A Comprehensive Solution Model for Complex FeTi Oxides Relevant to Thermobarometry

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AbstractEAR-0207475 Ghiorso and EvansUniversity of WashingtonThis project will use experimental phase equilibria combined with thermodynamic modeling to revise and extend the capabilities of the FeTi oxide thermobarometer. This thermobarometer is a primary tool used to infer conditions under which certain rocks form. It uses compositions of equilibrium pairs of Fe-Ti oxides (a rhombohedral oxide coexisting with a spinel) to derive estimates of temperature and oxygen fugacity (fO2) conditions under which a mineral assemblage last equilibrated. Recent experimental studies have demonstrated the failure of current formulations of the thermobarometer for conditions that exceed the nickel nickel-oxide oxygen buffer. In addition, subsequent to the formulation and calibration of the thermodynamic models that underlie the existing thermobarometer, new experimental work has been published revising estimates of long-range cation order and greatly clarifying the nature of short-range ordering as well as the character of the symmetry-driven phase transition in the rhombohedral phase. Together, these studies motivate a revision of the thermodynamic formulation of the thermobarometer to place it on sounder theoretical footing and to address the practical necessity of estimating T-fO2 in more oxidized parageneses, in particular orogenic arc magmas. The revised geothermometer will be distributed as a standalone and web-based software product.