METAMORPHIC ZIRCON TH/U REFINED (Invited Presentation)

Metamorphic rocks commonly contain multiple populations of zircon, both inherited and newly grown. Due to zircon Th and U concentration collection simultaneously with geochronology, the Th/U ratio is widely available and routinely used to help discriminate between these different zircon growth mechanisms. Despite the wide application of zircon Th/U ratios in informing the U–Pb geochronology of metamorphic rocks, the main controls on this ratio are poorly understood.

Here, phase equilibria modeling coupled with solubility expressions for monazite, zircon and apatite is used to investigate the controls on the Th/U ratio of zircon during suprasolidus metamorphism. Several factors are investigated, including the bulk rock concentrations of Th and U, the amounts of monazite and apatite in the system, the partition coefficient of Th between monazite and melt, and open versus closed system behavior.

This modeling predicts that the main controls on the Th/U ratio of metamorphic zircon are the concentrations of Th and U in the system and the breakdown and growth of monazite in equilibrium with zircon. The relative timing of zircon and monazite growth during cooling and melt crystallization plays a critical role controlling the Th/U ratio of zircon. High Th/U ratios in zircon are expected to be recorded early during cooling and melt crystallization in the absence of monazite. Low Th/U ratios in zircon are expected to occur late in the cooling and crystallization history near the solidus in the presence of monazite. For closed systems, low-temperature crystallization of zircon at or near the wet solidus is likely to lead to Th/U ratios <0.1. For open system melting, melt loss will result in elevated solidus temperatures and zircon is predicted to have higher Th/U ratios. Our modelling provides a new framework within which to interpret metamorphic zircon Th/U ratios.