Disclinations provide the missing mechanism for deforming olivine-rich rocks in the mantle

摘要

Mantle flow involves large strains of polymineral aggregates. The strongly anisotropic plastic response of each individual grain in the aggregate results from the interactions between neighbouring grains and the continuity of material displacement across the grain boundaries. Orthorhombic olivine, which is the dominant mineral phase of the Earth's upper mantle, does not exhibit enough slip systems to accommodate a general deformation state by intracrystalline slip without inducing damage. Here we show that a more general description of the deformation process that includes the motion of rotational defects referred to as disclinations can solve the olivine deformation paradox. We use high-resolution electron backscattering diffraction (EBSD) maps of deformed olivine aggregates to resolve the disclinations. The disclinations are found to decorate grain boundaries in olivine samples deformed experimentally and in nature. We present a disclination-based model of a high-angle tilt boundary in olivine, which demonstrates that an applied shear induces grain-boundary migration through disclination motion. This new approach clarifies grain-boundary-mediated plasticity in polycrystalline aggregates. By providing the missing mechanism for describing plastic flow in olivine, this work will permit multiscale modelling of the rheology of the upper mantle, from the atomic scale to the scale of the flow.%橄榄石是最丰富的上层地幔矿物质，同时在一系列热力学条件下也是最“软弱”的，所以它在应力下的变形是对地幔行为的一个重要约束条件。Patrick Cordier等人发表了关于橄榄石聚集体变形的一个模型，它将旋转缺陷(被称为“旋错”）的运动也包括在内，这是在以前的地幔流变学研究中被忽略的一个因素。作者的结论是，这种“旋错”运动有可能为描述橄榄石中的“塑流”提供所缺失的机制，这将使研究人员能够对上层地幔的流变学进行从原子尺度到大尺度地幔流的多尺度模拟研究。