ABSTRACT

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.