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

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 Sn 2+ has mineral-melt partition coefficients (D min/melt ) ≈ 1 in plagioclase and clinopyroxene, whereas it is highly incompatible in all phases save for ilmenite, attesting to the sole presence of Sn 4+ 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 Sn 4+ 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 d 122 Sn 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).