Mantle redox state and its effect on trace-element partitioning during basalt petrogenesis

Abstract

The redox states of planetary interiors can be inferred from the abundance, partitioning behaviour or direct valence determination of redox-sensitive elements in the mineral-melt assemblages of mantle rocks and their partial melting products. An element that has received considerable attention due to its large number of possible valence states in silicate melts and minerals is vanadium (V2+, V3+, V4+ and V5+). The transition from V2+ to V5+ covers the entire spectrum of redox conditions of the inner solar system, hence there is potential for V to be used as a universal redox sensor in geological and cosmochemical materials. A comprehensive experimental study aimed at investigating the speciation and partitioning behaviour of V using LA-ICP-MS and XANES techniques is presented. The results indicate that the interpretation of direct valence determinations in basaltic glasses at room temperature may be complicated by the effect of charge-transfer reactions, particularly between V and Cr at more reducing conditions. Modelling the bulk-rock abundances of V relative to homovalent elements also appears not to be effective as a redox indicator in basalt suites, because the influences of variables other than oxygen fugacity and source heterogeneity overwhelm the redox signal. As an alternative to both methods, we propose that the direct determination of V partition coefficients be- tween basaltic matrix and olivine phenocrysts can be used effectively as a redox sensor in planetary olivine-phyric mafic volcanics, recording the oxygen fugacity at the pre-eruptive stage of the magma’s history and thus avoiding effects from degassing, reaction with the atmosphere and subsequent alteration. This method is formulated, tested and applied to 79 recent olivine-phyric terrestrial mafic volcanics. The results indicate that MORBs and OIBs record a restricted and statistically identical range of redox conditions, from QFM to QFM+1. In contrast, IABs record significantly more oxidizing conditions, from QFM+1 to QFM+2.5. Additional partitioning experiments carried out over a large range of redox conditions were used to constrain the behaviour of Re during basalt petrogenesis. Oxygen fugacity was shown to have a strong effect on the bulk geochemical behaviour of Re because Re4+ is compatible and Re6+ is highly incompatible in most minerals. Therefore, Re is expected to be compatible during the genesis of lunar basalts, moderately incompat- ible during the genesis of MORBs, and highly incompatible during the genesis of IABs. Oxygen fugacity should therefore be considered a critical variable in the interpretation of Re-Os isotopic systematics applied to mantle and mantle-derived rocks.