A study of pressure shadows formed in torsion experiments of olivine-basalt systems

As a two-phase, solid-melt material flows around rigid particles, melt-depleted and melt-enriched regions (i.e. pressure shadows) develop due to the coupled fluxes of melt and solid driven by pressure gradient around the particles. To study this compaction-decompaction process, samples made from fine-grained San Carlos olivine and mid-ocean ridge basalt (MORB) containing dispersed sub-millimeter-sized beads of stabilized zirconia and single crystals of San Carlos olivine were deformed in torsion at a temperature of 1200°C and a confining pressure of 300 MPa in a gas-medium apparatus. Samples were sheared to a maximum strain of γ ≈ 10 at constant strain rates. Indicated by melt distribution maps obtained from reflected-light optical and backscattered electron microscopy, melt-depleted and melt-enriched regions around the beads, which became observable at a shear strain of γ ≈ 1, are oriented ~45° relative to the shear direction. From crystallographic preferred orientations (CPOs) generated from electron-backscattered diffraction (EBSD) analysis of olivine grains, the changes in the directions of (010) planes revealed the changes in stress field around the rigid beads. The shear direction was also constrained by the alignment of melt pockets and the formation of melt-rich bands, which orient ~20° to the shear plane, antithetic to the shear direction. The goal of this research is to study the relationship between melt fraction and bulk and shear viscosities in the two-phase system based on the theoretical analysis by McKenzie and Holness (2000).