Probing the water content of Earth’s mantle: hydrogen mobility under extreme conditions

Project background

How much water is in Earth’s mantle? Several ocean volumes of water could be stored in the form of structurally-incorporated hydrogen (H) in nominally anhydrous minerals. This H probably has a controlling influence on mantle properties and processes. In theory, electromagnetic induction data can be used to ‘map out’ the amount of H present in the mantle due to its influence on electrical conductivity. However, difficulties in measuring conductivity in wet (H-bearing) samples and the fact that traditional diffusion experiments do not provide thermodynamic data on H mobility, mean that the influence of H remains poorly constrained. Novel, state-of-the-art experiments will provide new insight into H mobility in Earth’s mantle. The data will be used to determine mantle water content and assess evidence for fabric development in the mantle in conjunction with existing geophysical data. Techniques, approaches and work to be undertaken during 3 years.

The main thrust of the project is to undertake a series of experiments using large-volume, high-pressure experimental facilities (piston-cylinder, multi-anvil) available in the SoG and CSEC. These will determine H mobility at high P/T in natural samples by novel isotope exchange (H-D) experiments followed by spectroscopic and ion microprobe analysis. This technique will provide first data on H mobility in mantle minerals directly under conditions of the deep Earth interior, including the effects of pressure, temperature, fO2, water content and crystallographic orientation. We hope they will resolve conflicting laboratory measurements of electrical conductivity in H-bearing minerals. The results can then be used in conjunction with existing geophysical data to determine H content of various parts of Earth’s mantle, and re-evaluate evidence for textural development (based on observed anisotropy in electrical conductivity) in parts of the mantle which in turn has implications for mantle dynamics.

Initial work will focus on determining H mobility in upper mantle phases such as olivine, progressing later to those appropriate for the mantle transition zone and lower mantle. There may be the opportunity to work at externally-funded large-scale (experimental, synchrotron) facilities.