Experimental measurements of zircon/melt trace-element partition coefficients

Abstract

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 D zircon/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, D Lu/ D La 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 D zircon/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.