High-uranium matrix effect in zircon and its implications for SHRIMP U-Pb age determinations

Abstract

A correlation has previously been noted between extremely high-uranium concentrations in zircon and apparent U/Pb age as measured in ion microprobe analysis. Here we present data from two zircon populations with extremely high-uranium concentrations in an attempt to quantify effects related to the nature of the samples and/or instrumentally induced fractionation (instrument, analytical setup, and/or matrix effect). The high-uranium matrix effect is apparent in zircons from the 180Ma Tasmanian Dolerite, which typically shows an increase in Pb/U age of around 3% per thousand μg/g of uranium when measured on SHRIMP RG. One session on SHRIMP RG shows a correlation with 8% per thousand μg/g of uranium. Data from SHRIMP II show a weak correlation and no correlation is evident in data collected on SHRIMP I. High-uranium zircons from the 98Ma Mt Dromedary monzonite show little to no correlation between uranium concentration and U/Pb age. Analyses of younger (~20-50Ma) high-uranium zircons show a correlation between uranium concentration and an increase in apparent age, but this is less pronounced than the results obtained from the older Tasmanian Dolerite. Raman spectroscopy of the Tasmanian Dolerite zircons shows that these zircons are metamict. The link between uranium concentration and apparent age appears to be related to the degradation of the zircon matrix from radiation damage. The change in matrix enhances the emission of Pb+ relative to U and U oxide species. As this effect is associated with matrix and machine parameters, an external correction cannot be made assuming a constant correction value. With a secondary high-uranium standard, it might be possible to quantify the effect, but the matrix of this standard needs to be closely matched in age (radiation damage) and chemistry (U concentration); this solution is impractical. We instead propose that workers be aware of this matrix effect in SHRIMP analyses of zircon and to use Raman spectroscopy before or after SHRIMP analyses to ensure that the zircon is crystalline. These findings have important consequences for the use of SHRIMP data for zircon geochronology.