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.
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.
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.
Publications
Fowler S
(2024)
Mineral-water reactions in Earth's mantle: Predictions from Born theory and ab initio molecular dynamics
in Geochimica et Cosmochimica Acta
Description | The primary objective of this project was to determine a fundamental property of water, its dielectric constant, at the extreme pressures and temperatures that are relevant to the deep Earth, up to approximately 700km depth. We aimed to do this in two ways: experimentally and using computer simulations. The experiments are still ongoing (see below) but the computer simulations are essentially complete. They indicate that the dielectric constant is higher than we originally thought and that existing models used to estimate this parameter at high pressure and temperature are not perfectly accurate. This is important because the dielectric constant determines how strong a solvent water is, i.e. how strongly it will react with the rocks that make up the deep Earth. We now know that water-rich fluids are present in the deep Earth, and so understanding this reactivity tells us what these fluids will do - how much rock will they dissolve, how efficiently will they strip metals from those rocks etc. This last implication is particularly important because that process is the first step on the pathway from the deep Earth to economically recoverable metal resources at the surface. In addition, we have also dscovered that water actually breaks down at high pressures and temperatures, i.e. rather than being made of H2O molecules, it increasingly splits into H and OH ions. This is also important for its reactivity in the deep Earth. As a side benefit, we have also determined the 'equation of state' for the density of water. Essentially this is an equation that allows you to determine the density of water at any given pressure and temperature and will tell you how buoyant water will be in the deep Earth, and how rapidly it will rise to the surface. |
Exploitation Route | Our new equations of state for the dielectric constant of water and its density will be used in geochemical models by academics to more accurately predict the solubility of different chemical compounds in aqueous fluids at the conditions of the deep Earth. |
Sectors | Environment |
Description | Diamond encapsulated thin film thermocouples |
Amount | £15,000 (GBP) |
Organisation | Engineering and Physical Sciences Research Council (EPSRC) |
Sector | Public |
Country | United Kingdom |
Start | 03/2024 |
End | 05/2024 |
Title | Electrochemical diamond anvil cell |
Description | As part of this project, we proposed to measure the dielectric constant of water at extreme pressures and temperatures. This has necessitated the development of a new diamond anvil cell high-pressure / high-temperature device housed within a specially disigned holder that enables the cell to be surrounded by inert gas (to prevent oxidation at high temperatures). The holder is also water cooled, to disspate heat from the resistive heaters surrounding the sample. The holder also contains carefully screened electrical connections for power to the heaters, and measurement cables from thermocouples mounted on the diamond anvils for temperature measurement. However, most important are the anvils themselves. Using a combination of UV photolithography, magnetron sputter coating and chemical vapour deposition we have developed techniques where we can 'print' complex interdigitated electrodes onto the surface of the anvils and electrically insulate them from the metallic gasket holding the sample using diamond precisely grown on their surfaces. In this way, the electrodes are only in contact with the fluid sample and diamond. In combination with carefully screened electrical connections to a frequency response analyser (also purchased using this grant) we will now be able to measure electical resistivity of fluids at extreme conditions and their complex dielectric properties. These measurements can be easily combined with x-ray absorption spectroscopy measurements at synchrotrons including diamond light source, as well as FTIR and Raman spectroscopy. |
Type Of Material | Improvements to research infrastructure |
Year Produced | 2024 |
Provided To Others? | No |
Impact | The tool is brand new, and so there are no impacts yet, bue we expect there to be soon. |
Title | Input parameters for ab initio molecular dynamics simulations of water at non-ambient pressure and temperature using the CP2K code |
Description | This deposit consists of a readme file, which describes the file 'simulationinput.in'. This is a simple text file that contains the information necessary to run any of the ab initio molecular dynamics computer simulations described in the paper that links to this deposit, using the CP2K software package. CP2K is open source. Paper in press: Mineral-water reactions in Earth's mantle: predictions from Born theory and ab initio molecular dynamics, Fowler, S. J. and Sherman, D. M. and Brodholt, J. P. and Sherman, D. M. Geochimica et Cosmochimica Acta. |
Type Of Material | Database/Collection of data |
Year Produced | 2024 |
Provided To Others? | Yes |
Impact | N/A this dataset relates to a paper that has not yet been accepted. |
URL | https://www2.bgs.ac.uk/nationalgeosciencedatacentre/citedData/catalogue/b435d6b4-3628-43cc-aa24-6175... |
Description | Brillouin spectroscopy with Professor Hauke Marquardt, University of Oxford |
Organisation | University of Oxford |
Country | United Kingdom |
Sector | Academic/University |
PI Contribution | Myself and PDRA Kirill Vlasov have added a Raman Spectroscopy system to the existing Brillouin Spectroscopy system at the University of Oxford. This allows pressure to be measured in diamond anvil cells during Brillouin measurements, which is necessary for useful experiments. |
Collaborator Contribution | Professor Marquardt has given us access to the Brillouin Spectroscopy system. The aim is to measure the densities of fluids as a function of pressure, relevant to the deep Earth but also ocean worlds within the solar system. In addition, we will use this system to determine the refractive index of pure water, which is another route to determining the dielectric constant of water, which is the main aim of the grant. |
Impact | None as yet. |
Start Year | 2023 |