Small-scale Sr isotope investigation of clinopyroxenes from peridotite xenoliths by laser ablation MC-ICP-MS—implications for mantle metasomatism

Abstract

Although numerous studies have shown that mantle xenoliths are isotopically heterogeneous on a hand specimen scale, experimental data imply that isotopic equilibrium should be attained within single mineral grains in the upper mantle. Until recently, this prediction has not been verified because of the difficulty of analyzing individual grains. We report in situ 87Sr/86Sr compositions of clinopyroxenes in garnet peridotite xenoliths from the Nikos kimberlite, Somerset Island (Arctic Canada) obtained by laser ablation MC-ICP-MS. Results for five different peridotites indicate the existence of large Sr isotope variations within individual xenolith samples, varying from 0.5‰ to as much as 1.1‰ (3 to 8 parts in 7000). This study is the first to document isotopic heterogeneity in peridotite xenoliths at the scale of individual grains. Multiple analyses of the same grain, however, indicate intra-grain Sr isotopic equilibrium.The Sr isotopic ratios for individual clinopyroxene grains correlate with major element abundances such as SiO2 and TiO2, and trend towards the composition of the host Nikos kimberlite. It thus appears likely that the Sr isotope heterogeneities recorded by the clinopyroxenes are the result of metasomatic interaction with the host kimberlite magma. The preservation of Sr isotopic variability in clinopyroxene separates from different peridotite samples and between clinopyroxene grains of individual xenoliths suggests that metasomatism occurred just prior to or during kimberlite transport. The Sr isotope heterogeneities documented here for a single peridotite phase suggest that isotopic compositions obtained on mineral separates from peridotite xenoliths likely represent weighted averages and could be less homogenous than previously assumed.