Towards a petrological explanation for the X-Discontinuity: The role of eclogite

将岩石学模型与地震波速度的不连续性结合起来，是对地球深部结构和成分施加约束的基本手段。在约300公里深度观察到的局部地震特征被称为X-不连续，但到目前为止，对这种不连续的岩石学解释尚不清楚。最新的有趣解释主要涉及辉石从榴辉岩矿物组合中存在的含钙(Ca0.5-0.5Si2O6)单斜辉石(CPx)中出溶。我们的目标是用两步实验方法验证这一假说：1)确定在模型和天然榴辉岩成分中低磷条件下可以并入cpx的最大Ca-Eskola组分，以及2)在较高磷条件下预先合成的组合在辉石稳定场内重新平衡。重点将放在确定辉锌矿析出的条件和监测cpx组成和摩尔体积的变化，以及相分布、微观结构和可能的择优取向。榴辉岩体和X-不连续面之间的联系对于地球动力学模拟和理解洋壳再循环进入地幔所涉及的物理和化学过程具有非常重要的意义。

Combining petrological models with discontinuities in seismic wave velocities is the essential means for placing constraints on the structure and composition in the deep Earth. A local seismic feature observed at ~300 km depth is referred to as the X-discontinuity, but to date, no clear petrologic explanation for this discontinuity is generally accepted. The most recent intriguing explanation primarily involves the exsolution of stishovite from Ca-eskola-bearing (Ca0.5□0.5Si2O6) clinopyroxene (cpx) that is present in an eclogitic mineral assemblage. Our goal is to test this hypothesis using a two-step experimental approach: 1) determining the maximum Ca-eskola-component that can be incorporated in cpx at lower P in model and natural eclogitic compositions, and 2) re-equilibration of pre-synthesised assemblages at higher P within the stishovite stability field. Emphasis will be placed on defining the conditions where stishovite exsolves and monitoring changes in cpx composition and molar volume, as well as in phase distributions, microstructures and possible preferred orientation of the exsolved stishovite. A linkage between eclogite bodies and the X-discontinuity has very important implications for geodynamic modelling and for understanding the physical and chemical processes involved in recycling of oceanic crust into the mantle.