Geochemical evidence for the melting of subducting oceanic lithosphere at plate edges

Most island-arc magmatism appears to result from the lowering of the meltin g point of peridotite within the wedge of mantle above subducting slabs owi ng to the introduction of fluids from the dehydration of subducting oceanic crust(1). Volcanic rocks interpreted to contain a component of melt (not j ust a fluid) from the subducting slab itself are uncommon, but possible exa mples have been recognized in the Aleutian islands, Baja California, Patago nia and elsewhere(2-4). The geochemically distinctive rocks from these area s, termed 'adakites', are often associated with subducting plates that are young and warm, and therefore thought to be more prone to melting(5). But t he subducting lithosphere in some adakite locations (such as the Aleutian i slands) appears to be too old and hence too cold to melt(6,7). This implies either that our interpretation of adakite geochemistry is incorrect, or th at our understanding of the tectonic context of adakites is incomplete. Her e we present geochemical data from the Kamchatka peninsula and the Aleutian islands that reaffirms the slab-melt interpretation of adakites(2), but in the tectonic context of the exposure to mantle flow around the edge of a t orn subducting plate. We conclude that adakites are likely to form whenever the edge of a subducting plate is warmed or ablated by mantle flow. The us e of adakites as tracers for such plate geometry may improve our understand ing of magma genesis and thermal structure in a variety of subduction-zone environments.