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-relat ed basalts, relative to their oceanic counterparts, arises by the pref erential transfer of lead to the mantle wedge (overlying the subductin g stab) by non-magmatic processes. Fluxing of the mantle wedge by low- Ce/Pb fluids, generated by the dehydration of subducted 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 dehyd ration of altered basalt. Our results show that the production of subd uction-zone magmas from mantle sources fluxed by basalt-derived fluid is a mechanism by which relatively lead-rich, cerium-poor, mantle-deri ved material is added to the continents. The lead enrichment of the Ea rth's continental crust is thus a continuing process occurring at conv ergent margins.