Effect of dehydrogenation on the electrical conductivity of Fe-bearing amphibole: Implications for high conductivity anomalies in subduction zones and continental crust

Abstract

Magnetotelluric measurements reveal the presence of high conductivity anomalies (up to ∼1 S/m) in both the forearc and backarc regions of subduction zones as well as the continental middle–lower crust. Such anomalies are commonly interpreted as a consequence of aqueous fluid released from the dehydration of hydrous minerals. Amphibole is an important constituent of the continental mid-crust and a major hydrous phase in subduction zones, such that its dehydration at high temperature has been suggested to provide a significant source of aqueous fluid. We performed electrical conductivity measurements of a natural Fe-bearing amphibole at 623–1173 K and 0.5–2.0 GPa using a multi-anvil apparatus and an impedance spectroscopy. Our results show that pressure has a very weak effect on conductivity compared with temperature. An abrupt variation of the impedance semicircular arc followed by a remarkable increase of electrical conductivity is observed at temperature of 843±20 K. However, the enhancement in conductivity is not attributed to conductive aqueous fluid but rather to amphibole oxidation–dehydrogenation, as confirmed by infrared spectroscopy and optical microscopy observations. A slight decrease in activation enthalpy from ∼0.80 eV to ∼0.70 eV suggests that the conduction mechanism does not change before and after dehydrogenation, and small polaron conduction (electron holes hopping between Fe2+ and Fe3+) is considered to dominate the conductivity of amphibole over the entire temperature range. Although amphibole dehydrogenation at high temperature cannot serve as a principal source of aqueous fluid, the enhanced electrical conductivity of amphibole after dehydrogenation is sufficient to account for the high conductivity anomalies observed in slab–mantle wedge interfaces and the continental lowermost mid-crust, particularly in local regions with high heat flow.