Thermal effects of metamorphic fluids in subduction zones

Abstract

In subduction zones, large volumes of fluid are released from the subducting slab during prograde metamorphism of hydrated oceanic crust. Once released, metamorphic fluids tend to migrate upward owing to buoyancy. Fluid migration can affect the thermal structure of a subduction zone by (1) advecting heat and (2) causing retrograde (hydration, carbonation) reactions in the hanging wall. Under most P - T conditions, the volumetric heat capacities of H 2 O and rocks are similar; therefore, the thermal effects of fluid migration and uplift are comparable. Calculations suggest that advective heat transport by fluid flow causes relatively minor thermal effects. Pervasive upward fluid flow results in thermal effects analogous to regional uplift rates on the order of 0.2 mm/yr. Fluid flow channelized along the subduction shear zone translates the local thermal structure of the shear zone updip at ∼1 mm/yr. A more important thermal effect results from exothermic retrograde reactions in the hanging wall that release ∼30 kJ/mol of H 2 O consumed. Retrograde reactions significantly retard cooling of the hanging wall just as prograde reactions retard warming of the subducting slab.