Electron Probe Microanalysis of Monazite and Its Applications to U-Th-Pb Dating of Geological Samples

Electron probe microanalysis (EPMA) dating of monazite has been developed over decades. However, limited by the detectability and analytical sensitivity of dating-related elements (Th, Pb, U and Y), the EPMA dating has been restricted to geological research. In this study, various probe currents, beam diameters and counting times have been utilized on a JEOL JXA-8230 electron microprobe to determine the optimal experimental conditions for measuring Th, Pb, U and Y in monazite. The optimal conditions are: (1) accelerating voltage is 15 kV; (2) probe current is 100 nA; (3) beam diameter is 1 mu m; (4) the peak and background counting time of U and Pb are 200 and 100 s; and (5) the peak and background counting time of Th and Y are 100 and 50 s. We apply this method to monazite from garnet-bearing biotite gneiss in the Zanhuang area of the Central Orogenic Belt of the North China Craton. The PbO-ThO2* isochron age calculated by EPMA data is 1 812 +/- 17 Ma (MSWD=2.06), which is similar to the weighted mean Pb-207/Pb-206 age (1 805 +/- 12 Ma, MSWD=1.07) obtained by LA-ICP-MS. This study suggests that EPMA dating of monazite as a powerful dating technique can be widely used in geochronological study.