Electrical properties of dry polycrystalline olivine mixed with various chromite contents: Implications for the high conductivity anomalies in subduction zones

Chromite, a crucial high-conductivity mineral phase of peridotite in ophiolite suites, has a significant effect on the electrical structure of subduction zones. The electrical conductivities of sintered polycrystalline olivine containing various volume percents of chromite (0, 4, 7, 10, 13, 16, 18, 21, 23, 100 vol.%) were measured using a complex impedance spectroscopic technique in the frequency range of 10⁻¹–10⁶ Hz under the conditions of 1.0–3.0 GPa and 873–1223 K. The relationship between the conductivities of the chromite-bearing olivine aggregates and temperatures conformed to the Arrhenius equation. The positive effect of pressure on the conductivities of the olivine–chromite systems was much weaker than that of temperature. The chromite content had an important effect on the conductivities of the olivine–chromite systems, and the bulk conductivities increased with increasing volume fraction of chromite to a certain extent. The inclusion of 16 vol.% chromites dramatically enhanced the bulk conductivity, implying that the percolation threshold of interconnectivity of chromite in the olivine–chromite systems is ~16 vol.%. The fitted activation enthalpies for pure polycrystalline olivine, polycrystalline olivine with isolated chromite, polycrystalline olivine with interconnected chromites, and pure polycrystalline chromite were 1.25, 0.78–0.87, 0.48–0.54, and 0.47 eV, respectively. Based on the chemical compositions and activation enthalpies, small polaron conduction was proposed to be the dominant conduction mechanism for polycrystalline olivine with various chromite contents. Furthermore, the conductivities of polycrystalline olivine with interconnected chromite (10^–1.5–10^0.5 S/m) provides a reasonable explanation for the high conductivity anomalies in subduction-related tectonic environments.