Monday, 6 October 2008
George R. Brown Convention Center, Exhibit Hall E
Experimentally measured D(zircon/melt) partition coefficients measured by us and others show less fractionation than natural phenocryst/matrix pairs. For example, D(Lu)/D(La) for a zircon/dacite pair (1) is 706,522 while our experimentally measured values range between 27 and 2062. D values from both studies show good agreement with the lattice strain model. However, 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 source rock composition when combined with compositions of "out-of-context" zircons such as the Eoarchean detrital zircons from Jack Hills, Australia and zircon xenocrysts in the Acasta gneisses: using D values from (1) yields LREE-enriched source rocks, while use of experimentally determined D values yields LREE-depleted source rocks. Experimentally measured D(zircon/melt) values may be in error due to development of diffusional boundary layers during growth of zircon from melt on the short timescales of experiments. Calculations suggest that concentration gradients are expected to develop and be maintained during our experiments. This causes incompatible elements to "pileup" and compatible elements to become depleted near the surface of the growing crystal. Pileup increases concentrations of incompatible elements in the boundary-layer melt and newly-grown zircon, while depletion reduces concentrations of compatible elements in the boundary-layer melt and zircon. D values are determined by measuring concentrations in melt pools outside of the boundary layers due to low spatial resolution of analytical techniques, so for incompatible elements equilibrium melt concentrations are underestimated and D values overestimated, and vice-versa for compatible elements, reducing the apparent fractionation. We conclude that analytical methods with spatial resolutions of < 1 um are required to accurately determine D(mineral/melt) values in experimental crystal growth studies. (1) Sano et al. (2002) Chem. Geol. v. 184, p. 217-230.