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

9526527 Rutherford建议的研究是从控制黑云母成分和黑云母稳定性开始，确定高硅流纹岩的相平衡。黑云母-碱性辉长岩-磁铁矿平衡可能是确定喷发前水逸度的有力工具，但由于黑云母的三价铁和二价铁组合物受压力、温度和流体组合物的影响而受到限制，因此使用有限。将进行两组实验：（1）用碱性长石和含钛磁铁矿对黑云母进行退火，（2）对来自Bishop凝灰岩和Toba凝灰岩的样品进行热液实验，所有实验都是在控制成分的混合流体（H2O、CO2、F）存在下进行的。黑云母的亚铁/三价铁组成将通过微量滴定和微量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、F含量。本研究将限制高硅流纹岩中黑云母的稳定性和成分，标定黑云母-磁铁矿-碱性长石平衡，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.