s-process Sr isotopic compositions in presolar SiC from the Murchison meteorite

We have used thermal ionization mass spectrometry to measure the isotopic compositions of Sr in size-sorted samples (the KJ series) of interstellar SiC separated from the Murchison meteorite. The Sr compositions are radically anomalous in comparison to those in normal solar system materials. As for other heavy elements in presolar SiC, the anomalies can be understood to represent enrichment of s-process (slow neutron capture) nucleosynthetic products, plausibly occurring in asymptotic giant branch stars. The inferred s-process ⁸⁸Sr/⁸⁶Sr ratios are variable and correlate with grain size: this ratio decreases (suggesting lower effective neutron exposure) with increasing grain size. The sense of this trend is the same as that previously observed in Ba, but opposite to that previously observed in Kr, in the same samples. The Sr and Ba trends indicate covariation of the factors governing neutron exposure and grain growth kinetics in a manner not yet understood. The contrasting trend for Kr suggests that the mechanisms for the introduction of volatile (Kr) and refractory (Sr and Ba) elements into SiC produce a biased sampling of the population of different nucleosynthetic products. The relative abundances of s-process ⁸⁶Sr, ⁸⁷Sr, and ⁸⁸Sr in Murchison SiC are very close to those for early solar system Sr, but in view of the different s-process superpositions and other contributions to all three of these isotopes, the close agreement must be regarded as coincidental.