Element partitioning and diffusion in silicate high pressure phases

地幔中的化学不均一性是由地表附近的岩浆分馏作用产生的，并在俯冲和对流循环过程中最终被深部地幔条件下矿物的扩散所消除。地幔物质的流变性质也是由扩散控制的微观机制决定的。下地幔的主要成分为斜方硅酸盐、钙钛矿和镁闪锌矿。因此，元素在这些矿物中的扩散研究对下地幔过程乃至整个地幔动力学具有重要的制约作用。然而，人们对这些矿物的扩散特性知之甚少。我们进行了高温高压实验，观察了镁硅钙钛矿与FeO黄铁矿之间的反应。用反应的方式和程度估算了镁-铁在硅酸铁、钙钛矿和镁辉石中的互扩散系数。在万吨级MA8型高压装置上进行了高温高压实验。近地表岩浆分异最显著的过程是大洋中脊洋壳的形成。在一些变质和火成岩分异作用后，它在俯冲带回到地幔。被俯冲的洋壳的命运是一个有相当大争议的问题。然而，可以想象，洋壳的化学特征最终将被对流环流中的机械变形(变薄)和扩散溶解所丧失。本文揭示的体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制约作用。

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.