Melt Geometry in Partially Molten Olivine: The Influence of Grain Size and Water

少量的熔融是由地球上地幔的动态过程不断产生的，例如在大洋中脊和俯冲带。熔体最初分布在部分熔融岩石的晶体颗粒之间。一旦熔体存在，岩石的物理性质，以及随之而来的持续熔化过程，都会受到熔体几何尺寸的影响。特别是，这种晶间熔体几何形状是部分熔融区强度的关键因素，是熔体最终通过多孔流动从基质中分离的关键因素，也是我们通过地震或电磁成像检测部分熔融区的能力的关键因素。由于在深部产生的熔体经常导致地表火山喷发，从而对我们的环境造成影响，了解熔融过程对于识别和减轻随之而来的危险非常重要。在这个项目中，研究人员建议通过在实验室模拟上地幔条件来确定部分熔融岩石中的熔体几何形状。实验的一个重要方面是，它们将在活塞缸装置中进行，其优点是可以在高温和高压下实现相对较长的持续时间。实验样品将通过重复进行平面切片的高分辨率成像(由场发射扫描电子显微镜进行)并去除薄层材料来检查，以重建亚微米级的三维孔隙几何结构。特别是，这个程序将解决是否存在润湿的双颗粒边界以及它们的数量有多多的问题。在熔体分数一定的情况下，润湿的双晶界对地震波速、衰减和流变性的影响最大。还将考察少量水的影响以及颗粒尺寸对熔体分布的影响。

Small amounts of melt are continuously generated by dynamic processes in the Earth's upper mantle, for example at mid-ocean ridges and subduction zones. The melt is initially distributed between the crystalline grains of the partially molten rock. Once melt is present, the physical properties of the rock, and with them the continuing melting process, are influenced by the grain scale melt geometry. In particular, this intergranular melt geometry is a key factor for the strength of partially molten regions, for the eventual segregation of the melt from the matrix by porous flow, and for our ability to detect partially molten regions by seismic or electromagnetic imaging. Since the melt generated at depth frequently leads to volcanic eruptions at the surface with consequences for our environment, understanding the melting process is important for recognizing and mitigating the attendant hazards.In this project, the investigator proposes to determine the melt geometry in partially molten rocks by simulating upper mantle conditions in the laboratory. An important aspect of the experiments is that they will be conducted in a piston cylinder apparatus, which has the advantage that relatively long durations at high temperatures and pressures can be achieved. The experimental samples will be examined by repeated high resolution imaging (by Field Emission Scanning Electron Microscope) of a planar section and removal of a thin layer of material in order to reconstruct the three-dimensional pore geometry at the sub-micron scale. In particular, this procedure will address the question of whether wetted two-grain boundaries exist and how numerous they are. At a fixed melt fraction, wetted two-grain boundaries will have the largest effect on seismic velocities and attenuation, and the rheology. The effects of small amounts of water, as well as grain size effects on the melt distribution will also be examined.