Mantle redox in Cordilleran ophiolites as a record of oxygen fugacity during partial melting and the lifetime of mantle lithosphere

We examine controls on mantle oxygen fugacity (fO2) during the partial melting process that forms mantle lithosphere at spreading centers. We compare the paleo fO2 at the time of melting inferred by V/Sc systematics of ophiolite peridotites, with the thermobarometric fO2 recorded by olivine–orthopyroxene–spinel assemblages during simple cooling in relatively young oceanic lithosphere. Modelling of the V/Sc in the ophiolite peridotites from Alaska, Yukon and British Columbia is permissive of only a narrow range in log fO2 during melting between NNO and NNO − 1 (where NNO is the nickel–nickel oxide buffer), depending on the choice of partition coefficients for Sc. This result is within uncertainty of the thermobarometric fO2 recorded by most samples (within 1 log unit of NNO − 1). The same cannot be said for more complex peridotite residues from continental mantle, where V/Sc systematics show a narrow paleo fO2 during formation but wide range of thermobarometric fO2 after equilibration in the lithosphere. In continental mantle with a complex history, thermobarometric fO2 is an ambiguous measure of that attendant during partial melting. Graphite-saturated melting in a system closed to oxygen controls melt Fe3+/Fe2+and CO2 content, and creates a shift in fO2 of about 2 log units [1] in a peridotite residue. In contrast, for the ophiolite mantle samples in this study, both paleo- and thermobarometric fO2 are near values predicted by carbon–fluid equilibria, yet show no relationship with depletion, suggesting the melt-residue system in the mantle may be open to oxygen during the partial melting process.