HR: 1340h
AN: V23A-1221    [Abstracts]
TI: Experimental Constraints on Al Solubility in Model Granitic Fluids at 700 °C and 1 GPa
AU: * Wohlers, A
EM: wohlers@ucla.edu
AF: Dept. of Earth & Space Sci., UCLA, 595 Charles Young Drive East, Los Angeles, CA 90095, United States
AU: Manning, C E
EM: manning@ess.ucla.edu
AF: Dept. of Earth & Space Sci., UCLA, 595 Charles Young Drive East, Los Angeles, CA 90095, United States
AB: High P and T fluids associated with metamorphism and magmatism play an important role in crustal evolution and mass transfer. To understand aqueous fluids associated with model granitic crust at high P and T, we investigated the solubilities of K-feldspar (Ksp), muscovite (ms) and corundum (co) in H₂O at 700°C and 10 kbar. Starting materials of natural microcline, single corundum crystals, reagent KSi₃O_6.5 (KS3) were equilibrated with H₂O in a piston-cylinder apparatus. Fluid compositions were determined by weight loss and phase-equilibrium bracketing methods. With increasing KS3 from 0.021 to 0.45 molal, Al increased from 0.012 to 0.170 molal and coexisting mineral assemblage changed from co, through ms, to Ksp. Our results locate two invariant points: I1, co + ms + fluid (0.082 molal Al, 1.32 molal KS3) and I2, Ksp + ms + fluid (0.16 molal Al, 2.69 molal KS3). Total solubility of co in the presence of dissolved KS3 is up to fifty times higher than in pure H₂O at the same P and T. Experiments at quartz saturation at the same P and T provide the pH buffering assemblages Ksp + ms + qtz (I1) and ky + ms + qtz (I2) and give nearly identical results, which implies that the presence or absence of quartz does not significantly effect Al solubility for the investigated mineral assemblages. Al enhancement due to a possible Al-K species was investigated by measuring co solubility in KOH solutions at 700°C and 10 kbar. Measured log_Al solubility increased from -2.55 to 0.43 molal with increasing log_KOH from -2.89 to 0.59 molal. Total Al solubility in the system Al-K-O-H assuming an Al-K species was calculated using thermodynamic properties of Pokrovskii and Helgeson (Chem. Geol. 137, 221, 1997) for Al species extrapolated to 10 kbar using the Guentelberg equation for activity coefficients. Calculated solubilities agree closely with the experimental results. Addition of silica species (SiO_{2, aq}, Si₂O₄, and HAlSiO₄) allowed prediction of Al solubilities of -1.87 molal log_Al (I1) and -1.82 molal log_Al (I2), which are substantially lower than measured. The underestimate of solubility using only ionic, monomeric and dimeric species points to the presence of more polymerized aqueous species in the fluid phase. Our experimental results show that Al solubility is much higher than previously appreciated in aqueous fluids equilibrated with granitic bulk compositions, and that substantial Al transfer may occur at deep-crustal metamorphic conditions.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1011 Thermodynamics (0766, 3611, 8411)
DE: 1020 Composition of the continental crust
DE: 1043 Fluid and melt inclusion geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting