Deformation-Driven Melt Segregation: Theoretical Predictions and Laboratory Observations

Deviatoric stress profoundly influences the distribution of melt in viscously deforming, partially molten rocks. Under hydrostatic stress melt forms an interconnected network, primarily along grain edges (triple junctions) to reach a state of minimum interfacial energy. Melt also wets a fraction of the grain boundaries due to anisotropy in solid-melt interfacial energy. Under a non-hydrostatic state of stress melt quickly redistributes at the grain scale, introducing a pronounced melt preferred orientation. Melt pockets align at the grain scale with their long axes 15 to 30^o to the maximum principal stress in triaxial compression experiments and at 15-40^o to the shear plane, antithetic to the shear direction, in shear experiments. This stress-induced, grain-scale alignment of melt gives rise to anisotropic viscosity of the solid/melt aggregate. In response, melt redistributes over distances larger than the grain scale. Two types of redistribution occur. First, in torsion experiments, a diffuse (base-state) migration of melt occurs from the outer radius toward the axis of the cylindrical sample. Second, in simple shear and torsion experiments, melt spontaneously segregates into melt-rich bands spaced at distances smaller than the compaction length. The occurrence of both base-state melt migration and melt-rich band formation are observed in laboratory experiments and modelled using two-phase flow theory. In intensely deforming regions of Earth’s upper mantle, melt-rich bands may provide high-permeability pathways that host rapid extraction of melt and zones of weakness that localize deformation. Moreover, they may be detected as regions of reduced seismic velocity and increased seismic anisotropy.