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 (δ¹⁸⁰) 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 δ¹⁸⁰ are produced by near-surface low-temperature processes. Here we show a significant linear relationship between δ¹⁸⁰ and stable iron isotope ratios (δ⁵⁷Fe) in a suite of pristine eclogite xenoliths. The δ¹⁸⁰ 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 δ¹⁸⁰ and δ⁵⁷Fe is unlikely to be inherited from oceanic crust, as δ⁵⁷Fe values determined for samples of hydrothermally altered oceanic crust do not differ significantly from the mantle value and show no correlation with δ¹⁸⁰. It is proposed that the correlated δ⁵⁷Fe and δ¹⁸⁰ 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 δ⁵⁷Fe and Cr content and bulk sample and garnet δ¹⁸⁰ 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 δ⁵⁷Fe and δ¹⁸⁰ by melting processes and specifically the production of low-δ¹⁸⁰ signatures in mafic rocks implies that some of the isotopically light δ¹⁸⁰ values observed in OIB and eclogite xenoliths may not necessarily reflect near-surface processes or components.