The fractionation of Ca isotopes by sputtering

Abstract

Using a double-spike, mass spectrometric technique, we have measured the absolute isotopic composition of Ca sputtered from several Ca minerals by normally incident, low energy, nitrogen beams (130 keV N^+ and 100 keV N^(+)_(2)). The precision in the ^(40)Ca/^(44)Ca ratio is 0.05% for samples as small as 10^(−7) g, corresponding to a resolution of possible mass dependent isotope fractionation effects of ∼0.01% per unit mass difference. In general, the material sputtered first was isotopically light by up to 2% in the isotopic ratio ^(40)Ca/^(44)Ca when compared with the measured isotopic composition of the target; as the bombardment progressed, the isotopic composition became less light and approached the initial composition of the target. We have demonstrated that, after prolonged bombardment, a surface layer develops on the target with a distinctly heavy isotopic composition; the thickness of the layer is a significant fraction of the range of the bombarding ions. Angular distributions of the sputtered material show that, throughout the bombardment, the material ejected back along the beam direction was isotopically light relative to the material ejected at oblique angles; the difference in the isotopic ratio ^(40)Ca/^(44)Ca over the angular range was ∼ 1.5%. This fractionation with angle of ejection persisted even when a quasi-steady state was reached after heavy bombardment, when the isotopic composition of the material averaged over angle had become essentially indistinguishable from that of the pristine target. These results indicate that sputtering produces isotope fractionation and that there is a marked angular dependence in the effects. This suggests that solar wind sputtering of the lunar surface may contribute to the isotopic and elemental enrichments observed for the surface layers of soil grains.