The Ti-in-zircon thermometer revised: The effect of pressure on the Ti site in zircon

The Ti content of zircon (ZrSiO₄) can be used as a geothermometer. Ti could substitute for Si or Zr in zircon, which would result in opposing responses of Ti solubility to (i) the activities of SiO₂ (aSiO₂^{quartz/coesite}) and TiO₂ (aTiO₂^rutile) and (ii) changes in pressure due to differences in the molar volumes of the components ZrTiO₄ and TiSiO₄ relative to ZrSiO₄. Previous calibrations of the Ti-in-zircon thermometer assumed that Ti occupies the Si-site. X-ray absorption near edge structure (XANES) spectroscopy has confirmed that Ti is tetrahedrally coordinated on the Si-site at atmospheric pressure, however, Ti is predicted to change to the Zr-site at higher pressures. Ti-bearing zircons were prepared at 1400 °C and pressures from 0.0001 to 6.5 GPa using a flux-method that produced crystals up to 50 μm in size. Ti K-edge XANES spectra of zircons prepared at < 4.0 GPa are characterised by an intense pre-edge peak (attributed to the 1s → 3d transition), which is consistent with Ti in tetrahedral coordination on the Si-site. The intensity of this peak is proportional to the amount of Ti on the Si-site, and decreases abruptly > 4.0 GPa (accompanied by a change in the energy and shape of the absorption edge), indicating a change in the coordination of Ti. Ti-O bond lengths determined from extended X-ray absorption fine structure (EXAFS) spectra of 1.788(7) Å at 1.0 GPa and both 1.87(4) Å and 2.21(2) Å at 6.5 GPa are consistent with Ti changing from the Si-site to the larger Zr-site. The proportion of Ti on each site varies systematically with pressure, with equal proportions of Ti on both sites at ∼ 4.4 GPa. Sole occupancy of Ti on the Zr-site is predicted to occur at pressures coinciding with the transition of zircon to reidite (∼ 8.0 GPa). This change in site is consistent with a systematic decrease in the Ti content of these zircons with increasing pressure when aSiO₂^{quartz/coesite} and aTiO₂^rutile = 1. As a result, an updated model for estimating zircon crystallisation temperatures from the Ti content was derived: logTif=5.847-[Formula presented]-0.121P-0.005615P³-logaSiO₂^{quartz/coesite}f+logaTiO₂^rutile where Ti is in ppm, f is the fraction of Ti on the Si-site given by 1/(1 + 10^{−(3.37(13) − 0.77(5)P)}), T is temperature in K, P is pressure in GPa, and the uncertainties are 1σ. For zircons with independent estimates of temperature, pressure, aSiO₂^{quartz/coesite} and aTiO₂^rutile this new model predicts crystallisation temperatures within 40 °C (or 8 %) of the independent temperature estimates at all pressures. This contrasts with previous models for which predicted temperatures are within certainty of nominal temperatures at ∼ 1.0 GPa, but underestimated (by up to 70 °C) at pressures < 1.0 GPa and underestimated (by up to 300 °C) at higher pressures where the proportion of Ti on the Zr-site should be significant.