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ZHANG Ya, LI Quan-zhong, YAN Jun, XIE Jian-cheng, YANG Qing-liang, GAO Ling. Analytical Conditions for U-Th-Pb Dating of Monazite by LA-ICP-MS[J]. Rock and Mineral Analysis, 2021, 40(5): 637-649. DOI: 10.15898/j.cnki.11-2131/td.202101130005
Citation: ZHANG Ya, LI Quan-zhong, YAN Jun, XIE Jian-cheng, YANG Qing-liang, GAO Ling. Analytical Conditions for U-Th-Pb Dating of Monazite by LA-ICP-MS[J]. Rock and Mineral Analysis, 2021, 40(5): 637-649. DOI: 10.15898/j.cnki.11-2131/td.202101130005
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Analytical Conditions for U-Th-Pb Dating of Monazite by LA-ICP-MS

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  • BACKGROUNDCompared with zircon U-Pb dating, monazite has more advantages in the dating of some metamorphic rocks or highly differentiated rocks. However, the U-Th-Pb isotopic dating of monazite has been limited, because of high Th concentration (commonly>7%), many mineral inclusions, and rare dating standard samples. Previous research on LA-ICP-MS U-Th-Pb dating of monazite mainly focused on laser parameters, and lacked attention on ICP-MS conditions.OBJECTIVESTo find suitable experimental LA-ICP-MS monazite dating conditions.METHODSUsing 193nm ArF excimer laser ablation system and Agilent 7500a inductively coupled plasma-mass spectrometer, laser parameters (laser beam spot diameter and laser frequency) and ICP-MS parameters(dwelling time of 232Th) were investigated. The laser beam spot diameter was set to 24μm, 16μm and 10μm, and the laser frequency was set to 3Hz, 4Hz and 5Hz. The dwelling time of 232Th was set to 10ms, 6ms, 3ms and 1ms for U-Th-Pb dating. Finally, the monazite sample RW-1 was used as the standard sample to calibrate the monazite sample Bananeira.RESULTSIt was suitable for monazite dating when the beam spot diameter was 16μm, the dwelling time of 232Th was 3ms or 1ms, the energy density was 4J/cm2, the laser frequency was 5Hz, the carrier gas (He) flow rate was 0.35L/min, and the carrier gas (Ar) flow rate was 0.95L/min. Under these conditions, Bananeira's weighted average ages of 207Pb/235U were 510.7±8.6Ma (MSWD=0.87), and 513.8±5.7Ma (MSWD=0.38), which were consistent with the recommended age of 507.7±1.3Ma with respective errors of 0.59% and 1.20%. 208Pb/232Th weighted average ages were 496.9±8.6Ma (MSWD=0.596) and 499.8±5.6Ma (MSWD=0.37), which were consistent with the recommended age of 497.6±1.6Ma, with respective errors of 0.14% and 0.44%. The 207Pb/235U weighted average age of Huangshan sample HS-1 was 128.3±2.4Ma (MSWD=0.73), which were close to the zircon age of 127.0±2.1Ma (MSWD=0.93) in the Huangshan area.CONCLUSIONSThe optimal laser and ICP-MS conditions are suitable for monazite U-Th-Pb isotopic age determination.

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