Nondestructive chemical dating of young monazite using XRF. 2. Context sensitive microanalysis and comparison with Th-Pb laser-ablation mass spectrometric data

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 separated from Pb-free polished petrographic thin sections. The nondestructive 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 Pb-208/Th-232 dating by Laser Ablation Plasma Ionisation Multi-collector Mass Spectrometry (LA-PIMMS) analysis. As an example, the 2sigma precision achieved with the XRF-microprobe for well characterised reference material, monazite FC-1 (TIMS age 54.3 +/- 1 Ma; mu-XRF age 55.3 +/- 2.6 Ma), doubly polished to 30 mum in thickness, is below 5% after 90 min integration time (50 kV; 30 mA) at a spatial resolution of 90 mum. At 38-mum, spatial resolution, the uncertainty is 35% for the same integration time. The sample characteristics are 200-300 ppm of Pb (mu-XRF), 3.8-5.1 wt.% of Th (EMPA), and 0.4-1.4 wt.% U (EMPA). Combined with an electron microprobe and conventional optical microscopy, the XRF-microprobe is thus a competitive low-cost and nondestructive alternative to more costly isotopic methods. The XRF-microprobe is easy to use and maintain. (C) 2002 Elsevier Science B.V All rights reserved