Electrical Conductivity of Omphacite as a Function of Water Content and Implications for High Conductivity Anomalies in the Dabie‐Sulu UHPM Belts and Tibet
2019
Zhang, Baohua | Zhao, Chengcheng | Ge, Jianhua | Yoshino, T. (Takashi)
Magnetotelluric surveys revealed high‐conductivity layers in the lower crust beneath Tibet and in the shallow part of the upper mantle relevant to continental collision extending to the Dabie‐Sulu ultrahigh‐pressure metamorphic (UHPM) belts of eastern China, which have been interpreted by the presence of aqueous fluids/partial melts or hydrous phases. However, these explanations are not consistent with their petrological features and seismic properties. Hydrogen‐bearing omphacite could be a probable candidate to explain such high‐conductivity anomalies due to its high water‐partitioning coefficient versus coexisting garnet in eclogite. In this study, we investigated electrical conductivities of Fe‐free and Fe‐bearing omphacite as a function of water content (0.005–0.122 wt.%) at 3 GPa and 500–1300 K. Our results show that water significantly enhances the electrical conductivity of omphacite, while iron facilitates conductivity by accelerating hydrogen diffusivity and lowering its activation enthalpy. Assuming a heat flow of 70 mW/m², the high electrical anomalies observed beneath the Dabie‐Sulu UHPM belts and the Tibetan Plateau can be reasonably explained by omphacite containing 0.07 wt.% water since water content higher than 0.07 wt.% in omphacite was frequently reported in naturally collected eclogite.
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