In-situ Stardust Searches in ALHA 77307

Recent measurements of silicate and oxide stardust grains have revealed large Mg-isotope anomalies in silicate stardust belonging to group 1, which has challenged the low-mass origins of these grains, among others. In this work, stardust searches were performed in a thin section of the CO3.0 carbonaceous chondrite meteorite Allan Hills (ALHA) 77307 with the NanoSIMS 50L. Several group 1 silicate and oxide grains were subsequently measured for their silicon and magnesium isotopes. Although several group 1 silicate grains were found to fall on the Galactic Chemical Evolution line for both Si and Mg isotopes, a significant fraction do not. These grains are therefore incompatible with their proposed low-mass Red Giant or Asymptotic Giant Branch stellar origins. These observations corroborate recent work and suggest that group 1 grains may have multiple stellar sources which might include pre-supernovae massive stars and supernovae. The silicate stardust abundance calculated from this study is 168 ppm, while the oxide abundance is 18 ppm in ALHA 77307, which is in good agreement with published literature. Additionally, three large silicate stardust grains were found which range in size from 0.8 x 0.6 µm2 to 1.6 x 0.6 µm2 and exhibit unusual “bi-lobed” or “ameboid” shapes. Several C-anomalous presolar grains were also identified in ALHA 77307, many of which were subsequently measured for their N and Si isotopes. These grains are important because in-situ measurements of N and Si isotopes in SiC stardust are rare and N in chemically isolated SiC grains is likely affected by the sample preparation procedure and/or contamination. A majority of SiC grains from this study belong to the “mainstream” group proposed to form in the circumstellar envelopes of low-intermediate mass AGB stars, while two rare SiC AB grains were found with possible origins in J-type carbon stars and/or supernovae. The calculated SiC abundance in ALHA 77307 ranged from 57-148 ppm, the upper limit of which would be the highest presolar SiC abundance so far reported for this meteorite.