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

Wenbin Ning, Junpeng Wang*, Deng Xiao, Fenfang Li, Bo Huang, Dong Fu
Hubei Key Laboratory of Critical Zone Evolution, State Key Laboratory of Geological Processes and Mineral Resources, Center for Global Tectonics, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China

ABSTRACT: 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 current, beam diameter and counting time 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 μm; 4) the peak and background counting time of U and Pb are 200 s and 100 s; and 5) the peak and background counting time of Th and Y are 100 s 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 1812±17 Ma (MSWD=2.06), which is similar to the weighted mean 207Pb/206Pb age (1805±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.

 

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Keywords:Monazite, EPMA dating, Garnet-bearing biotite gneiss, Zanhuang area, North China Craton

DOI:
10.1007/s12583-019-1020-8
CLC number:
U469.72
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