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 $a_{{\text{TiO}}_2}$ and $a_{{\text{SiO}}_2}$ during zircon crystallization. Thermodynamic software programs such as rhyolite-MELTS or Perple_X permit the estimation of $a_{{\text{TiO}}_2}$ and $a_{{\text{SiO}}_2}$ 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 2 kbar, 5 kbar, and H₂O = 3 wt% show $a_{{\text{SiO}}_2}$ during zircon crystallization close to 1 (0.75-1) and $a_{{\text{TiO}}_2}$ 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 $a_{{\text{TiO}}_2}$ 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 °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 $a_{{\text{TiO}}_2}$ (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 °C. Using a set of Ti-in-zircon measurements from a Variscan granite of the Bohemian Massif, we introduce a novel T-dependent $a_{{\text{TiO}}_2}$ and $a_{{\text{SiO}}_2}$ correction of Ti-in-zircon calculated temperatures which is based on $a_{{\text{TiO}}_2}$ -, $a_{{\text{SiO}}_2}$ -T functions modelled with rhyolite-MELTS. This method takes into account that early and late zircons in granitic systems may crystallize at different $a_{{\text{SiO}}_2}$ and $a_{{\text{TiO}}_2}$ . 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 °C above T _med, as can be modelled using zircon crystallization temperature distributions.