Influence of Titanium on Water Incorporation, Rheology and Seismic Properties of Olivine

地球内部的水可能与海洋中的水一样多，但它以微量的硅酸盐矿物形式存在，这些矿物构成了地球的地幔。在上地幔中，橄榄石是主相，至少占其体积的60%。它控制着物理性质，如与对流和板块构造有关的粘度。变形实验表明，水对橄榄石的粘度影响很大，使其降低一个数量级或更多。橄榄石中含有多少水取决于晶体结构将其作为一种微量元素结合的能力，也取决于水的可用性。除俯冲板块外，橄榄石吸收水的能力大大超过地幔中的可用水，因此橄榄石远未达到含水饱和度。相比之下，已发表的变形实验只在干燥或饱和水的条件下进行。拟议的实验将在水不饱和的条件下进行，这些条件代表了地球上的那些条件。用红外光谱研究了氢在橄榄石晶格中的成键环境。水不饱和包体的光谱与水饱和实验的光谱比较显示出明显的差异。当将微量的钛添加到包裹在贵金属(如铂)中的合成橄榄石中时，捕虏体光谱可以重现。谱图证实了这些合成样品中氢的含量和成键环境与天然样品相匹配。探索性变形实验表明，这些样品比干燥样品弱，说明含水饱和度不是水弱化的必要条件。如果得到证实，这些发现意味着在钛存在的情况下，氢在橄榄石结构中相对稳定，即使是少量的水也会导致减弱。这不仅对地球的流变性有影响，而且对月球或火星等更干燥的行星体也有潜在的影响。

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.