U and Th zonation in Fish Canyon Tuff zircons: Implications for a zircon (U-Th)/He standard

The inability to accurately determine the distribution of U and Th in zircon crystals analysed using (U-Th)/He is a major source of error, and limits the confidence in subsequent data interpretation. The Fish Canyon Tuff (FCT) zircon standard shows (U-Th)/He age reproducibility in excess of +/- 10%. We have tested the extent to which this is due to U and Th zonation using a combination of cathodoluminescence and ion probe analysis of a population of FCT zircons. Primarily we find that FCT zircons exhibit extreme U and Th zonation, and the population has large inter-crystal line variability. Furthermore, the net cathodoluminescence intensity from the FCT zircons is negatively correlated with U and Th concentration, allowing cathodoluminescence emission to be used as a proxy for U and Th zonation. This correlation was exploited to constrain the U and Th zonation within the crystal population, and to show that the poor age reproducibility of the FCT zircons is consistent with the observed zonation. (C) 2008 Elsevier Ltd. All rights reserved.