Element partitioning and diffusion in silicate high pressure phases

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Chemical heterogeneity in the Earth's mantle is produced by the magmatic fractionation near the surface and is ultimately erased out by diffusion in minerals in the deep mantle conditions during subduction and convective circulation processes. Rheological properties of the mantle materials are also determined by the diffusion- controlled micro-mechanism. The major constituents of the lower mantle are orthorhombic silicate perovskite and magnesiowustite. Therefore, studies on diffusion of elements in these minerals provide important constraints on the processes in the lower mantle and, thereby, the whole mantle dynamics. However, properties of diffusion of these minerals are poorly known. We have made high pressure and high temperature experiments to observe reaction between MgSiO_3 perovskite and FeO wustite. Mode and extent of the reaction are used to estimate Mg-Fe inter-diffusion coefficients in ferromagnesian silicate perovskite and magnesiowustatite. High pressure and high temperature experiments have been made by an MA8 type high pressure apparatus driven by 3k ton. The most prominent process of the magmatic differentiation near the surface is the formation of the oceanic crust at the mid oceanic ridges. It returns back into the mantle at the subduction zones after some metamorphic and igneous differentiation. The fate of the subducted oceanic crust is an issue under considerable debate. However, it is conceivable that the chemical characteristics of the oceanic crust would be ultimately lost by the mechanical deformation (thinning) in the convective circulation and the diffusive dissolution. The bulk diffusion coefficients and element partitioning relations revealed in the present studies surely give important constraints on the geochemical evolution and mantle convection.