Studies of calcium isotope fractionation found in nature and produced during ion sputtering

High-precision analytical techniques have been developed for the measurement of calcium isotopic abundances in samples as small as 2 x 10-9 mole. Isotopic differences between samples arising from mass fractionation can be resolved to 1 x 10-4 per unit mass difference. Variation of Ca isotope compositions in the solar system has been studied through analysis of meteorites, lunar samples, and terrestrial samples. Sputtering by the solar wind is one mechanism that might have led to the thin, isotopically heavy coatings on lunar soil grains. Terrestrial samples were sputtered with relatively low-current 130-keV N+ or 100-keV N2+ beams. Sputtered Ca was collected over a wide range of angles and later analyzed. At 130 keV, the first Ca removed is isotopically light, ranging from delta(40Ca/44Ca) = +11 to +13. Further sputtering causes the fractionation of the material being collected to decrease. There are large differences in the fractionation of Ca sputtered in different directions. These variations with ejection angle also change with increasing ion bombardment. It is concluded that fractionation intrinsic to sputtering was a major mechanism contributing to the lunar soil isotopic fractionation