Fractionation of oxygen and iron isotopes by partial melting processes: Implications for the interpretation of stable isotope signatures in mafic rocks

Abstract

Recycling of oceanic crust into the deep mantle via subduction is a widely accepted mechanism for creating compositional heterogeneity in the upper mantle and for explaining the distinct geochemistry of mantle plumes. The oxygen isotope ratios (δ 18O) of some ocean island basalts (OIB) span values both above and below that of unmetasomatised upper mantle (5.5 ± 0.4‰) and provide support for this hypothesis, as it is widely assumed that most variations in δ 18O are produced by near-surface low-temperature processes. Here we show a significant linear relationship between δ 18O and stable iron isotope ratios (δ 57Fe) in a suite of pristine eclogite xenoliths. The δ 18O values of both bulk samples and garnets range from values within error of normal mantle to significantly lighter values. The observed range and correlation between δ 18O and δ 57Fe is unlikely to be inherited from oceanic crust, as δ 57Fe values determined for samples of hydrothermally altered oceanic crust do not differ significantly from the mantle value and show no correlation with δ 18O. It is proposed that the correlated δ 57Fe and δ 18O variations in this particular eclogite suite are predominantly related to isotopic fractionation by disequilibrium partial melting although modification by melt percolation processes cannot be ruled out. Fractionation of Fe and O isotopes by removal of partial melt enriched in isotopically heavy Fe and O is supported by negative correlations between bulk sample δ 57Fe and Cr content and bulk sample and garnet δ 18O and Sc contents, as Cr and Sc are elements that become enriched in garnet- and pyroxene-bearing melt residues. Melt extraction could take place either during subduction, where the eclogites represent the residues of melted oceanic lithosphere, or could take place during long-term residence within the lithospheric mantle, in which case the protoliths of the eclogites could be of either crustal or mantle origin. This modification of both δ 57Fe and δ 18O by melting processes and specifically the production of low-δ 18O signatures in mafic rocks implies that some of the isotopically light δ 18O values observed in OIB and eclogite xenoliths may not necessarily reflect near-surface processes or components.