Variable Tl, Pb, and Cd concentrations and isotope compositions of enstatite andordinary chondritesEvidence for volatile element mobilization and decay of extinct Pb-205

作者：Palk Carl; Andreasen Rasmus; Rehkamper Mark; Stunt Alison; Kreissig Katharina; Coles Barry; Schonbachler Maria; Smith Caroline

来源：Meteoritics & Planetary Science, 2018, 53(2): 167-186.

DOI：10.1111/maps.12989

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 +/- 6Ma. The enstatite chondrites (ECs) have highly variable epsilon Cd-114/110 of between about +3 and +70 due to stable isotope fractionation from thermal and shock metamorphism. Furthermore, nearly all enstatite meteorites display epsilon Tl-205 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 epsilon Tl-205 with epsilon Cd-114/110, which suggests that at least some of the Tl isotope variability reflects stable isotope fractionation rather than radiogenic ingrowth of Tl-205 from Pb-205 decay. Considering L chondrites, most epsilon Tl-205 values range between -4 and +1, while two outliers with epsilon Tl-205-10 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 Pb-205 on the parent body, with an initial Pb-205/Pb-204=(1.5 +/- 1.4)x10(-4), which suggests late equilibration of the Pb-Tl system 26-113Ma after carbonaceous chondrites (CCs). The LL chondrites display highly variable epsilon Tl-205 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 epsilon Tl-205 and Pb-204/Tl-203. This defines an initial Pb-205/Pb-204 of (1.4 +/- 0.3)x10(-4), equivalent to a Pb-205-Tl-205 cooling age of 55+12/-24Ma (31-67Ma) after CCs.

• 出版日期2018-2