Water in the mantle

The amount and site of hydrogen in the upper mantle is important for understanding the geochemical recycling of volatiles, and the evolution of the mantle, atmosphere, and oceans. It has been proposed that the entire water budget of the upper mantle may be accommodated at defect sites in nominally anhydrous minerals such as olivine and orthopyroxene. The presence of hydrogen at defects influences mantle properties ranging from partial melting, to rheology to electrical conductivity.

We investigate the mechanism of water incorporation in nominally anhydrous mantle olivine and orthopyroxene using a combination of experimental petrology, the petrology of natural samples; advanced analytical methods such as infra-red spectroscopy and ion probe and ab-initio calculations. We are now able to distinguish between different types of hydrogen defects in olivine and we have demonstrated that in spinel peridotites H is accommodated at point defects associated with the trace element Ti.

This result allows primary mantle water to be distinguished from that incorporated during exhumation or retrogression. Water must be present at the Ti defect site for partitioning, seismic wave speed, and deformation experiments on hydrous olivine to be relevant to the Earth's uppermost mantle. We have developed a new method of quantifying water in anisotropic minerals using unpolarised infra-red spectroscopy on un-oriented grains. This permits now to fully assess water incorporation in olivine in experiments as a function of pressure, temperature and coexisting phases.

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