MELT SEGREGATION DURING SHEAR DEFORMATION OF PARTIALLY MOLTEN ROCKS

Theoretical analyses, numerical models, field observations, geophysical probes, and laboratory experiments combine to provide evidence for a close coupling between deformation and melt distribution resulting from stress-driven melt segregation. A large reduction in shear wave velocity marks the lithosphere-asthenosphere boundary, consistent with an asthenosphere characterized by horizontally aligned melt-enriched layers that provide a large viscosity contrast with the overlying lithospheric plate. In shear deformation experiments, melt segregation occurs at two scales. Stress-driven alignment of melt develops at the grain scale and spontaneous segregation of melt into melt-enriched networks evolves at a larger scale. In turn, this redistribution of melt influences not only the viscosity but, importantly, also the viscous anisotropy of the rock. Microstructural analyses of samples deformed in torsion experiments on rocks composed of olivine plus basalt and of olivine plus basalt plus chromite reveal segregation at both scales. In samples composed of olivine plus basalt, melt segregates radially from the outer radius toward the middle of the sample. If chromite is added to reduce the permeability and thus the compaction length, melt segregates into an anastomosing network of melt-enriched bands that extend from the outer radius toward the center of the sample. Both scales of melt segregation arise because (i) rock viscosity decreases with increasing melt fraction and (ii) alignment of melt introduces viscous anisotropy. In Earth’s mantle, such networks of melt-enriched bands facilitate a significant decrease in rock viscosity as deformation localizes in melt-enriched regions, and they enable a substantial enhancement in the rate of melt extraction as melt flows through the melt-enriched high permeability channels.