Biotite Equilibria in High Silica Rhyolites: Controls on Volatile Fugacities and Abundance in Bishop and Toba Tuff Magmas

建议的研究是从控制黑云母组成和黑云母稳定性开始确定高硅流纹岩的相平衡。黑云母-碱长石-磁铁矿平衡可能是确定喷发前水逸性的有力工具，但由于对黑云母的铁和亚铁组成作为压力、温度和流体组成的函数的约束较差，因此其用途有限。将进行两组实验：(1)用碱长石和含钛磁铁矿退火黑云母，(2)在控制成分的混合流体（H2O, CO2， F）存在下，对Bishop和Toba凝灰岩样品进行热液实验。黑云母的铁/铁组成可以用微滴定法和微x射线光电子能谱法测定。用离子探针和电子探针测定O、H和F的含量。本研究将限制高硅流纹岩中黑云母的稳定性和组成，校准黑云母-磁铁矿-碱长石平衡，并将这些结果应用于确定Bishop和Toba岩浆的喷发前条件。

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.