Experimental measurements of zircon/melt trace-element partition coefficients

Zircon was grown from trace-element doped hydrous peralkaline rhyolite melts with buffered oxygen fugacities in cold-seal experiments at 0.1 and 0.2 GPa and 800 °C and piston-cylinder experiments at 1.5 GPa and 900–1300 °C. Zircon and glass were present in all run products, and small monazite crystals were present in eight of the 12 experiments. Average diameters of zircon crystals ranged from 5 to 20 μm at 800 °C to 30–50 μm at 1300 °C. Zircon crystals have thin rims, and adjacent glass has a narrow (∼1 μm thick) compositional boundary layer. Concentrations obtained through in-situ analysis of cores of run product zircon crystals and melt pools were used to calculate trace-element partition coefficients Dzircon/melt for P, Sc, Ti, V, Y, La, Ce, Pr, Nd, Eu, Gd, Ho, Yb, Lu, Hf, Th, and U. In most cases Lu was the most (D 12–105) and La the least (0.06–0.95) compatible elements. D values from this study fall within the range of previously measured values for Rare Earth Elements (REE). However, D values measured experimentally show less fractionation than those recently measured using natural phenocryst/matrix pairs. For example, DLu/DLa measured experimentally in this study range between 27 and 206 compared to a value of 706,522 for a natural zircon/dacite pair [Sano, Y., Terada, K., and Fukuoka, T. 2002 High mass resolution ion microprobe analysis of rare earth elements in silicate glass, apatite and zircon: lack of matrix dependency. Chem. Geol.184, 217–230]. Although D values from this study show good agreement with the lattice strain model, D values from natural phenocryst/matrix pairs combined with measured zircon compositions better reproduce host-rock (magma) compositions of igneous rocks. They also yield more reasonable estimates of magma compositions when combined with compositions of ‘‘out-of-context” zircons. For example, compositions of the Hadean detrital zircons from Jack Hills, Australia yield LREE-enriched magmas when combined with D values from phenocryst/matrix pairs yields, but yield LREE-depleted magmas when experimentally determined D values are used. We infer that experimentally measured Dzircon/melt values represent disequilibrium partitioning resulting from rapid zircon growth during short laboratory timescales. Rapid growth causes development of observed diffusive boundary layers in the melt adjacent to zircon crystals. D values from phenocryst/matrix pairs are therefore recommended for petrogenetic modeling.