Vanadium K-Edge XANES and its Application to Microscale Oxygen Barometry

A promising proxy for oxygen fugacity is the valence of vanadium because it has a large number of possible states (2+, 3+, 4+ and 5+) and is ubiquitous in earth and planetary materials. A non-destructive method for valence determinations of vanadium and oxygen fugacity estimates with spatial resolution of a few micrometers is synchrotron x-ray absorption near edge structure (XANES) spectroscopy. The valence determination is based on the intensity of a pre-edge peak near 5470 eV in the vanadium K-edge XANES spectrum and much of our development work has focused on igneous glasses. A key to this method is calibration with standards of known valence and experimental charges produced under known conditions. Vanadium pre-edge peak intensity correlates well with the known valence states for glasses whose valence was established independently. Using this calibration curve, the effective valences of other glasses can be determined. Measurements on isobaric experimental couples were used to determine the magnitude of valence variations due to temperature differences. In this way, a calibration curve for peak intensity versus oxygen fugacity can be produced for the appropriate liquidus temperature of each unknown glass. The method has been successfully applied to natural glasses from the Earth, Moon and Mars.