Testing the Ti-in-Zircon Thermometer

Results of an empirical thermometer based on the concentration of Ti in zircon recently led Watson and Harrison (2005) to propose the existence of wet, minimum melting throughout the Hadean Eon - a hypothesis with far ranging implications. Tests of this hypothesis include: assessment of the range of validity of the thermometer, documentation of crystallization temperatures of post-Hadean granitoid zircons, and replication of the ca. 680°C} peak temperature of Hadean zircons. We have probed the energies of the isovalent substitution of Ti on the Si and Zr sites in zircon using the SIESTA implementation of density functional theory which uses pseudopotentials to describe the core electrons and nuclei, and a numerical scheme to describe valence electrons which allows large numbers of atoms to be assessed. Results show that the substitution of Ti on the Si site is highly favoured over the competing mechanism by approximately 32 kJ/mol. Thus essentially all Ti is expected to occupy the Si site in the zircon crystal structure in the temperature range over which the thermometer has been calibrated thus explaining in part the observed linear behaviour. We have developed a SHRIMP multi-collector protocol for zircon utilizing 49Ti/SiO which yields <0.5% precision for Ti concentrations down to the ppm level. Analysis of zircons from rutile-bearing metmorphic rocks constrained by thermobarometry to have formed between 550 and 800°C} yield calculated temperatures within uncertainty of the known value, further confirming the general validity of the thermometer in that temperature range. Analyses of a variety of granitoids including tonalite yield calculated temperatures ranging up to 820°C} that broadly correlate with zircon saturation temperature. Thus we infer that calculated temperatures of <700°C} from granitoid zircons are generally restricted to water-saturated melting conditions. We have revisited the Hadean zircons of Jack Hills, Western Australia, and more than doubled the number of Ti analyses. Of the 136 total analyses of zircons ranging in age from 3.91-4.35 Ga, only seven yield calculated temperatures in excess of 800°C}. The vast majority of analyses instead plot in a normal distribution with a peak at 680 ± 20°C}. Together with results from other studies (e.g., O isotopes, Lu-Hf, inclusion mineralogy) we conclude that the Hadean Earth supported wet, minimum melting, the formation of clay minerals under ambient conditions, and peraluminous melting of pelitic protoliths. Current analytical developments involve ion imaging Ti concentration variations using 48Ti at high mass resolving power.