Determination of PVT properties and vapor-liquid phase separation in the H2O-NaCl system with in-situ experiments

Brines play a significant role in magmatic hydrothermal systems by controlling metal transport, phase equilibria and the type and degree of ore enrichment. ^{[1, 2, 3]} Accurate and precise constraints on the volumetric and compositional properties of the fluids along with the pressure-temperature (P-T) conditions of vapor–liquid (V–L) phase separation in the H₂O–NaCl system are critical to better understand the behaviour of magmatic-hydrothermal fluids in natural systems. ^[4] From previous work, it is clear that there are gaps in the investigated P-T conditions of the existing experimental studies that show various discrepancies especially at higher pressures and concentrations where the location of the V–L boundary is more complicated to observe. Therefore, semi-empirical thermodynamic models based on existing experimental datasets lack validation and need to be re-evaluated. ^{[5, 6]}

In this contribution, we present ongoing in situ experimental work aimed at precisely constraining the onset of V–L phase separation in the H₂O–NaCl system and determining the densities of coexisting phases over a broad range of pressure, temperature, and compositional conditions i.e. 100-800°C, 200-1500 bars and 0.2-4 molal NaCl. Experiments are conducted in-situ using high-pressure, high-temperature vessel ^[7] combined with techniques such as X-ray radiography and X-ray absorption to visualise the phase separation changes. In addition,we aim at developing a novel image-based density calculation method to extract a density map for each phase directly from the radiographic data based on the Beer-Lambert attenuation law. Once validated by classical transmission measurements, this approach would enable simultaneous determination of phase proportions and densities, avoiding relying on indirect model assumptions. Preliminary results indicate systematic differences in the P-T conditions of V–L separation compared to earlier experimental and modelling studies, highlighting potential uncertainties in commonly used equations of state.

This work is conducted within the framework of the ANR MAGBRINES project, which investigates the role of magmatic brines in mobilizing and concentrating economically valuable metals in magmatic systems of the Lesser Antilles. From a broader perspective, the new experimental dataset will provide improved constraints for development of new equations of state (EOS) and thermodynamic models for H₂O-NaCl, with implications for simulating magmatic degassing, hydrothermal circulation, and ore-forming processes in volcanic arcs. Future work will extend this methodology to more complex brine compositions relevant to natural magmatic–hydrothermal systems.

Keywords: Magmatic-hydrothermal fluids; In-situ experimentation; aqueous sodium chloride; high pressure; high temperature

References:

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[6] Driesner. T, GCA. 71, 20, 4902-19, 2007.

[7] D. Testemale, R. Argoud, O. Geaymond and J. Hazemann, Review of Scientific Instruments, 76, 4, 2005.