Systematic iron isotope variations in mantle rocks and minerals: The effects of partial melting and oxygen fugacity

Iron isotopic compositions potentially provide a powerful new tracer of planetary formation and differentiation processes and of secular and spatial changes in mantle oxidation state. However, the processes governing iron isotope fractionation in igneous rocks remain poorly understood. Here we show that there are significant variations in the iron isotope compositions (δ57/54Fe) of mantle rocks (0.9‰) and minerals (olivines 0.6‰, clinopyroxenes 0.9‰ and orthopyroxenes 0.8‰), with spinels showing the greatest total variation of 1.7‰. Positive linear functional relationships with slopes that are, within error, equal to unity are found between the δ57/54Fe values of coexisting orthopyroxene, clinopyroxene and olivine, strongly suggesting that the δ57/54Fe values of these minerals reflect intra-sample mineral-mineral isotopic equilibrium. Positive correlations between the δ57/54Fe values of silicate minerals and spinels also exist, although they are more scattered, which could be caused by late disturbance of mineral-spinel isotopic equilibrium. Bulk-rock, clinopyroxene and spinel δ57/54Fe values correlate with chemical indices of both melt extraction and oxidation. Iron isotope fractionation during spinel-facies partial melting is investigated using simple models, which demonstrate that the maximum expected fractionation between melt and residue will be ∼0.5‰, with the residue becoming isotopically light relative to the melt and to the initial source region. Hence melt extraction, in combination with significant changes in mantle oxidation state, may be an explanation for Fe isotopic variations in mantle peridotites. Metasomatism of the sub-arc mantle by iron-rich silicate melts originating from the subducting slab may also explain the light bulk-sample δ57/54Fe values of some arc peridotites (− 0.2‰ to − 0.6‰), but mass-balance calculations require these metasomatic agents to have extreme δ57/54Fe values (e.g. − 3.0‰). The large differences in the δ57/54Fe values of garnet and spinel facies rocks are likely to be caused by the contrasting behaviour of Fe3+ during melting in the spinel and garnet facies. However, there is little difference in the δ57/54Fe values of MORB and OIB, despite the fact that OIB are considered, on the basis of incompatible element abundances, to arise dominantly by melting in the garnet stability field. Given that iron is a relatively compatible element, the similarities in the δ57/54Fe values of MORB and OIB provide strong evidence that MORB and OIB are both dominated by melting in the spinel facies.