Partitioning of tin between mafic minerals, Fe-Ti oxides and silicate melts: Implications for tin enrichment in magmatic processes

Abstract

The partition coefficients of Sn (tin) between minerals and silicate melts (DSnmin/melt) are vital for understanding the Sn enrichment during magmatic processes related to tin-granites. However, experimentally determined DSnmin/melt values remain scarce due to the difficulty in avoiding severe Sn loss to noble metal capsules. In this study, we performed mineral/melt Sn partitioning experiments at 0.5–1.0 GPa, 850–1000 °C, and fO2 of FMQ + 8 to ∼ FMQ − 1. The fO2s were imposed by solid buffers of Ru–RuO2, Re–ReO2, Ni–NiO, Co–CoO and graphite using three improved capsule designs: 1) a single sample capsule (Pt) for the Ru–RuO2 buffered runs, 2) double capsules with Pt95Rh05 as outer capsule and Re as inner sample capsule for the graphite and Re–ReO2 buffered runs, and 3) triple capsules for the Co–CoO and Ni–NiO buffered runs, with Pt95Rh05 (or Au) as outer capsule, Re lined Pt as inner sample capsule. These new capsule designs avoided significant Sn loss and enabled us to obtain accurate DSnmin/melt at a controlled fO2. The experimental results show that DSnmin/melt values are 0.08–12.66 for amphibole, 0.01–5.55 for biotite, 0.09–10.39 for clinopyroxene, 0.004–0.97 for orthopyroxene, < 0.01 for olivine, 1.34–108.43 for Ti-magnetite, 0.04–4.17 for spinel and 0.10–0.64 for ilmenite. The large variation of DSn for each mineral was mainly ascribed to the effect of fO2, which results in an arresting decrease of DSn with decreasing fO2. Under the reducing conditions (fO2 < FMQ), Sn is highly incompatible (DSn < 0.1) in almost all the minerals. Modeling results indicate that partial melting at low fO2 conditions results in Sn enrichment in the derived magma, while subsequent high degree of fractional crystallization is also important for the Sn enrichment in the residual melt. The experimental and modeling results thus explain why major primary tin deposits are always related to reducing and highly fractionated granites.