MOLYBDENUM ISOTOPIC COMPOSITION OF INDIVIDUAL PRESOLAR SILICON-CARBIDE GRAINS FROM THE MURCHISON METEORITE

We report the isotopic composition of molybdenum in twenty-three preso lar SiC grains from the Murchison meteorite which have been measured b y resonant ionization mass spectrometry (RIMS). Relative to terrestria l abundance land normalized to s-process-only Mo-96), the majority of the analyzed grains show strong depletions in the p-process isotopes M o-92 and Mo-94 and the r-process isotope Mo-100. Sixteen of these grai ns have delta-values <-600% for these three isotopes. The observed iso topic patterns of Mo from mainstream SiC grains clearly reveal the sig nature of s-process nucleosynthesis. Three-isotope plots of all grain data (delta(i)Mo vs. delta(92)Mo) show strong linear correlations with characteristic slopes. This finding suggests mixing of solar-like mat erial and pure s-process material in the parent stars. Comparison with evolutionary calculations of nucleosynthesis and mixing in red giants suggests that low-mass thermally-pulsed symptotic giant branch (TP-AG B) stars are the most likely site for the observed s-process nucleosyn thesis. Copyright (C) 1998 Elsevier Science Ltd.