Large 18O Excesses in Circumstellar Graphite Grains from the Murchison Meteorite: Indication of a Massive-Star Origin
Forty-seven graphite round grains from the lowest density fraction extracted from the Murchison carbonaceous chondrite were analyzed for their C and N isotopic ratios by ion microprobe mass spectrometry, and subsets of the grains were analyzed for their O, Mg, Si, Ca, and Ti isotopic ratios. The 12C/13C ratios of the grains range from 6.3 to 3140 (solar ratio is 89). Most grains have 15N excesses (up to 2.5 times solar), 18O excesses (up to 100 times solar), and large inferred 26Al/27Al ratios (up to 0.15), and Si isotopic compositions range from 28Si excesses (up to 30%) to 29Si and 30Si excesses (up to 50%). All seven grains analyzed for Ti show large 49Ti excesses (up to 170%), and some show 42Ca and 43Ca excesses and 25Mg excesses. The large 18O excesses point to an origin in massive stars, either Wolf-Rayet stars or Type II supernovae, where 18O is produced from 14N at the onset of He burning. Most of the other isotopic signatures are consistent with such an origin: the Ti, Ca, and Mg anomalies can be explained by slow neutron capture during He burning, the large 26Al/27Al ratios by admixture of material that has experienced H burning. The large variations in the Si isotopic compositions, including both 29Si and 30Si excesses and deficits, favor a supernova origin with 28Si coming from oxygen burning in an inner layer of the star.