Zinc partitioning between minerals and melts: Implications for the origins of the iron-depletion trend in convergent-margin magmatism

Zinc shows a marked preference for tetrahedral coordination in oxide and silicate minerals. The only phases among those suggested to be involved in the petrogenesis of basaltic-andesitic-dacitic magmas that has a tetrahedral site suitable for 2+ cations are the spinel group minerals, including magnetite. Zn therefore has the potential to be a geochemical indicator for magnetite in the same way that heavy rare earth elements are for garnet. In this capacity it has the advantage over vanadium in that it has only one oxidation state in silicate melts, namely 2+, making its mineral/melt partitioning dependent on oxygen fugacity only insofar as this variable affects the compositions of the minerals and melts. Here we report experiments on the partitioning of Zn between hydrous andesitic-dacitic melts and all the mineral phases hypothesized to be involved in their petrogenesis, including olivine, orthopyroxene, augitic clinopyroxene, amphibole (hornblende), garnet, ilmenite and magnetite-rich spinel. We combine these results with those available in the literature to show that Zn is indeed highly compatible in the spinels, including magnetite, but incompatible in amphibole and garnet, confirming its usefulness for detecting magnetite fractionation. The behaviour of Zn in a number of convergent-margin magmatic suites shows that magnetite entered the cotectic assemblage when the suites had differentiated to 5 ± 1 wt% MgO. It is magnetite + amphibole crystallization that causes the depletion of Fe and concomitant enrichment of SiO2 of the calc-alkaline trend. The correlated depletion of V with Zn suggests that this happened under moderately oxidizing conditions, at approximately FMQ +2 (±1), where FMQ is the fayalite-magnetite-quartz equilibrium. Thermodynamic analysis of the Zn partitioning data shows that Zn behaves similarly to Mg, and is thus a member of a group of mid-sized divalent cations comprising Mg, Fe2+, Mn, Co, Ni, and Cr2+, whose oxide activity coefficients in silicate melts all vary with melt composition in a remarkably similar manner, despite the differences in their electronic configurations and chemical bonding characteristics, which are so obvious in their crystal chemistry in ferromagnesian silicates and spinels.