Abstract:
Solubilities of corundum (Al2O3) and wollastonite (CaSiO3) were measured in H2O-NaCl solutions at 800 °C and 10 kbar and NaCl concentrations up to halite saturation by weight-loss methods. Additional data on quartz solubility at a single NaCl concentration were obtained as a supplement to previous work. Single crystals of synthetic corundum, natural wollastonite or natural quartz were equilibrated with H2O and NaCl at pressure (P) and temperature (T) in a piston-cylinder apparatus with NaCl pressure medium and graphite heater sleeves. The three minerals show fundamentally different dissolution behavior. Corundum solubility undergoes large enhancement with NaCl concentration, rising rapidly from Al2O3 molality (mAl2 O3) of 0.0013(1) (1σ error) in pure H2O and then leveling off to a maximum of ~0.015 at halite saturation (XNaCl ≈ 0.58, where X is mole fraction). Solubility enhancement relative to that in pure H2O, XAl2 O3 / XAl2 O3°, passes through a maximum at XNaCl ≈ 0.15 and then declines towards halite saturation. Quenched fluids have neutral pH at 25 °C. Wollastonite has low solubility in pure H2O at this P and T(mCaSiO3 = 0.0167 (6)). It undergoes great enhancement, with a maximum solubility relative to that in H2O at XNaCl ≈ 0.33, and solubility >0.5 molal at halite saturation. Solute silica is 2.5 times higher than at quartz saturation in the system H2O-NaCl-SiO2, and quenched fluids are very basic (pH 11). Quartz shows monotonically decreasing solubility from mSiO2 = 1.248 in pure H2O to 0.202 at halite saturation. Quenched fluids are pH neutral. A simple ideal-mixing model for quartz-saturated solutions that requires as input only the solubility and speciation of silica in pure H2O reproduces the data and indicates that hydrogen bonding of molecular H2O to dissolved silica species is thermodynamically negligible. The maxima in XAl2 O3 / XAl2 O3° for corundum and wollastonite indicate that the solute products include hydrates and Na+ and/or Cl- species produced by molar ratios of reactant H2O to NaCl of 6:1 and 2:1, respectively. Our results imply that quite simple mechanisms may exist in the dissolution of common rock-forming minerals in saline fluids at high P and T and allow assessment of the interaction of simple, congruently soluble rock-forming minerals with brines associated with deep-crustal metamorphism. © 2006 Elsevier Inc. All rights reserved.
