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激光剥蚀-扇形磁场电感耦合等离子体质谱法同时测定锆石U-Pb年龄和微量元素含量

Simultaneous Determination of U-Pb Age and Trace Elements of Zircon by Laser Ablation Sector Field Inductively Coupled Plasma-Mass Spectrometry

  • 摘要: 激光剥蚀-扇形磁场电感耦合等离子体质谱(LA-SF-ICP-MS)具有高灵敏度特征,被广泛应用于锆石等含U矿物原位微区U-Pb定年研究,但磁偏转式质量分析器的使用导致该质谱仪扫描速度相对较慢,可能影响U-Pb同位素与其他关键微量元素的同时采集。本文通过优化仪器信号稳定性和实验方法,对目前常用的7种锆石U-Pb标准样品进行U-Pb定年和Ti、REEs、Hf等关键元素同时定量分析,探讨了多元素同时分析方法的可行性及对于U-Pb定年结果的影响。实验结果表明,相对于LA-SF-ICP-MS仅检测U-Pb同位素方法,同时开展多元素含量检测可能会使U-Pb同位素信号强度稳定性下降,导致单点U-Pb年龄结果误差及离散程度增大。与仅测定U-Pb同位素年龄的测定结果相比较,根据不同锆石样品中U-Pb同位素含量高低,多元素同时检测获得分析点的206Pb/238U年龄和207Pb/235U年龄变化范围不同程度地增大,其中207Pb/235U年龄受影响明显,单点207Pb/235U年龄误差从~1.5%增大至~2.0%,单点年龄的相对标准偏差(RSD)从0.5%~1.3%增大至1.2%~3.3%。尽管如此,多元素同时检测方法对于各样品最终测定年龄没有明显的影响,相对于TIMS年龄,各样品的谐和年龄和206Pb/238U加权平均年龄偏差分别小于1.0%和0.7%,完全满足U-Pb同位素地质年代学测试要求。同时测定锆石样品中的关键微量元素含与其推荐值相对误差小于10%。因此,采用LA-SF-ICP-MS可以同时准确地测定锆石U-Pb年龄和微量元素含量,该方法亦可用于其他副矿物U-Pb年龄与关键微量元素同时测定。

     

    Abstract: Laser ablation sector field inductively coupled plasma-mass spectrometry (LA-SF-ICP-MS) is widely applied in U-Pb dating of zircon due to its remarkable sensitivity. However, the utilization of a magnetic sector mass analyzer imposes constraints on its scanning speeds, potentially affecting the concurrent acquisition of U-Pb isotopes and other trace elements. Here a method for simultaneous zircon U-Pb dating and key trace elements quantifying by LA-SF-ICP-MS were developed. Seven zircon U-Pb standard samples were measured to assess the method's feasibility. Experimental data indicate that simultaneous collection of U-Pb isotopes and other trace elements may decrease signal stability, particularly for low-content isotopes like 207Pb, which in turn leads to an increased age uncertainty and dispersion for single analyses. However, the accuracy of the concordance age and weighted mean 206Pb/238U age of each sample, and the statistical results of all data points, are not affected significantly. Compared to TIMS ages, the discordance in these ages across all samples remains below 1.0% and 0.7%, respectively, meeting the requirements of U-Pb geological dating. Furthermore, the determination of key trace elements in zircon samples shows relative errors to recommended values of less than 10%. LA-SF-ICP-MS can accurately determine both zircon U-Pb ages and trace element contents simultaneously. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202309110151.

     

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