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

Abstract. The electrical conductivity of gneiss samples with different chemical compositions (W_A = Na₂O + K₂O + CaO  =  7.12, 7.27 and 7.64 % weight percent) was measured using a complex impedance spectroscopic technique at 623–1073 K and 1.5 GPa and a frequency range of 10⁻¹ to 10⁶ Hz. Simultaneously, a pressure effect on the electrical conductivity was also determined for the W_A = 7.12 % gneiss. The results indicated that the gneiss conductivities markedly increase with total alkali and calcium ion content. The sample conductivity and temperature conform to an Arrhenius relationship within a certain temperature range. The influence of pressure on gneiss conductivity is weaker than temperature, although conductivity still increases with pressure. According to various ranges of activation enthalpy (0.35–0.52 and 0.76–0.87 eV) at 1.5 GPa, two main conduction mechanisms are suggested that dominate the electrical conductivity of gneiss: impurity conduction in the lower-temperature region and ionic conduction (charge carriers are K⁺, Na⁺ and Ca²⁺) in the higher-temperature region. The electrical conductivity of gneiss with various chemical compositions cannot be used to interpret the high conductivity anomalies in the Dabie–Sulu ultrahigh-pressure metamorphic belt. However, the conductivity–depth profiles for gneiss may provide an important constraint on the interpretation of field magnetotelluric conductivity results in the regional metamorphic belt.