Effect of temperature, pressure and iron content on the electrical conductivity of olivine and its high-pressure polymorphs

The electrical conductivity of olivine and its high-pressure polymorphs with various iron contents [X_Fe = Fe/(Fe + Mg) = 0.1, 0.2, 0.3, 0.5, 0.7 and 1.0] was measured over a wide range of pressure (P) and temperature (T) conditions covering the stability field of olivine, wadsleyite and ringwoodite in a Kawai-type multianvil apparatus. The pressure was determined using in situ X-ray diffraction of MgO as a pressure marker in SPring 8. Molybdenum electrodes were used so that oxygen fugacity is similar to that for the iron-wstite buffer. The transition from low-pressure phase to high-pressure phase led to an increase of conductivity. In the stability field of each phase, the electrical conductivity slightly increased with increasing pressure at a constant temperature, suggesting a negative activation volume. The conductivity increased with increasing total iron content for each phase. All electrical conductivity data fit the formula for electrical conductivity σ= σ₀ X_Feexp-[E₀-αX_Fe^1/3 + P(δV₀-βX_Fe)]/kT}, where ₀ is the pre-exponential term, E₀ and V₀ are the activation energy and the activation volume at very low total iron concentration, respectively, and k is the Boltzmann constant. The activation energy decreased with increasing total Fe content in olivine and ringwoodite. Dependence of the activation energy on the total Fe content suggests that the dominant mechanism of charge transport is Fe²⁺-Fe³⁺ hopping (small polaron). The activation volume for small polaron conduction in olivine and its high-pressure polymorphs tends to decrease with total Fe content. For olivine with low Fe content, the activation volume for small polaron conduction still is negative and very small. Assuming constant Fe content (X_Fe = 0.1) and oxygen buffer condition, the conductivity will increase with depth mainly due to the increase of the temperature along the mantle adiabat.