Low‐Density Graphite Grains and Mixing in Type II Supernovae

Primitive meteorites contain presolar grains that originated in stellar outflows and supernova ejecta. Low-density graphite grains from the Murchison carbonaceous meteorite were analyzed for the isotopic compositions of C, N, O, Mg, Si, K, Ca, and Ti by ion microprobe mass spectrometry. The grains are characterized by a large range of 12C/13C ratios (from 3.6 to 7200 compared to the solar ratio of 89), excesses in 15N (15N/14N up to 10 times solar) and 18O (18O/16O up to 185 times solar), large inferred 26Al/27Al ratios (from 26Mg excesses) ranging up to 0.15, a large range in Si isotopic ratios (from 50% deficits in 29Si and 30Si relative to 28Si up to more than 120% excesses), large excesses in 41K and 44Ca from the prior presence of now-extinct 41Ca (T½=105 yr) and 44Ti (T½=59 yr), respectively, and excesses in 42Ca, 43Ca relative to 40Ca, and 49Ti, and 50Ti relative to 48Ti.