Probing the water content of the Earth's mantle: an experimental study of hydrogen mobility under extreme conditions

Abstract

Previous research has established that the majority of nominally anhydrous minerals (NAMs) in Earth’s mantle can incorporate water in the form of structurally bound hydrogen and, correspondingly, the mantle is thought to contain a substantial volume of water. Water has been shown to play a key role in the geodynamics of the Earth’s interior and quantifying the amount, and distribution, of water in the mantle is an important step in understanding many deep-Earth processes. One of the parameters highly sensitive to the incorporation of water in the mantle is electrical conductivity, as hydrogen is highly mobile and acts as the dominant charge-carrying species. In theory, this relationship can be used in conjunction with geophysical techniques that measure mantle-scale electrical conductivity to ‘map-out’ the deep Earth’s water content – but accurate interpretation of such data requires full understanding of hydrogen mobility in NAMs under extreme conditions, which remains poorly constrained. The aim of this project is to contribute to the reconciliation of geophysical observations with laboratory measurements of electrical conductivity, by considering hydrogen-deuterium exchange in single crystals. In a novel experimental design, hydrogen in crystals synthesised under mantle conditions (such that the hydrogen defects present correspond to the conditions being studied) exchanges with deuterium from a liquid source under controlled (mantle) pressure and temperature conditions for a specified time period. This results in hydrogen-deuterium exchange profiles that can be characterised by SIMS and subsequently fitted to Fick’s law to calculate hydrogen diffusion coefficients – which in turn can be related to electrical conductivity through the Nernst-Einstein equation. Analysis of the experimental results underlines the complexity of the influence of hydrogen on electrical conductivity in NAMs, and emphasises the need for careful consideration when interpreting and applying the results of diffusion studies. Ultimately, the data obtained in this study provides a useful contribution to understanding hydrogen diffusion in mantle minerals, but the non-trivial nature of both the experimental and analytical aspects mean that the method cannot easily be applied to other mantle phases.