Clinopyroxene‐liquid partitioning for vanadium and the oxygen fugacity during formation of cratonic and oceanic mantle lithosphere

Abstract

The distribution coefficient for vanadium Dvcpx/liq between clinopyroxene and silicate liquid in the system Di‐Ab‐An has been measured at trace levels as a function of oxygen fugacity (ƒO2) at 100 kPa and 2 GPa. With increasing ƒO2 vanadium changes from compatible to incompatible in clinopyroxene at about one log ƒO2 unit below the nickel‐nickel oxide (NNO) buffer. The trend in the Di‐Ab‐An system represents a reasonable upper limit for Dvcpx/liq versus ƒO2 in natural systems. The Dvcpx/liq versus fo2 systematics from this study are combined into a melting model to evaluate the V budget during partial melting in the mantle. It is shown that fo2 has a very measurable effect on bulk Dvsource/liq along the spinel peridotite solidus, and at 7 GPa along the garnet peridotite solidus. Isobaric melting models are formulated and used to examine the distinctly different covariation of V with degree of depletion in mantle lithosphere from various tectonic settings. The spectrum of oceanic lithosphere can be explained by partial melting in the spinel peridotite stability field at an fo2 of ∼NNO‐3, identical to that recorded by midocean ridge basalts. Cratonic peridotites are not well explained by melting in the garnet peridotite stability field at any fo2 but this may be a shortcoming of both the isobaric melting model and the poor constraints for Dvgt/liq with fo2. If cratonic peridotites formed by depletion in the spinel stability field, then fo2 during melting was above ∼NNO‐2.5, requiring that mantle lithosphere in the Archean formed under more oxidizing conditions than hitherto realized. The V abundances of cratonic peridotites are collinear with subduction zone peridotites, possibly linking the mantle roots beneath Archean cratons to a more oxidized subduction zone setting.