Rutile saturation in hydrous siliceous melts and its bearing on Ti-thermometry of quartz and zircon

The TiO2 solubility of rutile-saturated hydrous siliceous melts has been investigated at P = 1 GPa and T = 650–1000 °C for several representative felsic compositions. The dissolution of a rutile crystal into a TiO2 undersaturated melt provides information on both TiO2 solubility and Ti diffusion. Results of this study confirm that TiO2 solubility is strongly dependent on both temperature and melt composition, but not on the amount of H2O present. For a given T, TiO2 content decreases as the melts become more felsic. The solubility of TiO2 is given by:log⁡(Ti,ppm)=7.95−5305T+0.124FMwhere T is in K and FM is a melt composition parameter,FM=1Si.Na+K+2(Ca+Mg+Fe)Alin which the chemical symbols represent cation fractions.Results of dissolution experiments yield an activation energy (E) for Ti transport in a hydrous felsic melt of 186 ± 27 kJ/mol and a frequency factor, Do, of 3.6 ± 1.2 m2/s. These results suggest an activation energy similar to that established for Zr diffusion in similar melts, but with Ti diffusion rates 2–3 orders of magnitude faster.Both TiO2 solubility and Ti diffusion have important applications in geothermometry, particularly in light of new thermometers calibrated for the incorporation of Ti into quartz and zircon. Rutile saturation is improbable in the types of melts where these thermometers are most likely to be useful, and therefore it is important that rutile solubility behavior in these melts to be well-constrained. TiO2 activities in silicic melts at typical magmatic temperatures are generally 0.6 or higher, implying that Ti thermometry of out-of-context zircons will rarely underestimate zircon crystallization temperature by more than ∼ 50 °C.