Fluids in metamorphic systems

There is abundant evidence for the presence of fluids during metamorphism, and they contribute significantly to mass transport and geochemical cycling, enhance melting, affect rheology, and influence seismicity in the crust. In this chapter there is a discussion of general aspects of fluid flow in different crustal environments, including contact metamorphism, regional metamorphism, and subduction metamorphism. There is also discussion of the mechanisms of fluid flow, the chemistry of metamorphic fluids, and the mathematical treatment of mass transport. Evidence for the direction of fluid flow in different settings, the time-integrated flux of metamorphic fluids, and duration of fluid flow events is presented. There is also a focused discussion of two examples of areas where evidence from metamorphic rocks connects with evidence from other disciplines related to subduction zone processes. These two examples are: oxygen fugacity in subduction zones and metamorphic fluids and seismicity.