Direct determination of strontium enrichment on grain boundaries in a garnet lherzolite xenolith by proton microprobe analysis

Abstract

The mechanisms by which trace and radiogenic elements are held and transported in the Earth's mantle are of prime importance in studies of basalt petrogenesis and the chemical processing of the mantle. While such mechanics include the migration of melts and the convective physical admixture of differentiated material¹, the role of fluids as transport agents has attracted much interest, especially, in regions affected by kimberlitic activity^2–4, continental basaltic volcanism^5,6, and in areas of subduction-related magmatic activity^7,8. The observation that many incompatible elements are readily leached from garnet lherzolites by dilute acid treatment³ has led to the assumption that a proportion of these elements is located on grain boundaries, rather than in solution in the various minerals present. This indirect evidence is reinforced by comparison of the compositions of mineral separates with their bulk parental material^4–8,11. Although careful electron microprobe analyses have made it possible to detect minor and trace elements within grains¹², the low concentrations involved, together with the high X-ray background of the electron microprobe, have made it impossible to investigate the supposed distribution of incompatible trace elements on grain boundaries. We have measured directly, using a high-resolution proton microprobe^9,10, the distribution of Sr and other elements in a garnet lherzolite xenolith. The analyses demonstrate that, in this sample, Sr is preferentially concentrated along grain boundaries.