Solubility of corundum in the system Al 2O 3–SiO 2–H 2O–NaCl at 800 °C and 10 kbar

Abstract

The solubility of corundum was measured at 800 °C and 10 kbar in NaCl–H 2O–SiO 2 fluids over NaCl mole fractions ( X NaCl) of 0 to 0.5 and SiO 2 concentrations of zero to quartz or albite + melt saturation, depending on X NaCl. Experiments were performed in a piston-cylinder apparatus. Dissolved Al 2O 3 and SiO 2 were determined by weight losses of corundum and quartz crystals, or by bulk compositions of bracketing experiments. Although kyanite and sillimanite are slightly more stable than corundum + quartz at these pressures ( P) and temperatures ( T), neither appeared in any of the experiments. Results indicate that the enhancement of corundum solubility by NaCl at this P and T is further promoted by the addition of SiO 2. At quartz saturation, significant enhancement occurs in initially pure H 2O and addition of NaCl yields yet higher Al 2O 3 concentrations. At 0.03 ≤ X NaCl ≤ 0.1, quartz saturation is replaced by albite + silicate melt. The anhydrous melt composition is nearly on the join NaAlSi 3O 8–SiO 2. Al 2O 3 molality rises rapidly with NaCl concentration to 0.038 at X NaCl = 0.1 and then increases more slowly to 0.052 at halite saturation. There is, in addition, a measurable reciprocal enhancement of Si solubility by virtue of dissolved Al at a fixed X NaCl. Quench pH in NaCl-bearing fluids is strongly acidic and Na/Cl < 1 in quench solutes, suggesting low pH at high P and T. These observations, combined with previous work indicating Si–Al and Na–Al complexing, lead to the hypothesis that the reciprocal solubility enhancement of Al and Si is due to formation of Na-aluminosilicate complexes. Mass balance consideration suggests that the bulk Si/Al ratio of the group of complexes ranges from 1 to 2 in the NaCl-free system, to > 3 at high X NaCl. Our data show that Al 2O 3 is a moderately soluble component in quartz + aluminosilicate-saturated rocks in the presence of intergranular salt solutions at deep-crustal metamorphic P– T conditions and commonly realized salinities. Al 2O 3 should not be regarded as a fixed reference component, because it may exhibit substantial mobility.