2020 Volume 54 Issue 6 Pages 393-410
Isotopic analyses of Sr and Ba and quantitative analyses of Rb, Sr, Cs, Ba, and rare earth elements from chemical separates of four chondrites, i.e., Cold Bokkeveld (CM2.2), Murray (CM2.4/2.5), Nogoya (CM2.2), and NWA 4428 (H4), were performed to investigate the origin of Sr and Ba isotopic variations. Most Ba isotopic deviations of the chemical separates showed positive isotopic anomalies of 135Ba that correlated with 137Ba and 138Ba, suggesting a heterogeneous distribution of s-process isotopes in the early solar system. The Sr and Ba isotopic data for the acid residues showed significant isotopic deficits of 84Sr, 130Ba, 132Ba, 135Ba, 137Ba, and 138Ba derived from the enrichments of s-process isotopes caused by the presence of presolar SiC grains. Furthermore, the correlation between the isotopic data for 135Ba, 137Ba, and 138Ba with the isotopic data for the acid residues suggested the contribution of n-process nucleosynthetic components. Assuming that the Ba isotopic compositions of the acid residues were a mixture of s- and n-process nucleosynthetic components and radiogenic components of 135Ba, the upper limits of the initial 135Cs isotopic abundance expressed as the 135Cs/133Cs ratio in the early solar system could be estimated as (2.4 ± 0.4) × 10–4 for Cold Bokkeveld, (1.6 ± 0.9) × 10–4 for Murray, and (4.6 ± 0.7) × 10–4 for Nogoya, respectively.