Sputtering rates of minerals and implications for abundances of solar elements in lunar samples

Sputtering rates of minerals have been investigated using the ion microprobe and confirmed by interferometry. Relative erosion rates in an ion microprobe indicate a variation in mineral specific sputtering of a factor of about 9. Evidence from investigation of pit depths give values similar to those from the ion probe. It is possible to compare results to lunar surface sputtering rates by using the theory of Smith¹ to convert from laboratory to lunar surface values. This calculation is, however, limited by the applicability of existing theory to complex silicate materials. We found apparent dose-dependent effects of sputtering rate as a function of primary beam intensity (30 keV O^- primary beam). These effects could be important at the lunar surface, where, because of the long (10⁵ y) exposure times, a very low incident flux of 2 × 10⁸ atoms/cm² s of H results in a very high total dose of 10²⁰ atoms/cm² H. Despite possible departures from linearity, calculations of saturation concentrations (assuming a sputter-equilibrium) for ³⁶Ar and carbon indicate surface abundances of these elements of saturation grains are similar to experimental values. Heavier solar wind elements appear to be influenced by sputtering effects, high surface concentrations (10¹⁵/cm²) of Fe measured in plagioclases²) require a lower sputter rate than that calculated for plagioclase at the lunar surface. This may be a result of departure from theory due to dose dependent or other effects. If these can be quantified, and corrected for, variations in solar wind flux, energy and H/He ratio may be observable by looking for mineral grains with anomalously high or low concentrations of these elements.