This project will continue an experimental program of determining the equilibrium solubilities of Al-silicate minerals at crustal conditions. Aluminum speciation in H2O-HCl and H2O-NaCl-HCl solutions will be investigated building on recent experiments measuring corundum solubilities in pure H2O, H2O-NaCl and H2O-CaCl2 solutions. Corundum and andalusite plus quartz, at elevated temperatures (400o to 650o C) and pressures (up to 3 kbar) in acid solutions and aqueous salt fluids (0.05, 0.1, 0.5, 1.0 molal HCl and NaCl) will be measured and interpreted. The experiments supported by this award will involve extracting fluid samples while they are at high pressure and temperature with a uniquely designed hydrothermal apparatus and then determining the concentration of all volatile and non-volatile constituents of the fluid through a battery of modern analytical techniques including a direct current plasma emission spectrophotometer.
An important part of the theoretical analysis of the measurements is that activity coefficients of neutral species, which are common in these solutions, must be considered when determining the equilibrium. Hopefully a better model of Al speciation as a function of pH and ionic strength at supercritical pressure and temperature can be constructed in the Al2O-SiO2-H2O-NaCl-HCl system and the extent of Na+Al and possible Na+Al+Si complexes in solution determined. The proposed experiments should put limits on the possible solution compositions from which aluminum can be transported and aluminum bearing phases precipitate during metamorphism of the earth's crust.
The project will undertake the training of 1 to 2 graduate students giving them the ability to run the instruments in a modern wet-chemical and hydrothermal laboratory. The PI will use concepts developed in this project in a graduate level course and include them in an undergraduate/graduate textbook he is developing.
