Application of Ti-in-zircon thermometry to granite studies: problems and possible solutions

The application of the Ti-in-zircon thermometer to granitic rock requires consideration of aTiO2 and aSiO2 during zircon crystallization. Thermodynamic software programs such as rhyolite-MELTS or Perple_X permit the estimation of aTiO2 and aSiO2 values from whole-rock geochemical data as a function of pressure and temperature. Model calculations carried out on a set of 14 different granite types at 2kbar, 5kbar, and H2O=3wt% show aSiO2 during zircon crystallization close to 1 (0.75-1) and aTiO2 generally far below unity (0.1-0.6). This would suggest that Ti-in-zircon temperatures for granites must be significantly upward corrected relative to the original TiO2- and SiO2-saturated calibration of the thermometer. Both the rhyolite-MELTS and Perple_X calculations indicate that aTiO2 is typically around 0.5 in ilmenite-bearing granites. Thus, for ilmenite-series granites (that is, almost all S-type and many I-type granites), it could be a reasonable first order approximation to apply a constant temperature correction of +70 degrees C to the Ti-in-zircon thermometer. Granites lacking the paragenesis zircon-ilmenite, that is, some A-type granites and a few special I-type granites may have even lower aTiO2 (0.1-0.5) and some of them may require a huge upward correction of Ti-in-zircon temperatures on the order of 100-200 degrees C. Using a set of Ti-in-zircon measurements from a Variscan granite of the Bohemian Massif, we introduce a novel T-dependent aTiO2 and aSiO2 correction of Ti-in-zircon calculated temperatures which is based on aTiO2-, aSiO2-T functions modelled with rhyolite-MELTS. This method takes into account that early and late zircons in granitic systems may crystallize at different aSiO2 and aTiO2. Furthermore, we highlight the usefulness of comparing the corrected results of Ti-in-zircon thermometry with bulk-rock-Zr-based zircon solubility thermometry and ideal zircon crystallization temperature distributions for granites, and we present a graphical method that enables this comparison. In addition, this paper addresses the problem that Ti-in-zircon measurements are commonly collected with only moderate spatial analytical resolution, which leads to an averaging effect and to difficulties in recording accurate crystallization temperatures. Therefore, we propose that Ti-in-zircon thermometry for granites should generally rely on the more representative median-T (T-med) value of a series of zircon analyses. Peak magma temperatures will be, in general, 35-50 degrees C above T-med, as can be modelled using zircon crystallization temperature distributions.