It is furthermore significant that bulk samples of carbonaceous chondrites were found to be devoid of nucleosynthetic isotope anomalies for other moderately and highly volatile elements such as Zn (Luck, 2005), Cd (Wombacher et al., 2008) and Te (Fehr et al., 2005). This may be either a direct consequence of volatility or the more labile nature of these elements during parent body alteration (Fehr et al., 2006). Similarly, cosmogenic spallation or neutron capture reactions are not expected to be important, due to the relatively small neutron capture cross-sections of the relevant nuclides (Huey and Kohman, 1972). Stable isotope fractionation of Tl is a more likely mechanism for introducing isotopic variability. Despite its high mass, Tl exhibits stable isotope variations of more than 30 ε-units in terrestrial environments (e.g., Rehkämper et al., 2004; Nielsen et al., 2006b). In meteorites, Tl isotope fractionations may be generated by alteration of the samples on Earth or by processes that occurred in the meteorite parent bodies and the solar nebula. Based on previous investigations, which showed that Cd isotope compositions are readily altered by partial evaporation and/or condensation (Wombacher et al., 2004, 2008), Cd isotope analyses were employed to screen the Tl isotope results for potential stable isotope effects from volatilization. This approach, which assumes that stable isotope fractionations of Cd and Tl are generally coupled, is reasonable, given that Cd is also very volatile with TC50% = 652 K and probably concentrated in troilite during nebular condensation (Lodders, 2003). A decoupled isotope fractionation behavior of these two elements cannot be excluded entirely, however. The Cd isotope data (Table 2) are in accord with the previous (but less precise) results of Wombacher et al. (2008). They confirm that most carbonaceous chondrites have nearly identical Cd isotope compositions, which are only marginally heavier than the terrestrial isotope standard (Fig. 4). The two exceptions are Kainsaz (CO3) and Leoville (CV3 reduced), which both have anomalously heavy ε114/110Cd values of more than + 15. It was previously shown that such large isotope fractionations probably reflect mobilization of Cd during repeated cycles of partial evaporation and condensation, as a consequence of parent body processes such as metamorphism and shock (Wombacher et al., 2003, 2008). The results of this study indicate that Cd is even more labile during such thermal processing than Tl. For example, Cd is depleted in Leoville by a factor of 10 relative to Allende (Rosman and De Laeter, 1974), whilst the Tl content is only 40% lower (Table 2). Hence, it is not surprising that stable isotope effects are also much more significant for Cd — the Cd isotope composition of Leoville is heavier by about 7.5 ε/amu mass difference compared to Allende, whereas Tl isotopes differ by only about 1 ε/amu (Fig. 4). In spite of this, it is prudent to exclude the data of both Leoville and Kainsaz from any chronological interpretation of the relationship seen in Fig. 3. For the remaining samples, the correlation of increasing ε205Tl with increasing Pb/Tl ratio could be explained by a depletion of Tl relative to Pb, and associated preferential loss of isotopically light 203 Tl, e.g., due to volatilization. Our data are not in accord with such a

Fig. 4. Plot of Cd vs. Tl isotope compositions for the carbonaceous chondrites. Note the anomalously high ε114/110Cd values of Leoville and Kainsaz, which appear to result from open system thermal metamorphism on the respective parent bodies. The grey field, which encompasses the majority of the samples, shows the range of Cd isotope compositions for the silicate Earth, which extends from ε114/110Cd values of about − 4 to + 6 (Wombacher et al., 2003; Schmitt et al., 2008).

scenario, however, as the lightest Tl isotope compositions are exhibited by two samples (NWA 801 and Allende NHM), which have relatively low concentrations of both Tl and Pb. Alternatively, it is conceivable that the correlation of Fig. 3 reflects mixing processes between isotopically fractionated components that (i) exhibit primary differences in stable Tl isotope compositions or (ii) were generated by aqueous alteration on the meteorite parent bodies. Neither scenario can be completely excluded but a number of considerations render them unlikely. The first scenario may encompass mixing between a refractory component (associated with CAIs and/or chondrules) with low ε205Tl and an isotopically heavy phase hosted in the matrix. Such a scenario is supported by the light stable Cd isotope compositions that were previously measured for CAIs and chondrules (Wombacher et al., 2008). However, the absence of significant Cd isotope fractionations in all but two of the samples strongly argues against such a mixing model. The second scenario may involve the formation of phases enriched in 203Tl by hydrothermal alteration, because such phases have been identified in basalts that were altered at temperature conditions (≤150 °C) that are relevant for the parent bodies of carbonaceous chondrites (Huss et al., 2006; Nielsen et al., 2006b). In this case, however, the lowest ε205Tl values should be associated with high Tl concentrations, which is not in accord with the observation that samples with relatively low Tl contents (NWA 801, Allende) exhibit the lightest Tl isotope compositions (Fig. 3, Table 2). In addition, mixing should generate linear correlations, when the results are plotted in a diagram of ε205Tl vs. 1/ Tl concentration (Fig. 5). The absence of any correlation in Fig. 5 thus indicates that mixing processes did not generate the correlation seen in Fig. 3. Alternatively, it is not unreasonable to speculate that the ε205Tl– 204 Pb/203Tl correlation of Fig. 3 was produced by terrestrial weathering and associated Tl isotope fractionation. This is of particular concern due to the characteristics of the two samples, which display the most extreme Pb/Tl ratios and Tl isotope compositions. The chondrite NWA 801 (which defines the lower end of the correlation; Fig. 3) may have been affected by alteration because it is a desert meteorite find, whilst Colony (which displays of the highest Pb/Tl ratio) appears to be pervasively contaminated with terrestrial Pb (Table 2). A number of observations argue, however, that the correlation of Fig. 3 is unlikely to be a product of secondary terrestrial processes.

