CHARACTERIZATION OF PRESOLAR SILICATE AND OXIDE GRAINS IN PRIMITIVE CARBONACEOUS CHONDRITES

Raster ion imaging of the oxygen isotopes with the NanoSIMS ion microprobe has been used to identify presolar grains in two primitive meteorites. Eleven presolar silicates and eight presolar oxides were identified in the primitive carbonaceous chondrite Acfer 094 for abundances of 325 and 360 parts per million (ppm), respectively. In addition, nine presolar silicates and five presolar oxide grains were identified in the CO3 chondrite ALHA 77307, for abun-dances of 320 and200ppm, respectively. These abundances,which arematrix-normalized and corrected for instrumental detection efficiencies, aremuch higher than those of other presolar phases,with the exception of nanodiamonds, although the latter may not all be presolar. The chemical compositions of six presolar silicate grains from ALHA 77307 were elucidated by Auger spectroscopy. Transmission electron microscopy (TEM) analysis of one presolar silicate grain revealed a nonstoichiometric composition and an amorphous structure as indicated by the diffuse electron diffraction pattern. The oxygen isotopic compositions of the presolar silicates indicate origins in red giant and asymptotic giant branch stars. Analysis of the Si isotopic compositions of 10 presolar silicates provides further constraints on the ef-fects of Galactic chemical evolution. Subject headinggs: circumstellar matter — dust, extinction — Galaxy: evolution — nuclear reactions, nucleosynthesis, abundances — stars: evolutio