Zoom Meeting ID: 96620344427
Subduction of oceanic lithosphere at convergent margins drives plate tectonic motion on Earth. Determining the petrophysical changes that occur to subducted crust along the downgoing slab–asthenospheric mantle interface is critical to defining mass and energy transport between the hydrosphere and deep interior, and for constraining long-term geochemical cycles. Here, I show via petrological modelling how phase assemblages, the P–T conditions of key devolatilization reactions, and the effect of densification with depth vary for typical MORB along newly defined “hotter” subduction zone geotherms for cold, warm, and average environments. The depth and extent of devolatilization of typical MORB is strongly dependent on the geotherm under which the oceanic crust subducts. At the onset of subduction along a warm geotherm, metabasites contain ~3 wt% of H2O, and release ~45% of the bulk-rock H2O in a fluid pulse at ~20 km. Below these depths, metabasites will dehydrate gradually liberating 45% of the bulk-rock H2O and complete dehydration will be achieved at ~70 km. Oceanic crust subducting along an average geotherm will contain ~3.5 wt% of H2O at the onset of subduction, and will release ~40% of the bulk-rock H2O in two fluid pluses occurring at ~30 and 50 km. Below these depths, the metabasite will dehydrate gradually liberating ~50% of the bulk-rock H2O and complete dehydration will be achieved at ~80 km. By contrast, at the onset of cold subduction, metabasites will typically be H2O undersaturated, and will dehydrate gradually at different depths, subducting ~0.6 wt% of H2O to sub-arc depths. Metabasites subducting along a warm and average geotherm will liberate most of the fluids at shallower depths, suggesting that these lithologies might preferentially exhume, yet MORB subducting along cold geotherms will not dehydrate until greater depths, inhibiting buoyancy-driven exhumation. While we show that metabasites formed along warmer geotherms are denser than metabasites from colder geotherms, buoyancy-driven exhumation provoked by fluids play a most important role than the overall metabasite bulk-rock density. We also put forward that slab melting occurs only along the hottest subduction geotherms. In these subduction zones, whereas melting is predicted to occur at subarc depths—allowing adakitic magmatism. We posit that in the hottest subduction zones, aqueous fluids and hydrous melts enhance chemical recycling.