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Fig. 5. Diagram of ε205Tl vs. 1/Tl concentrations for the carbonaceous chondrites. The absence of a linear correlation suggests that the Tl isotope variability does not reflect (two-component) mixing processes. Uncertainties in 1/Tl are smaller than the size of the symbols.

(i) A previous study demonstrated that surficial weathering typically does not generate Tl isotope fractionations of greater than 0.5 to 1 ε (Nielsen et al., 2005), whilst NWA 801 and Colony differ in their Tl isotope compositions from most other chondrites by about 1 to 3 ε205Tl. (ii) Very similar Tl contents were obtained for samples from a single group (e.g., CV or CO) or meteorite (Allende NHM) despite markedly different levels of Pb contamination (Fig. 2a, Table 2). This indicates that the Tl concentrations, and by inference isotope compositions, were not significantly altered by terrestrial processes. (iii) It would be very fortuitous for weathering and associated Tl isotope fractionation to generate the ε205Tl–204Pb/203Tl correlation seen in Fig. 3. In particular, this would require that different but well correlated changes in Pb/Tl and ε205Tl were generated for both Colony and NWA 801. Such changes cannot be ruled entirely but are highly improbable. 4.5. Live

205

Pb in the early solar system

The above discussion leaves open the likelihood that the correlation of ε205Tl with Pb/Tl (Fig. 3) reflects mainly radiogenic variations in Tl isotope composition from the decay of formerly live 205 Pb. A chronological evaluation of the results is therefore warranted. Data obtained for the chondrites Leoville and Kainsaz will be disregarded, however, as these samples exhibit fractionated Cd (and therefore potentially Tl) isotope compositions. For Allende, only the results obtained for Allende NHM are utilized, as this sample is essentially uncontaminated with terrestrial Pb (Table 2). An isochron plot of the remaining data for 8 carbonaceous chondrites (Fig. 6) is then utilized to estimate the abundance of 205 Pb and the initial Tl isotope composition, at the time of parent– daughter fractionation. Bulk rock isochrons for carbonaceous chondrites were previously obtained for the extinct 53Mn–53Cr (Shukolyukov and Lugmair, 2006; Moynier et al., 2007; Yin et al., 2009; Qin et al., 2009) and 107Pd–107Ag (Schönbächler et al., 2008) decay systems and these were interpreted to record nebular fractionation processes, which occurred at about 4567 Ma. It is likely that the Pb–Tl system dates the same events, which suggests that the isochron of Fig. 6 records the initial Tl isotope composition and 205Pb abundance of the solar system (SS). The slope of the Pb–Tl isochron (MSWD = 2.0) yields 205Pb/204PbSS,0 = (1.0 ± 0.4) × 10− 3 and an initial Tl isotope composition of ε205TlSS,0 = −7.6 ± 2.1 (all uncertainties quoted here and below are at the 2 sd level). It is notable that essentially identical

45

Fig. 6. Pb–Tl isochron diagram for the carbonaceous chondrites. Not shown are the data for two meteorites that exhibit stable Cd isotope variations as well as Allende Smithsonian, which is contaminated with terrestrial Pb (Table 2).

