EAR-0207475 Ghiorso and Evans University of Washington
This 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.
