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

少量的融冰是由地球上地幔的动态过程持续产生的，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结晶颗粒之间。一旦熔体存在，岩石的物理性质，以及随之而来的持续熔融过程，都受到颗粒尺度熔体几何形状的影响。特别是，这种晶间熔体几何形状是部分熔融区域强度的关键因素，是通过多孔流动最终使熔体从基体中分离出来的关键因素，也是我们通过地震或电磁成像检测部分熔融区域的关键因素。由于深层产生的融化经常导致地表火山爆发，对我们的环境造成影响，因此了解融化过程对于认识和减轻随之而来的危害非常重要。在这个项目中，研究者建议通过在实验室模拟上地幔条件来确定部分熔融岩石的熔融几何形状。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。实验样品将通过平面切片的重复高分辨率成像（通过场发射扫描电子显微镜）和去除薄层材料来检查，以便在亚微米尺度上重建三维孔隙几何结构。特别地，这个程序将解决湿润的两晶界是否存在以及它们有多少的问题。在一定的熔体分数下，湿化的两晶界对地震速度和衰减以及流变学的影响最大。还将研究少量水的影响以及晶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.