STABILITY OF ZIRCONS AT ULTRAHIGH-TEMPERATURE CONDITIONS
We explore the behavior of zircons during ultrahigh-temperature metamorphism (UHTM) along representative P-T paths with the Theriak/Domino software package (de Capitani & Petrakakis, 2010), modified to incorporate the thermodynamic treatment of Kelsey & Powell (2011; KP11) for Zr-bearing systems. We use the representative metapelite composition of KP11, but vary the Zr and H2O contents. The model uses common UHT P-T paths: isothermal decompression, clockwise from high pressures, counterclockwise, and isobaric cooling.
The results show that zircon is continuously being dissolved (mainly into melt and rutile, less so into garnet and ilmenite) during prograde UHTM. Growth of zircon only occurs during cooling and/or exhumation, and may commence at temperatures well below peak. For instance, with a peak temperature of 1000 °C, zircons in hydrous metapelite with 200 ppm Zr will not grow until the rock has cooled to <~915 °C regardless of path. The durability of zircon is greatly dependent on bulk Zr and H2O contents. For example, adding ~50 ppm Zr to the KP11 composition increases zircon stability by ~30 °C at a given pressure. On the other hand, doubling the H2O content decreases zircon stability by ~50 °C due to the increased formation of melt and zircon dissolution into it. Thus, fluid influx at high temperatures can partially or completely destroy zircons, including any portions that recorded cooling from UHT conditions. If the rock is dry, some Zr will dissolve into rutile on the prograde path but zircon will typically remain stable during UHTM. Kinetic effects, however, may impede new zircon growth upon cooling and exhumation such that UHTM isn’t recorded. For most H2O-bearing compositions and P-T paths, zircon doesn’t begin growing until cooling to near the lower limit of UHTM (~915 °C), or at sub-UHT conditions. This results in zircon U-Pb ages corresponding to cooling and/or exhumation rather than the peak UHTM. Ti-in-zircon geothermometry combined with U-Pb geochronology is needed to determine what part of the retrograde P-T path is being dated.