Electrical conductivity of rock samples subjected to high temperatures and pressures

• Main Author: Glover, Paul W. J.
• Published: University of East Anglia 1989
• Subjects: 551.11; Crustal electrical properties
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.236438

The field determinations of crustal electrical conductivity/depth profiles show anomalously high conductivities in the lower crust. This has yet to be explained by a consistent theory and interpretation of field results is difficult due to the lack of laboratory conductivity measurements of saturated rocks at lower crustal temperatures. A cell was designed to measure the electrical conductivity of saturated rocks up to the lower crustal conditions of; confining pressures of 1 GPa, pore-fluid pressures of 1 GPa and temperatures of 900°C. This complex exercise required the use of a metal sleeve and the use of guard-ring techniques to remove leakage currents induced by the sleeve. The development of the cell involved several years work and is a breakthrough in measurement techniques as it has enabled the measurement of saturated rock conductivities at lower crustal temperatures and high pressures for the first time. The conductivity of 14 samples of acidic and metabasic rocks was measured at a variety of confining pressures (<0.2 GPa), porefluid pressures (<0.2 GPa), temperatures (<900°C) and saturation fluids. The pressure variations showed that the basic rocks had a conductivity too high to be explained by conduction through saturating electrolyte alone. The temperature variation showed a dramatic difference between acidic and basic rocks. The acidic rocks showed large decreases in conductivity above 350°C after initial rapid increases in conductivity. The basic rocks showed no such reduction in conductivity indicating that a conduction mechanism in addition to pore-fluid conduction was present. Results indicate that saturated -rocks at high temperatures have 7/ conductivities sufficient to explain the high conductivity layers in the upper lower crust whereas acidic rocks do not. The results also demonstrate that graphite may be an important additional conduction mechanism possibly accounting for the moderately high conductivities in the lower lower crust even in the absence of electrolyte conduction