Vanadium partitioning between orthopyroxene, spinel and silicate melt and the redox states of mantle source regions for primary magmas

The partitioning of V between orthopyroxene-liquid and spinel-liquid has been investigated in synthetic and natural mafic and ultramafic compositions as a function of temperature and oxygen fugacity (fO(2)) at 100 kPa and in one experiment at higher pressure. The purpose of the experiments was to understand redox relationships for V in silicate melts with a view to deriving an empirical oxygen barometer for geochemically altered mafic and ultramafic magmas in the geologic record. Partitioning data for both orthopyroxene-liquid and spinel-liquid show profound changes at an fO(2), approximately 3 orders of magnitude below the nickel-nickel oxide (NNO) buffer, suggesting changes in the dominant valence state of V in silicate liquids from V3+ to V4+, near this fO(2). The results of the experiments on orthopyroxene-liquid are combined with published data for olivine liquid and are applied to suites of mafic and ultramafic magmas that have equilibrated with a harzburgite residue in the mantle. The results show that Archean alumina-undepleted komatiites could have formed at fairly high oxygen fugacities, near Delta NNO similar to 0, somewhat higher than Cretaceous komatiites and related picrites in the Caribbean region (between Delta NNO similar to -1 to -3), and plume-related picrites from West Greenland (Delta NNO similar to -3), Picrites and boninites from convergent margins record the highest fO(2)'s by this method, (Delta NNO = +1 to +2), consistent with other petrological estimates of their redox states. The approach developed in this study can thus provide estimates for the redox states of altered, mantle-derived magmas in the geological record, to which more conventional methods of oxygen barometry cannot be applied. Copyright (C) 1999 Elsevier Science Ltd.