Variable Tl, Pb, and Cd concentrations and isotope compositions of enstatite and ordinary chondrites—Evidence for volatile element mobilization and decay of extinct 205Pb

1Carl Palk,1Rasmus Andreasen,1Mark Rehkämper,1Alison Stunt,1Katharina Kreissig,1Barry Coles,2Maria Schönbächler,3Caroline Smith
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12989]
1Department of Earth Science & Engineering, Imperial College London, London, UK
2Institute of Geochemistry and Petrology, ETH Zürich, Zürich, Switzerland
3Department of Mineralogy, Natural History Museum, London, UK
Published by arrangement with John Wiley & Sons

New 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.

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