Dielectric properties of aqueous fluids at depth

Lead Research Organisation: University of Bristol
Department Name: Earth Sciences

Abstract

The importance of aqueous fluids as a driver of geochemical change in the Earth's crust has long been known, but these supercritical, highly saline fluids are increasingly recognized as key agents of mass transfer at greater depths, from subducting oceanic slabs, via the mantle wedge and back to the surface, where they are critical in forming economically important ore bodies and geothermal energy resources. However, our understanding of the physical and chemical properties of Earth's primary solvent is limited by significant technological barriers encountered when studying them both experimentally and theoretically. These limitations include the difficulty of containing, uncontaminated, these fluids in high pressure apparatus, given that they can be highly corrosive. At the same time, first principles molecular dynamics (FPMD) simulations are challenging because of the difficulties inherent in describing the hydrogen bonding between water molecules and the lack of experimental data for benchmarking.

The most significant limiting factor in our ability to model the properties of aqueous fluids is our lack of knowledge of the dielectric properties of water, encapsulated in the dielectric constant, which ultimately determines the ability of water to carry solutes - including economically important strategic metals. The dielectric constant is a function of pressure (P), temperature (T) and composition (X) and is used to determine the contribution to the thermodynamic properties of dissolved aqueous species due to their solvation. It is a primary input into the Helgeson-Kirkham-Flowers Equation of State that underpins many of the models used to study fluid composition, mineral solubility and the speciation and complexation of trace elements during fluid-rock interactions at depth in the Earth. However, measurements of the dielectric constant are restricted to a limited range of P-T-X (~0.5 GPa and ~800 K), leaving most conditions at which aqueous fluids operate unexplored with respect to this key parameter. Advances have been made in estimating the dielectric constant via empirical correlations with other parameters, notably density, and a recent FPMD study produced estimates far beyond the current experimental P-T range (~12 GPa and 2000 K). These efforts have led to a profusion of electrostatic models for water, but these models deviate significantly beyond ~15 km depth along a subduction zone geotherm leading to inevitable uncertainties in the outputs of models designed to describe the effects of interactions between supercritical fluids and the rocks of the crust and mantle.

In this proposal, we intend to extend measurements of the dielectric constant of water and dilute H2O-NaCl mixtures by a factor of 20 in P to 10 GPa and a factor of 2 in T to 1500 K by using electrical impedance spectroscopy in the diamond anvil cell, with custom electrodes printed directly onto the anvils. At the same time, we will develop new, state-of-the-art FPMD protocols for the accurate description of the molecular interactions of water at the P-T conditions found throughout subduction zones, benchmarked against our new dielectric constant dataset and existing data on the effect of P and T on the density of water. We will then extend these simulations to include H2O-NaCl mixtures encompassing the full range of salinities found in natural systems, from which we can extract the dielectric constant, density, compressibility, solute speciation and liquid structure etc. These new data will provide a rigorous test of existing models of the geochemical properties of saline geofluids and provide new constraints on the solvent behaviour of H2O during mantle and lower crustal fluid fluxing. This will allow us to significantly improve our understanding of the solvation of mineral species, the speciation of other volatile components and ultimately the composition of high-T fluids during fluid-rock-melt interactions throughout the Earth's crust and mantle.

Planned Impact

The primary beneficiaries of this research will be academics working across a broad range of Earth science related fields, including experimental and computational petrology, reactive transport modelling and those studying the broader geochemical and geodynamic evolution of the planet, from the process of subduction to the deep water cycle. The experimental and computational techniques that this research will enable will be useful to those studying the electrical properties of materials more generally, including physicists and materials scientists. This builds on PI-Lord's history of using his capabilities and facilities in high pressure experimentation, partly funded by NERC, to collaborate with materials scientists and physicists, both at Bristol and at other UK and international institutions, to study the effect of pressure and temperature on a broad range of materials with economic and industrial importance, including thermal barrier coatings used in jet turbines and the graphite material used in the UK's fleet of nuclear reactors[1-3]. In addition, current interdisciplinary work, yet to be published, with CoI-Friedemann, on the synthesis and characterisation of high critical temperature hydride superconductors ultimately led to the realisation that in combination, we had the necessary capabilities to tackle to objectives of the current proposal. If funded, this project will expand those capabilities to include the electrical properties of materials at non-ambient conditions which will enhance opportunities for cross-disciplinary studies in future. For example, we intend to use our new experimental capabilities to study the process of molecular dissociation and proton conduction in solid water ice and liquid water, which is both a fundamental question in condensed matter physics as well as being a key process in areas as disparate as neurobiology, electrolytic batteries and hydrogen based technology, but is not fully understood[4].

The advancements we intend to make to our ability to simulate the behaviour of water using first principles molecular dynamics will also be of use to anyone studying the molecular properties of water or other hydrogen bonded fluids over a broad range of P-T conditions, including ambient conditions. This would be of benefit those working on the properties of H2O-CO2 mixtures that are critical to the success of carbon sequestration efforts and those working on aqueous fluids at shallow depths that are a key step in the precipitation of economically important and strategic metal rich minerals and ore bodies. There is therefore a likelihood that this work, through its technical advances, will benefit the extractive industries in their drive to find new mineral resources critical to the decarbonation of our economy. The School of Earth Sciences at the University of Bristol is an ideal place to develop links between our research and potential industrial partners and communicate our findings to them given the industry funded and focussed research already underway here, including a field and experimental collaboration with BHP Billiton to track magmatic fluids from the slab to the site of ore mineral precipitation, and the NERC funded FAMOS grant (From Arc Magmas to OreS).

[1] Liu, D., Lord, O. T., Stevens, O., Flewitt, P. F. J. (2012) The role of beam dispersion in Raman and photo-stimulated luminescence piezo-spectroscopy of yttria-stabilized zirconia in multi-layered coatings. Acta Mat. 61, 12-21.
[2] Liu, D., Lord, O. T., Flewitt, P. E. J. (2012) Calibration of Raman spectroscopy in the stress measurement of air-plasma-sprayed yttria-stabilized zirconia. J. App. Spectro. 66(10), 1204-1209.
[3] Liu, D., Mingard, K., Lord, O. T., & Flewitt, P. (2017). On the damage and fracture of nuclear graphite at multiple length-scales. J Nuclear Mat., 493, 246-254.
[4] Cassone G. et al. 2014 J. Phys. Chem. B. 118, 4419-4424.

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