Oxygen-isotype evidence for recycled crust in the sources of mid-ocean-ridge basalts

Mid-ocean-ridge basalts (MORBs) are the most abundant terrestrial magmas an d are believed to form by partial melting of a globally extensive reservoir of ultramafic rocks in the upper mantle(1). MORBs vary in their abundances of incompatible elements (that is, those that partition into silicate liqu ids during partial melting) and in the isotopic ratios of several radiogeni c isotope systems(2-4). These variations define a spectrum between 'deplete d' and 'enriched' compositions, characterized by respectively low and high abundances of incompatible elements(5,6). Compositional variations in the s ources of MORBs could reflect recycling of subducted crustal materials into the source reservoir(7), or any of a number of processes of intramantle di fferentiation(8-10). Variations in O-18/O-16 (principally sensitive to the interaction of rocks with the Earth's hydrosphere) offer a test of these al ternatives. Here we show that O-18/O-16 ratios of MORBs are correlated with aspects of their incompatible-element chemistry. These correlations are co nsistent with control of the oxygen-isotope and incompatible-element geoche mistry of MORBs by a component of recycled crust that is variably distribut ed throughout their upper mantle sources.