At relatively high oxygen fugacities (>NNO+1), magmas crystallize a rhombohedral FeTi-oxide of intermediate, supersolvus composition. Current formulations of the FeTi-oxide thermobarometer do not correctly model these compositions in the 700-900^oC range of phenocryst growth in hydrous, silicic and intermediate volcanic rocks (Evans and Scaillet, 1997). The tendency is to give too high values of fO₂ and T.

We have conducted hydrothermal crystallization experiments at 800^oC and H₂–membrane-controlled logfO₂ from NNO to NNO+3 on haplorhyolite glass doped with Fe and Ti, and in some cases also with Mg and Mn. These experiments are intended to support a revised thermodynamic model of the thermobarometer applicable to complex natural oxide compositions explicitly accomodating the components Al, Mg and Mn. At 800^oC the spinel phase crystallizes homogeneously whereas rhombohedral oxide in the range Ilm₅₅ to Ilm₈₅ is inhomogeneous. In this range the activities of Ilm and Hem components are little dependent on composition. This behavior is manifested in Roozeboom plots of X_Ilm vs. X_Usp by an inflection of isotherms at supersolvus rhombohedral compositions, accompanied by reduced values of dlogfO₂/dX_Ilm along the isotherm.

Published projection schemes fail to recover consistent binary FeTi compositions at one T, P and fO₂. Instead, they produce trend-lines subparallel to buffer curves on the T - logfO₂ diagram. With increasing fO₂ (and X_Hem), Mg and Mn show decreasing preference for the rhombohedral phase, eventually reversing the sense of partition.