Essentially all anhydrous minerals, including feldspars, contain minor to trace amounts of hydrogen incorporated into their structures. It is crucial to understand the mechanisms for incorporation and retention of hydrogen in nominally anhydrous minerals if hydrous species in these minerals are to be used to understand geochemical and geological processes in nature. This work constrains the mechanisms for incorporation and retention of hydrogen in feldspars through two lines of research. The first project characterizes the rate and mechanism of diffusion of hydrogen in OH-bearing feldspars at magmatic temperatures (800-1000 degrees C) through modeling of experimental diffusion data obtained under dry conditions and at a fixed oxygen fugacity. The diffusion rate of hydrogen at magmatic temperatures determines the amount of OH preserved in a feldspar phenocryst during a typical eruptive event, and the extent to which zoning of OH in feldspar phenocrysts with complex magmatic histories should be preserved. Models of integrated loss and diffusion profiles across heated slabs of feldspar will determine if diffusion is dependent on the concentration of hydrous species. The second project is to determine hydrogen (OH and H2O) concentrations in plagioclase phenocrysts and their melt inclusions from the 1980-1982 eruption sequence of Mount St. Helens. The eruption temperature, oxygen fugacity, and bulk chemical composition were all fairly constant through this time period while the water content of successive eruptions decreased by several weight percent. Plagioclase feldspar is the most abundant phenocryst in these rocks. Assuming a preliminary determination of a diffusion coefficient of hydrogen in volcanic feldspar, the plagioclase from wet eruption should contain higher OH concentrations than the plagioclase from the dryer eruptions. The partition coefficient of hydrogen between the plagioclase and their melt inclusions can be evaluated by measuring the water content of the feldspar and its melt inclusions. Phenocrysts with longer crystallization histories should preserve OH zoning on characteristic length scales which may aid our understanding of eruptions induced by increased magmatic volatile content due to progressive crystallization in the magma reservoir. The results from this work should be applicable to the many high-temperature geologic systems in which water plays an important role.
