Electrical resistivity of Fe3O4 to 48 GPa: Compression-induced changes in electron hopping at mantle pressures

We have measured the electrical resistivity of Fe3O4 to pressures of 48 GPa at temperatures between 258 and 300 K in order to evaluate compression-induced changes in electron exchange between divalent and trivalent iron ions. At ambient pressures, inverse spinel-structured magnetite is well known for its electron hopping between divalent and trivalent iron octahedral sites, and our results thus provide constraints on the role of intervalence charge transfer in altering the electrical resistivity of iron-rich phases. The resistivity of Fe3O4 decreases by more than an order of magnitude between 0 and similar to 20 GPa, where it reaches a minimum, subsequently increasing by a factor of 2 between similar to 20 and 48 GPa. This finding implies that the electronic exchange between Fe2+ and Fe3+ ions is notably enhanced by the initial 7% of volumetric compression but is marginally impeded at higher compressions. We associate this discontinuity in slope with a phase transition from the inverse spinel structure to a monoclinic structure at similar to 20 GPa. Both previous Mossbauer work and the small change in magnitude of the resistivity indicate that electron hopping persists as the mechanism of charge transport through the transition. The resistivity at these pressures is within an order of magnitude of characteristic metallic values, but measurements of the temperature dependence of resistivity demonstrate that the high pressure phase remains semiconducting. However, the resistivity of Fe3O4 is 1-2 orders of magnitude less than that of Fe-0.94 at 48 GPa, illustrating the profound effect of intervalence charge transfer (particularly at high pressures) on the electrical properties of iron-rich oxides. By measuring electrical resistivity at pressures between 48 and 12 GPa, at temperatures between 258 and 300 K, we also constrain the pressure dependence of the activation enthalpy for conduction of the high-pressure phase of Fe3O4. We demonstrate that the activation volume of conduction is pressure dependent for the low-pressure (magnetite) phase. Such pressure dependent behavior implies that simulations of mantle resistivity that use a constant activation volume may underestimate the resistivity of the lower mantle. We also evaluate the shifts in the iron-wustite, wustite-magnetite, magnetite-hematite, and wustite-hematite oxygen fugacity buffers with depth in order to assess the oxygen fugacity conditions that could generate mixed valence compounds in the lower mantle.