Large thallium isotopic variations in iron meteorites and evidence for lead-205 in the early solar system
Introduction
Investigations of extinct radionuclides present in the early solar system can provide information on the presolar production sites of elements and the timescales of early solar system processes. The nuclide 205Pb, which decays to 205Tl with a half-life of 15.1 ± 0.4 Myr (Pengra et al., 1978), is of particular significance in this regard. For astrophysicists, 205Pb offers a unique opportunity to study nucleosynthetic mechanisms because it is the only short-lived radionuclide that is produced solely by the s-process (Wasserburg et al., 1994), whereby heavier elements are formed by slow neutron capture in the interior of stars. Both Pb and Tl are somewhat chalcophile and/or siderophile and the elements are volatile with half mass condensation temperatures of 727 and 532 K, respectively (Lodders, 2003). Thus, the Tl and 204Pb concentrations of the bulk silicate Earth (BSE) are a function of the volatile element depletion history of the Earth as well as their respective partitioning into the core. The 205Pb–205Tl decay system may therefore also prove to be useful for dating volatile loss and planetary differentiation.
A number of previous studies have attempted to find evidence of formerly live 205Pb in meteorites and establish the initial solar system abundance of this isotope (Anders and Stevens, 1960, Chen and Wasserburg, 1994b, Huey and Kohman, 1972). All of these investigations were unsuccessful, mainly because they utilized thermal ionization mass spectrometry, which is unable to provide sufficiently precise Tl isotope data. Only an upper limit for the initial solar system 205Pb/204Pb ratio of less than 9 × 10−5 is, therefore, currently available (Anders and Stevens, 1960, Huey and Kohman, 1972, Chen and Wasserburg, 1994b) but this figure may be too low if the high Pb/Tl ratios that were reported for some samples are due to contamination with terrestrial Pb. More recent analyses (Chen and Wasserburg, 1987) revealed a “hint” of excess 205Tl for an iron meteorite, but this effect was barely resolved. Moreover, a sulfide analyzed by Chen and Wasserburg (1987) was reported to have a very low (unradiogenic) 205Tl/203Tl isotope ratio that was difficult to reconcile with the other results.
In the present study, we have utilized the technique of multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) because this enables us to determine the isotopic composition of less than 1 ng of Tl, to a precision over an order of magnitude better than previously possible (Rehkämper and Halliday, 1999, Nielsen et al., 2004b). The low initial 205Pb/204Pb ratio estimated by Huey and Kohman (1972) implies that only samples with high Pb/Tl should exhibit resolvable Tl isotope anomalies, even with our improved analytical techniques. We have therefore, concentrated our efforts on the analyses of iron meteorites because these are known from Pd–Ag and Hf–W isotope studies to have formed early (Chen and Wasserburg, 1996, Lee and Halliday, 1996, Horan et al., 1998) and because they display highly fractionated Pb/Tl ratios (Chen and Wasserburg, 1994a).
We have analyzed both metal fragments and troilite inclusions from iron meteorites of groups IAB, IIAB, and IIIAB. These samples were selected so as to obtain as large a spread in Pb/Tl ratio as possible and to enhance the probability of observing Tl isotope variation from the decay of 205Pb. Moreover, we have analyzed six separate metal fragments of the IAB iron Toluca. By analogy with studies of Ag isotopes in iron meteorites (Chen and Wasserburg, 1990), it was anticipated that these samples could yield an internal Pb–Tl isochron that would provide improved constraints on the initial abundance of 205Pb in the early solar system.
Section snippets
Sample preparation
The iron meteorite samples, about 7–40 g of metal or 1–3 g of sulfide, were cut and alteration surfaces removed using a diamond-coated saw blade. The samples were then cleaned by repeated leaching with aqua regia followed by washing with 18 MΩ-grade water, until only a shiny central piece remained. This material, equivalent to about 70–95% of the original mass was weighed, dissolved in aqua regia, dried down, and taken up in an appropriate volume of 1 M HCl and bromine water (Rehkämper and
General features
The Tl isotope compositions as well as Pb and Tl concentrations have been determined for metal and sulfide phases from iron meteorites of groups IAB, IIAB, and IIIAB and the unclassified iron Mundrabilla (Table 3). It can be seen that the samples display a broad range of primordial 204Pb and 203Tl contents. In general terms, the concentrations are similar to those reported in the latest published Pb–Tl study of iron meteorites (Chen and Wasserburg, 1994b). All sulfides except one display higher
Astrophysical implications of the solar system initial 205Pb/204Pb
The inferred value of (205Pb/204Pb)SS,0 ≈ 1.5 × 10−4 can be compared with predictions based on stellar models of proposed s-process nucleosynthetic sites. Astrophysical studies indicate that Wolf-Rayet (WR) and asymptotic giant branch (AGB) stars are feasible sites of s-process nucleosynthesis (Wasserburg et al., 1994, Arnould et al., 1997), as both feature suitable neutron fluxes and they are also known to expel large amounts of material into the interstellar medium (ISM). Wolf-Rayet stars are
Conclusions and outlook
In this study, we have determined the Tl isotope compositions and the Pb/Tl ratios of a number of iron meteorites, in search of evidence for the former existence of radioactive 205Pb in the early solar system. Seven metal fragments from the IAB iron meteorites Toluca and Canyon Diablo define an isochron with a slope equivalent to 205Pb/204Pb = 7.4 ± 1.0 × 10−5 at the time of metal crystallization. Adoption of an I–Xe crystallization age for the IAB iron meteorites of 10–20 Ma after formation of the
Acknowledgments
We thank T. McCoy (Smithsonian Institution) and R. Wieler (ETH) for providing samples, T. Kleine and H. Williams for fruitful discussions, and M. Meier, U. Menet, D. Niederer, B. Rütsche, C. Stirling, A. Süsli, S. Woodland, H. Williams, and the rest of the IGMR group at the ETH for keeping the mass spectrometers and clean labs functioning at all times. We are also grateful to M. Boyet, K. Rankenburg, and an anonymous referee for insightful reviews and R. Gallino, G. Wasserburg, and B. Wood for
References (54)
- et al.
