Temperature-enhanced electrical conductivity anisotropy in partially molten peridotite under shear deformation

Abstract

The high conductivity anomalies observed in the oceanic asthenosphere have shown anisotropic signature parallel to the plate motion. Anisotropic alignment of partial melt has been considered to one of probable explanations for the observed anisotropic conductivity structure, but the effect of temperature on the distribution of melt, the composition of melt, and the magnitude of electrical anisotropy for partial molten peridotite is unknown under shear deformation. In this study, the electrical conductivity of partially molten peridotite (KLB-1) under shear deformation was measured at 1 GPa in a DIA type apparatus with a uniaxial deformation facility to provide new constraints on the anisotropic signature in the oceanic asthenosphere. The conductivity measurements were performed simultaneously in two directions of three principal axes: parallel and normal to the shear direction on the shear plane, and perpendicular to the shear plane, by using impedance spectroscopy at temperature ranges of 1483-1548 K. Our results indicate that the total melt fraction, the absolute conductivity values, and the magnitude of electrical anisotropy of partially molten peridotite increase with increasing temperature. Although the Na2O content varies widely at constant temperature in the recovered melt, very small changes of shear-parallel conductivities (σx) before and after shear deformation suggest that the electrical conductivity of partially molten peridotite is mainly controlled by temperature, rather than alkali content in partial melt. Microstructural observations of the recovered samples reveal that the development of conductivity anisotropy was caused by the realignment of melt pockets parallel to the shear direction, which forms two melt-rich regions. Furthermore, we estimate how much melt fraction is partitioned into melt-rich regions by calculating the area ratio of two melt-rich regions to the whole area. Our calculations show that once melt segregation occurs, more than 50% of the total melt fraction will partition into the melt-rich regions, and this proportion will continue to increase with the increase of temperature. This finding suggests that development of electrical anisotropy in partially molten peridotite under shear deformation will increase with increasing temperature, which may provide new constraints on interpretation of high conductivity anomalies observed in the oceanic asthenosphere.