Stress-driven melt segregation in deforming partially molten rocks (Invited)

Stress-driven melt segregation is a likely mechanism for simultaneously nucleating shear zones and forming high permeability pathways for melt extraction from Earth's upper mantle and lower crust. Due to the dependence of rock viscosity on melt fraction, pressure gradients develop during deformation of partially molten rocks. These pressure gradients lead to amplification of spatial perturbations in melt fraction and thus to segregation of melt into distinct melt-rich bands in response to stress. To systematically explore the evolution of melt distribution during deformation and the influence of melt segregation on the rheological properties of the partially molten rocks, we performed deformation experiments on partially molten rocks in direct shear and in torsion geometries in a high-pressure, high-temperature, gas-medium testing apparatus. Results demonstrate that, under the conditions imposed in our experiments, melt-rich bands form at a shear strain of γ ≈ 1. A drop in viscosity of the partially molten rock occurs at the onset of melt segregation, weakening the rock by a factor of ~2. In samples deformed to different strains at the same stress, networks of melt-rich bands maintained an average steady-state orientation ~20o to the shear plane, antithetic to the shear direction. Offsets along strain markers in sheared samples demonstrate that deformation localizes on melt-rich bands. To explore the dependence of band spacing on compaction length, a critical length scale in two-phase flow, several samples were deformed to the same shear strain at different shear stresses. In this way, the compaction length is systematically varied, and experimental results can be scaled to estimates of compaction length in Earth's upper mantle. The spacing between bands increases with increasing compaction length such that band spacing is about 1/5 the compaction length. The scale of melt-rich bands predicted from this relationship is consistent with the range of observed spacing between tabular dunite bodies in ophiolites, which are interpreted to be relict melt channels. Furthermore, experimentally produced and naturally observed morphologies of self-organized melt-rich structures share first-order similarity, suggesting similar dynamical origin.