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激烈地对流。这种“地幔对流”是控制地球演化和大陆运动的最重要的过程。当物质在地球深处移动时，由于地球深处巨大的压力，矿物的晶体结构会改变成新的结构。这些相变可能会改变材料的变形性(粘度)，本项目的主要目标是评估这些相变可能改变变形性的程度。我们的重点是相变引起的矿物尺寸的变化，这可能会使材料的粘度改变几个数量级。这项研究的结果将对地球深部相变在改变地幔对流性质中的作用提供新的见解。发生在地球过渡带的相变可能对地幔对流有很大的影响。相变可以通过对密度和粘度的影响来影响对流。在改变材料粘度的各种机制中，我们关注的是颗粒大小的变化，它可以使粘度变化高达~10个数量级。我们将根据不同的P-T-t路径(P：压力、T：温度、t：时间)对橄榄石到水滑石的转变进行系统的实验，并确定新颗粒的大小和空间分布。我们将根据相变过程中微结构演化的模型来解释这些结果，并将这些结果应用于评估过渡区俯冲板块的粘度变化。

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.