An experimental study on grain-size evolution during phase transformations in the mantle transition zone and its influence on rheological properties

现在人们很好地认识到，地球内部在地质时间尺度上正在发生剧烈的对流。这种“地幔对流”是控制地球演化和大陆运动方式的最重要过程。当物质在地球内部深处移动时，矿物的晶体结构会因地球深处的巨大压力而发生变化，形成新的晶体结构。这些相变可能会改变材料的变形能力（粘度），本项目的主要目标是评估这些相变可能改变变形能力的程度。我们的重点是相变引起的矿物尺寸的变化，这可能会改变材料的粘度几个数量级。这项研究的结果将提供一个新的洞察地球内部深部的相变在修改地幔对流的性质的作用。相变发生在地球的过渡区可能有很大的影响地幔对流。相变可以通过其对密度和粘度的影响来影响对流。在材料粘度变化的各种机制中，我们关注的是粒度的变化，它可以使粘度变化高达~10个数量级。我们将进行系统的实验，对橄榄石的wadsleyite转变以下各种P-T-t路径（P：压力，T：温度，t：时间），并确定新的颗粒的大小和空间分布。我们将解释这些结果在相变过程中的微观结构演变的模型，并应用这些结果来评估在过渡区的俯冲板的粘度变化。

It is now well appreciated that Earth's interior are vigorously convecting at the geological time scale. This "mantle convection" is the most important process that controls the way in which Earth evolves and continents move. As materials move in the deep interior of Earth, crystal structure of minerals changes to new ones caused by a large pressure inside of the deep Earth. These phase transitions likely change the deformability (viscosity) of materials and the main goal of this project is to evaluate the degree to which these phase transitions might change the deformability. Our focus is the changes in the size of minerals caused by phase transitions, that could modify the viscosity of materials by several orders of magnitude. The results of this study will provide a new insight into the role of phase transitions in the deep interior of Earth in modifying the nature of mantle convection.Phase transitions that occur in Earth's transition zone likely have a large influence on mantle convection. A phase transition could affect convection through its effect on density and viscosity. Among the various mechanisms by which viscosity of materials change, we focus on the change in grain-size that could change the viscosity up to ~10 orders of magnitude. We will conduct systematic experiments on the olivine to wadsleyite transitions following various P-T-t paths (P: pressure, T: temperature, t: time) and determine the size and spatial distribution of new grains. We will interpret these results in terms of a model of microstructural evolution during a phase transition, and apply these results to evaluate the change in viscosity of a subducting slab in the transition zone.