Influence of temperature, pressure, and oxygen fugacity on the electrical conductivity of dry eclogite, and geophysical implications

Abstract

The electrical conductivity of eclogite was measured at temperatures of 873–1173 K and pressures of 1.0–3.0 GPa within a frequency range of 0.1–106 Hz using a YJ‐3000t multianvil press and Solartron‐1260 impedance/gain‐phase analyzer. Three solid‐state oxygen buffers (Cu + CuO, Ni + NiO, and Mo + MoO2) were employed to control the oxygen fugacity. Experimental results indicate that the electrical conductivity of the samples tended to increase with increasing temperature, conforming to an Arrhenius relation. Under the control of a Cu + CuO oxygen buffer, the electrical conductivity of the eclogite decreased with a rise in pressure, and its corresponding activation volume and activation energy at atmospheric pressure were calculated as −2.51 ± 0.29 cm3/mole and 0.86 ± 0.12 eV, respectively. At 2.0 GPa, the electrical conductivity of the eclogite increased with increasing oxygen fugacity, and the preexponential factor increased while the activation enthalpy decreased. The observed positive exponential factor for the dependence of electrical conductivity on oxygen fugacity, as well as the negative activation volume, confirm that the hopping of small polarons is the dominant conduction mechanism in eclogite at high temperatures and pressures. Our results suggest that the electrical conductivity of dry eclogite under various redox conditions cannot explain the high anomalies in conductivity under stable midlower continental crust and under the Dabie‐Sulu ultrahigh‐pressure metamorphic belt of eastern China.

Effect of chemical composition on the electrical conductivity of gneiss at high temperatures and pressures

“…10). A similar transformation was also conducted for granulite by Fuji-ta et al (2004) and eclogite with different oxygen fugacity (Cu + CuO, Ni + NiO and Mo + MoO 2 ) by Dai et al (2016). Figure 10 makes clear that the high conductivity anomaly of 10 −1.5 -10 −0.5 S m −1 from the field MT results in the Dabie-Sulu UHPM belt occurs at 12-21 km compared with three dominant constituent rock conductivities of gneiss, granulite and eclogite in the region.…”

“…Therefore, we deduced that the conduction mechanism for gneiss samples may be related to ions. The activation enthalpy is one crucial piece of evidence for the conduction mechanism of minerals and rocks (Dai et al, 2016). The activation enthalpies for gneiss samples are 0.35-0.58 eV in the lower-temperature region and 0.77-0.87 eV in the higher-temperature region (Table 3).…”

“…We can compare gneiss with the electrical conductivity of eclogite, another representative metamorphic rock. Recently, Dai et al (2016) measured the electrical conductivity of dry eclogite and the negative activation volume for eclogite was −2.51 cm 3 mole −1 under 1.0-3.0 GPa and 873-1173 K. It was proposed that the main conduction mechanism for dry eclogite is intrinsic conduction (Dai et al, 2016). In addition, Fig.…”

“…3 and based on a surface heat flow of 75 mW m −2 in the Dabie-Sulu UHPM belt. The dashed blue lines represent the conductivity-depth profiles based on the conductivities of eclogite, and the dashed brown line represents the conductivity-depth profiles based on the conductivities of granulite (Fuji-ta et al, 2004;Dai et al, 2016). The green region represents the MT data derived from a high conductivity anomaly in the Dabie-Sulu UHPM belt (Xiao et al, 2007;He et al, 2009).…”

“…Metamorphic rocks (e.g., slate, schist, gneiss, granulite and eclogite) with different degrees of metamorphism play an important role because of their widespread distribution in regional metamorphic belts. Dai et al (2016) measured the electrical conductivity of dry eclogite at 873-1173 K, 1.0-3.0 GPa and different oxygen partial pressures (using Cu + CuO, Ni + NiO and Mo + MoO 2 solid oxygen buffers) and found that the hopping of small polaron is the dominant conduction mechanism for dry eclogite at high temperature and pressure. The electrical conductivity of natural eclogite is much lower than the high conductivity anomaly in the Dabie-Sulu ultrahighpressure metamorphic (UHPM) belt of eastern China.…”