In situ simultaneous Fe K-edge XAS spectroscopy and resistivity measurements of riebeckite: Implications for anomalous electrical conductivity in subduction zones

Abstract

In this paper, we address two key features of the behaviour of Fe-rich amphibole at high temperatures: (1) the Fe²⁺ → Fe³⁺ + e⁻ exchange within the crystal bulk, and (2) the consequent rise in electrical conductivity. Cycling heating-cooling experiments were done in situ up to 542 °C (815 K) at beamline B11 of the Diamond Synchrotron Laboratory (UK). X-ray absorption spectra at the Fe K-edge and electrical resistivity were measured simultaneously on a single crystal of riebeckite with a composition very close to the ideal formula ^A□^BNa₂^C(Fe²⁺₃Fe³⁺₂)^TSi₈O₂₂^W(OH)₂. The Fe³⁺/Fe_tot ratio was monitored via analysis of the pre-edge feature in the XANES spectra. Our data show slight oscillations of the oxidation state of Fe with temperature cycling up to around 400 °C (673 K), followed by a substantial gradual increase in Fe²⁺ → Fe³⁺ oxidation that starts at ~450 °C (~723 K) and is completed at ~525 °C (~798 K). The conductivity (σ) measured along the crystallographic c-axis oscillates strongly with cycling temperature allowing us to conclude that it is intrinsically related to the electron hopping induced by thermal treatment. The activation-energy derived from the σ(T) trend is E_a = 74.4 ± 0.6 kJ/mol (0.77 ± 0.01 eV), in agreement with small-polaron conduction. This study provides direct and robust support of the conduction mechanisms in Fe-amphibole previously inferred from indirect methods. Given that riebeckite is a significant component in the glaucophanitic amphiboles common in blueschists associated with subducted oceanic crust, our data provide a link between atomic-scale processes and Earth-scale anomalous conductivity observed via geophysical measurements.