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

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Abstract

Since its calibration, the Ti-in-quartz thermobarometer 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 temperature 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 during 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 activities are calculated from these dependent potentials, with resultant activities used to correct for Ti isopleth projection in the Thomas et al. (2010) solubility equation. The modeling results are in good agreement with previous studies 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 activity 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 reactions, deformation-driven solution-transfer processes and other Si-mobilization events occur during the prograde and retrograde history of metapelites, and assuming TiO2 activities associated with the peak metamorphic paragenesis may be misleading and result in significant errors in P–T calculations.

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Acknowledgments

We thank reviewer Bruce Watson and handling editor Chris Ballhaus for their constructive comments that greatly improved the clarity and discussion of this paper. We thank Frank Spear for insightful discussions pertaining to thermodynamic components of this work. This work is supported by the National Science Foundation under Grant No. EAR-1220345 (awarded to R.D. Law).

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  1. Department of Geosciences, Virginia Tech, 4044 Derring Hall (0420), Blacksburg, VA, 24061, USA

    Kyle T. Ashley & Richard D. Law

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  1. Kyle T. Ashley
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  2. Richard D. Law
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Correspondence to Kyle T. Ashley.

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Communicated by Chris Ballhaus.

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Ashley, K.T., Law, R.D. Modeling prograde TiO2 activity and its significance for Ti-in-quartz thermobarometry of pelitic metamorphic rocks. Contrib Mineral Petrol 169, 23 (2015). https://doi.org/10.1007/s00410-015-1118-7

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