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

Trustees Of Boston University, Boston

Investigators

Abstract

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.