Identification of an Extremely O-rich Presolar Silicate Grain in Acfer

Introduction: Presolar silicate grains have been abundantly identified since their first discovery less than a decade ago [1,2,3]. The O isotopic compositions of both silicate and oxide stardust indicate the vast majority (>90%) condensed around O-rich asymptotic giant branch (AGB) stars. Though both presolar phases have average sizes of ~300 nm, grains larger than 1 μm are extremely uncommon for presolar silicates. Thus, while numerous isotopic systems have been measured in presolar oxide grains [4], very few isotopic analyses for presolar silicates exist outside of O and Si [2,5]. And still, these measurements suffer from isotopic dilution with surrounding matrix material [6]. We conduct a search for presolar silicates in the primitive carbonaceous chondrite Acfer 094 and in some cases obtain high spatial resolution, high precision isotopic ratios. Experimental: A grain-size separate of Acfer 094 containing matrix silicate grains of diameters ~100-500 nm was analyzed by raster ion imaging in the JSC NanoSIMS 50L ion microprobe. Raster fields of 20 × 20 μm 2 were first scanned for all O and Si isotopes, and 24 Mg 16 O was also measured for the majority of these regions. The primary beam size (spatial resolution) was ~120 nm. Eleven anomalous grains were subsequently analyzed for O and Si isotopes with higher spatial resolution (~70 nm) to reduce contributing signal from surrounding grains. Results: A total of 47 presolar silicate grains were identified. As expected, all of them likely condensed in AGB stars save for 6 grains that are enhanced in one or both of the heavy O isotopes. Supernova are favored over high metallicity stars as the parent sources of these rare grains [4,7], as in the case for three presolar silicates that have extreme 18 O enrichments but solar or sub-solar 17 O/ 16 O [4,5,8,9]. We identified one grain with one of the highest enhancements of 18 O (18 O/ 16 O = 0.013) found in a presolar silicate to date and normal 17 O/ 16 O. A SN origin for this grain is highly probable, but unfortunately its Si isotopic composition is not diagnostic. The NanoSIMS images show the grain contains Mg and we plan to measure Mg/Al isotopes in this unusual grain to better constrain its source. We note that two SN oxide grains were observed to be depleted in 25 Mg and enhanced in 26 Mg [4]. The O isotopic anomalies of grains re-measured under high spatial …