Dynamics of Geofluid Generation and Migration in Subduction Zones

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The generation and migration of geofluids in subduction zones are discussed for the subducting slab and the overlying mantle wedge and crust in terms of theoretical models and observations. Theoretical models include several mechanisms of fluid migration, e.g., Rayleigh-Taylor instability, Stokes ascent, channel flow, and porous flow, whose characteristic lengths and velocities differ significantly. As a result, these mechanisms may occur in different settings within subduction zones. We compare seismic and geochemical observations with the model of fluid migrations, based on which a typical fluid fraction within the mantle wedge is estimated to be 0.1 to 1 vol.%. Accordingly, it is suggested that fluid migration within the mantle wedge is driven by the buoyancy of the fluid, rather than being dragged by the flow of solid matrix. This suggests the fluid rises vertically. In the shallow part of the mantle wedge and within the arc crust, in particular the upper crust, the channel flow seems to be dominant. However, the relationship between these channels and the surface exits observed as volcanoes and hot spring systems is unclear. To better understand fluid distribution and migration, we need to incorporate more observations (e.g., electrical conductivity structure) and models (e.g., models of petrological and thermal structures).