The nature of NaCl-H2O deep fluids from ab initio molecular dynamics at 0.5-4.5 GPa, 20-800 °C, and 1-14 m NaCl
The nature of NaCl-H2O deep fluids from ab initio molecular dynamics at 0.5-4.5 GPa, 20-800 °C, and 1-14 m NaCl
复制标题
从头算分子动力学在 0.5-4.5 GPa、20-800 °C 和 1-14 m NaCl 条件下从头计算 NaCl-H2O 深层流体的性质
DOI：
10.1016/j.gca.2020.03.031
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发表时间：
2020
影响因子：
5
通讯作者：
Fowler S
中科院分区：
文献类型：
--
作者：
Fowler S
NaCl-H2O二元系是地壳深部和上地幔流体的第一近似。这些流体是理解地球物理性质、金属迁移和地球内矿石成因的基础。因此，最近的实验研究试图确定的状态方程的NaCl-H2O二元组合物的功能。然而，在极端条件下，实验表征并不简单，而且迄今为止对相关流体的分子性质了解甚少。水的结构、离子溶剂化和离子缔合的变化影响诸如离子强度和电导率的性质。这些性质反过来影响地球物理特征和金属溶解度在高压和高温（P，T）。我们进行了一系列的NPTab初始分子动力学模拟作为压力（0.5-4.5 GPa），温度（20-800 °C）和组成（1-14 m NaCl）的函数，以发展高P，T NaCl盐水的分子水平的理解。模拟结果与实验和理论密度的比较提供了测试当前理论水平的基础。模拟使我们能够确定离子溶剂化，离子缔合的性质，以及溶质对溶剂的影响在高P，T.这里使用的PBE功能是已知的过度结构化的水和高估的密度在环境条件下的水性流体。这是由于氢键和分散的处理不充分。然而，我们发现在升高的P，T的理论和实验密度之间的良好的协议。因此，我们已经将现有的状态方程外推到尚未通过实验测量的区域。从O-O对分布函数中，我们解释了在高P，T下氢键断裂的结果。在环境条件下，Na和Cl的存在对水的结构有很强的影响。但在高磷、高温条件下，即使14 mNaCl对水体结构也没有影响.然而，Na和Cl高度相关。我们建议，NaCl盐水在高P，T最好描述为含水熔体，而不是作为含水电解质。
The NaCl–H2O binary is a first approximation to fluids in the deep crust and upper mantle. Such fluids are fundamental to understanding geophysical properties, metal transport, and ore genesis within the Earth. Consequently, recent experimental studies have sought to determine the equation of state of the NaCl–H2O binary as a function of composition. However, experimental characterization is not straightforward at extreme conditions and there is so far little understanding of the molecular nature of associated fluids. Variations in the structure of water, ion solvation, and ion association affect properties such as ionic strength and electrical conductivity. These properties in turn influence geophysical signatures and metal solubilities at high pressure and temperature (P,T).We performed a series of NPTab initiomolecular dynamics simulations as a function of pressure (0.5–4.5 GPa), temperature (20–800 °C), and composition (1–14 m NaCl) to develop a molecular-level understanding of high P,T NaCl brines. Comparison of simulation results with experimental and theoretical densities provides a foundation for testing current levels of theory. The simulations have allowed us to determine the nature of ion solvation, ion association, and the effect of solutes on the solvent at high P,T.The PBE functional used here is known to over-structure water and overestimate the density of aqueous fluids at ambient conditions. This is due to an inadequate treatment of hydrogen bonding and dispersion. However, we find excellent agreement between theory and experimental densities at elevated P,T. Accordingly, we have extrapolated the existing equation of state to regions that have not been measured experimentally. From the O–O pair distribution functions, we interpret the improved agreement as resulting from the breakdown of hydrogen bonding at high P,T. At ambient conditions, the presence of Na and Cl has a strong effect on the structure of water. However, at high P,T, even 14 m NaCl has no effect on water structure. Nevertheless, Na and Cl are highly associated. We propose that NaCl brines at high P,T are best described as hydrous melts rather than as aqueous electrolytes.