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

Abstract

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 (Tmed) value of a series of zircon analyses. Peak magma temperatures will be, in general, 35–50 °C above Tmed, as can be modelled using zircon crystallization temperature distributions.