Electron-phonon coupling in amphiboles contributes to the anomalous conductivity in subduction zones

Abstract Anomalous high-conductivity layers (HCL) are typical of subduction zones, the largest recycling systems of our planet. Understanding the underlying physics of rock high conductivity has paramount implications for several planetary-scale processes, including the global water cycling, earthquake activity, and arc volcanism. Here we provide a direct proof for the activation of anisotropic electron-phonon excitations (small polarons) and delocalized H+ in riebeckite, a Fe-rich sodic amphibole typical of blue-schist metamorphic facies. This phenomenon occurs above 500 K, regardless of the oxygen fugacity, whereas the expulsion of H+ from the crystal surface occurs above 650 K only in the presence of exO2. Notably, the expected depth of activation of charge carriers, for modelled warm and cold subduction zones, fits the identified HCL providing the atomic-scale evidence for the observed anomalous electrical conductivity. This work thus gives new insights into the link between atomic-scale phenomena and Earth-scale processes interacting at convergent plate margins.