The NaCl-H2O geological fluid system is pervasive in and on the Earth, playing a crucial role in fluid-rock interactions, metamorphic and magmatic processes, and ore deposit formation. Over the past fifty years, extensive research has been conducted on this system, resulting in more than 40 sets of experimental data and the development of over ten models. However, the majority of these studies have been limited to pressures below 5 to 6 kbar, thereby restricting geochemical investigations in the lower crust and upper mantle. To address this limitation, we first conducted molecular dynamics (MD) simulations of the pressure-volume-temperaturecomposition (PVTx) properties of NaCl-H2O solutions. Utilizing a non-rigid, flexible RWK2 potential, we explored an extensive P-T range, from approximately 1 to 30 kbar and from 573 K to 1573 K. Based on the simulated PVTx data and supplemented by a limited number of experimental data points, we developed an equation of state (EOS) for the NaCl-H2O system. This EOS covers a wide range of conditions, extending from 1 kbar to 30 kbar and from 573 K to 1573 K. It also spans the compositional range from pure water to pure NaCl melts. The accuracy of this EOS is comparable to the most reliable existing models for pressures from 1 to 5 kbar, and it closely aligns with experimental data from 5 kbar up to 45 kbar. From the EOS, we have derived several important thermodynamic models that offer potential applications in geochemical and geophysical studies. These include a fugacity model for calculating H2O activities, which is derived from the EOS of the NaCl-H2O system and should have broad applications in fluid-rock interactions and mineral solubility studies. Additionally, we developed an electrical conductivity model for investigating crustal and upper mantle fluids. This model is simpler, more precise, and applicable over a wider P-T range compared to existing models. To demonstrate the practical utility of these models, we provide several illustrative examples of their application in geochemical and geophysical contexts, highlighting their versatility and reliability in addressing complex geological phenomena. A computer program implementing the EOS is available on our new website: https://efs.idsse.ac.cn/module1/ binary/h2o-nacl/EOS.html.
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