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

The extinct radionuclide Pb-205, which decays to (TI)-T-205 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 Pb-205 in the early solar system has only recently been obtained from analyses of IAB iron meteorites, but significant uncertainties remain about the initial Pb-205 abundance and TI isotope composition of the solar system. In an attempt to better constrain these values, a comprehensive Pb-205-(TI)-T-205 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 TI isotope compositions from thermal processing. The analyses revealed only limited variation in epsilon(TI)-T-205, with values of between -4.0 and +1.2, but nonetheless the TI isotope compositions correlate with Pb/TI 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 Pb-205 to (TI)-T-205. Previous Mn-53-Cr-53 and Pd-107-Ag-107 studies of bulk carbonaceous chondrites provide evidence that the Pb-TI isochron records volatile fractionation in the solar nebula at close to 4567 Ma. The isochron thus yields the initial 205 Pb abundance and TI isotope composition of the solar system, with values of Pb-205/Pb-204(ss.0) = (1.0 +/- 0.4) x 10(-3) and epsilon(TIss.0)-T-205 = -7.6 +/- 2.1, respectively. These results confirm the previous Pb-TI data for IAB iron meteorites, which provided the first clear evidence for the existence of live 205 Ph in the early solar system. The initial Pb-205(ss.0) abundance inferred from carbonaceous chondrites demonstrates that the Pb-205-(TI)-T-205 decay system is well suited for chronological studies of early solar system processes that produce fractionations in Pb/TI ratios, including core crystallization and the mobilization of volatiles during thermal processing. The Pb-205(ss.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. (c) 2010 Elsevier B.V. All rights reserved.