Dielectric properties of aqueous fluids at depth

点击翻译按钮获取中文摘要

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.