Modeling prograde TiO2 activity and its significance for Ti-in-quartz thermobarometry of pelitic metamorphic rocks

Abstract

other Si-mobilization events occur during the prograde and retrograde history of metapelites, and assuming TiO2 activities associated with the peak metamorphic paragen- esis may be misleading and result in significant errors in P-T calculations. Abstract Since its calibration, the Ti-in-quartz thermo- barometer has been applied to a wide variety of geologic scenarios. The abundance of quartz in the continental crust and the involvement of silica in metamorphic reactions, deformation, and fluid flux processes make it a particularly powerful tool for constraining the pressure and tempera- ture evolution of rocks, which is essential for developing tectonic models. Being able to quantitatively determine the solubility of Ti in quartz, however, is dependent upon being able to determine the activity of TiO2 in the rock dur- ing quartz growth or re-equilibration. Here we calculate TiO2 chemical potentials of the system relative to that of rutile (as a standard state), projected in P-T space for an average sub-aluminous pelite composition. Titania activi- ties are calculated from these dependent potentials, with resultant activities used to correct for Ti isopleth projec- tion in the Thomas et al. (2010) solubility equation. The modeling results are in good agreement with previous stud- ies that suggest ilmenite-bearing assemblages buffer high TiO2 activities and titanite-bearing assemblages have much lower activities (≥0.5). At elevated temperatures, however, significant deviation from an assumed average pelite activ- ity of 1.0 occurs, where the projected Ti concentration in quartz is up to 400 % different when assuming a dynamic system activity. This is due, in part, to the sequestering of Ti in biotite during heating and the destabilization of Ti- oxides at higher temperatures. With quartz-producing reac- tions, deformation-driven solution-transfer processes and