The Origin and Evolution of Nucleosynthetic Sr Isotope Variability in Calcium and Aluminum-rich Refractory Inclusions

Abstract

Abstract Nucleosynthetic isotope anomalies in meteorites are useful for investigating the origin of materials in the protoplanetary disk and dynamical processes of planetary formation. In particular, calcium and aluminum-rich inclusions (CAIs) found in chondrites are key minerals for decoding the initial conditions of the solar system before the accretion of small planetary bodies. In this study, we report isotopic analyses for three Allende CAIs, fluffy type A (FTA), type B, and fine-grained spinel rich (FS) inclusions, with a specific emphasis on the measurements of 84Sr/86Sr ratios. It was found that the average μ 84Sr values (106 relative deviations from a standard material) were 175, 129, and 56 ppm for the samples of FTA, type B, and FS inclusions, respectively. Additionally, the FTA samples exhibited heterogeneous μ 84Sr values, while those for the type B and FS inclusions were homogeneous within individual inclusions. The elevated μ 84Sr values were most likely explained by the relative enrichment of r-process nuclides in the CAI formation region. The variation of μ 84Sr values between the FTA and type B inclusions, as well as within the FTA inclusion, suggests the presence of multiple CAI source reservoirs with distinct isotopic compositions, which is either inherited from isotopic heterogeneity in the molecular cloud or caused by the selective destruction of r-process-enriched supernova grains via nebular thermal processing. On the other hand, the reaction between a refractory precursor of the FS inclusion and a gaseous reservoir enriched in Mg, Si, and 16O resulted in the lowest μ 84Sr values for the FS inclusion.