Solubility of corundum + kyanite in H2O at 700 C and 10 kbar: evidence for Al-Si complexing at high pressure and temperature

The solubility of the assemblage corundum + kyanite in H2O was determined at 700 C and 10 kbar, using a piston-cylinder apparatus and rapid-quench/fluid-extraction techniques. Weighted mean concentrations of total Al and Si were 5.80 ± 0.03 mmol kg H2O and 0.308 ± 0.003 mol kg )1 H2O, respectively (1r errors). The Al concentration is nearly five times higher than that of corundum solubility in pure H2O. This difference is interpreted to arise from complexing between Si and Al to form HAlSiO4,aq species. Charged or more polymerized species are also possible, but their abundance cannot be constrained based on these experiments. Assumption of a single aqueous aluminosilicate complex permits calculation of the thermodynamic consequences of Al-Si interaction in high-pressure fluids, as well as phase diagrams for the system Al-Si-O-H. Formation of Al-Si complexes leads to a large increase in dissolved Al with increasing Si in solution, such that Al concentration in equilibrium with kyanite + quartz is predicted to be 7.1 mmolal, higher than with kyanite + corundum. The elevated concentration of Si in deep-crustal and mantle aqueous fluids suggests that Al must readily be dissolved and transported by Al-Si complexing in high-pressure metamorphic and metasomatic environments. The results provide a simple explanation for the common observation of kyanite + quartz segregations in eclogites and Barrovian metamorphic