Aqueous fluids have a profound influence on the evolution of the crust, both as agents of chemical mass transfer and mineral reactions, and through modifying its rheology. This paper is particularly concerned with the composition and role of fluids through the crustal cycle of burial, metamorphism and uplift. Information now comes from both conventional geological studies and from geophysics, which has documented both the presence of fluid in active areas of the crust, and also its migration in response to faulting. This contrasts with the overall slow rate of some fluid processes: for example, devolatilization reactions are endothermic and limited by the rate of heat supply. There is no clear distinction between diagenetic and metamorphic fluids, and extensive equilibration with host rocks means that few chemical or isotopic characteristics survive to provide tracers for deep fluid origins. After the metamorphic peak, remaining pore fluid is consumed in hydration reactions resulting in strong, dry rock. Subsequently, infiltration may reintroduce fluids but these normally survive only briefly until consumed by hydration reactions. Metamorphic rocks are strongly overpressured during prograde metamorphism and therefore have a low permeability. Considerable doubt is therefore cast on some claims of widespread and pervasive large fluid fluxes accompanying metamorphism.

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