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
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摘要: 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32 μm束斑直径、5.0 J/cm2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，29Si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，16~44 μm的剥蚀直径可以获取可靠的锆石U-Pb年龄，但32~44 μm相比16~24 μm小斑束直径所测得的年龄更加精准。Abstract: 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 in-situ 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 32 μm beam spot diameter, 5.0 J/cm2 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 29Si 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 16~44 μm 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 32~44 μm beam spot diameters are more accurate than those obtained by 16~24 μm beam spot diameters.
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Key words:
- LA-ICP-MS /
- zircon /
- U-Pb dating /
- trace element
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图 1 不同时间段Plešovice锆石U-Pb谐和图和年龄加权平均值
Figure 1. U-Pb concordia plot and weighed mean age of the zircon Plešovice in different periods
图 2 Temora1锆石U-Pb谐和图及年龄加权平均值
(a)U-Pb谐和图；(b)年龄加权平均值
Figure 2. U-Pb concordia plot and weighed mean age of the zircon Temora1
(a) U-Pb concordia plot; (b) Weighed mean age
图 3 Qinghu锆石U-Pb谐和图及年龄加权平均值
(a)U-Pb谐和图；(b)年龄加权平均值
Figure 3. U-Pb concordia plot and weighed mean age of the zircon Qinghu
(a) U-Pb concordia plot; (b) Weighed mean age
图 4 样品11-5不同实验室U-Pb年龄对比图
a—自然资源部古地磁与古构造重建重点实验室测试结果；b—中国地质大学(武汉)地质过程与矿产资源国家重点实验室测试结果
Figure 4. Comparison of U-Pb concordia plot for sample 11-5 from different Laboratories
(a) Results from the Key Laboratory of Paleomagnetism and Tectonic Reconstruction; (b) Results from the State Key Laboratory of Geological Processes and Mineral Resources
图 5 锆石91500和NIST SRM612标样稀土元素球粒陨石标准化配分模式图(球粒陨石标准值据Sun and McDonough, 1989；91500锆石的微量元素平均含量的参考值据王岚等，2012)
Figure 5. Chondrite-normalized REEs patterns of the standard sample 91500 and NIST SRM612 (Chondrite-normalized values are cited from Sun and McDonough, 1989; reference values of trace elements for zircon 91500 are cited from Wang et al., 2012)
图 6 不同斑束直径剥蚀取样获得的Plešovice U-Pb谐和图及年龄加权平均值
Figure 6. U-Pb concordia plot and weighed mean age of the zircon Plešovice obtained by different beam spot diameters
图 7 不同时间段内24 μm斑束直径获得的Plešovice U-Pb年龄加权平均值
Figure 7. Weighed U-Pb mean ages of the zircon Plešovice with the 24 μm beam spot diameter in different periods
表 1 LA-ICP-MS主要工作参数
Table 1. Major working parameters of the LA-ICP-MS
激光剥蚀系统
(GeoLasHD 193 nm ArF准分子)电感耦合等离子质谱系统
(Agilent 7900)分析参数 工作条件 分析参数 工作条件 工作波长 193 nm 等离子气流 15 L/min 能量密度 5 mJ/cm2 RF功率 1550 W 剥蚀频率 5 Hz RF匹配 1.8 V 采样时间 50 s 采样深度 6.0 mm 束斑直径 44/32/24 μm 载气流速 0.6~0.9 L/min 载气类型 100% 氦气 分析同位素 202、204、206~208、232、238 载气流速 0.4 L/min 积分时间 25~30 ms/同位素 
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