Numerical simulation of metamorphic pressure‐temperature‐time paths and fluid production in subducting slabs

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Abstract
In order to investigate metamorphic reactions and fluid production in subduction zones, a 2‐dimensional finite difference model has been constructed. The model simulates heat transfer in a subduction zone with a 27° dip. A spatial resolution of 1 km is used in the region of the subduction thrust in order to carefully monitor metamorphic reactions; elsewhere the spatial resolution is 5 km. In these calculations, the oceanic crust is assumed to contain 2 wt % bound volatiles, distributed homogeneously within the 7.5‐km‐thick crust. Fluids are released by continuous and discontinuous end‐members of three different dehydration models: pressure‐sensitive, temperature‐sensitive, and amphibole (negative dP/dT) dehydration. Subduction zone pressure‐temperature‐time (P‐T‐t) paths predicted by the model intersect the wet basaltic solidus only at the initiation of subduction. In mature subduction zones, and in the absence of significant frictional heating and induced mantle convection, P‐T‐t paths encounter subsolidus conditions at depths of 100–150 km beneath magmatic arcs, suggesting that slab dehydration reactions play an important role in arc magma genesis. Metamorphic dehydration reactions that consume 50 kJ/kg result in P‐T‐t paths that are only 5 K cooler than models that neglect such reactions. In both the temperature and amphibole dehydration models, the fluid production region moves to greater depths over time as the subduction zone cools. Within the oceanic crust, fluid production starts at the top of the crust and migrates downward in the temperature and amphibole dehydration models. This phenomenon, coupled with the weakening effect of devolatilization reactions, may result in the accretion of oceanic crust to the hanging wall of the subduction zone. For a convergence rate of 3 cm/yr, average fluid fluxes out of the subducting slab range from ∼0.1 kg fluid/(m² yr) for the continuous reaction models to >1 kg fluid/(m² yr) for the discontinuous pressure and amphibole dehydration models. Vertical fluid fluxes on the order of 1 kg fluid/(m² yr) can substantially perturb the thermal structure of overlying rocks and can cause large‐scale hydration and metasomatism of the overlying mantle wedge. In contrast, the thermal effect of fluid flow parallel to the subduction thrust zone is insignificant compared to the thermal effect of subducting oceanic lithosphere.