Investigating H (hydrogen) self-diffusion in olivine and upper mantle electrical conductivity
Abstract
Considerable amounts of H2O could be present deep within Earth's interior, where H (hydrogen) atoms can enter natural minerals as defect in their nominally anhydrous structures. Up to several wt.% H2O have been observed in natural minerals that were originally formed in the deep mantle and brought to the surface through volcanic eruptions. The range of observed H2O contents in mantle minerals collected at Earth's surfaces would drastically influence the physical and chemical properties of mantle material. As a consequence, hydrogen has likely played a critical role in the evolution of the Earth, its atmosphere and the oceans. H2O contained in upwelling plumes and downwelling slabs complete a potentially deep water cycle between Earth's mantle and oceans. Such a cycle would traverse through the upper mantle, where H diffusion through the predominant mineral olivine would control its transport and distribution. Chemical diffusion (diffusion due to a chemical gradient) in olivine has been widely investigated over the past few decades, but only limited data have been reported on self-diffusion (the intrinsic transport of H in the mineral structure). This gap in information needs to be investigated more extensively to fully understand how H2O transports throughout the mantle. We will discuss results of H self-diffusion in San Carlos olivine single crystal that were obtained in experiments performed at high-pressure and high-temperature using a piston cylinder apparatus, reproducing conditions encountered in the deep upper mantle. H self-diffusion coefficients were investigated by tracking deuterium diffusion profiles in oriented, single crystals of olivine that were saturated by hydrogen. Diffusion coefficients along the 3 principal orientation of olivine are highly anisotropic, with an over 1 order of magnitude difference between the `fast' [100], and the `slower' [010] and [001] orientations. Diffusion coefficients are used to calculate a substantial contribution of hydrogen to the overall electrical conductivity of olivine. We compare these results to magnetotelluric profiles for various regions and mantle settings, and discuss the implications for the presence of H2O in the Earth's interior.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMMR31A2657N
- Keywords:
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- 1030 Geochemical cycles;
- GEOCHEMISTRYDE: 3924 High-pressure behavior;
- MINERAL PHYSICSDE: 5422 Ices;
- PLANETARY SCIENCES: SOLID SURFACE PLANETSDE: 5470 Surface materials and properties;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS