S-PROCESS BA, ND, AND SM IN PRESOLAR SIC FROM THE MURCHISON METEORITE

Six grain size fractions in the size range 0.2-3.0-mu-m of interstellar SiC from the Murchison carbonaceous meteorite have been analyzed by ion microprobe mass spectrometry for Ba isotopes (Ba-134 through Ba-138), one fraction (0.3-0.7-mu-m) also for Nd (Nd-142 through Nd-146) and Sm (Sm-147 through Sm-149) isotopes. All elements show isotopic abundance patterns characteristic for the s-process. The Ba-134/Ba-136 ratios are distinct from solar, indicating that s-Ba in SiC is different from that in the solar system. Ba-138/Ba-136 ratios decrease with grain size; if interpreted as being due to different neutron exposures, this trend is opposite of that shown by the Kr-86/Kr-82 ratios (Lewis, Amari, & Anders). Although the other isotopic ratios for Ba and those for Nd and Sm differ in detail from theoretical predictions, the disagreements probably being due to uncertainties in the n-capture cross sections and simplifications in the models, the general agreement of the data with models of s-process nucleosynthesis support an AGB star origin for the relatively fine-grained SiC of this study. Ba-136 in SiC is up to 85% pure s-process. If the (isotopically normal) remainder is from the AGB envelope, Ba/Si is almost-equal-to 170-300 x solar in the He-burning shell where the s-Ba was synthesized.