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

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 minimum melt fraction estimated from laboratory-based conductivity measurements disagrees with that estimated from the MT observations in the upper mantle, and the effect of melt fraction on the magnitude of electrical anisotropy of partial molten peridotite remains poorly constrained. The quantitative interpretation of such an anisotropic structure revealed by magnetotelluric profiles requires robust laboratory characterization under controlled conditions.

In this study, Zhang and Yoshino (2020) conducted in situ electrical conductivity measurements acquired on partially molten peridotite with KLB-1 composition at various temperatures and strain rates 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. 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 (Fig. 1). The present 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 new 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.

Fig. 1. Comparison of laboratory data on electrical anisotropy (Δ(logs)) ofpartially molten KLB-1 peridotite as a function of temperature with the geophysically observed electrical anisotropy in the oceanic asthenosphere beneath the oceanic plate.

For Details: Zhang, B.H., Yoshino, T., 2020. Temperature-enhanced electrical conductivity anisotropy in partially molten peridotite under shear deformation. Earth and Planetary Science Letters, 530, 115922.

LINK: https://doi.org/10.1016/j.epsl.2019.115922