Snapshots of an Evolving Solar Nebula Recorded in Nucleosynthetic Sr and Ba Signatures of Early Condensates

Abstract

The discovery of extreme strontium isotope anomalies (μ 84Sr) in refractory leachates from Allende fine-grained calcium-aluminum-rich inclusions (CAIs) is at odds with long-standing predictions regarding the homogenization of presolar components in the CAI-forming region. Elucidating the stellar source(s) of these phases and the mechanisms for their preservation holds potential significance in understanding the dynamics and evolution of the protoplanetary disk. Here we present barium isotope data for the same set of leachates previously analyzed for μ 84Sr. Our results show fairly homogeneous Ba isotope anomalies across leachates (∼100–200 ppm variability), in contrast to the observed μ 84Sr variations (up to ∼8%). Secondary phases extracted in earlier leaching steps (L1 and L3) reveal trends in μ 137Ba and μ 138Ba akin to that of mainstream SiC and a second nucleosynthetic component. We show that SiC X grains from Type II supernovae are good end-member candidates for explaining the intra-leachate spread in L1 and L3 μ 13x Ba. Notably, neither s-variability nor X-variability appears to contribute to trends in the barium isotope anomalies of the most refractory components (L4 and L5). We propose that the contrast in isotope anomaly systematics between the labile and refractory leachates could reflect a shift in the nucleosynthetic signatures of reservoirs sampled by these components. These observations are consistent with extreme 84Sr p-excesses manifesting only in L4 and L5 leachates. Finally, the decoupled Sr and Ba isotope anomalies point to a nucleosynthetic source that significantly overproduces strontium relative to barium, such as electron-capture supernovae or the collapse of rotating massive stars.