Hydrogen diffusion in spinel grain boundaries and consequences for chemical homogenization in hydrous peridotite |
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Authors: | Sylvie Demouchy |
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Institution: | 1. Geosciences Montpellier UMR 5243, Université Montpellier 2 and CNRS, Place Eugène Bataillon, 34095, Montpellier Cedex 5, France
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Abstract: | Hydrogen can be stored in the structure of nominally anhydrous minerals as point defects, and these impurities substantially
modify many physical properties of Earth’s mantle minerals. However, mantle rocks are composed of mineral grains separated
by grain boundaries and interphase grains boundaries. Therefore, as a potential hydrogen reservoir, grain boundaries should
be given proper attention. Here, I report an experimental investigation into hydrogen diffusion through grain boundaries in
polycrystalline aggregates. Sintering and diffusion experiments were performed using a gas-medium high-pressure vessel at
under pressure of 300 MPa and over a temperature range of 900–1,250°C. The diffusion assembly consisted of a polycrystalline
cylinder of aluminous spinel + olivine crystals with a talc cylinder as the main hydrogen source. A Ni capsule was used to
buffer the oxygen fugacity at Ni–NiO. Experimental durations varied from 5 min to 5 h. The presence of hydrogen in the crystals
was measured by Fourier-transform infrared spectroscopy. The calculation of the diffusion coefficients was based on the estimation
of the characteristic distance. The absence or presence of hydrogen recorded by the ‘hydrogen sensor’ olivines embedded in
the aggregate allows the estimation of bounds on this characteristic distance. Results presented here suggest that hydrogen
effective diffusion coefficients are only one order of magnitude faster (~10−9 m2s−1 at 1,000°C) than in an olivine single crystal along the 100] axis. Resulting diffusion coefficients for hydrogen in grain
boundary are four orders of magnitude faster than in a single crystal, but this diffusivity is not fast enough to affect hydrogen
mobility in mantle rocks with grain sizes greater than ~1 mm. Thus, very limited chemical homogenization would occur using
grain boundaries diffusion in mantle hydrous peridotite for incompatible and volatile element, such as hydrogen. |
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