The fugacities of hydrogen and oxygen in silicate melts limit the water concentration of melts, and hence are a major control of viscosity, explosivity, crystallization temperature, and order of crystallization of minerals. Both H and O are key to the crystallization of hydrous minerals, and their abundance may drive magmas along calc-alkaline or tholeiitic differentiation paths. The goal of this project is to assess the Fe3+/Fe2+ and H contents of felsic melts and their crystallization products, and to relate those contents to the fo2 and fH2O of the starting melt. We plan to 1) measure the H concentrations in minerals and glasses from magmatic systems for which independent constraints on fH2O are available, using both synchrotron and conventional (globar) FTIR spectroscopy; and 2) measure the Fe3+ contents of naturally-produced melts, feldspars, and coexisting oxide and ferromagnesian phases (such as amphibole) for which independent constraints on fO2 are available, using micro-XANES spectroscopy. From these data, we can independently derive oxygen and hydrogen fugacity values from the measured Fe3+/SFe and compare these results to previously determined values. Samples from the Bishop Tuff (California), the Timber Mountain Tuff (Nevada), Mt. Erebus (Antarctica), the Purico complex (Chile), and the Katmai complex (Alaska) will be used in this study. To associate oxygen and hydrogen fugacity values with measured Fe3+/SFe, we will use coexisting hydrous phases, either hornblende or biotite, for which experimental data are available that provide a calibration between Fe3+/SFe and H2O deficiency as a function of fH2, T, and P. Because we will know H2O concentration from independent work on melt inclusions, and we will measure Fe3+/SFe, we will be able to calculate fH2. From the calculated fH2 and fH2O derived from water concentrations in melt inclusions, we will be able to calculate fO2. We also plan to compare our results to results with the oxygen barometers developed by Sugawara (2001) based on thermodynamic models to calculate oxygen fugacity in the crystallizing system.
