Experimental evidence for the origin of lead enrichment in convergent-margin magmas

IT has been proposed 1–5 that the low Ce/Pb ratio of subduction-related basalts, relative to their oceanic counterparts, arises by the preferential transfer of lead to the mantle wedge (overlying the subducting slab) by non-magmatic processes. Fluxing of the mantle wedge by low-Ce/Pb fluids, generated by the dehydration of sub-ducted oceanic crust, is one mechanism favoured for this process (see, for example, ref. 5). Here we report the results of a series of high-pressure experiments, which confirm that low-Ce/Pb fluids coexist with the dominant mineral phases (garnet and clinopyroxene) produced during high-pressure dehydration of altered basalt. Our results show that the production of subduction-zone magmas from mantle sources fluxed by basalt-derived fluid is a mechanism by which relatively lead-rich, cerium-poor, mantle-derived material is added to the continents. The lead enrichment of the Earth's continental crust is thus a continuing process occurring at conver-gent margins.