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相结合是限制地球深层结构和成分的重要手段。在约 300 公里深度处观察到的局部地震特征被称为 X 不连续性，但迄今为止，对于这种不连续性还没有明确的岩石学解释被普遍接受。最近有趣的解释主要涉及辉石从存在于榴辉岩矿物组合中的含钙 (Ca0.5□0.5Si2O6) 单斜辉石 (cpx) 中溶出。我们的目标是使用两步实验方法来测试这一假设：1）确定模型和天然榴辉岩成分中较低 P 下可纳入 cpx 的最大 Ca-eskola 成分，以及 2）在 stishovite 稳定场内较高 P 下重新平衡预合成组合。重点将放在定义 Stishovite 溶出的条件并监测 cpx 组成和摩尔体积的变化，以及溶出的 Stishovite 的相分布、微观结构和可能的优选方向。榴辉岩体和 X 不连续面之间的联系对于地球动力学建模以及了解洋壳再循环到地幔中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.