The gentle separation of presolar SiC grains from meteorites

This paper describes the development of a new, effective, and non-destructive method of SiC isolation from meteorites by freeze-thaw disaggregation, size, and density separation. This new method is important because there is evidence that current methods, which use strong acids and chemical treatments to dissolve silicates and separate out the interstellar grains, may alter the surfaces of the grains chemically and isotopically. Furthermore, any non-refractory coating present on the grains would be destroyed. Using our new separation method, SiC grains were enriched from similar to 6 ppm abundance in Murchison whole rock to 0.67% abundance in the 0.4-1.4 mu m size range and 0.27% abundance in the 1.4-17 mu m size range. Individual SiC grains were easily identified using electron probe microanalysis (EPMA) mapping of grains distributed thinly on gold foil; a small aliquot from these fractions has so far yielded > 150 SiC grains for isotopic analysis. The method separates out SiC grains efficiently, is applicable to very small or rare samples, and avoids the harsh acid treatments that may alter possible amorphous or non-refractory coats on the grains. The procedure also preserves the remainder of the original sample and it is hoped that it may be extended to other micron-sized presolar grains found in meteorites such as corundum, graphite, and silicon nitride.