Fig. Two sets of laboratory-based conductivity–depth profiles as a function of different oxygen partial pressures (using Cu + CuO, Ni + NiO, and Mo + MoO2 buffers), compared with the magnetotelluric field results for granulite and the anomalous high conductivity zones (HCZ) under the stable mid–lower continental crust and Dabie–Sulu UHPM belts, respectively. (Image by IGCAS)
The field magnetotelluric surveys have demonstrated that the anomalous high electrical conductivity zone (conductivities ≥ 10–1 S/m) in the world-famous Dabie–Sulu ultra-high pressure metamorphic belts of eastern China exists at depths of 12–21 km. As a representative high-temperature and high-pressure metamorphic rock, eclogite forms as a result of high-pressure metamorphism of gabbro or basalt at depths of 50–300 km during subduction and the consequential continental collision of blocks. It would be usual for eclogite to exist at a depth of ~100 km in the deep interior of the Earth, and the typical eclogite would be composed of approximately equal volume fractions of garnet and omphacite. Therefore, there have been many studies on the electrical conductivity of natural eclogite. However, all these earlier studies of the electrical conductivity of eclogite were performed at a defined pressure, and the effects of pressure and oxygen fugacity on the electrical conductivity of eclogite remain unclear.
Professor DAI Lidong’s group at Institute of Geochemistry, Chinese Academy of Sciences (IGCAS) measured the electrical conductivity of eclogite by virtue of electrochemical AC impedance spectroscopy at temperatures of 873–1173 K, pressures of 1.0–3.0 GPa, and controlled the oxygen fugacity by three solid buffers (Cu + CuO, Ni + NiO, and Mo + MoO2).
The obtained results from researchers confirm that the experimental conditions including temperature and pressure have a relatively large influence on the electrical conductivity of eclogite. The values of activation enthalpy, activation energy, the pre-exponential factor and activation volume have been successfully obtained.
On the basis of obtained parameters and pre-exponential factor of electrical conductivity along with oxygen fugacity, the researchers can reasonably extrapolate the conduction mechanism of small polaron.
Furthermore, by comparing the field magnetotelluric surveys results, the researchers conclude that electrical conductivity of dry eclogite under various redox conditions cannot be used to interpret the high conductivity anomaly in these typical regions.
Moreover, the conductivity-depth profiles constructed by the researchers for eclogites under various redox conditions may provide important constraints on the interpretation of field magnetotelluric conductivity results for a wide range of oxidation-reduction boundaries at various depths in the interior of the Earth.
This research was financially supported by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB 18010401), Key Research Projects of the Frontier Science of the Chinese Academy of Sciences (QYZDB-SSW-DQC009), “135” Program of the Institute of Geochemistry of CAS, Hundred Talents Program of CAS, Youth Innovation Promotion Association of CAS，and NSF of China (41474078, 41304068 and 41174079).
The study titled “Influence of temperature, pressure, and oxygen fugacity on the electrical conductivity of dry eclogite, and geophysical implications.” has been published in Geochemistry, Geophysics, Geosystems.
Institute of Geochemistry, Chinese Academy of Sciences
(By Prof. DAI Lidong’s group)