Electromagnetic and seismic results from various subduction zones reveal the presence of anomalies of electrical conductivity and seismic wave attenuation in the mantle wedge and fore-arc regions, characterized by electrical conductivity up to 1 S/m. For the young and hot subduction zones with high thermal gradient, partial melting in the mantle wedge is extensively considered to be the cause of anomalously high conductivity.

However, based on the petrological studies, epidote/zoisite reaches 30-35 vol % in abundance occurring in epidote-blueschist facies in the subduction slab, and it with huge stability field could contribute to fluid flux to the mantle wedge at depths of 100-120 km. Therefore, epidote is very likely to make critical contribution to the high-conductivity anomalies in deep mantle wedge. Nevertheless, no previous study has been devoted to the electrical conductivity of epidote under the conditions prevailing in subduction zones and its possible contribution to the anomalously high conductivity regions in the deep mantle wedge.

Dr. HU Haiying measured the electrical property of epidote at conditions of 0.5-1.5 GPa and 573-1273 K on the AC impedance spectroscopy experiment platform established by Prof. DAI Lidong’s group at Institute of Geochemistry, Chinese Academy of Sciences (IGCAS).

The results indicate that the pressure dependence on the electrical conductivity of epidote is relatively weak compared to temperature, and the remarkable dehydration reaction of epidote happens around 1073 K through the observation of the abrupt variation of electrical conductivity. The discontinuous variation of electrical conductivity in high temperature regimes can be ascribed to the presence of aqueous fluid in sample and the simultaneous escape of fluids from sample cell. The existence of free fluid was verified by the optical microscope and scanning electron microscope images on the recovered sample after measurement.

Moreover, taking into account the petrological and geothermal structures of subduction zones, the aqueous fluid produced by epidote dehydration could be responsible for the anomalously high conductivity in deep mantle wedges observed at depths of 70-120 km, particularly in hot subduction zones setting such as central Cascadia and central New Zealand. The study offers another new insight into the cause of high-conductivity anomalies in the subduction zone.

This research was financially supported by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB 18010401), Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (QYZDB-SSW-DQC009), “135” Program of the Institute of Geochemistry of CAS, Hundred Talents Program of CAS, NSF of China (41474078, 41304068 and 41174079) and the Special Fund of the West Light Foundation of CAS.

This work has been published in the top journal of Nature Index as follows,
Haiying Hu, Lidong Dai*, Heping Li, Keshi Hui, Wenqing Sun. Influence of dehydration on the electrical conductivity of epidote and implications for high-conductivity anomalies in subduction zones. Journal of Geophysical Research: Solid Earth, 2017, 122, 2751-2762, doi: 10.1002/2016JB013767.