Electrical Conductivity of Omphacite as a Function of Water Content and Implications for High Conductivity Anomalies in the Dabie‐Sulu UHPM Belts and Tibet

Abstract

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.

Influence of water on the physical properties of olivine, wadsleyite, and ringwoodite

“…2b for hydrogen diffusion in (Mg,Fe) 2 SiO 4 ringwoodite, the experimental data reported by Kudo et al (2006) are comparable with that calculated by the cB model (Zhang, 2012). Sun et al (2015) reported a hydrogen self-diffusion coefficient in Fe-bearing ringwood- (Hae et al, 2006); Z12 (Zhang, 2012); S18 (Sun et al, 2018); K06 (Kudo et al, 2006); K04 (Kubo et al, 2004); H09 (Holzapfel et al, 2009); S04 (Shimojuku et al, 2004); S09 (Shimojuku et al, 2009); S10 (Shimojuku et al, 2010); Z11 (Zhang et al, 2011); Z15a (Zhang and Shan, 2015a); Z15b (Zhang and Shan, 2015b); Z19b (B. H. Zhang et al, 2019b); Z21 (Zhang et al, 2021).…”

“…2a). In addition, the activation energy (∼ 113 kJ/mol) obtained by Zhang et al (2021) was much lower than that (229-289 kJ/mol) determined by Holzapfel et al (2009) and Zhang and Shan (2015a).…”

“…However, the activation energy for O diffusion in ringwoodite reported by Shimojuku et al (2009) is inconsistent with that determined by Zhang et al (2011). (Fei et al, 2013); F14 (Fei et al, 2014); F18 (Fei et al, 2018); H05 (Hier-Majumder et al, 2005); S19 (Sun et al, 2019); Z19b (B. H. Zhang et al, 2019b); Z21 (Zhang et al, 2021).…”

“…By combining all existing results, Holzapfel et al (2009) proposed a new expression to constrain the effects of pressure and composition on D Fe−Mg and found that the interdiffusivity of Fe-Mg in wadsleyite decreases with increasing pressure. Recently, Zhang et al (2021) measured Fe-Mg interdiffusion rates in polycrystalline wadsleyite aggregates as a function of water content (up to ∼ 0.44 wt. % H 2 O) at 16 GPa and 1373-1773 K by using the diffusion couple method.…”

“…These different characteristics of diffusion response to water may be related to the hydrogen incorporation mechanism in host minerals. Experimental observations (Purevjav et al, 2016;Zhang et al, 2021) and theoretical calculations (Wright and Catlow, 1996;Caracas and Panero, 2017) demonstrated that, for Fe-free hydrous wadsleyite, all hydrogen cations in the crystal should have been fully concentrated into a vacant M3 octahedral sites bonded with oxygen O1. In the case of Fe-bearing hydrous wadsleyite, reduction from ferric to ferrous iron probably drives the formation of Mg vacancies in wadsleyite, which in turn contributes to Fe-Mg interdiffusion.…”