Development of new oxybarometers for silicic magmas

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The oxygen fugacity of intermediate to silicic magmas is most commonly reconstructed by Fe-Ti two-oxide thermobarometry. However, many silicic igneous rocks lack coexisting magnetite and ilmenite or other mafic minerals that could be used to constrain fO2. Furthermore, in many samples in which such phases originally were present they subsequently were destroyed or reset during subsolidus processes. The aim of this project is to develop three new mineral-melt partitioning oxybarometers that can be applied to both volcanic and slowly-cooled, intrusive rocks. The project is divided into three independent subprojects that deal with the mineral-melt partitioning of three different multivalent elements: (1) vanadium partitioning between magnetite and silicate melt, (2) uranium partitioning between zircon (or apatite) and silicate melt, and (3) iron partitioning between feldspar and silicate melt. Each of these equilibria will be investigated experimentally under controlled temperature and fO2 conditions, and the resulting calibrations will be tested on natural samples that formed at known T-fO2 conditions. The latter will be done via laser ablation ICP-MS analysis of silicate melt inclusions and solid inclusions within magmatic quartz phenocrysts, which has the advantage that the trapped phases were protected from alteration by hydrothermal fluids, and that re-equilibration processes that occurred within the inclusions during magma cooling are "reversed" during the analysis of entire, unexposed inclusions.