Effects of Stress-Driven Melt Segregation on Melt Orientation, Melt Connectivity and Anisotropic Permeability

Stress-driven melt segregation may have important geochemical and geophysical effects but remains a poorly understood process. Few constraints exist on the permeability and distribution of melt in deformed partially molten rocks. Here, we characterize the 3D melt network and resulting permeability of an experimentally deformed partially molten rock containing several melt-rich bands based on an X-ray microtomography data set. Melt fractions range from 0.08 to 0.28 in the similar to 20-mu m-thick melt-rich bands, and from 0.02 to 0.07 in the intervening similar to 30-mu m-thick regions. We simulated melt flow through subvolumes extracted from the reconstructed rock at five length scales ranging from the grain scale (3 mu m) to the minimum length required to fully encompass two melt-rich bands (64 mu m). At grain scale, few subvolumes contain interconnected melt, and permeability is isotropic. As the length scale increases, more subvolumes contain melt that is interconnected parallel to the melt bands, but connectivity diminishes in the direction perpendicular to them. Even if melt is connected in all directions, permeability is lower perpendicular to the bands, in agreement with the elongation of melt pockets. Permeability parallel to the bands is proportional to melt fraction to the power of an exponent that increases from similar to 2 to 5 with increasing length scale. The permeability in directions parallel to the bands is comparable to that for an isotropic partially molten rock. However, no flow is possible perpendicular to the bands over distances similar to the band spacing. Melt connectivity limits sample scale melt flow to the plane of the melt-rich bands. When a mixture of solid olivine grains and basaltic melt is subjected to differential stress, the melt can collect into sheet-like bands. Melt transport properties, including melt trajectories, chemical diffusion, and ductile creep, may all become anisotropic in the presence of these melt-rich bands. We present here the first quantification of the melt network properties in three dimensions based on synchrotron X-ray microtomography of experimentally deformed olivine/basalt aggregates. We show that the melt network presents no connection between melt-rich bands, restricting flow to the directions contained in the planes of these bands. Even at grain scale, melt pockets are preferentially oriented along the plane of the bands. The relationship between melt content and permeability derived from undeformed rocks is not valid at sample scale in sheared aggregates. 3D microtomography images of sheared partially molten rocks reveal scale-dependent melt network properties The permeability parallel to melt-rich bands is similar to that of rocks with textural equilibrium melt geometries Melt cannot flow over long distances perpendicular to melt-rich band due to lack of melt connectivity in that direction and scale