Origin of the high conductivity anomalies in the mid-lower crust of the Tibetan Plateau: Dehydration melting of garnet amphibolites

Abstract

High-conductivity layers of 0.1–1 S/m distributed in the mid-lower crust of the Tibetan Plateau are traditionally explained as the fluid/melt related crustal flow. Dehydration of amphibole-bearing rocks may play an important role in explaining these anomalies. To survey the anomalies' origin, the electrical conductivities of amphibole-bearing samples are measured at 1.5 GPa and 600–1300 K. Our experiments show that dehydration melting of amphibole occurs at about 1100 K. Before dehydration melting of amphibolites, proton conduction together with small polaron conduction dominate the conduction process, whereas ionic conduction plays a more important role after the dehydration melting of amphibolites. The dehydration melting of polycrystalline amphibole yields a conductivity of up to 1 S/m under the lower-crust conditions beneath the Tibetan Plateau. Combining the calculated thermal gradient in the crust of Tibetan Plateau with the petrology proof and seismic data, dehydration melting of garnet amphibolites likely contributes to the high conductivity anomalies within the Tibetan crust. MW and cube model simulation indicates that the melt fraction is 3.8–36 vol% in the partial molten region.