Deformation-Driven Melt Segregation: Theoretical Predictions and Laboratory Observations

Abstract:

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.