NUMERICAL-SIMULATION OF SUBDUCTION ZONE PRESSURE TEMPERATURE TIME PATHS - CONSTRAINTS ON FLUID PRODUCTION AND ARC MAGMATISM

The location and sequence of metamorphic devolatilization and partial melting reactions in subduction zones may be constrained by integrating fluid and rock pressure-temperature-time (P-T-t) paths predicted by numerical heat-transfer models with phase diagrams constructed for metasedimentary, metabasaltic, and ultramafic bulk compositions. Numerical experiments conducted using a two-dimensional heat transfer model demonstrate that the primary controls on subduction zone P-T-t paths are: (1) the initial thermal structure; (2) the amount of previously subducted lithosphere; (3) the location of the rock in the subduction zone; and (4) the vigour of mantle wedge convection induced by the subducting slab. Typical vertical fluid fluxes out of the subducting slab range from less than 0.1 to 1 (kg fluid) m-2 a-1 for a convergence rate of 3 cm a-1. Partial melting of the subducting, amphibole-bearing oceanic crust is predicted to only occur during the early stages of subduction initiated in young (less than 50 Ma) oceanic lithosphere. In contrast, partial melting of the overlying mantle wedge occurs in many subduction zone experiments as a result of the infiltration of fluids derived from slab devolatilization reactions. Partial melting in the mantle wedge may occur by a two-stage process in which amphibole is first formed by H2O infiltration and subsequently destroyed as the rock is dragged downward across the fluid-absent 'hornblende-out' partial melting reaction.