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

• Highly saline fluids have profound effects on high-grade metamorphism.
• Mixing thermodynamics differ greatly from H2O–CO2.
• New experiments indicate alkali consumption by granitoid liquid yields HCl-rich fluids.
• Mineral solubilities depend strongly on salt concentration in the coexisting fluid.
• Brines are effective metasomatic agents for REE.

A number of observations point to the participation of brines in high-grade metamorphic processes. These include findings of alkali and alkaline-earth halides as daughter crystals in fluid inclusions, appreciable concentrations of Cl measured in amphiboles, biotite, scapolite and apatite, and direct observations on high-temperature halides present in the intergranular space in high-grade rocks. This paper reviews some thermodynamic, petrologic and geochemical effects of these brines. Thermodynamic mixing properties of concentrated water–salt fluids at high pressure (P) and temperature (T) differ greatly from those of water–non-polar gas mixtures: the former are characterized by a large negative deviation from ideal solutions, while the latter exhibit positive deviation from ideality. The contrasting behavior has three major petrologic implications. First, compared to mixtures of water and non-polar gases, brines more strongly increase the melting temperature of quartzofeldspathic rocks and more strongly decrease dehydration temperature of water-bearing minerals. This allows a wide P–T window in which subsolidus deep-crustal metasomatism may take place at relatively low H2O activity (aH2OaH2O) via migrating fluids. In addition, above 2–3 kbar, brine-saturated solidi for simple granite melting show positive dP/dT at constant H2O mole fraction (XH2OXH2O), favoring ascent of fluid-saturated liquids. Finally, a large miscibility gap exists in H2O–CO2–salt ternaries at lower crustal conditions, which may concentrate salts in a separate phase and help explain the common observation of CO2-rich inclusions in high-grade minerals. We discuss three geochemical consequences of high-grade brines. We report new experimental data on melting of a model granitoid liquid (69 wt% NaAlSi3O8, 31 wt% SiO2) at 2 kbar in the presence of aqueous NaCl solutions ranging in concentration from a salt mole fraction (XNaCl) of 0.1–0.3. The results show that Na preferentially partitions into the silicate liquid, enriching the coexisting fluid in HCl. This hydrolysis effect, known previously for granite melts equilibrated with dilute solutions, therefore also extends to more saline brines. Mineral solubilities depend strongly on salt concentration in the coexisting fluid. Below 5 kbar at 700 °C, quartz initially salts in with addition of NaCl to H2O, reaches a maximum, and then declines; however, it salts out at all XNaCl at higher P. At granulite-facies P–T conditions, the solubilities of other oxide and silicate minerals (corundum, wollastonite, grossular) salt in and then either reach a plateau or salt out slightly at high XNaCl. In contrast, the solubility of Ca–salt minerals increases exponentially with rising XNaCl. The solubility patterns reflect variations in complexing and H2O activity in the brine. Partitioning of REE between rock forming minerals and brines, and between felsic melts and brines, differs strongly from that between minerals, melts and water (±non-polar gas). Brines extract REE from the melts much more efficiently, and LREE are extracted more efficiently than HREE. This effect may contribute to the decreased La/Yb ratio that accompanies the overall decrease in bulk REE concentration in leucosomes relative to their parental rocks (paleosomes) that has been documented from amphibolite- and granulite-grade migmatite complexes.