Element recycling from subducting slabs to arc crust: a review

Abstract

Subduction zones not only return oceanic lithosphere into the mantle, but are also sites where chemical components are transferred from the downgoing plate back to the surface in arc crust and, to a lesser extent, fore-arc and back-arc basins. Understanding of subduction-zone processes has evolved significantly over a relatively brief 40-year research history, thanks to combined insights from experimental petrology, geophysics, numerical and thermodynamic modelling, arc magma geochemistry and studies of high- and ultrahigh-pressure metamorphic rocks. Early models considered aqueous fluids produced by metamorphic devolatilisation of the slab to be responsible for directly transferring chemical components of the slab into the overlying mantle wedge, as well as fluxing melting of the mantle wedge to produce arc basalt. Subducting crustal rocks were considered too cold to melt under most circumstances. However, the latest generation of thermal modelling combined with improved understanding of the chemistry and phase petrology of subduction-zone fluids and melts indicates that conditions for deep slab melting are likely met in subduction zones, provided that free fluid is available at sub-arc depths. 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 mélange 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 mélange 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.