Empirical thermometer of TiO₂ in quartz for ultrahigh-temperature granulites of East Antarctica

Two preliminary experiments, heating of rutilated quartz grains with 0.082 wt% TiO₂ on average from Bunt Island, Napier Complex, East Antarctica and a synthetic TiO₂-SiO₂ (rutile-cristobalite) system in air at 1300 °C for 39 days, showed increasing solubility of TiO₂ in silica minerals with temperature. Bunt quartz was converted to cristobalite and traversed by many transparent seams with the disappearence of needles and spots of rutile. Unreacted host grains retained many fine needles of rutile. The seams are homogeneous and slightly enriched in TiO₂ up to 0.149 wt% on average, which is about one-fifth lower than that of the synthesized TiO₂-SiO₂ cristobalite (0.767 wt% on average). Area analyses with an electron beam in the raster mode at a magnification of × 5000 gave 0.308 wt% TiO₂ for the bulk composition of the Bunt quartz. This indicates that needles of rutile exsolved from the TiO₂-saturated quartz at the cooling stage, or during retrograde metamorphism. Natural examples of quartz in geologically and petrologically well-characterized metamorphic rocks were chemically analysed to examine the temperature controls on the Ti saturation level in quartz. The TiO₂ content of quartz in equilibrium with rutile increases sensitively with the metamorphic temperature, which can be expressed as where XQtz TiO₂ is the mole fraction of TiO₂, or the number of Ti atoms per formula unit based on a two-oxygen atom normalization. This empirical equation is very useful to evaluate the metamorphic temperatures for ultrahigh-temperature granulites. The temperatures calculated by the existing Ti-in-quartz thermometer are about 200 °C higher than those estimated by the present thermometer, potentially because of underestimates of Ti solubility in quartz in the previous calibration.