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3.4 a. Hydrogen diffusion and defect equilibration in olivine (S.J. Mackwell)

Experiments have been performed to incorporate water-derived species into single crystals of San Carlos olivine. These experiments were performed at temperatures of 800 to 1000°C, fluid pressures of 200 to 300 MPa, times from 0.5 to 8 hours, and with the oxygen activity in the fluid buffered by either the Ni/NiO or Fe/FeO oxygen buffers.

After the samples had been hydrothermally treated they were sectioned into thin slices. Series of infrared spectra were measured across each slice in order to determine the diffusion coefficient for the mobile hydrous species parallel to each of the crystallographic directions for each temperature and time.

At shorter times and lower temperatures the incorporation rate of the hydrous species into olivine is very fast, indicating that diffusion occurs by the motion of protons. Charge balance for the mobile protons is provided by a counterflux of electron holes, which exist at concentrations of up to several hundred ppm in olivine as the excess charge on ferric iron in octahedral cation sites. Diffusion of the protons is anisotropic, with diffusion parallel to [100] being faster than parallel to [001], which is faster than parallel to [010]. Under the conditions of these lower-temperature and shorter-duration experiments, little equilibration of the other point defect species in the olivine can occur, so that the internal activity of oxygen, for example, is out of equilibrium with the environment during the experiments.

At longer times and higher temperatures, partial equilibration of the olivine internal defect structure is expected to occur, based on previous creep and electrical conductivity studies that showed a gradual equilibration to new steady state behaviour after a change in the externally imposed oxygen fugacity. Such equilibration in a water-rich environment might be expected to include inward transport of electron holes and associated vacant octahedral sites, compensating for the loss of holes due to proton incorporation. Thus, one might anticipate higher concentrations of hydrous species near the sample edges, where there would be higher concentrations of possible sites for protons - the octahedral vacancies. After experiments for longer times and higher temperatures, higher concentrations of hydrous species were observed using infrared spectroscopy near the sample surface. Analysis of these concentration profiles yielded diffusivities that were several orders of magnitude slower than the known proton diffusion rates in olivine. Clearly, the hydrous species are "decorating" defect species that diffuse more slowly -- probably the octahedral vacancies. In this way diffusion rates for octahedral vacancies could be measured. While the diffusivities are anisotropic, their anisotropy is different from that for protons, with diffusion parallel to [001] being significantly faster than that parallel to [010] and [100], which are fairly similar. While these results are in excellent agreement with previous diffusion measurements for octahedral vacancies in olivine based on equilibration rates, these data are the first using actual diffusion profiles and the first to demonstrate the anisotropy of diffusion.

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