SOLUBILITY OF ALUMINIUM IN SiO2-H2O FLUIDS AT 800-1000°C AND 5-15 KBAR

Low solubility of Al in pure H₂O suggests that Al is immobile in environments such as the lower crust, upper mantle and subduction zones. However, veins with Al-rich minerals found in the same settings imply significant Al mobility. SiO₂ is among the most abundant solutes in vein fluids, and this suggests that SiO₂-Al₂O₃ complexing may lead to elevated Al solubility. To investigate this hypothesis we studied (1) the solubility of corundum in fluids of varying initial SiO₂/H₂O ratios at 800°C, 10 and 15 kbar, (2) the effect of P on corundum solubility at fixed SiO₂/H₂O ratio, and (3) the effect of temperature on the solubility of corundum at 15 kbar and SiO₂ saturation. Synthetic corundum, natural high-purity quartz, and ultrapure H₂O were equilibrated using hydrothermal piston-cylinder methods. Initial SiO₂ concentrations were varied between zero and quartz saturation by adding small quartz crystals. Solubility was determined by the weight loss from the corundum crystals. The solubility of Al in pure H₂O was 0.0028 molal, consistent with previous work. We find that at 800°C, 10 kbar, the concentration of Al increases with increasing dissolved SiO₂, to a maximum of 0.0152 molal at quartz saturation. The increase in Al concentration with increasing Si requires Al-Si complexing, consistent with recent predictions of Al-Si oligomerization in high P-T fluids. At constant SiO₂ molality, Al solubility rises with increasing P. At 15 kbar and quartz saturation, Al molality is described by the equation m_Al = 0.0001P^2.1235. Also at 15 kbar, a vesiculated aluminosilicate melt forms at temperatures of 958±8°C and higher. These results suggest that Al-Si complexing is a simple mechanism for enhancing Al solubility in metamorphic settings.