The Earth's interior may contain as much water as in the oceans, but it occurs in trace amounts in silicate minerals which make up the Earth's mantle. In the upper mantle, olivine is the major phase, occupying at least 60% of its volume. It dominates the physical properties such as the viscosity involved in convection and plate tectonics. Deformation experiments show that water has a large effect on the viscosity of olivine, lowering it by an order of magnitude or more. How much water is contained in olivine depends on the capacity of the crystal structure to incorporate it as a trace element, but also on the availability of water. Except above subducting plates, the capacity of olivine to incorporate water substantially exceeds the water available in the mantle, so that the olivine is far from water saturation. By contrast, published deformation experiments have only been carried out either dry or under water-saturated conditions. The proposed experiments will be conducted under water-undersaturated conditions representative of those in the Earth.
The bonding environment of hydrogen in the crystal lattice of olivine can be investigated with infrared spectroscopy. Comparison of spectra from water-undersaturated xenoliths with those from water saturated experiments show significant differences. The xenolith spectra can be reproduced when trace amounts of titanium are added to synthetic olivine encapsulated in noble metals like platinum. The spectra confirm that the amount and bonding environment of hydrogen in these synthetic samples matches the natural samples. Exploratory deformation experiments show that these samples are weaker than dry samples, suggesting that water saturation is not necessary for water-weakening. If confirmed these findings imply that in the presence of titanium hydrogen is comparatively stable in the olivine structure, and that even small amounts of water will induce weakening. This has implications not only for the rheology of the Earth, but potentially also for drier planetary bodies such as the Moon or Mars.
