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 AlF_n³⁻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.

Introduction

Because of the ubiquity of aluminum-bearing minerals in most geological environments, knowledge of the thermodynamic properties of Al aqueous species to high temperatures and pressures is essential for understanding and modeling mass transfers and mineral equilibria in the earth’s crust. On the basis of textural relations (i.e., Carmichael, 1969) and on the low solubility of boehmite and corundum in pure water Ragnarsdottir and Walther 1985, Verdes et al 1992, Walther 1997b, aluminum has been long regarded as an inert element in geological processes. However, this is inconsistent with the common presence of sapphire or aluminum silicate minerals (i.e., andalusite, kyanite, sillimanite) in metamorphic veins (e.g., Moine et al., 1998) and the loss of aluminum rock-forming components during acid metasomatism (e.g., Zaraisky, 1994). Aluminum mobility during hydrothermal and metamorphic reactions implies its interaction with the ligands present in crustal fluids to form soluble complexes. Chemical compositions of crustal fluids reported in the literature (e.g., see the data reported by Yardley and Shmulovich, 1994) for modern well fluids and ancient fluids preserved in fluid inclusions, and by Stefánsson and Arnórsson (2000) for geothermal waters) indicate that besides the hydroxide complexes, the main species likely to be involved in aluminum transport are fluorine, sodium, and silica, assuming that Al, a “hard” metal, should prefer the hardest donor atoms. Sulfate and carbonate, and the softer chloride ion should not contribute significantly to aluminum transport, as confirmed by Arnórsson’s (1999) speciation calculations.

During the last 15 yr, numerous studies have been devoted to characterizing aluminum complexing with the hydroxide ion Kuyunko et al 1983, Couturier et al 1984, Ragnarsdottir and Walther 1985, Apps et al 1989, Palmer and Wesolowski 1992, Palmer and Wesolowski 1993, Wesolowski 1992, Bourcier et al 1993, Castet et al 1993, Wesolowski and Palmer 1994, Walther 1997b, Bénézeth et al 2001, Palmer et al 2001, chloride Korzhinsky 1987, Baumgartner and Eugster 1988, fluoride Couturier 1986, Sanjuan and Michard 1987, Zaraisky and Soboleva 1997, and sodium Pascal and Anderson 1989, Azaroual et al 1996, Diakonov et al 1996. Moreover, recent studies have addressed aluminum complexation with aqueous silica Pokrovski et al 1996, Pokrovski et al 1998, Salvi et al 1998 and Al-F complexation in alkaline solutions (Tagirov et al., 2000). This large and growing body of data makes possible, probably for the first time, prediction of aluminum speciation in hydrothermal and orthomagmatic fluids.

The purpose of this article is to build on the experimental data available for Al³⁺, Al hydroxide complexes, and Al complexes formed with Na⁺, F⁻, SO₄²⁻, and SiO₂(aq) to compute Al speciation in crustal fluids ranging from moderate-temperature thermal waters to high-temperature/pressure metamorphic fluids. We hope that this work will lead to a more accurate characterization of the aqueous species that control aluminum transport at the conditions encountered in the Earth’s crust.