Abstract
AbstractNew Tl, Pb, and Cd concentration and Tl, Pb isotope data are presented for enstatite as well as L‐ and LL‐type ordinary chondrites, with additional Cd stable isotope results for the former. All three chondrite suites have Tl and Cd contents that vary by more than 1–2 orders of magnitude but Pb concentrations are more uniform, as a result of terrestrial Pb contamination. Model calculations based on Pb isotope compositions indicate that for more than half of the samples, more than 50% of the measured Pb contents are due to addition of modern terrestrial Pb. In part, this is responsible for the relatively young and imprecise Pb‐Pb ages determined for EH, L, and LL chondrites, which are hence only of limited chronological utility. In contrast, four particularly pristine EL chondrites define a precise Pb‐Pb cooling age of 4559 ± 6 Ma. The enstatite chondrites (ECs) have highly variable ε114/110Cd of between about +3 and +70 due to stable isotope fractionation from thermal and shock metamorphism. Furthermore, nearly all enstatite meteorites display ε205Tl values from −3.3 to +0.8, while a single anomalous sample is highly fractionated in both Tl and Cd isotopes. The majority of the ECs thereby define a correlation of ε205Tl with ε114/110Cd, which suggests that at least some of the Tl isotope variability reflects stable isotope fractionation rather than radiogenic ingrowth of 205Tl from 205Pb decay. Considering L chondrites, most ε205Tl values range between −4 and +1, while two outliers with ε205Tl ≤ −10 are indicative of stable isotope fractionation. Considering only those L chondrites which are least likely to feature Pb contamination or stable Tl isotope effects, the results are in accord with the former presence of live 205Pb on the parent body, with an initial 205Pb/204Pb = (1.5 ± 1.4) × 10−4, which suggests late equilibration of the Pb‐Tl system 26–113 Ma after carbonaceous chondrites (CCs). The LL chondrites display highly variable ε205Tl values from −12.5 to +14.9, also indicative of stable isotope effects. However, the data for three pristine LL3/LL4 chondrites display an excellent correlation between ε205Tl and 204Pb/203Tl. This defines an initial 205Pb/204Pb of (1.4 ± 0.3) × 10−4, equivalent to a 205Pb‐205Tl cooling age of 55 + 12/−24 Ma (31–67 Ma) after CCs.
Highlights
Extinct radionuclide systems are powerful cosmochemical tools that provide quantitative information on the presolar production sites of elements as well as the time scales of early solar system processes
Received attention for many years. This stems from the suggestion that it is unique among extinct radionuclides, as it may be produced solely by the s-process of nucleosynthesis (e.g., Blake et al 1973; Yokoi et al 1985; Wasserburg et al 1994, 2006)
The Tl and Cd contents of all three suites typically vary by 1–2 orders of magnitude and for the enstatite and L chondrites, the concentrations are well correlated with petrographic grade, whereby less metamorphosed samples have the highest abundances of both elements. This feature was most likely established during parent body metamorphism and associated mobilization of volatile elements
Summary
Extinct radionuclide systems are powerful cosmochemical tools that provide quantitative information on the presolar production sites of elements as well as the time scales of early solar system processes. This stems from the suggestion that it is unique among extinct radionuclides, as it may be produced solely by the s-process of nucleosynthesis (e.g., Blake et al 1973; Yokoi et al 1985; Wasserburg et al 1994, 2006). Aside from its possible role as a unique nucleosynthetic tracer, the 205Pb-205Tl decay system has many potential uses in chronometry. Both Pb and Tl are volatile and moderately siderophile elements, so that the Pb/Tl ratios of solar system materials are affected by numerous processes, including partial evaporation and condensation, metal-silicate equilibration, and fractional crystallization of solid metal during the cooling of planetary cores. The 205Pb-205Tl decay system may, in principle, be useful for dating a range of early solar system processes, including volatilization during thermal processing, planetary differentiation, and core crystallization
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