CHEMICAL ANALYSIS AND AGE DATING BY ELECTRON MICROPROBE OF SIX MONAZITES DATED BY THERMAL IONISATION MASS SPECTROMETRY (TIMS)

Following the pioneering work of Suzuki and Adachi (1991), chemical dating of monazite by electron probe microanalyzer (EPMA) has been widely used to obtain contextural age information for metamorphic and felsic igneous rocks. As with all EPMA analyses, the accuracy of chemical ages is strongly dependent on the standards used for determining the U, Th and Pb content of monazites. Tests of the overall accuracy of this technique have been conducted in a number of studies by comparing chemical ages with U-Pb isotopic ages obtained for the same rocks by thermal ionization mass spectrometry (TIMS). However, the detailed TIMS data on which these comparisons are based have not reported nor are the monazite samples used in these comparisons widely available for inter-laboratory study. To fill this gap, we have selected a suite of large monazite crystals or crystal aggregates from six different localities worldwide. The samples range in age from Mesozoic to NeoArchean. Two EPMA's were used, a JEOL JXA-8900 at the Univ. of Alberta and a JEOL JXA-8200 at the Univ. of Calgary. Prior to analysis, a systematic assessment of peak overlaps and a complete characterisation of standards was performed. X-ray maps of samples and standards were prepared to search for zoning. Some samples show significant chemical zonation. However, with the exception of one Archean-age sample which yielded more scatter, all samples showed a unimodal distribution of chemical ages. This suggests that each monazite sample largely represents a single age population, which facilitates direct comparison between chemical and TIMS ages. Chemical ages obtained with the two EPMA's are in reasonably good agreement, although there are some systematic differences likely due to the use of different standards for Pb analyses.