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

The distribution coefficient for vanadium D v cpx/liq between clinopyroxene and silicate liquid in the system Di‐Ab‐An has been measured at trace levels as a function of oxygen fugacity (ƒ O 2 ) at 100 kPa and 2 GPa. With increasing ƒ O 2 vanadium changes from compatible to incompatible in clinopyroxene at about one log ƒ O 2 unit below the nickel‐nickel oxide (NNO) buffer. The trend in the Di‐Ab‐An system represents a reasonable upper limit for D v cpx/liq versus ƒ O 2 in natural systems. The D v cpx/liq versus f o 2 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 f o 2 has a very measurable effect on bulk D v source/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 f o 2 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 f o 2 but this may be a shortcoming of both the isobaric melting model and the poor constraints for D v gt/liq with f o 2 . If cratonic peridotites formed by depletion in the spinel stability field, then f o 2 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.