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.