Identification of small presolar spinel and corundum grains by isotopic raster imaging

The oxygen isotopic compositions of densely packed submicron oxide grains in two grain separates of different grain size from the CM2 carbonaceous chondrite Murray were determined by multidetection raster imaging on the NanoSIMS ion microprobe. This led to the identification of 81 presolar spinel and 3 presolar corundum grains among similar to 51 700 grains in the CF residue (mean diameter 0.15 mum) and 171 presolar spinel and 29 presolar corundum grains among similar to 21 500 grains in the CG residue (mean diameter 0.45 mum). Previous NanoSIMS analysis of individual grains from the same residues has led to the discovery of 15 presolar spinel and 3 presolar corundum grains among 628 CF grains, and 9 presolar spinels among 753 CG grains. The oxygen isotopic compositions of the presolar oxides found by raster imaging are comparable to those of the presolar oxides measured individually. While the abundance of presolar spinel among the (larger) grains of the CG residue is the same for both techniques, the detection efficiency for presolar spinel by imaging among the (smaller) grains in CF is lower due to the small size of these grains. Nonetheless, it is possible to identify presolar grains of this size range. Though single grain measurements are effective for determining the precise isotopic compositions and abundances of presolar grains, raster ion imaging is the method of choice in searches for rare presolar grain types such as presolar silicates.