Collaborative Research: Effects of Composition and Cooling Rate on Fe XANES Glass Calibrations

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Magmas are probes of the physical conditions and chemistry of the Earth?s interior. Exploring the mineralogy of these erupted melts provides insight into the continuing geologic evolution of the planet. In addition to basic geochemistry, parameters such as temperature and pressure are important to quantify to constrain the conditions present within the Earth. Of equal importance to the above physical parameters is oxygen fugacity (fO2), a measure of the oxidative environment a magma crystallized under. Of all the elements in magmas, iron is the only major rock-forming element that commonly exists in multiple valence states. Thus, accurate knowledge of the valence state (Fe3+/Fe2+) provides the best proxy for the amount of oxygen that was present when the minerals crystallized, and allows for estimation of magmatic oxygen fugacity. The oxygen fugacity of a magma is one of the principal parameters, along with temperature, pressure, and water content, that determines a magma?s crystallization path, as well as the composition of the resulting minerals. In order to decipher the origin of any igneous rock, it is essential to first understand the compositional differences, phase changes, and crystallization sequence variations that can be caused by magmatic processes in a closed system at a given fO2. Oxybarometers have been developed to quantify the fO2 of a magmatic system using the chemistry or partitioning behavior of multivalent elements such as iron within a mineral (e.g., FeTi oxides, pyroxene) or glass. The best-calibrated methods for evaluating Fe3+/Fe2+ are wet chemistry and Mössbauer spectroscopy, both of which use utilize bulk samples that obfuscate small-scale variations. Fe XANES allows for in situ Fe3+ measurements in glass, but there are significant analytical issues primarily related to changes in melt structure arising from composition and cooling history. Therefore composition-specific calibrations and sophisticated multivariate analysis techniques are necessary for robust microanalyses of Fe3+/Fe2+ in glasses. The goal of this proposal is thus to provide a broad range of independently-analyzed Fe3+/ΣFe standards for glasses spanning a broad range of geologically-feasible conditions. The investigators propose a wide ranging series of experiments under carefully controlled fO2 conditions with the resulting products subjected to multiple analytical techniques to quantify Fe3+. The results of these experiments will provide a robust calibration of the Fe XANES technique for a wide range of geologically-relevant compositions and cooling histories.