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 (aSiO(2)(quartz/coesite)) and TiO2 (aTiO(2)(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 degrees C and pressures from 0.0001 to 6.5 GPa using a flux-method that produced crystals up to 50 mu m 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) & Aring; at 1.0 GPa and both 1.87(4) & Aring; and 2.21(2) & Aring; 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 similar to 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 (similar to 8.0 GPa). This change in site is consistent with a systematic decrease in the Ti content of these zircons with increasing pressure when aSiO(2)(quartz/coesite) and aTiO(2)(rutile) = 1. As a result, an updated model for estimating zircon crystallisation temperatures from the Ti content was derived: log(Ti f) = 5.847 - 4800(86)/T - 0.12(1)P-0.0056(15)P-3 -log(aSiO(2)(quartz/coesite))f + log(aTiO(2)(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 1 sigma. For zircons with independent estimates of temperature, pressure, aSiO(2)(quartz/coesite) and aTiO(2)(rutile) this new model predicts crystallisation temperatures within 40 degrees 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 similar to 1.0 GPa, but underestimated (by up to 70 degrees C) at pressures < 1.0 GPa and underestimated (by up to 300 degrees C) at higher pressures where the proportion of Ti on the Zr-site should be significant.