Experimental studies on rheological properties of transition zone minerals

过渡带矿物的流变学性质对地幔对流的作用方式有很大的影响。地震层析成像显示，地幔过渡带深部冷物质的强烈变形是一个明显的证据。然而，冷材料的强烈变形与流变特性的温度敏感性的传统观点不一致。该项目的主要目的是提供一个新的数据集，基于新开发的高压变形技术，我们可以解决这个矛盾。解决这个悖论的一个假设是，在过渡区发生的相变将使材料的晶粒尺寸减小而变弱。然而，迄今为止，还没有关于过渡带矿物流变特性对粒度敏感性的定量数据。在该项目中，研究人员将使用RDA（旋转Drickamer装置）对wadsleyite（过渡带矿物之一）进行一系列实验室实验，以确定粒度敏感变形机制的流动规律。使用的样品是具有受控粒度和含水量的合成多晶骨料。将使用RDA以恒定应变率对样本进行变形，并在同步加速器设施（Brookhaven的NSLS）中原位获得力学数据（应力-应变曲线）。在恢复后，将对变形样品进行非原位检查，以使用一系列技术研究微观结构，包括光学显微镜、EPMA、SEM（EBSD）、FTIR、拉曼和TEM。研究结果将揭示地幔对流如何在地球上发生，以及地球如何热演化。

The rheological properties of transition zone minerals have a strong influence on how mantle convection works. There is clear evidence, from seismic tomography, for intense deformation of deep sinking cold materials in the mantle transition zone. However, intense deformation of cold materials is at odds with the conventional view of temperature sensitivity of rheological properties. The main purpose of this project is to provide a new data set, based on newly developed high-pressure deformation techniques, by which we can solve this paradox. One hypothesis to resolve this paradox is that the phase transformations that occur in the transition zone will make materials weak by grain-size reduction. However, so far there is no quantitative data on the sensitivity of rheological properties of transition zone minerals on grain-size. In this project, the investigator will perform a series of laboratory experiments on wadsleyite (one of the transition zone minerals), using RDA (rotational Drickamer apparatus), to determine the flow laws for grain-size sensitive deformation mechanisms. Samples to be used are synthetic polycrystalline aggregates with controlled grain-size and water content. Specimens will be deformed at constant strain-rates using RDA, and the mechanical data (stress-strain curves) will be obtained in-situ at a synchrotron facility (NSLS at Brookhaven). Deformed samples will be examined ex-situ after the recovery to study the microstructures using a range of techniques including optical microscope, EPMA, SEM (EBSD), FTIR, Raman and TEM. The results will shed light on how mantle convection occurs in Earth and how Earth evolves thermally.