Influence of temperature, pressure, and chemical composition on the electrical conductivity of granite

Abstract

The electrical conductivities of granites with different chemical compositions [ X A = (Na 2 O + K 2 O + CaO)/SiO 2 = 0.10, 0.13, 0.14, and 0.16 in weight percent] were measured at 623–1173 K and 0.5 GPa in a multi-anvil high-pressure apparatus using a Solartron-1260 Impedance/Gain Phase analyzer within a frequency range of 10 −1 –10 6 Hz. The conductivity of the granite sample with X A = 0.13 was also measured at 0.5–1.5 GPa. The results indicate that pressure has a very weak influence on the electrical conductivity in the stability field of granite, whereas increases in temperature and the value of X A produce dramatic increases in the electrical conductivity. For the granite samples with X A = 0.16 and 0.13, the activation enthalpies are 1.0 eV above 773 K and 0.5 eV below 773 K, suggesting that impurity conduction is the dominant conduction mechanism in the lower-temperature region. For the granites with X A = 0.14 and 0.10, the activation enthalpy is 1.0 eV over the whole temperature range, suggesting that only one conduction mechanism dominates the conductivity. Based on the value of activation enthalpy (~1.0 eV) and the dependence of electrical conductivity and activation enthalpy on X A at high temperatures, we propose that intrinsic conduction is the dominant conduction mechanism in all samples, and that K + , Na + , and Ca 2+ in feldspar are the probable charge carriers controlling the conductivity. All conductivity data at high temperatures can be fitted to the general formula σ = σ 0 X A α exp ( - Δ H 0 + β X A γ k T ) where σ 0 is the pre-exponential factor; α, β, and γ are constants; Δ H 0 is the activation enthalpy at very small values of X A ; k is the Boltzmann constant; and T is the temperature. The present results suggest that the granite with various chemical compositions is unable to account for the high conductivity anomalies under stable mid- to lower-crust and southern Tibet.