Ionization of H2O, HCl, and NaCl in low-density crustal fluids: thermodynamic modeling

Thermodynamic modelling of H₂O, HCl and NaCl ionization at magmatic-hydrothermal conditions provides essential basis for understanding speciation, acidity changes, metal complexation and ore-deposit formation. We critically evaluate modelling of H₂O, HCl and NaCl ionization using pre-existing approaches in literature, that include: (i) electrostatic models [1,2]; (ii) models based on semi-empirical logarithmic correlations with water density [3]; (iii) models based on virial expansion [4]; (iv) approaches based on stepwise hydration of solute [5], and (v) various statistical-mechanics-based theories [6]. Simulations of H₂O, HCl and NaCl ionization from individual models are consistent up to 400 °C and pressures above 3 kbar. However, significant discrepancies emerge with increasing temperatures and decreasing pressures. Electrostatic and virial models at low pressures (below 300 bar) and at high fluid density poorly perform partly because loose physical significance. Hydration models and approaches based on mean spherical approximation inaccurately describe pressure-temperature dependence. Density models emerge as the most accurate in ionization predictions and therefore, as promising approach for constructing new equations of state for ionic species. We develop a new low-parametric density model to depict the thermodynamic properties of aqueous species in low-density hydrothermal fluids with a more rigorous theoretical framework that incorporates intrinsic properties of aqueous solute (entropy, enthalpy, heat capacity and hardcore volume) and accounts for solute-solvent interactions (via volume compression). Our new density model offers improved accuracy and performance in the temperature-pressure space requiring fewer equation parameters in comparison to existing models in literature. The new density model is applied for the prediction of H₂O, HCl and NaCl speciation along four fluid-flow paths representing distinct crustal settings, specifically: transcrustal metamorphic devolatilization, intrusion-related lateral, vertical and adiabatic flow. Simulations reveal that metamorphic fluids have ionization capability by four orders of magnitude greater than the upper-crustal magmatic fluids. This demonstrates the superior effectiveness of high-pressure fluids in the transport of ionic species and acidity generation. Fluids exsolved from upper-crustal magmatic sources during lateral or vertical flow exhibit mutually comparable behavior upon cooling and progressively ionize species and produce significant acidity from 400 °C.  By contrast, speciation occurring during adiabatic flow is mainly controlled by decreasing fluid density, and as the fluid cools and expands solute species remain completely associated. Overall, the simulation of the four thermal gradients highlights the major impacts that pressure and fluid density have on H₂O, HCl and NaCl ionization and the various efficiency for acidic alteration and mineralization of hydrothermal fluids along their specific pathways and hydrodynamic conditions.

 

References:

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[6] Lvov S N et al. (2018) J Molecul Liq 270: 62-73