Fluid processes in subduction zones: two-phase flow numerical simulation and observational constraints

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Subduction zones are places of intense mass exchange between the Earth’s surface and its interior, and consequently are associated with extensive volcanism and earthquakes. Geophysical fluids, especially water, are deeply involved in this exchange process but the physics underlying the fluid transport from the dehydrating subducting lithosphere to the surface is still matter of debate. Indeed, several modes of transportation (dykes, diapirs, Darcy-type percolation) have been suggested but their degree of implication has still to be determined. The two-phase flow theory in its new formulation (Bercovici and Ricard 2003) is used to investigate the consequence of water exsolution and percolation on the dynamics of the surrounding matrix (subducting lithosphere and mantle wedge). In the first phase of the project an appropriate 2D code (fluid & rocky matrix) including compaction with variable bulk viscosity has been developed, and a semi-analytical new solution of a solitary porosity wave has been found to benchmark such codes. After including the energy equation and poro-viscoelasticity numerical models will be carried out in a 2D geometry adapted to subduction zone settings. The effect of matrix compaction and water exsolution on the mantle wedge dynamics and thus on the arc-magma generation and water recycling into the deep mantle will be studied. The cooperation partner Prof. H. Iwamori of the Tokyo Institute of Technology will provide his expertise on subduction zone petrology and focus on water effects on phase change thermodynamics. Numerical results will be compared to the available seismic and petrological data, especially from the Japanese arc.