Constraints on oxygen fugacity within metal capsules

Abstract

Experiments were conducted with olivine encapsulated or wrapped in five different metals (Pt, Ni, $$\hbox {Ni}_{70}\hbox {Fe}_{30}$$ , Fe, and Re) to determine the oxygen fugacity in the interior of large capsules used for deformation and seismic property experiments. Temperature ( $$1200\,^{\circ }\hbox {C}$$ ), 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 $$\hbox {fO}_2$$ 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 $$\sim$$ 100 $$\upmu \hbox {m}$$ , (2) $$\hbox {fO}_2$$ gradients at the sample–capsule interface up to 2 mm into the sample, and (3) constant interior $$\hbox {fO}_2$$ values with an ordering corresponding to the capsule material. The inferred diffusive processes are: Fe diffusion in olivine with a diffusivity $$\sim \,10^{-14}\,\hbox {m}^2\hbox {/s}$$ , diffusion possibly of oxygen along grain boundaries with a diffusivity $$\sim \,10^{-12}\,\hbox {m}^2\hbox {/s}$$ , and diffusion possibly involving pre-existing defects with a diffusivity $$\sim \,10^{-10}\,\hbox {m}^2\hbox {/s}$$ . The latter, fast adjustment to changing $$\hbox {fO}_2$$ 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 $$\hbox {fO}_2$$ requires atomic diffusion.