A NEW CALIBRATION OF THE FE-TI, TWO-OXIDE GEOTHERMOMETER AND OXYGEN BAROMETER

The thermodynamic formulation of the Fe-Ti oxide geothermometer developed by Ghiorso & Sack (1991, CMP 108, 485-510) is extended and recalibrated in light of recently published experimental measurements of cation ordering in ilmenite-hematite solid solutions (Harrison et al., 2000 AM 85, 194-205) and new two-oxide phase equilibrium experiments conducted over the temperature range 800°-1300°C and oxidation state conditions characterized by NNO-3 to NNO+3. Previous incarnations of the Fe-Ti oxide geothermometer (op. cit.; Andersen & Lindsley, 1988, AM 73, 714-26) were calibrated from experimental data collected largely at oxidation states below NNO, and consequently these calibrations extrapolate poorly to highly oxidized conditions. The new calibration models the experimental constraints equally well over the entire experimental range of temperature and oxidation state conditions. Two-oxide Fe-Ti partitioning data collected at 1200° and 1300°C confirm the necessity for including cation ordering in the expression for the configurational entropy of the rhombohedral phase. Within the framework of the adopted thermodynamic models for these oxides, the newer experimental data are internally consistent with previously published phase equilibrium measurements. Our thermodynamic models for the cubic and rhombohedral oxides permit evaluation of activity-composition relations in the systems (Fe²⁺,Mg)₂TiO₄-(Mg,Fe²⁺)(Al,Cr,Fe³⁺)₂O₄ and (Fe³⁺,Al)₂O₃-(Fe²⁺,Mg,Mn)TiO₃, respectively. This allows us to evaluate chemical potentials of endmember components of the Fe-Ti exchange [FeTiO₃(rhm) + Fe₃O₄(spn)=Fe₂O₃(rhm) + Fe₂TiO₄(spn)] and Fe-oxidation [6Fe₂O₃(rhm)=4Fe₃O₄(spn) + O₂(gas)] reactions without the necessity of projecting compositions into the Fe-Ti subsystem. Phase equilibrium experiments at 800° and 900°C (collected over a broad range of oxidation state conditions) have been designed to systematically evaluate the effects of Al, Mg and Mn on the thermodynamics of the Fe-Ti exchange reaction. These data are utilized in the calibration and are internally consistent with Fe-Ti subsystem experiments.