Numerical modeling of subduction parameters' influence on partial melting

A bi-dimensional finite element model has been developed in order to study simultaneously the influence of water and geometrical parameters on the onset and amount of partial melting in subduction zone. This model incorporates classical fluid mechanics equations and the simultaneous computation of partial melting and water transfers between minerals in the mantle and in the crust. Energy conservation equation is solved using a Galerkin procedure while mass and momentum equations are solved by a penalty method. Fluid transfers are incorporated with a tracer cell method. Each tracer includes compositional and water content information. The released water content which takes part in partial melting is computed from the temperature and pressure conditions resulting from the thermo-mechanical model. We also introduce a water-dependent term in the viscosity computations, corresponding to the decrease of rocks strength with water content. Water is introduced in the model according to phase diagrams relevant to the mantle and to the crust, respectively. The studied subduction zones parameters are the convergence rate, the dip angle and the crustal thickness of the overriding plate. The latter parameter influences to some extent the thermal regime and the amount of partial melting in the mantle wedge. Water released by the oceanic crust contributes to hydrate the overlying mantle. A thin layer of hydrated mantle at the crust mantle interface appears and reduces the viscosity. Above this layer, water contributes to melt production. Two main magmatic production areas are located above the slab corresponding to the maximum of plate dehydration. For a convergence rate of 5 cm/yr, a dip angle of 45° and an overriding crust of 30 km thick, these two areas correspond to a distance of 80 km and 100 km from the trench and 70 km and 90 km depth, respectively. The results are in line with geochemical data recorded in lavas from volcanic arcs. Thereby, an increasing overriding crustal thickness is related to a decreasing magmatic production, while high partial melting rates are correlated with high convergent rates. The location of partial melting is mainly controlled by the dip angle and to a lesser extent by the convergence rate.