Keshi Hui

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

Abstract The electrical conductivities of granites with different chemical compositions [XA = (Na2O + K2O + CaO)/SiO2 = 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 10-1-106 Hz. The conductivity of the granite sample with XA = 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 XA produce dramatic increases in the electrical conductivity. For the granite samples with XA = 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 XA = 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 XA at high temperatures, we propose that intrinsic conduction is the dominant conduction mechanism in all samples, and that K+, Na+, and Ca2+ in feldspar are the probable charge carriers controlling the conductivity. All conductivity data at high temperatures can be fitted to the general formula where σ0 is the pre-exponential factor; α, β, and γ are constants; ΔH0 is the activation enthalpy at very small values of XA; 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.

The Influence of Dehydration on the Electrical Conductivity of Trachyandesite at High Temperatures and High Pressures

The electrical conductivity of trachyteandesite was measured in situ under conditions of pressure range from 0.5-2.0 GPa and temperature range from 773-1,323 K using a YJ-3000t multi-anvil press and a Solartron-1260 Impedance/Gain-phase Analyzer. The experimental results indicate that the electrical conductivity of trachyteandesite increases with increasing temperature and decreases with a rise in pressure. The relationship between the electrical conductivity (σ) and temperature (T) conforms to the Arrhenius equation within a certain temperature range. When the temperature rises to 923 K, the electrical conductivity of trachyandesite abruptly increases. This result demonstrates that trachyandesite begins to dehydrate at ~923 K and produces magnetite with a high-conductivity mineral phase after dehydration. The intergrowth of interconnected magnetite is the cause for the ~2 orders of magnitude increase in the electrical conductivity after dehydration. The interconnected high-conductivity mineral phase of magnetite in the dehydration product of the trachyandesite sample can be used to reasonably explain the high-conductivity anomalies in the South-Central Chilean subduction zone beneath the Andes.