Mineral-melt vanadium oxybarometry for primitive arc magmas: effect of hydrous melt composition on fO2 estimates

If primitive arc magmas are primarily oxidized or if they acquire their oxidizing character during crustal evolution remains debated. Mineral-melt V partitioning is extremely redox sensitive and has the potential to resolve this debate, but critical low-temperature, hydrous experimental partitioning data are limited. We present new experimental V partitioning data for olivine-melt, spinel-melt, and clinopyroxene-melt based on the ≤ 1200 °C, hydrous basalt phase equilibrium experiments of Pichavant et al. (Geochim Cosmochim Acta 66:2193–2209, 2002) and Pichavant and Macdonald (Contrib Mineral Petrol 154:535–558, 2007). Combined with published experimental data, we use our olivine-melt V partitioning coefficients to show that—contrary to previous conclusions—hydrous melt composition, i.e. melt H₂O concentration in addition to the concentration of other melt components, affects V partitioning and thus calculated fO₂, systematically overestimating fO₂ for olivine-melt equilibrated at high melt H₂O concentration. False absolute fO₂ values and false oxidation or reduction trends may be inferred. Based on these findings, we have calibrated a set of new olivine-melt oxybarometers applicable to hydrous arc magmas partially crystallized at ~ 1025–1350 °C. In a case study on a high-Mg basalt from St. Vincent, Lesser Antilles, we show that olivine-melt V oxybarometry records oxidizing near-liquidus conditions (~ QFM + 1.8) and possibly limited oxidation (~ 0.3 log units) during intermittent crustal magma storage.