Melt segregation during Poiseuille flow of partially molten rocks

Abstract:

Studies of the dynamics of partially molten regions of the Earth’s mantle provide the basis necessary for understanding the chemical and physical evolution of our planet. Since we cannot directly observe processes occurring at depth, we rely on models and experiments to constrain the rheological behavior of partially molten rocks. Here, we present the results of an experimental investigation of the role of viscous anisotropy on melt segregation in partially molten rocks through Poiseuille flow experiments. Partially molten rock samples with a composition of either forsterite or anorthite plus a few percent melt were prepared from vacuum sintered powders and taken to 1200ºC at 0.1 MPa. The partially molten samples were then extruded through a channel of circular cross section under a fixed pressure gradient at 1200^o to 1500^oC. The melt distribution in the channel was subsequently mapped through image analyses of optical and backscattered electron microscopy images. In these experiments, melt segregates from the center toward the outer radius of the channel with the melt fraction at the outer radius increasing to twice that at the center. These results are consistent with base-state melt segregation as predicted by Takei and Holtzman (JGR, 2009), Takei and Katz (JFM, 2013) and Allwright and Katz (GJI, 2014) for sheared partially molten rocks for which viscosity is anisotropic due to the stress-induced, grain-scale alignment of melt.