The nature of NaCl–H2O deep fluids from ab initio molecular dynamics at 0.5–4.5 GPa, 20–800ºC, and 1–14 m NaCl

Sarah Jane Fowler*, David M Sherman

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

Abstract

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 NPT ab initio molecular 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.
Original languageEnglish
Pages (from-to)243-264
Number of pages23
JournalGeochimica et Cosmochimica Acta
Volume277
Early online date4 Apr 2020
DOIs
Publication statusE-pub ahead of print - 4 Apr 2020

Keywords

  • Molecular dynamics
  • ab initio
  • equation of state
  • saline fluid
  • aqueous

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