Aqueous fluids at elevated pressure and temperature

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AbstractThe general major component composition of aqueous fluids at elevated pressure and temperature conditions can be represented by H2O, different non‐polar gases like CO2 and different dissolved metal halides like NaCl or CaCl2. At high pressure, the mutual solubility of H2O and silicate melts increases and also silicates may form essential components of aqueous fluids. Given the huge range of P–T–x regimes in crust and mantle, aqueous fluids at elevated pressure and temperature are highly variable in composition and exhibit specific physicochemical properties. This paper reviews principal phase relations in one‐ and two‐component fluid systems, phase relations and properties of binary and ternary fluid systems, properties of pure H2O at elevated P–T conditions, and aqueous fluids in H2O–silicate systems at high pressure and temperature. At metamorphic conditions, even the physicochemical properties of pure water substantially differ from those at ambient conditions. Under typical mid‐ to lower‐crustal metamorphic conditions, the density of pure H2O is , the ion product Kw = 10−7.5 to approximately 10−12.5, the dielectric constant ε = 8–25, and the viscosity η = 0.0001–0.0002 Pa sec compared to , Kw = 10−14, ε = 78 and η = 0.001 Pa sec at ambient conditions. Adding dissolved metal halides and non‐polar gases to H2O significantly enlarges the pressure–temperature range, where different aqueous fluids may co‐exist and leads to potential two‐phase fluid conditions under must mid‐ to lower‐crustal P–T conditions. As a result of the increased mutual solubility between aqueous fluids and silicate melts at high pressure, the differences between fluid and melt vanishes and the distinction between fluid and melt becomes obsolete. Both are completely miscible at pressures above the respective critical curve giving rise to so‐called supercritical fluids. These supercritical fluids combine comparably low viscosity with high solute contents and are very effective metasomatising agents within the mantle wedge above subduction zones.