Stress‐induced anisotropy of partially molten rock analogue deformed under quasi‐static loading test

Abstract

Two quasi‐static loading tests one with and one without repeated changes in the principal stress directions were performed on a partially molten rock analogue. In both runs, development of stress‐induced anisotropy was observed through a continuous, nondestructive monitoring of the sample microstructure using ultrasonic shear waves. The direction of the shear wave anisotropy showed that the area of the grain‐to‐grain contact faces whose normals are nearly parallel to the σ3 direction decreased, while the area of the other contact faces remained almost unchanged. Large viscous anisotropy caused by this microstructural anisotropy could be detected through the strong coupling between shear and isotropic stress components. The large amplitude of viscous anisotropy relative to the small amplitude of shear wave (elastic) anisotropy is consistent with the prediction from theoretical models. Amplitude and direction of this anisotropy almost synchronized with those of shear stress, indicating the existence of steady state microstructure under a given shear stress. A small hysteresis, however, was also observed, which led to a significant grain boundary wetting due to the repeated changes in the shear direction. The present results show the validity of the theoretical models for elasticity and viscosity. The coupling between shear and isotropic components produced by viscous anisotropy significantly affects the dynamics of partially molten mantle, because it enhances shear‐induced melt segregation.