Dielectric properties of aqueous fluids at depth

水流体作为地壳地球化学变化的驱动因素的重要性早已为人所知，但这些超临界、高含盐量的流体越来越多地被认为是在更深的地方进行质量转移的关键因素，从俯冲的洋板，通过地幔楔子，再回到地表，在那里它们对形成具有重要经济意义的矿体和地热能源资源至关重要。然而，我们对地球主要溶剂的物理和化学性质的了解受到在实验和理论上研究它们时遇到的重大技术障碍的限制。这些限制包括难以在高压设备中包含无污染的这些流体，因为它们可能具有高度腐蚀性。同时，第一性原理分子动力学(FPMD)模拟具有挑战性，因为描述水分子之间的氢键存在固有的困难，并且缺乏用于基准的实验数据。我们建模水流体性质的最重要的限制因素是我们缺乏对水的介电性质的知识，这些介电性质被封装在介电常数中，这最终决定了水携带溶质的能力-包括经济上重要的战略金属。介电常数是压力(P)、温度(T)和组成(X)的函数，用来确定溶解的水物种由于溶剂化而对热力学性质的贡献。它是Helgeson-Kirkham-Flowers状态方程的主要输入，该方程是许多用于研究地球深处流体-岩石相互作用过程中流体组成、矿物溶解度和微量元素的形态和络合的模型的基础。然而，介电常数的测量被限制在P-T-X(~0.5 Gpa和~800K)的有限范围内，留下了关于这一关键参数的大多数水相流体运行条件的探索。通过与其他参数，特别是密度的经验关联来估计介电常数已经取得了进展，最近的一项FPMD研究给出了远远超出当前实验P-T范围(~12 Gpa和2000K)的估计。这些努力已经导致了大量的水的静电模型，但这些模型在俯冲带地热的深度上偏离了约15公里，导致用于描述超临界流体与地壳和地幔岩石之间相互作用的模型的输出不可避免地存在不确定性。在这项提议中，我们打算通过使用钻石顶锤单元中的阻抗频谱，将水和稀释的H2O-NaC l混合物的介电常数的测量范围扩大到P的20倍到10 GPA，T到1500 K的2倍，并将定制的电极直接打印到砧座上。同时，我们将开发新的、最先进的FPMD协议，以我们新的介电常数数据集和关于P和T对水密度影响的现有数据为基准，准确描述在整个俯冲区发现的P-T条件下水的分子相互作用。然后，我们将把这些模拟扩展到涵盖自然系统中发现的各种盐度的H2O-NaCl混合物，从中我们可以提取介电常数、密度、可压缩性、溶质形态和液体结构等。这些新数据将对现有的含盐地球流体地球化学性质的模型进行严格的测试，并对地幔和下地壳流体流动过程中H2O的溶剂行为提供新的约束。这将使我们能够显著提高我们对矿物物种的溶剂化、其他挥发性成分的形态以及在整个地壳和地幔的流体-岩石-熔体相互作用过程中最终高T流体的组成的理解。

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.