Presolar spinel grains from the Murray and Murchison carbonaceous chondrites

With a new type of ion microprobe, the NanoSIMS, we determined the oxygen isotopic compositions of small (<1 mum) oxide grains in chemical separates from two CM2 carbonaceous meteorites, Murray and Murchison. Among 628 grains from Murray separate CF (mean diameter 0.15 mum) we discovered 15 presolar spinel and 3 presolar corundum grains, among 753 grains from Murray separate CG (mean diameter 0.45 mum) 9 presolar spinel grains, and among 473 grains from Murchison separate KIE (mean diameter 0.5 mum) 2 presolar spinel and 4 presolar corundum grains. The abundance of presolar spinel is highest (2.4%) in the smallest size fraction. The total abundance in the whole meteorite is at least I ppm, which makes spinel the third-most abundant presolar grain species after nanodiamonds.(if indeed a significant fraction of them are presolar) and silicon carbide. The O-isotopic distribution of the spinel grains is very similar to that of presolar corundum, the only statistically significant difference being that there is a larger fraction of corundum grains with large O-17 excesses (O-17/O-16 > 1.5 x 10(-3)), which indicates parent stars with masses between 1.8 and 4.5 M-circle dot. Copyright (C) 2003 Elsevier Ltd.