Possible isotopic fractionation effects in material sputtered from minerals

Abstract

We discuss in detail a model which makes definite predictions for the fractionation of isotopes in sputtered material. The fractionation patterns can be nonlinear, and the pattern for a particular set of isotopes depends on the chemical matrix within which those isotopes are contained. Calculations are presented for all nonmonoisotopic elements contained in the minerals perovskite, anorthite, ackermanite, enstatite, and troilite. All isotopes are fractionated at the level of approximately 4-6%o per atomic mass unit. O is always positively fractionated (heavier isotopes sputtered preferentially), and heavier elements are generally negatively fractionated (lighter isotopes sputtered preferentially). The value of $(1sO: 160) is always less by about 1.8%o than a linear extrapolation based upon the calculated $(170:160) value would suggest. The phenomenon of both negative and positive fractionation patterns from a single target mineral can be used to make an experimental test of the proposed model. The sums are taken over all target atomic species, but k and l refer only to isotopes of a single chemical element. We emphasize that (2) applies only to the material actually sputtered away from the target and not to the composition of the target surface subsequent to sputtering. Modification of the surface composition can be described by models that incorporate both preferential sputtering effects and subsurface diffusion processes, but such a project is beyond the scope of the work discussed here. The abundance factors n• appearing in (2) refer to the instantaneous atomic abundances. In general, these are not constant in time if/• • 0, and thus/• --/•(0. The results which are reported here, therefore, refer only to low dose experiments where a limited amount of material is sputtered from the sample.To illustrate the results one would expect if fractionation occurs according to (2), we first specialize to several idealized cases. Consider a target composed of a single element, which in turn is composed of only two isotopic species, 1 and 2. The total cross sections %. are all equal to a common value, hence 9553