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The relative roles of crust and upper mantle in the generation of oceanic island arc magmas

The oceanic island arcs should represent the least complicated type of subduction related magmatism. Theoretically they represent an environment in which contamination by continental crustal materials does not occur. Basaltic lavas from most island arc systems have Sr, Nd and Pb isotope characteristics that do not deviate substantially from the normal arrays of mantle derived magmas. However, their distinctive trace element geochemistry requires a distinctive mantle source composition which is most readily achieved by metasomatism of the lherzolite of the mantle wedge by fluids ascending from the upper surface of the subducted slab. Such fluids may be variably enriched in 87S r/86Sr, in which case they will induce deviations from the Nd-Sr mantle array. In marked contrast, a range of basic to intermediate lavas from the Sunda-Banda arc of Indonesia and the Lesser Antilles island arc have a significant continental fingerprint to their isotopic compositions and show marked deviations from the Sr-Nd and Pb isotope mantle arrays. These data could be explained by the involvement of a terrigenous sedimentary component in the genesis of the slab derived fluids. However, they could equally reflect high level contamination of the ascending magmas by sediments in situ at the base of the island arc crust.