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

作　　者：Wenbin Ning Junpeng Wang Deng Xiao Fenfang Li Bo Huang Dong Fu 

机构地区：[1]State Key Laboratory of Geological Processes and Mineral Resources, Hubei Key Laboratory of Critical Zone Evolution, Center for Global Tectonics, School of Earth Sciences, China University of Geosciences

出　　处：《Journal of Earth Science》2019年第5期952-963,共12页地球科学学刊（英文版）

基　　金：supported by the National Natural Science Foundation of China(No.41602234);the Fundamental Research Funds for the Central Universities,China University of Geosciences,Wuhan,China(Nos.CUGL180406,CUGCJ1707);Open Fund(No.GRMR201901)from the State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Wuhan

摘　　要：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 k V;(2) probe current is 100 n A;(3) beam diameter is 1 μm;(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 Pb O-Th O2* isochron age calculated by EPMA data is 1 812±17 Ma(MSWD=2.06), which is similar to the weighted mean 207 Pb/206 Pb 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.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 k V;(2) probe current is 100 n A;(3) beam diameter is 1 μm;(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 Pb O-Th O2* isochron age calculated by EPMA data is 1 812±17 Ma(MSWD=2.06), which is similar to the weighted mean 207 Pb/206 Pb 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.

关 键 词：MONAZITE EPMA DATING garnet-bearing BIOTITE GNEISS Zanhuang area North China CRATON geochemistry 

分 类 号：P[天文地球]