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

将岩石学模型与地震波速度中的不连续性相结合是对地球深部的结构和组成施加约束的基本手段。在~300 km深度处观察到的局部地震特征被称为X-不连续，但迄今为止，对该不连续没有明确的岩石学解释被普遍接受。最新的解释主要涉及到榴辉岩矿物组合中含钙埃斯科拉（Ca0.5□0.5Si2O6）单斜辉石（cpx）的斯氏石英的出溶。我们的目标是使用两步实验方法来测试这一假设：1）确定最大的钙-eskola-组件，可以在较低的P在模型和天然榴辉岩组合物中的cpx，和2）重新平衡的预合成组合在较高的P内的stishovite稳定性领域。重点将放在定义的条件下，斯石英exsolves和监测的cpx组合物和摩尔体积的变化，以及在相分布，显微结构和可能的exsolved斯石英的择优取向。榴辉岩体和X-不连续面之间的联系对于地球动力学建模和理解洋壳再循环进入地幔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.