The effect of anisotropic electrical conductivity of amphiboles on geophysical anomalies observed in subduction zones

Electrical-conductivity anomalies in subduction zones are believed to be strongly connected with global water cycling, volcanism and seismicity. However, the causal atomic-scale processes related to conductivity of rock-forming minerals in subducting rocks are virtually unknown. Here, in situ simultaneous high-temperature Raman spectroscopy and resistivity measurements on riebeckite as a model Fe-rich amphibole in subduction zones show that (1) electronic small polarons, with high mobility along the c-axis of the amphibole structure, activate above 500 K; (2) H⁺ starts diffusing within the crystal above 650 K, although electron transport via polaron hopping is still the dominant mechanism of charge transfer; (3) the anisotropy in the conductivity is enhanced with increasing temperature, emphasizing the dominant role of e^- over H⁺ in causing the high conductivity (above 0.01 S/m) of Fe-rich amphiboles. We show that conductivity data obtained via magnetotelluric measurements are best modelled by considering the effect of stress-driven alignment of amphiboles during plate motion. Our results thus link atomic- and Earth-scale conductivity processes, significantly improving our understanding of subduction processes.