Effect of water on the electrical conductivity of lower crustal clinopyroxene

Abstract

[1] The electrical conductivity of lower crustal clinopyroxene was measured at 6-12 kbar, 250-1000°C, and Ni-NiO buffer conditions. The dependence of electrical conductivity on water content was studied using both natural and preannealed samples separated from a fresh xenolith granulite, with water contents from 0 to 375 ppm. An end-loaded piston cylinder apparatus and a Solarton-1260 Impedance/Gain Phase Analyzer were used in the study over a frequency range of 0.01-10 6 Hz to obtain the complex impedance spectra. The results show that the influence of pressure is very weak relative to temperature and water content and that two distinct mechanisms with different activation enthalpies dominate electrical conduction under dry and wet conditions. For the dry sample, the activation enthalpy is ∼102 kJ/mol and the main charge carriers are small polarons, i.e., hopping of electron holes between ferrous and ferric irons. For wet samples, the electrical conductivity is significantly enhanced with an activation enthalpy of ∼70 kJ/mol and the charge carriers are likely to be protons. Under hydrous conditions, the activation enthalpies are nearly independent of water content and the conductivity is a function of water content with an exponent of ∼1. As a major constituent of granulites with >60% modal volume in some regions, clinopyroxene containing minor amounts of water may contribute significantly to the high electrical conductivity in the lower crust, especially under stable continental regions.

The effect of chemical composition and oxygen fugacity on the electrical conductivity of dry and hydrous garnet at high temperatures and pressures

“…The bulk electrical conductivity of eclogite shows a linear decrease with increasing volume percentage of garnet, and this relation is strengthened at higher temperatures. Considering the result for clinopyroxene reported by Yang et al (2011a), we found that the electrical conductivity of garnet is lower than that of clinopyroxene at a given temperature, pressure, and oxygen fugacity. In conclusion, the electrical conductivity of eclogite is affected mainly by garnet content.…”

“…In fact, many previous studies have examined the electrical conductivity of clinopyroxene (Huebner and Voigt 1988;Alekseev and Galanov 1990;Wang et al 1999;Xu and Shankland 1999;Yang et al 2011a). Here, we use the electrical conductivity data on clinopyroxene reported by Yang et al (2011a), mainly because their experimental temperatures and pressures are similar to those of the current work, and because they adopted a measurement technique of alternating current impedance spectroscopy, which is consistent with our measurements of the electrical conductivity of garnet. Anisotropy may influence measurements of the electrical conductivity of clinopyroxene; however, the results obtained for diopside by Dai et al (2005) indicate that this effect is feeble and can be ignored.…”

“…The clinopyroxene data are fromYang et al (2011a). The dashed line shows the electrical conductivity of eclogite during cooling at room pressure, as reported byLaštovičková (1975) Fig.…”

Grain size effect on the electrical conductivity of clinopyroxene

“…The uncertainty of electrical conductivity, including those from fitting the complex spectra and from sample dimensions, is \20% (often \10%). The effect of Fe diffusion loss from sample to Pt electrodes is not significant due to the similar electrical conductivity between different heating/cooling cycles and the low temperature measurement (Yang et al 2011b). The assembly background electrical conductivity, assessed by replacing the sample with a piece of dense ceramic Al 2 O 3 in Yang et al (2011b), is usually 2-4 orders of magnitude lower than that of the samples except at *1,000°C where the background conductance is relatively higher.…”

Influence of pressure and temperature on the electrical conductivity of dolomite

“…When the calcium-dominated carbonate rocks, such as marine sediments, in the subducted slabs are dragged into the deep mantle, the electrical conductivity of the aragonitedominated rock bodies is higher than that of the surrounding rocks in the upper mantle. It is known that the electrical conductivity of wet rock, which contains a hydroxyl ion component in its minerals and/or a fluid phase, is higher than that of dry mineral (e.g., Wang et al 2006;Yoshino et al 2009;Yang et al 2011). The calcium carbonates could contribute to the higher electrical conductivity in the same way as the water effect.…”