OXIDATION-DEHYDROXYLATION AND ELECTRON-PHONON COUPLING IN AMPHIBOLES AT HIGH T: THE ORIGIN OF HIGH ELECTRICAL CONDUCTIVITY IN SUBDUCTING ROCKS

Oxidation-dehydroxylation of Fe-bearing amphiboles has long been associated with the behaviour and properties of subducting oceanic crust. However, the mechanistic details of the processes involved were known only in the most general way. Here, we present recent work on the oxidation-dehydroxylation and conductivity behaviour of riebeckite and synthetic potassic-ferro-richterite concerning the development of mobile charge-carriers as a function of temperature and amphibole composition.

The occurrence of resonance Raman scattering at high temperature revealed the presence of temperature‑activated small polarons arising from coupling between polar optical phonons and electron transitions within Fe²⁺O₆ octahedra. The FeO₆‑related polarons coexist with delocalised H⁺; thus at elevated temperatures, Fe‑bearing amphiboles are conductive and have two types of charge carriers: highly anisotropic mobile polarons and H⁺ ions. Reversible oxidation of Fe (under oxidizing conditions), and the consequent development of electrical conductivity were studied in-situ up to 823 K (550 °C) on a single crystal of riebeckite of almost ideal composition. X-ray absorption spectra at the Fe K-edge and electrical resistivity were measured simultaneously and the Fe³⁺/Fe_tot ratio was monitored via analysis of the pre-edge feature in the XANES spectra. The data show cyclic reversible oxidation of Fe²⁺ up to around 673 K (400 °C), and irreversible Fe²⁺ → Fe³⁺ oxidation starts at ~723 K (~450 °C) and is complete at ~798 K (~525 °C). The resistivity along the crystallographic c-axis is also cyclic and concordant with reversible Fe²⁺ ↔ Fe³⁺ oxidation, allowing us to conclude that electrical conduction is connected with electron hopping induced by thermal treatment. The measured activation energy (E_a = 0.45 eV) is in agreement with small-polaron conduction. Temperature- and time-dependent Raman scattering on synthetic potassic-ferro-richterite shows that the presence of A-site K reduces the temperature of polaron activation and slows polaron mobility along the ribbons of MO₆ octahedra. The expected depth of activation of charge carriers for warm and cold subduction zones fits the identified high-conductivity layers, providing the atomic-scale mechanism for the observed anomalous electrical conductivity.