CORRELATED STRONTIUM AND BARIUM ISOTOPIC COMPOSITIONS OF ACID-CLEANED SINGLE MAINSTREAM SILICON CARBIDES FROM MURCHISON

We present strontium, barium, carbon, and silicon isotopic compositions of 61 acid-cleaned presolar SiC grains from Murchison. Comparison with previous data shows that acid washing is highly effective in removing both strontium and barium contamination. For the first time, by using correlated Sr-88/Sr-86 and Ba-138/Ba-136 ratios in mainstream SiC grains, we are able to resolve the effect of C-13 concentration from that of C-13-pocket mass on s-process nucleosynthesis, which points toward the existence of large C-13 pockets with low C-13 concentrations in asymptotic giant branch stars. The presence of such large C-13 pockets with a variety of relatively low C-13 concentrations seems to require multiple mixing processes in parent asymptotic giant branch stars of mainstream SiC grains.