Electrical conductivity anisotropy of deformed talc rocks and serpentinites at 3 GPa

The electrical conductivity anisotropy of deformed natural talc rocks and serpentinites was investigated using a Kawai-type multi-anvil press in the temperature range of 500–1000 K at 3 GPa. The electrical conductivities of the samples were measured by an impedance analyzer with a frequency range of 10−3–106 Hz along three sample directions: the direction parallel to the lineation of oriented minerals (x direction), the direction perpendicular to the lineation on the foliation plane (y direction), and the direction perpendicular to the foliation (z direction). For both rock types, electrical conductivities parallel to the x and the z directions were the highest and the lowest, respectively. Electrical conductivities of talc rocks and serpentinites were at least two orders of magnitude lower than previous reports. Electrical conductivities of the serpentinites were higher than those of the talc rocks. Electrical conductivity anisotropy for the talc rocks was stronger than that for the serpentinites at the same oxygen buffer condition. Electrical conductivity and its anisotropy of the serpentinites become higher with higher oxygen fugacity. The activation enthalpy of talc rocks was the lowest (0.59 eV) in the x direction and highest (0.68 eV) in the z direction. The activation enthalpies of the serpentinites in different directions show similar values: about 0.74 eV and 0.68 eV for the experiments using Mo and Ni electrodes, respectively. Impedance spectra for both rock types show the presence of two conduction paths: grain interior and grain boundary conductions. The total electrical conductivities were reduced by grain boundary conduction. Electrical conductivities of hydrous minerals show strong dependence on hydrogen concentration and hydrogen mobility. The conduction mechanism probably was proton migration through extrinsic vacancies derived from a presence of ferric iron perpendicular to the c axis of the crystals and through interstitial mechanisms parallel to the c axis. In both warm and cold subduction zones, high electrical conductors (∼10−1.5 S/m) cannot be explained by the presence of talc or serpentinites. The electrical conductivity anisotropy in a subduction zone would be inconspicuously small (within one order) unless we consider other well aligned high conductive phases.

► EC anisotropy of talc and serpentinites depends on the reorientation of minerals. ► EC of hydrous minerals is dependent on the hydrogen concentration and diffusivity. ► Proton conduction mechanism is dominant for hydrous minerals. ► Serpentinization cannot explain the high EC anomalies in the subduction zone.