Experimental zircon/melt and zircon/garnet trace element partitioning and implications for the geochronology of crustal rocks

Abstract

Garnet is the most commonly used mineral in thermobarometry, whereas zircon is the most robust chronometer to date high-grade metamorphic rocks. To provide a basis for correlation of zircon and garnet growth, we determined experimentally the trace element partitioning between zircon, a hydrous granitic melt and garnet at 20 kbar and 800-1000 °C for P, Y, rare earth elements (REE), Zr, Hf, Th and U. In respect to melt, zircon preferentially incorporates all investigated elements apart from REE with atomic number lower than Sm. At this pressure and in the chosen composition, the distribution coefficient between zircon and melt (DZrn/Melt) for REE increases with increasing atomic number of the REE and with decreasing temperature. DYbZrn/Melt is ∼ 20 at 1000 °C, but more than an order of magnitude higher at 800 °C. The solubility of Zr in hydrous granitic melts buffered by zircon is about a factor of two lower at 20 kbar than what has been previously established for mid-crustal pressures. Large garnet produced in the experiments allowed determination of garnet/melt trace element partitioning (DGrt/Melt) at temperatures of 800-1000 °C, conditions relevant for partial melting of crustal rocks. There is a systematic increase in DREEGrt/Melt with decreasing temperature. Zircon contains significantly more heavy-REE than garnet at temperatures of 800-850 °C. Zircon/garnet partition coefficients of heavy-REE decrease with increasing temperature from DLuZrn/Melt of 12 at 800 °C to 1.4 at 1000 °C. Middle-REE partitioning is close to unity for the whole investigated temperature range. Different empirically determined zircon/garnet partition coefficients from granulites and ultra-high temperature granulites can potentially be explained by the experimentally determined change of partitioning as a function of temperature. These data can assist in establishing equilibrium between garnet and zircon zones in natural rocks, and in the construction of detailed pressure-temperature-time paths in high-grade metamorphic rocks.