results are obtained if the isochron is constructed without the strongly Pb contaminated CO3 chondrite Colony. Excluding this sample, the chondrite isochron (MSWD = 2.3) yields 205Pb/204PbSS,0 = (1.0 ± 0.5) × 10− 3 and ε205TlSS,0 = −7.7 ± 2.5. These results can be further evaluated in the context of the only other published Pb–Tl isochron data, which are for metal samples from IAB iron meteorites (Nielsen et al., 2006a). The latter analyses yielded an initial 205Pb/204PbIAB,0 = (7.3 ± 0.9) × 10− 5, which suggests that the IAB isochron was established 57 + 10/−14 Ma after the start of the solar system. This result can be compared with the relative age that was inferred from recent Pd–Ag isochron studies of the same meteorite groups (Carlson and Hauri, 2001; Woodland et al., 2005; Schönbächler et al., 2008). Both chronometers are expected to record metal crystallization ages for iron meteorites but the Pd–Ag analyses indicate that the Pd/Ag fractionations in the IAB irons are only 19 + 24/−10 Ma younger than carbonaceous chondrites. The Pb–Tl and Pd–Ag ages are thus only barely consistent, within the combined uncertainties, and this hints at a possible discrepancy between the ages. This conclusion is supported by the 10–20 Ma I–Xe ages for silicate inclusions from the IAB iron Toluca (Niemeyer, 1979; Pravdivtseva, 2000), which are in accord with the older Pd–Ag isochron age for IAB metal samples. Inaccuracies in the slopes of the four relevant isochrons provide a possible, albeit speculative, explanation for this age discrepancy. All Pd–Ag and Pb–Tl isochrons (for carbonaceous chondrites and IABs) are rather imprecise, with the possible exception of the Pb–Tl data for the IAB irons. An alternative interpretation is provided by differences in the closure temperatures of the two decay systems. A blocking temperature of about 1100 K was previously inferred for the Pd–Ag system in iron meteorites (Sugiura and Hoshino, 2003). This is comparable to the N1000 K closure temperatures that have been quoted for the extinct I–Xe and Hf–W systems in silicates (Bogard et al., 2005; Kleine et al., 2008). Notably, Hf–W ages that post-date CAI formation by up to ∼ 11 Ma have been reported for IAB silicate inclusions (Schulz et al., 2009), in agreement with the Pd–Ag metal age. In contrast, a recent Ar–Ar study of such inclusions (Vogel and Renne, 2008), which evaluated new and published results, observed a large range of ages (between about 0 and 250 Ma post-CAI), which were interpreted to reflect the prolonged accretional and thermal evolution of the IAB parent body (Benedix et al., 2000). This evolution was inferred to culminate in the catastrophic breakup and reassembly of the parent asteroid at about 4470 Ma (Vogel and Renne, 2008). It is thus conceivable that the relatively young Pb–Tl isochron age of the

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IAB metal samples reflects late closure of this isotope system, due to a blocking temperature that is significantly lower than 1000 K and potentially similar to that of the K–Ar system at b650 K (Renne, 2000; Trieloff et al., 2003). This interpretation is supported by the observation that Tl is typically one of the most labile elements (and more labile than Ag) during thermal processing of chondritic matter (Lipschutz and Woolum, 1988). In summary, these considerations indicate that the apparent discrepancy between the Pb–Tl and Pd–Ag ages of IAB iron meteorites are in accord with differences in the blocking temperatures of the two isotope systems. At present, this provides the most reasonable explanation for the age discrepancy. Based on this assumption, the carbonaceous chondrite isochron of Fig. 6 affords the best currently available constraints for the initial 205Pb abundance and Tl isotope composition of the solar system. It is important, however, that these results are further verified in the future, through Pb–Tl analyses of other suitable meteorites.

observed abundances of extinct radionuclides are in accord with an AGB star contribution. Using a new generation of stellar models, they inferred an upper bound of 5.3 × 10− 4 for 205Pb/204PbSS,0. This value could be higher, as the processes that produce and destroy 205Pb within stars are highly sensitive to fluctuations in temperature and density, thus making the amount of 205Pb that reaches the stellar surface difficult to quantify (Mowlavi et al., 1998). Given these uncertainties, the higher 205Pb/204PbSS,0 determined in this study may be in accord with the models of Wasserburg et al. (2006). This result, however, should be treated with some caution in terms of the constraints it places on the amount of freshly synthesized material, that was derived primarily from an AGB star. More precise constraints are desirable but these will have to await further Pb–Tl isochron studies of meteorites and more detailed modeling of AGB stars and other stellar sources.

