The current status of titanite–rutile thermobarometry in ultrahigh-pressure metamorphic rocks: The influence of titanite activity models on phase equilibrium calculations

Titanite, an important accessory mineral in ultrahigh-pressure (UHP) rocks, commonly deviates significantly from its ideal composition (CaTiSiO4O) by the substitution of Al and (F, OH) for Ti for O. This compositional variability of titanite could be used in phase equilibrium calculations, if the activity–composition relations in (Al + F)-bearing titanites were sufficiently known. Experimental investigations by Troitzsch and Ellis ([Troitzsch, U. and Ellis, D.J. (2002) Thermodynamic properties and stability of AlF-bearing titanite CaTiOSiO4–CaAlFSiO4. Contributions to Mineralogy and Petrology, 142, 543–563]) and Tropper et al. ([Tropper, P., Manning, C., Essene, E.J. 2002. The substitution of Al and F in Titanite at high pressure and temperature: experimental constraints on phase relations and solid solution properties. Journal of Petrology 43, 1787–1814.) derived non-ideal-mixing models for solid solutions along the join CaTiSiO4O–CaAlSiO4F. Tropper et al. [Troitzsch, U. and Ellis, D.J. (2002) Thermodynamic properties and stability of AlF-bearing titanite CaTiOSiO4–CaAlFSiO4. Contributions to Mineralogy and Petrology, 142, 543–563] derived a preliminary regular model in the T range 900–1100 °C, in which the T-dependent interaction parameter, WG, was negative. In contrast, Troitzsch and Ellis ([Troitzsch, U. and Ellis, D.J. (2002) Thermodynamic properties and stability of AlF-bearing titanite CaTiOSiO4–CaAlFSiO4. Contributions to Mineralogy and Petrology, 142, 543–563]) favored a regular activity model with positive WG. The different signs of the interaction parameter strongly influence calculated non-ideal titanite activities. Comparing available simple ideal ionic (coupled-ionic, non-coupled-ionic) activity models with the non-ideal models shows that aCaTiSiO4O, calculated with the ideal ionic models is substantially lower at almost all T. Calculation of two suitable titanite–rutile-involving equilibria for thermobarometry applied to literature data from rocks from four UHP terranes shows that ideal ionic models yield the best convergence with independently established P estimates. Although the literature data have to be treated with caution (e.g. retrogression, compositional disequilibrium etc.), the calculations nonetheless indicate that in terms of T estimates, high-Al (XAl > 0.5) titanites yield a large variation of up to 300 °C in T and low-Al (XAl < 0.2) titanites yield the best convergence with independently established T estimates. This study shows that P and T estimates derived from ideal models and experimentally constrained non-ideal models for titanite activity may show large deviations at UHP conditions, ranging from 0.05 to 3.0 GPa and up to 300 °C. Therefore the current status of titanite–rutile-involving thermobarometry allows it only to be applied to Al-rich (XAl > 0.2) titanites from UHP rocks if independent, more robust P–T estimates are available. Until better activity constraints are available, it is recommended that the user employ either the ideal coupled-ionic model for titanite solid solutions involving mixing on the Ti and the O1-site and compare the results to an experimentally derived activity model, or adopt a range of different activity models (ionic and regular) to obtain a range of P–T conditions.