LA-ICP-MS锆石微区U-Pb定年方法及不同束斑直径对年龄结果的影响作用探讨

In-situ zircon U-Pb dating method by LA-ICP-MS and discussions on the effect of different beam spot diameters on the dating results

利用自然资源部古地磁与古构造重建重点实验室新引进的GeoLas HD型193nm ArF准分子激光剥蚀系统和Agilent 7900型四极杆电感耦合等离子质谱仪,成功建立了LA-ICP-MS锆石微区U-Pb定年及微量元素分析测试方法。以标准锆石91500为外标,在32μm束斑直径、5.0 J/cm^(2)能量密度和5 Hz剥蚀频率等实验条件下,对Plešovice、Temora1和Qinghu锆石标样开展了U-Pb定年实验,所测年龄结果与各标样推荐值在误差范围允许的条件下一致,并且Plešovice年龄结果在不同时间段内保持稳定。同时对未知年龄样品11-5开展了不同实验室测年结果对比研究,所测结果与中国地质大学(武汉)地质过程与矿产资源国家重点实验室所测年龄在误差允许范围内一致。以NIST SRM 610为外标,^(29)Si为内标,分析测试了锆石91500和NIST SRM 612标准样品的微量元素含量,实验测试结果与推荐值一致。在此基础上探索总结了不同剥蚀斑束直径对U-Pb年龄结果的影响,认为在同样的能量密度和剥蚀频率条件下,16~44μm的剥蚀直径可以获取可靠的锆石U-Pb年龄,但32~44μm相比16~24μm小斑束直径所测得的年龄更加精准。

By using the newly introduced GeoLas HD 193 nm ArF excimer laser ablation system and the Agilent 7900 quadrupole inductively coupled plasma mass spectrometer(ICP-MS)in the Key Laboratory of Paleomagnetism and Tectonic Reconstruction at the Institute of Geomechanics,we successfully established the insitu zircon U-Pb dating method and trace element analytical method by LA-ICP-MS.U-Pb dating experiments were carried out on Plešovice,Temora1 and Qinghu zircons under the experimental conditions of 32μm beam spot diameter,5.0 J/cm^(2) energy density and 5 Hz denudation frequency using the standard zircon 91500 as the external standard.The ideal age results are consistent with their recommended values,and the measured U-Pb ages for Plešovice zircon remain stable in different periods.Meanwhile,a comparative study of the U-Pb dating results in different laboratories was carried out on an unknown sample 11-5,and the dating results are consistent with that obtained in the State Key Laboratory of Geological Processes and Mineral Resources at China University of Geosciences(Wuhan)within analytical error.Trace elements in zircon standard 91500 and glass standard NIST SRM 612 were analyzed by using NIST SRM 610 as an external standard and ^(29)Si as an internal standard,and the measured values are consistent with the recommended values of trace element contents.In addition,the influence of different beam spot diameters on the zircon U-Pb age results was explored by using the Plešovice zircon as an example.The results show that a denudation diameter of 16~44μm can obtain reliable zircon U-Pb ages similar to the recommended age,and under the same energy density and denudation frequency conditions,U-Pb ages obtained by using 32~44μm beam spot diameters are more accurate than those obtained by 16~24μm beam spot diameters.