Isothermal heating experiments on Bjurbole—implications for the release mechanisms of radiogenic Xe-129
Geochim. Cosmochim. Acta
(1995) - et al.
Extending the Pd-107–Ag-107 chronometer to low Pd/Ag meteorites with multicollector plasma-ionization mass spectrometry
Geochim. Cosmochim. Acta
(2001) - et al.
The isotopic composition of Ag in meteorites and the presence of Pd-107 in protoplanets
Geochim. Cosmochim. Acta
(1990) - et al.
Classification and origin of IAB and IIICD iron meteorites
Geochim. Cosmochim. Acta
(1995) - et al.
U–Pb systematics in iron meteorites: uniformity of primordial lead
Geochim. Cosmochim. Acta
(1985) - et al.
A comparison of metallographic cooling rate methods used in meteorites
Geochim. Cosmochim. Acta
(1994) - et al.
182W and 187Re–187Os systematics of iron meteorites: chronology for melting, differentiation, and crystallization in asteroids
Geochim. Cosmochim. Acta
(1998) - et al.
Search for extinct natural radioactivity of Pb-205 via thallium-isotope anomalies in chondrites and lunar soil
Earth Planet. Sci. Lett.
(1972) - et al.
Experimental partitioning studies near the Fe–FeS eutectic, with an emphasis on elements important to iron meteorite chronologies (Pb, Ag, Pd, and Tl)
Geochim. Cosmochim. Acta
(1993) - et al.
The composition of the Earth
Chem. Geol.
(1995)
The precise and accurate determination of thallium isotope compositions and concentrations for water samples by MC-ICPMS
Chem. Geol.
The thallium isotope composition of the upper continental crust and rivers—an investigation of the continental sources of dissolved marine thallium
Geochim. Cosmichim. Acta
Orbital electron capture ratios in the decay of 205Pb
Nucl. Phys.
The precise measurement of Tl isotopic compositions by MC- ICPMS: application to the analysis of geological materials and meteorites
Geochim. Cosmochim. Acta
Thallium isotope variations in seawater and hydrogenetic, diagenetic, and hydrothermal ferromanganese deposits
Earth Planet. Sci. Lett.
Cenozoic marine geochemistry of thallium deduced from isotopic studies of ferromanganese crusts and pelagic sediments
Earth Planet. Sci. Lett.
An evaluation of the methods to determine the cooling rates of iron meteorites
Geochim. Cosmochim. Acta
Precise Re–Os determinations and systematics of iron meteorites
Geochim. Cosmochim. Acta
Isotopic effects on diffusion in MgO melt simulated by the molecular-dynamics (Md) method and implications for isotopic mass fractionation in magmatic systems
Geochim. Cosmochim. Acta
The IAB iron-meteorite complex: a group, five subgroups, numerous grouplets, closely related, mainly formed by crystal segregation in rapidly cooling melts
Geochim. Cosmochim. Acta
Iron isotope fractionation during planetary differentiation
Earth Planet. Sci. Lett.
Stable isotope compositions of cadmium in geological materials and meteorites determined by multiple-collector ICPMS
Geochim. Cosmochim. Acta
Accurate measurement of silver isotope composition in geological materials including low Pd/Ag meteorites
Geochim. Cosmochim. Acta
Search for extinct lead 205 in meteorites
J. Geophys. Res.
Short-lived radionuclide production by non-exploding Wolf-Rayet stars
Astron. Astrophys.
A petrologic study of the IAB iron meteorites: constraints on the formation of the IAB-winonaite parent body
Meteorit. Planet. Sci.
Ar–Ar and I–Xe ages and the thermal history of IAB meteorites
Meteorit. Planet. Sci.
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