Tin isotopic fractionation during igneous differentiation and Earth's mantle composition

Abstract

Tin exists both under the 2+ and 4+ oxidation states in igneous systems, and thus its geochemical behaviour changes as a function of oxygen fugacity. To characterise the redox state of Sn during magmatic differentiation and how this affects its isotope composition, we have measured Sn isotopic and elemental abundances in a suite of samples from the Kilauea Iki lava lake. Sn behaves as a highly incompatible element during fractional crystallisation. Lattice strain modelling shows that Sn2+ has mineral-melt partition coefficients (Dmin/melt) ≈ 1 in plagioclase and clinopyroxene, whereas it is highly incompatible in all phases save for ilmenite, attesting to the sole presence of Sn4+ in basaltic liquid at the Fayalite-Magnetite- Quartz (FMQ) buffer. Furthermore, Sn isotopes are unfractionated during crystallisation of silicates, but decrease to lighter values upon ilmenite precipitation. Isotopic fractionation is onset by the coordination change between Sn4+ in the melt (6- to 8-fold) and ilmenite (6-fold). The Sn isotope composition of komatiites, which are high degree, high temperature partial melts are used to estimate the Sn isotope composition of the bulk silicate Earth (BSE). Komatiites have δ122Sn within the range of the basalts (before ilmenite precipitation) and together provide the best estimate of the BSE of 0.49 ± 0.11 ‰ (2 s.d., n = 9).