Thermally-activated electron hopping in Fe-rich amphiboles: Implications for the high-conductivity anomalies in subduction zones

Anomalous high-conductivity layers are typical of subduction zones, the largest recycling systems of the Earth. Understanding the mineral physics underlying the high conductivity of rocks has paramount implications for several planetary-scale processes, including global water cycling, earthquake activity, and arc magmatism.Here, by using in situ polarized Raman spectroscopy, we provide a direct proof for the development of anisotropic electron-phonon excitations (polarons) and delocalized H⁺ in riebeckite, a Fe-bearing sodic amphibole typical of blue-schist metamorphic facies. The activation of polarons starts at 500 K (227 °C) and is complete at 650 K (377 °C) under both reducing and oxidizing conditions. At higher temperatures external oxygen triggers the expulsion of H⁺ and e⁻ from the crystal bulk. The temperature range observed for the development of charge carriers is in excellent agreement with the conductivity trends measured for riebeckite in previous studies, and nicely fits the temperatures for the development of high-conductivity layers in warm and cold subduction zones.The study directly demonstrates the activation of polarons at temperatures characteristic of convergent plate margins provides the atomic-scale picture whose macroscopic-scale expression is the anomalous conductivity measured in subduction zones.