LARGE-SCALE HYDRATION OF THE LITHOSPHERE ABOVE SUBDUCTING SLABS

During subduction, oceanic lithosphere, capped by variably hydrated uppermost mantle, oceanic crust, and sediments, undergoes progressive metamorphism and devolatilization. The petrologic evolution of subducting oceanic lithosphere and the overriding continental/oceanic lithosphere can be predicted by integrating thermal models with petrologic phase equilibria. In mature subduction zones that have achieved thermal steady state, the subducting slab lies at subsolidus conditions for depths < 100- 1 50 km. Most subducting oceanic crust passes through the blueschist to eclogite facies transition where large amounts of H2O are released by the breakdown of Na-amphibole, lawsonite (or clinozoisite), and chlorite. The large amounts of H2O released by continuous blueschist - eclogite reactions can cause large-scale hydration of the overriding lithosphere and can trigger partial melting of the mantle wedge (lowermost lithosphere and asthenosphere). Upward migration of slab-derived fluids add H2O, CO2, SiO2, and incompatible elements to the overlying lithospheric plate that result in the formation of new phases (e.g., serpentine, amphibole, and mica) and chemical modification of existing phases (e.g., trace element enrichment of clinopyroxene). Evidence for large-scale lithospheric hydration above subducting slabs includes serpentinite diapirs observed in the Marianas forearc, low P-wave velocities observed in the hanging wall of the Alaskan and Japanese subduction zones, and studies of high-pressure metamorphic terrains.