AUTOMATED NanoSIMS MEASUREMENTS OF SPINEL STARDUST FROM THE MURRAY METEORITE

We report new O isotopic data on 41 presolar oxide grains, 38 MgAl2O4 (spinel) and 3 Al2O3 from the CM2 meteorite Murray, identified with a recently developed automated measurement system for NanoSIMS. We have also obtained Mg-Al isotopic results on 29 of the same grains (26 spinel and 3 Al2O3). The majority of the grains have O isotopic compositions typical of most presolar oxides, fall well into the four previously defined groups, and are most likely condensates from either red giant branch or asymptotic giant branch stars. We have also discovered several grains with more unusual O and Mg compositions suggesting formation in extreme astrophysical environments, such as novae and supernovae (SNe). One of these grains has massive enrichments in O-17, Mg-25, and Mg-26, which are isotopic signatures indicative of condensation from nova ejecta. Two grains of SN origin were also discovered: one has a large O-18/O-16 ratio typical of Group 4 presolar oxides; another grain is substantially enriched in O-16, and also contains radiogenic Ca-44 from the decay of Ti-44, a likely condensate from material originating in the O-rich inner zones of a Type II SN. In addition, several Group 2 presolar spinel grains also have large Mg-25 and Mg-26 isotopic anomalies that are difficult to explain by standard nucleosynthesis in low-mass stars. Auger elemental spectral analyses were performed on the grains and qualitatively suggest that presolar spinel may not have higher-than-stoichiometric Al/Mg ratios, in contrast to SIMS results obtained here and reported previously.