COMPARATIVE STUDY OF DATING MONAZITES USING XRF-MICROPROBE, ELECTRON MICROPROBE AND LASER-ABLATION MASS SPECTROMETRY

A newly developed XRF-microprobe at the Institute of Mineralogy and Petrology, University of Bern, Switzerland, has been applied for precise chemical Th-U-Pb dating of individual monazite grains from the Monte Rosa nappe, Switzerland. Monazites were microdrilled from Pb-free polished petrographic thin sections. The non-destructive nature of the XRF-measurement permitted a comparative study of dating methods by sequentially applying chemical dating by electron microprobe analysis (EMPA), chemical dating by XRF-microprobe analysis, and isotopic 208Pb/232Th dating by LA-PIMMS (Laser Ablation Plasma Ionisation Multi-collector Mass Spectrometry) analysis. For the Variscan (~280Ma) Monte Rosa monazites chemical ages are within error identical with isotope ages, whereas for Alpine monazites (~35Ma) low common lead content leeds in few cases to up to 20% too old age estimates. For comparison, the two sigma precision achieved with the XRF-microprobe for well characterised reference material, monazite FC-1 (TIMS age 54.3 ± 1 Ma; µ-XRF age 55.3 ± 2.6 Ma). Combined with an electron microprobe and conventional optical microscopy, the XRF-microprobe dating is thus a competitive low-cost and non-destructive alternative to more costly isotope dating methods.