Aqueous fluids at elevated pressure and temperature

Abstract

The general major component composition of aqueous fluids at elevated pressure and temperature conditions can be represented by H 2 O, different non-polar gases like CO 2 and different dissolved metal halides like NaCl or CaCl 2 . At high pressure, the mutual solubility of H 2 O 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 H 2 O at elevated P-T conditions, and aqueous fluids in H 2 O-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 H 2 O is q H2O ¼ 0:6À1:0 g cm À3 , the ion product K w = 10 )7.5 to approximately 10 )12.5 , the dielectric constant e = 8-25, and the viscosity g = 0.0001-0.0002 Pa sec compared to q H2O ¼ 1:0 g cm À3 , K w = 10 )14 , e = 78 and g = 0.001 Pa sec at ambient conditions. Adding dissolved metal halides and non-polar gases to H 2 O 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.

Fluids in the Continental Crust

“…the critical temperature of H 2 O), or low greenschist grade conditions, metamorphic fluids are unlikely to be composed of pure H 2 O and CO 2 . Commonly, they contain significant amounts of salts that expand the region of immiscibility compared to H 2 O-CO 2 fluids (Bowers and Helgeson, 1983a,b;Skippen and Trommsdorf, 1986;Tromsdorff and Skippen, 1986;Schmidt and Bodnar, 2000;Heinrich, 2007;Liebscher, 2007Liebscher, , 2010Shmulovich et al, 1995). Figure 2.5 shows the migration of critical points in the H 2 O-CO 2 -NaCl system, compared to the critical points in the H 2 O-NaCl and H 2 O-CO 2 binary systems ( Schmidt and Bodnar, 2000).…”

Dielectric properties of water under extreme conditions and transport of carbonates in the deep Earth

“…Several models correlating experimental data suggested extrapolations of e 0 to ∼1 GPa and ∼1,300 K (e.g., refs. [13][14][15], which corresponds to only very shallow mantle conditions under the oceans; however, deeper mantle conditions relevant to plate tectonic processes could not be reached and different extrapolations showed poor agreement with each other (1). The current lack of knowledge of the dielectric constant of water under the P and T of the mantle hampers our ability to model water-rock interactions, to study the solubility of minerals, and hence our understanding of geochemical processes involving aqueous fluids below the Earth's crust.…”

“…water solvation properties | carbon cycle | ab initio simulations | supercritical water W ater, a major component of fluids in the Earth's mantle (1,2), is expected to play a substantial role in hydrothermal reactions occurring in the deep Earth at supercritical conditions (3,4). Pressure (P) and temperature (T) increase with increasing depth (5) and at ∼400 km, where seismic discontinuities define the bottom boundary of the upper mantle, the pressure can reach ∼13 GPa and the temperature can be as high as 1,700 K (6)(7)(8).…”

Brines at high pressure and temperature: Thermodynamic, petrologic and geochemical effects

“…The third consequence of H 2 O-CO 2 and H 2 O-NaCl activity relations is an extensive region of immiscibility in H 2 O-CO 2 -salt ternaries at lower crustal conditions (e.g., Heinrich, 2007;Liebscher, 2010;Manning et al, 2013). Most work has been conducted at low pressure (Gehrig et al, 1979;Aranovich et al, 2010;Anovitz et al, 2004), but extension of these results to lower crustal conditions (Frantz et al, 1992;Johnson, 1991;Kotelnikov and Kotelnikova, 1990;Graham, 1999, 2004;Joyce and Holloway, 1993;Gibert et al, 1998) yields a ternary miscibility gap between NaCl-H 2 O-rich brine and CO 2 -H 2 O-rich vapor.…”