We outline a model to explain element transfer out of subducting slabs that involves serpentinite subduction and slab partial melting. Serpentinite is likely to comprise part of the subducting slab, either as downgoing oceanic lithosphere that was hydrated at, or near, the seafloor, or as down-dragged fore-arc mantle wedge that was initially hydrated at shallow levels by aqueous fluids emanating from underthrust crustal rocks. Slab coupling with convecting asthenospheric mantle at sub-arc depths leads to slab heating and devolatilisation of deep slab serpentinite and/or hydrated m¨¦lange atop of the slab. Interaction between these fluids and coesite-phengite eclogite at the top of the slab produces hydrous slab melts, which then migrate out of the slab to ultimately contribute to arc magma generation. In this scenario hydrous slab melts dominate element transfer from the slab to arc magmas, although serpentinites (and/or related hybrid m¨¦lange rocks) are the initial source of H2O and some trace elements (e.g., B, Cl, As, and Sb). This model conforms to petrological and geophysical constraints on deep subduction conditions, and in general is consistent with the geology of blueschist-and eclogite-facies terranes and key geochemical and isotopic features of arc lavas.
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