A Model for the Density of H2O-CO2-NaCl Fluids Based on High-Pressure Experimental Data to 800 K and 1500 bars

Fluids in the H₂O-CO₂-NaCl system are of particular geological importance, not only regarding the potential geological storage of CO₂ in deep saline aquifers, but more generally to the evolution of metamorphic, magmatic and volcanic systems [1]. The development of geochemical models describing fluid-rock interactions at high P-T metamorphic conditions or the chemical exchanges associated with magmatic degassing requires an accurate knowledge of the volumetric (ie., density, partial molar volume, compressibility) and thermodynamic properties of theses fluids at the adequate pressure and temperature of formation and circulation.

Up to now, experimental data describing the PVTx properties of H₂O-CO₂, H₂O-NaCl and H₂O-CO₂-NaCl are extremely scattered and mostly limited to T < 500 K and P < 1000 bars, to the exception of few synthetic fluid inclusion and diamond-anvil cell studies [2,3]. Consequently, the validity of thermodynamic models for CO₂ and NaCl-bearing fluids to metamorphic and magmatic-hydrothermal conditions remains highly questionable [4,5].

This limitation (or lack of validation) of thermodynamic models to T < 500 K and P < 1000 bars mostly arises from experimental difficulties in the study of high P-T fluids. Here we wish to present new in-situ experiments that were conducted in a dedicated hydrothermal autoclave [6] at the ESRF synchrotron (Grenoble, France) to determine the density of H₂O-NaCl, H₂O-CO₂ and H₂O-CO₂-NaCl fluids up to 800 K and 1500 bars. The experimental data are used to test the validity of available models for the binary H₂O-CO₂ and H₂O-NaCl systems [7,8] and extend the P-T range covered for the ternary H₂O-CO₂-NaCl to the high-temperature conditions that may prevail during metamorphic processes or late stage evolution of crustal magmatic systems.

References: [1] Lowenstern, 2001. Mineralium Deposita 36, 490-502. [2] Mantegazzi et al., 2013. Geochimica et Cosmochimica Acta 121, 263-290. [3] Schmidt et al., 1995. Geochimica et Cosmochimica Acta 59, 3953-3959. [4] Li et al., 2011. Energia Procedia 4, 3817-3824. [5] Mao et al., 2015. Applied Geochemistry 54, 54-64. [6] Louvel et al., 2017. Chemical Geology 466, 500-511.[7] Duan et al., 2008. Energy Fuels 22, 1666-1674. [8] Driesner, 2007. Geochimica et Cosmochimica Acta 71, 4902-4919.