TOWARDS A NEW CALIBRATION OF FE-TI, TWO-OXIDE GEOTHERMOMETER AND OXYGEN BAROMETER: EXPERIMENTS AT 1 BAR, 1000-1300°C IN THE FE-TI-O SYSTEM

Since the classical study of Buddington & Lindsley (1964, JPET 5, 310-357), the titanomagnetite-ilmenite (Tmt-Ilm_ss) thermo-oxybarometer has been widely used to estimate temperature and oxygen fugacity (fO₂) in magmatic rocks. Experimental studies in complex, nature relevant systems (Toplis & Carroll, 1995, JPET 36, 1137-1170; Scaillet & Evans, 1999; JPET 40, 381-411) have revealed that the current calibrations (Andersen & Lindsley, 1988, AM 73, 714-726; Ghiorso & Sack, 1991, CMP 108, 485-510) are not reliable at high fO₂ (>NNO) or high temperatures (>1000°C). This is partly due to deficiencies in the calibration in the pure system.

Titanomagnetite-ilmenite pairs were synthesized or re-equilibrated in sub-solidus conditions in the Fe-Ti-O system at 1 bar and 1000-1300°C. The oxygen fugacities (in the range FMQ -4 to +5) were fixed either with CO/CO₂ gas mixtures or by using solid oxygen buffers placed together with the sample in evacuated silica glass ampoules. Run lasted > 24 h at 1300°C and up to 240 h at 1000°C and were terminated by drop-quenching in water. The products were analyzed with the electron microprobe.

Tmt compositions are broadly in accordance with the current models at moderate DFMQ (-1 to +2), but significantly richer in Ti at DFMQ <-2 and T>1100°C, due to cationic vacancies. Ilm_ss compositions depart from the predicted values practically at all fO₂ and T conditions. At DFMQ <-2 and T>1100°C they are enriched in Ti, whereas around FMQ they tend to be poorer in Ti than predicted. Ilm_ss equilibrated above FMQ+1.5 have X_Ilm<0.64 with long-range disordered R-3c structure and their compositions are independent of temperature, in contradiction with the model of Andersen & Lindsley (1988). In Roozeboom plots of X_Ilm vs. X_Usp this translates into closely spaced isotherms for oxidized conditions. At higher X_Ilm the isotherms spread out and converge again at very high X_Ilm and X_Usp.

The present data support the revised thermodynamic model of Ghiorso et al. (following abstract).