Unusually high electrical conductivity of phlogopite: the possible role of fluorine and geophysical implications

Abstract

Phlogopite is an accessory mineral often found in mantle samples from various tectonic settings of continental regions. Considerable effort has been expended on the effect of phlogopite on some key chemical and petrological processes of the upper mantle, such as the recycling of K, F and H2O and the generation of K-rich fluids/melts; in contrast, less attention has been devoted to its physical properties. In this study, the orientation-related electrical conductivities of phlogopite single crystals have been experimentally determined at 1 GPa and 200–900 °C with an end-loaded piston cylinder press and a Solartron-1260 Impedance/Gain-Phase Analyzer in the frequency range of 106–0.1 Hz. The results demonstrate that phlogopite can have unusually high conductivity, >0.01 S/m above ~600 °C and ~1 S/m at ~900 °C, significant electrical anisotropy, by a factor of >6 above ~900 °C, and large activation enthalpies, ~134–204 kJ/mol along different directions. The main charge carriers are probably K+ and F−, and fluorine may play a critical role in electrical conduction. The regional enrichment of K- and F-rich phlogopite, above subduction zones or in normal shields for example, could result in remarkable anomalies of electrical conductivity. This provides a new mechanism for explaining some locally resolved electrical anomalies in the continental upper mantle.