Stress-driven Melt Segregation and Strain Partitioning in Partially Molten Rocks: Effects of Stress and Strain

Abstract

The evolution of melt segregation in deforming partially molten olivine-rich rocks has been studied in a series of laboratory experiments. During deformation, melt segregates into networks of anastamosing channels (or ‘bands’) surrounding lenses of melt-depleted material. We quantify the nature of the melt distribution in the samples, including thickness, angle, spacing, volume fraction, and melt fraction of melt-rich bands, to understand the dynamics of melt-network organization. Two series of experiments were designed to isolate the effects of (1) increasing shear strain (at similar stress levels), and (2) varying stress levels (deformed to similar shear strains). Melt-rich bands develop by a shear strain of unity. In samples deformed at varying stress levels, higher stress produces smaller characteristic band spacings. We relate these variations to the compaction length, dc, which varies only as a result of the reduction of matrix viscosity with increasing stress. Simple approaches to scaling from experimental to mantle conditions suggest that stress-driven melt segregation can occur in the asthenosphere; if so, it will significantly affect rheological, transport and seismic properties, with enticing consequences for our understanding of plate^mantle interactions.