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

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 (delta O-18) of Some ocean island basalts (OIB) span values both above and below that of unmetasomatised upper mantle (5.5 +/- 0.4 parts per thousand) and provide support for this hypothesis, as it is widely assumed that most variations in delta O-18 are produced by near-surface low-temperature processes. Here we show a significant linear relationship between delta O-18 and stable iron isotope ratios (delta Fe-57) in a suite of pristine eclogite xenoliths. The delta O-18 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 delta O-18 and delta Fe-57 is unlikely to be inherited from oceanic crust, as delta Fe-57 values determined for samples of hydrothermally altered oceanic crust do not differ significantly from the mantle value and show no correlation with delta O-18. It is proposed that the correlated delta Fe-57 and delta O-18 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 0 isotopes by removal of partial melt enriched in isotopically heavy Fe and O is supported by negative correlations between bulk sample delta Fe-57 and Cr content and bulk sample and garnet delta O-18 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 delta Fe-57 and delta O-18 by melting processes and specifically the production of low-delta O-18 signatures in mafic rocks implies that some of the isotopically light delta O-18 values observed in OIB and eclogite xenoliths may not necessarily reflect near-surface processes or components. (C) 2009 Elsevier B.V. All rights reserved.