Element partitioning and diffusion in silicate high pressure phases

地幔的化学非均质性是由地表附近的岩浆分选作用产生的，并最终在俯冲和对流环流过程中被深部地幔条件下矿物的扩散所消除。地幔材料的流变特性也是由扩散控制的微观机制决定的。下地幔主要成分为正方晶硅酸盐钙钛矿和镁武矿。因此，对这些矿物中元素扩散的研究对下地幔的过程，从而对整个地幔动力学提供了重要的约束。然而，这些矿物的扩散特性却鲜为人知。对MgSiO_3钙钛矿与FeO浮氏体的反应进行了高压和高温实验。用反应的模式和程度来估计镁铁在镁铁硅酸盐钙钛矿和镁钨酸盐中的相互扩散系数。用3k吨驱动的MA8型高压装置进行了高压高温实验。近地表岩浆分异最突出的过程是洋中脊洋壳的形成。它经过变质和火成岩的分异作用，在俯冲带回到地幔中。俯冲的海洋地壳的命运是一个备受争议的问题。然而，可以想象，海洋地壳的化学特性最终会由于对流环流和扩散溶解的机械变形（变薄）而丧失。本研究揭示的体积扩散系数和元素分配关系对地球化学演化和地幔对流具有重要的约束作用。

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.