Aluminum speciation in crustal fluids revisited

Abstract

Aluminum speciation in crustal fluids is assessed by means of standard thermodynamic properties at 25°C, 1 bar, and revised Helgeson-Kirkham-Flowers (HKF) (Tanger J. C. IV and Helgeson H. C., “Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: Revised equations of state for the standard partial molal properties of ions and electrolytes,” Am. J. Sci. 288, 19–98, 1988) equations of state parameters for aqueous species in the system Al-O-H-Na-Si-Cl-F-SO4 derived from recent experimental data with the help of isocoulombic reactions and correlations among parameters in the HKF model. In acidic to neutral hydrothermal solutions and for fluorine concentrations in excess of 1 ppm, the fluoride complexes AlFn3−n dominate Al speciation at temperature (T) < 100°C, whereas the hydroxide fluoride species Al(OH)2F(aq)0 and AlOHF20(aq) are dominant up to ∼400°C. In high-temperature (T > 300°C) hydrothermal and metamorphic fluids, aluminum mobility is considerably enhanced by formation of NaAl(OH)3F(aq)0 and NaAl(OH)2F20(aq) ion paired mixed species. NaAl(OH)2F20(aq) controls Al transport in granite-derived fluids and during greisenization. At alkaline pH, Al(OH)4−, Al(OH)3H3SiO4−, and the NaAl(OH)40(aq) ion-pair are the dominant Al species. Thermodynamic calculations show that as a result of strong interactions of Al(aq) with NaOH, NaF, HF, and SiO2(aq) present in crustal fluids, the concentrations of aluminum in equilibrium with Al-bearing minerals can be several orders of magnitude higher than those calculated assuming that only Al hydroxyde complexes are formed. Interactions with these components are likely to be responsible for aluminum mobility during hydrothermal and metamorphic reactions.