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

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。

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.