9526527 Rutherford The proposed research is to determine the phase equilibria for high silica rhyolite beginning with the controls on biotite composition and biotite stability. The biotite-alkali feldspar-magnetite equilibrium is potentially a powerful tool for determining the pre-eruptive water fugacities, but has been of limited use because of the poor constraints on the ferric and ferrous iron composition of biotite as a function of pressure, temperature, and fluid composition. Two sets of experiments will be performed: (1) annealing biotite with alkali feldspar and titaniferous magnetite, and (2) hydrothermal experiments on samples from the Bishop and Toba Tuffs, all in the presence of mixed fluids (H2O, CO2 , F) of controlled composition. The ferrous/ferric composition of biotite will be determined by microtitration and micro x-ray photoelectron spectroscopy. The O, H and F contents will be determined by ion probe and electron microprobe. This research will constrain the stability and composition of biotite in high silica rhyolite, calibrate the biotite-magnetite-alkali feldspar equilibrium, and apply these results to determine the pre-eruptive conditions of the Bishop and Toba magmas.
