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

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Abstract
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 mantle1. 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 systems2,3,4. These variations define a spectrum between ‘depleted’ and ‘enriched’ compositions, characterized by respectively low and high abundances of incompatible elements5,6. Compositional variations in the sources of MORBs could reflect recycling of subducted crustal materials into the source reservoir7, or any of a number of processes of intramantle differentiation8,9,10. Variations in 18O/16O (principally sensitive to the interaction of rocks with the Earth's hydrosphere) offer a test of these alternatives. Here we show that 18O/16O 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.

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