Experimental studies on rheological properties of transition zone minerals

过渡带矿物的流变性对地幔对流的作用有很大影响。地震层析成像有明确的证据表明，深沉的冷物质在地幔过渡带发生了强烈的变形。然而，冷材料的剧烈变形与流变性的温度敏感性的传统观点是不一致的。这个项目的主要目的是提供一个新的数据集，基于最新发展的高压变形技术，通过它我们可以解决这一悖论。解决这一悖论的一个假设是，发生在过渡区的相变将使材料因颗粒尺寸的减小而变得脆弱。然而，到目前为止，还没有关于过渡带矿物的流变性对粒度的敏感性的定量数据。在这个项目中，研究人员将使用RDA(旋转Drickamer装置)对过渡带矿物之一的wadsleyite进行一系列实验室实验，以确定颗粒尺寸敏感变形机制的流动规律。要使用的样品是粒度和水分受到控制的合成多晶集合体。试件将使用RDA在恒定应变率下变形，力学数据(应力-应变曲线)将在同步加速器设施(布鲁克海文的NSLS)中原位获得。变形后的样品将在恢复后进行异地检查，以使用光学显微镜、电子探针、扫描电子显微镜(EBSD)、傅立叶变换红外光谱(FTIR)、拉曼光谱和透射电子显微镜等一系列技术来研究微观结构。这一结果将有助于阐明地幔对流是如何在地球上发生的，以及地球是如何以热的方式演化的。

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.