Do SiC grains in Orgueil differ from those in Murchison?

Abstract

Studies of individual presolar SiC grains have shown that most are enriched in Si-29, Si-30, and C-13, and depleted in N-15, compared to solar-system abundances and that many have large excesses of Mg-26, most plausibly from in situ decay of Al-26. Stone et al., observed that Si from a family of platy SiC grains define a linear array on a 3-isotope plot that does not pass through normal solar-system Si. In contrast, Si-isotope data from over 100 3-4 micron SiC grains from Murchison from an elongate ellipse enclosing the Stone et al. linear array but also including 'normal' solar-system Si. To investigate whether this difference in Si isotopes indicates different populations of SiC in the two meteorites and to improve the characterization of Orgueil SiC, we used the PANURGE ion microprobe to measure Si, C, N, and Mg isotopes and Al and Na concentrations in a suite of 2-5 micron SiC grains from a new sample of Orgueil.