变形和部分熔融共同作用下的上地幔岩石物性实验研究

At plate boundaries, partial melting and deformation occur simultaneously over large region of the Earth’s upper mantle. Recent studies have also demonstrated that small melt fractions are present in the asthenosphere, which is constantly deforming due to the convection of the Earth upper mantle. Therefore, the study of the relationship between partial melting and deformation is key for our understanding of melt formation, segregation and migration up to the surface. Our tools to study melt distributions in the Earth’s upper mantle over large scale are based on geophysical observations, mainly seismic and magnetotelluric. However, the interpretation of these data requires an excellent knowledge of the physical properties of these deformed and partially molten mantle rocks. High pressure and temperature experiments provides important information on the melt organization when deformation occurs, while numerous experiments on partially molten aggregate, but without deformation, deliver constraints on the effect of melt on seismic properties and electrical conductivity of the mantle rocks. These previous experiments were always performed by directly mixing a solid and a liquid phase but never generate melting during the experiment. In this project, we propose a new kind of experiments where partial melting will be triggered while deformation is occurring. We will deform at Earth’s upper mantle conditions a mixture of olivine and phlogopite that will undergo partial melting. During the deformation, temperature will be raised progressively up to the melting temperature of the phlogopite and the experiments will be stop at variable strains after the melting occurred. We will them performed 3D analysis of the melt microstructures. On the same recovered samples, we will also measure in-situ the seismic velocities and electrical conductivities at the pressure and temperature conditions of the deformation experiments. These data will allow us to compare how the melt distribution controlled by the deformation has influenced the rheology, seismic properties and electrical conductivity in our sample. This project involves state of the art methods for high-pressure and temperature deformation experiments and in-situ physical properties measurements as well as newly developed methods for synchrotron X-ray micro- and nano-computed tomography. Our results will be used to address several important questions: How does this melt organizes when partial melting occurs simultaneously with deformation? How does this melt distribution affect rheology, seismic properties and electrical conductivity of the deformed partially molten mantle rocks? Finally, we will be able to bring better constraints on mantle rock physical properties and therefore allow better use of the geophysical data used to interpret the structure of the large partially molten regions in the Earth’s upper mantle.

软流圈和板块边界的上地幔岩石多同时发生部分熔融和变形，研究部分熔融和变形之间的联系对了解熔体的产生、分离和迁移的物理过程具有重要意义。对上地幔中熔体分布的研究主要是通过地震学和大地电磁观测，但地质解析需要对部分熔融地幔岩石的物理性质有准确的认知。前人相关地幔岩物性实验基本都是在无变形条件下进行的，实验起始物质采用熔体和矿物的混合物，熔体并不是原位生成的，与实际地质过程并不一致。本项目拟研究部分熔融和变形共同作用下地幔岩的物性，以流变学实验、原位岩石物性测量和新近发展的同步辐射X光显微和纳米层析成像技术为主要手段，以橄榄石和金云母混合物作为实验起始物质，进行脱水熔融条件下的变形实验，解析熔体三维结构，原位测量高温高压条件下变形样品的波速和导电性，解决同变形条件下熔体拓扑结构和对地幔流变、地震波速和导电性影响这一关键基础科学问题，为上地幔中部分熔融区的地球物理观测和地质解释提供更合理的约束。

开展了橄榄石和金云母组成的起始材料的高温高压变形实验研究由。前人对 橄榄石-金云母系统的研究程度高，上地幔条件下金云母的稳定域和熔融温度已 被精确测定(Yoder and Kushiro, 1969)。我们也对橄榄石-金云母集合体熔融 做过了初步的实验测试，结果与前人工作一致。另外，我们还与 Geeth Manthilake 博士合作，开展了部分熔融变形实验后样品的高温高压下的地震波速和电导率的原位测量，采用软流圈 的温压实验条件，并将温度控制在金云母的熔融温度之上。