Constraints on oxygen fugacity within metal capsules

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

Constraints on oxygen fugacity within metal capsules

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Publisher
Springer Journals
Copyright
Copyright © 2017 by Springer-Verlag GmbH Germany, part of Springer Nature
Subject
Earth Sciences; Mineralogy; Crystallography and Scattering Methods; Geochemistry; Mineral Resources
ISSN
0342-1791
eISSN
1432-2021
D.O.I.
10.1007/s00269-017-0937-7
Publisher site
See Article on Publisher Site

Abstract

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

Journal

Physics and Chemistry of MineralsSpringer Journals

Published: Dec 28, 2017

References

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