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

Mid-ocean-ridge basalts (MORBs) are the most abundant terrestrial magmas and 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 liquids during partial melting) and in the isotopic ratios of several radiogenic isotope systems 2 , 3 , 4 . These variations define a spectrum between ‘depleted’ and ‘enriched’ compositions, characterized by respectively low and high abundances of incompatible elements 5 , 6 . Compositional variations in the sources of MORBs could reflect recycling of subducted crustal materials into the source reservoir 7 , or any of a number of processes of intramantle differentiation 8 , 9 , 10 . Variations in 18 O/ 16 O (principally sensitive to the interaction of rocks with the Earth's hydrosphere) offer a test of these alternatives. Here we show that 18 O/ 16 O ratios of MORBs are correlated with aspects of their incompatible-element chemistry. These correlations are consistent with control of the oxygen-isotope and incompatible-element geochemistry of MORBs by a component of recycled crust that is variably distributed throughout their upper mantle sources.