Oxygen and Hydrogen Fugacity of the Purico Ignimbrite Magmas, Northern Chile

University Of Massachusetts Amherst, Amherst MA

Investigators

Abstract

EAR-0001081 Seaman Felsic magmas are the sources of the keystone components of continental crust, and of the most explosive volcanic eruptions that commonly occur on Earth. The major factors in the evolution and eruptive behavior of felsic magmas are their water concentrations and their degree of oxidation. In this study, synchrotron spectroscopy will be used to explore the possibility that plagioclase crystals preserve information about the oxidation state and the water concentration of the magmas from which the crystals grew. Both Fe 3+/G Fe and H2 concentrations in the nominally anhydrous plagioclase will be measured and their use as proxies, for, respectively, oxygen fugacity and water concentration of the magmas that produced the crystals, will be explored. Synchrotron microXANES (X-ray absorption near-edge structure) spectroscopy of oxidation state of Fe in plagioclase crystals is based on measuring small energy differences in cation/oxygen bonds between ferrous and ferric ions that bond to oxygen. Synchrotron microFTIR (Fourier transform infrared) spectroscopy, like conventional FTIR spectroscopy, provides a correlation between the wavelength of light absorbed and the nature of chemical bonds in the mineral. Synchrotron microFTIR analysis of H2 in plagioclase will utilize a very bright source, eliminating the need to remove and polish oriented single crystals, and allowing for the analysis of crystals in their spatial context. These techniques will be tested on the Lower Purico ignimbrite (~1 Ma) in the Andean Altiplano Puna Volcanic Complex, for which independent measurements of maximum magmatic water concentrations have already been determined by SIMS and FTIR, and oxygen fugacity determinations have been made on the basis of crystallization temperatures and mineral equilibria by deSilva (1989, 1991) and Schmitt et al. (1998). Along with testing these new techniques, goals of this study include acquiring insight into the interplay of oxygen fugacity, water concentration, mineral assemblage, and the eruption style of siliceous magmas across a large range of geologic time.