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

俯冲带是地球表面和内部物质交换强烈的地方，因此与广泛的火山活动和地震有关。地球物理流体，特别是水，深入参与了这一交换过程，但流体从脱水俯冲岩石圈向地表输送的物理学原理仍存在争议。事实上，已经提出了几种运输方式（堤、底辟、达西型渗透），但它们的影响程度仍有待确定。新提出的两相流理论（Bercovici and Ricard 2003）用于研究水的溶解和渗透对周围基质（俯冲岩石圈和地幔楔）动力学的影响。在项目的第一阶段，已经开发了一个适当的二维代码（流体和岩石矩阵），包括可变体积粘度的压实，并且已经找到了一个孤立孔隙度波的半解析新解来对这些代码进行基准测试。在包括能量方程和孔隙粘弹性数值模型之后，将在适应俯冲带设置的二维几何结构中进行数值模型。研究基质压实作用和水溶蚀作用对地幔楔体动力学的影响，进而对弧岩浆生成和深部水循环的影响。合作伙伴东京工业大学的H. Iwamori教授将提供他在俯冲带岩石学方面的专业知识，并专注于水对相变热力学的影响。数值结果将与现有的地震和岩石学数据，特别是日本弧的数据进行比较。

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.