4.6. Cosmochemical implications

Eight carbonaceous chondrites with nearly identical Cd isotope compositions display a positive correlation between Tl isotope compositions and 204Pb/203Tl. This cannot be readily explained by stable isotope fractionations from terrestrial weathering or early solar system processes such as volatilization. The correlation therefore most likely represents an isochron that was generated by in situ decay of 205Pb to 205Tl. Evaluation of the isochron yields initial solar system values of 205Pb/204PbSS,0 = (1.0 ± 0.4) × 10− 3 and ε205TlSS,0 of −7.6 ± 2.1. The new data thus confirm the results of a previous Pb–Tl study of IAB iron meteorites, which provided the first clear evidence for the existence of live 205Pb in the early solar system (Nielsen et al., 2006a). The new estimate for the initial 205PbSS,0 abundance renders the 205 Pb–205Tl decay system suitable for chronological studies of early solar system processes that generate Pb–Tl fractionations. In addition, the results also provide novel constraints for models of terrestrial accretion and investigations that seek to identify the stellar production sites, which provided freshly synthesized nucleosynthetic matter to the nascent solar system. In particular, the initial 205PbSS,0 abundance inferred in this study is similar to the upper limit of recent production estimates for AGB stars (Wasserburg et al., 2006). As such, it is in accord with the conclusion that the early solar system budget of short-lived radioactivities was influenced by a late AGB star contribution. For this and all other applications of the 205Pb–205Tl decay system it is desirable that the inferred values for the initial 205Pb abundance and Tl isotope composition of the solar system are further verified and more precisely defined through additional investigations of independently dated extraterrestrial materials, with a suitable range of Pb/Tl ratios.

The results of this study are relevant to the application of the Pb–205Tl decay system for early solar system chronology. In particular, the new initial solar system abundance of 205Pb/204PbSS,0 = (1.0 ± 0.4) × 10− 3 is about an order of magnitude higher than the previous estimate of 205Pb/204PbSS,0 = (1–2)× 10− 4, which was inferred from the IAB isochron, assuming a metal crystallization age of 10 to 20 Ma after the formation of the solar system (Nielsen et al., 2006a). Clearly, the higher initial abundance of 205Pb renders the Pb–Tl decay system more precise for studies of early solar system processes that generate Pb–Tl fractionations. This includes chronological investigations of metal crystallization for iron meteorites and thermal events that are associated with the mobilization of labile trace elements on meteorite parent bodies. The new estimates obtained for 205Pb/204PbSS,0 and ε205TlSS,0 also provide important constraints for models of terrestrial accretion and core formation. This is because reasonable accretion models need to reproduce the observations made for both the U–Th–Pb and the Pb–Tl decay systems, whilst also satisfying Hf–W constraints (e.g., Wood et al., 2008). A bulk silicate Earth (BSE) with a Pb concentration of 150 to 185 ng/g (McDonough and Sun, 1995; Palme and O'Neill, 2005) and a Tl content of 5 ± 2 ng/g (Nielsen et al., 2006a) is more volatiledepleted than carbonaceous chondrites but it features a 204Pb/203Tl ratio of 1.9 ± 0.9, which is essentially identical to the chondrite value of ∼ 1.4 ± 0.1. The BSE and carbonaceous chondrites are also both characterized by ε205Tl ≈ −2 (Nielsen et al., 2006a, Figs. 3 and 6). Taken together, this places critical constraints on models that account for the radiogenic Pb isotope compositions of the BSE, by partitioning of Pb into the core (Wood et al., 2008) and/or late loss of Pb by volatilization (Lagos et al., 2008). The constraints require, for example, that the processes responsible for the depletion of Pb in the BSE did not significantly fractionate the Pb/Tl ratio, if the bulk Earth is also characterized by a chondrite-like 204Pb/203Tl ratio of ∼ 1.4. The new result obtained for 205Pb/204PbSS,0 is also of interest as 205 Pb is the only known short-lived radionuclide in the early solar system that was solely produced by s-process nucleosynthesis. Hence, it can be used to evaluate whether asymptotic giant branch (AGB) stars and Wolf–Rayet stars are the most likely sources of s-process material, as has been suggested in a number of studies (e.g., Wasserburg et al., 1994; Arnould et al., 1997). Given the relatively long half-life of 205Pb, it is possible that the inferred initial solar system abundance reflects primarily the steady-state galactic production rate (Meyer and Clayton, 2000). The abundances of other very short-lived radionuclides in the early solar system (e.g., 26Al, 60Fe), however, require the late injection of freshly produced nucleosynthetic matter (Lee et al., 1976; Shukolyukov and Lugmair, 1993). Wasserburg et al. (2006) investigated the extent to which the 205

5. Conclusions

Acknowledgements We thank Barry Coles for assistance with mass spectrometry, as well as Terry Williams and Teresa Jeffries for helping to keep the mass spec lab running smoothly. We are much indebted to Tim McCoy (Smithsonian Institution), Cecilia Satterwhite (JSC), and particularly Caroline Smith (NHM) for providing the samples. We also thank Sara Russell for her advice on sample selection, Gretchen Benedix, Phil Bland, Tim Elliott and Sune Nielsen for helpful discussions, and Carsten Münker, Editor Rick Carlson and an anonymous referee for persuasive comments that led to significant improvements of the manuscript. The extraterrestrial studies at the Imperial College (MAGIC) and Oxford isotope laboratories are supported by grants from STFC; RB was funded by a NERC studentship. Appendix A. Supplementary data Supplementary data associated with this article can be found, in online version, at doi:10.1016/j.epsl.2009.12.044.

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