Constraints on oxygen fugacity within metal capsules

The oxygen fugacity in the interior of solid experimental charges is only infrequently assessed. In order to determine the oxygen fugacity in the interior of relatively large samples used for deformation and to measure seismic properties, experiments were conducted with olivine encapsulated or wrapped in five different metals (Pt, Ni, Ni70Fe30, Fe, and Re). Temperature (1200°C), pressure (300 MPa), and duration (24 h) were chosen to represent the most common conditions in these experiments. The oxygen fugacity was determined by analysing the Fe content of initially pure Pt particles that were mixed with the olivine powder prior to the experiments. Oxygen fugacities in the more oxidizing metal containers are substantially below their respective metal-oxide buffers, with the fO₂ of sol-gel olivine in Ni about 2.5 orders of magnitude below Ni-NiO. Analysis of olivine and metal blebs reveals three different length-, and hence diffusive time scales: (1) Fe loss to the capsule over ∼ 100 μm, (2) fO₂ gradients at the sample-capsule interface up to 2 mm into the sample, and (3) constant interior fO₂ values with an ordering corresponding to the capsule material. The inferred diffusive processes are: Fe diffusion in olivine with a diffusivity ∼ 10^-14 m²/s, diffusion possibly of oxygen along grain boundaries with a diffusivity ∼ 10^-12 m²/s, and diffusion possibly involving pre-existing defects with a diffusivity ∼ 10^-10 m²/s. The latter, fast adjustment to changing fO² may consist of a rearrangement of pre-existing defects, representing a metastable equilibrium, analogous to decoration of pre-existing defects by hydrogen. Full adjustment to the external fO² requires atomic diffusion. These experiments enabled the determination of a systematic dependence of seismic properties of olivine on oxygen fugacity. A potential mechanism for the sensitivity to oxygen fugacity is the concentration of Fe³⁺ in grain boundaries, affecting grain boundary viscosity and diffusivities.