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

地球上地幔的动态过程不断产生少量的熔体，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融过程就会受到颗粒尺度熔体几何形状的影响。特别是，这种晶间熔体的几何形状是一个关键因素，部分熔融区域的强度，最终偏析的熔体从基质的多孔流，并为我们的能力，以检测部分熔融区域的地震或电磁成像。由于在深处产生的熔体经常导致地表火山爆发，对我们的环境产生影响，因此了解熔融过程对于认识和减轻随之而来的危害非常重要。在该项目中，研究人员建议通过在实验室中模拟上地幔条件来确定部分熔融岩石中熔体的几何形状。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长的持续时间。将通过对平面部分重复进行高分辨率成像（通过场发射扫描电子显微镜）并去除薄层材料来检查实验样品，以便在亚微米尺度下重建三维孔几何形状。特别是，这一程序将解决的问题，是否润湿的两个晶界存在，以及有多少。在一定的熔体分数下，润湿的双颗粒边界对地震波速度和衰减以及流变性的影响最大。少量的水的影响，以及对熔体分布的晶粒尺寸的影响也将被检查。

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.