岛弧下方榴辉岩粒间流体分布的三维成像

The distribution and connectivity of aqueous fluid within mineral grains directly affect the physical properties of the area. It is also closely related to the migration mechanism of fluid in the Earth interior and is especially important to the processes within subduction zone, such as partial melt under island arc volcanoes. Most of earlier researches characterize the connectivity of fluid by measuring the apparent dihedral angle on 2D sections of the sample. Due to the inherit problem of this method, the results of distribution and connectivity are still controversial in certain areas。This limitation hinder our understanding to the migration of fluid in the subduction zone. We proposed to adopt the improved FIB/SEM tomography technique to reveal the distribution of fluid in the eclogite (the subducted ocean crust). By analyzing the quenched sample prepared by high- pressure apparatus, we may use FIB/SEM to directly measure the dihedral angle and distribution of aqueous fluid within grains of eclogite at sub-arc depths and find its relationship to temperature, pressure and fraction of the fluid. The minimum fraction of fluid to maintain the connectivity will be determined. We will also try to build a numerical model and discuss the migration of aqueous fluid within Earth's mantle, especially in the mantle wedge.It will help us to get a better understanding on the material and energy exchange in subduction zone. In addition, the develop of FIB/SEM tomography will provides new insight and new method to the study of partial melting and rheology of rocks.

流体在岩石矿物颗粒之间的存在形态和连通关系不但直接影响着区域的物理性质，而且与它在地球内部的运移过程和富集机制密切相关，尤其是对俯冲带部分熔融和岛弧火山活动具有重要意义。受到分析方法的限制，前人多只能通过二维切片上视二面角的测量来间接评估流体连通状况，因此对关键区域流体连通性的识别还存在极大的争议,严重妨碍了我们对俯冲带流体运移过程的认识。本项目拟通过率先引入FIB/SEM 三维成像手段，发展新的二面角测量技术，结合高温高压实验手段重现岛弧下方俯冲洋壳-榴辉岩中粒间流体的三维空间分布形态,定量研究三维条件下二面角真值与流体连通性之间的关系，探索流体连通性随温度，压力和流体含量的变化，揭示完成流体连通所需的最小含量，构建流体分布数值模型，探讨俯冲带尤其是弧下俯冲板片范围内的流体运移机制。此外， FIB/SEM 三维成像技术的发展和完善也可以为部分熔融和岩石流变研究提供新的思路和技术手段。

流体运移是俯冲带中物质和能量交换的重要媒介。流体在岩石矿物颗粒之间的二面角直接影响其分布和连通关系，是影响流体运移的关键因素。本项目通过发展和改进聚焦离子束/扫描电镜（FIB/SEM）三维成像技术，实现了矿物/岩石-流体结构平衡的三维观测；通过开发和优化分析程序，实现了不同矿物-流体之间的空间二面角准确测量。在此基础之上，结合高温高压淬火样品，对弧下榴辉岩矿物单斜辉石，石榴子石等与流体之间的空间二面角进行了测量。结果显示斜辉石-单斜辉石-流体二面角随压力的增加而降低，但在常规弧下压力区间内均不能实现连通。石榴子石-石榴子石-流体二面角受温度的影响较大，相对高的温度有利于其粒间流体的连通。本项目建立了利用FIB/SEM三维成像技术直接测量高温高压样品固液空间二面角的新方法，为流/熔体形态学提供新的研究工具。