Dielectric properties of aqueous fluids at depth

含水流体作为地壳地球化学变化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被认为是更深处物质转移的关键因素，从俯冲洋板块，通过地幔楔，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理解受到实验和理论研究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容纳未污染的这些流体，因为它们可能具有高度腐蚀性。与此同时，第一性原理分子动力学（FPMD）模拟是具有挑战性的，因为在描述水分子之间的氢键和缺乏实验数据的基准固有的困难。在我们的能力，模拟含水流体的性质的最重要的限制因素是我们缺乏水的介电性质的知识，封装在介电常数，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，并且用于确定由于其溶剂化作用而对溶解的水性物质的热力学性质的贡献。它是Helgeson-Kirkham-Flowers状态方程的主要输入，该状态方程支持用于研究地球深处流体-岩石相互作用期间流体成分、矿物溶解度以及微量元素的形态和络合的许多模型。然而，介电常数的测量仅限于P-T-X的有限范围（~0.5 GPa和~800 K），使得水性流体在此关键参数方面的大多数操作条件未被探索。在通过与其他参数（特别是密度）的经验相关性估计介电常数方面取得了进展，最近的FPMD研究产生的估计值远远超出了当前的实验P-T范围（~12 GPa和2000 K）。这些努力已经导致了大量的水的静电模型，但是这些模型沿着俯冲带地热显著偏离超过~15 km深度，导致设计用于描述超临界流体与地壳和地幔的岩石之间的相互作用的影响的模型的输出中不可避免的不确定性。我们打算通过在金刚石砧座单元中使用电阻抗谱法将水和稀释的H2O-NaCl混合物的介电常数的测量在P至10GPa时扩展20倍，在T至1500K时扩展2倍，定制电极直接印刷在砧座上。与此同时，我们将开发新的，最先进的FPMD协议，用于准确描述整个俯冲带中发现的P-T条件下的水分子相互作用，以我们新的介电常数数据集和现有的P和T对水密度影响的数据为基准。然后，我们将扩展这些模拟，包括H2O-NaCl混合物，包括自然系统中发现的所有盐度范围，从中我们可以提取介电常数，密度，压缩性，这些新的数据将为现有的含盐流体地球化学性质模型提供严格的检验，并为地幔和地壳中H_2O的溶解行为提供新的约束。下地壳流体流动这将使我们能够显着提高我们的理解的溶剂化的矿物物种，物种的其他挥发性成分，并最终在整个地球的地壳和地幔的流体-岩石-熔体相互作用的高T流体的组成。

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.