[C]

Abstract

We performed combined focused ion beam /transmission electron microscopy studies to investigate the chemistry and structure of eight presolar silicate grains that were previously detected by NanoSIMS oxygen isotope mapping of the carbonaceous chondrite Acfer 094. The analyzed presolar silicates belong to the O isotope Groups I/II (17O-enriched and 18O-depleted) and therefore come from 1–2.5 M asymptotic giant branch stars of close-tosolar or slightly lower-than-solar metallicity. Three grains are amorphous, Mg-rich, and show a variable, but more pyroxene-like composition. Most probably, these grains have formed under circumstellar low-temperature conditions below the crystallization temperature. Three grains are Fe-bearing glasses similar to the “glass with embedded metal and sulfides” (GEMS) grains found in interplanetary dust particles. However, two of the meteorite GEMS grains from this study lack comparatively large ( 20 nm) Fe-rich inclusions and have sulfur contents <1 at.%, which is different than observed for the majority of GEMS grains. These grains likely condensed under strong non-equilibrium conditions from an Si-enriched gas. One olivine is characterized by a crystalline core and an amorphous, more Fe-rich rim, which is probably the result of interstellar medium sputtering combined with Mg removal. The detection of another olivine with a relatively high Fe content (Mg# 0.9) shows that circumstellar crystalline silicates are more Fe-rich than astrophysical models usually suggest. The overall predominance of olivine among the crystalline silicate stardust population compared to pyroxene indicates preferential formation or survival of this type of mineral. As pyroxene is indeed detected in circumstellar outflows, it remains to be seen how this result is compatible with astrophysical observations and experimental data.