Abstract: LA-ICP-MS(laser ablation-inductively coupled-mass spectrometry)has been recently used for rapid, accurate and precise U-Pb geochronology on zircon grains. In this paper, we adopted an Agilent 7900 quadrupole ICP-MS coupled with a Resolution M50-LR 193nm excimer laser system to establish integrated measurement procedures. Before analysis, the system is tuned to achieve sensitivities better than 30000 cps/s for ²³⁸ U with a 40μm spot size, at~3.5J/cm². Detailed parameters for laser system and ICP-MS are presented here. Then, we analyzed five reference zircons(91500, GJ-1, Plesovice, FCT, Penglai)with a wide range in age from~1064 to~4.4Ma. Two standard zircons, 91500 and GJ-1, are employed as external reference standards. Generally, second zircon standard is analyzed in an effort to ensure accuracy and evaluate reproducibility. A typical analysis sequence includes one international glass standard(NIST610), two external reference standards, five grains of unknown zircon with every eight ablations. Laser induced time-dependent elemental fractionation is corrected using the intercept method, whereas instrument drift, mass bias and elemental fractional caused by ionization differences are corrected by external reference standard 91500 or GJ-1. Compared with 91500 and GJ-1, common Pb content of Plesovice, FCT, Penglai can't be ignored. Thus, we did common Pb correction for the above three standard zircons. The performance of the established procedure was assessed by analyzing zircon range in age from~1 064 to~4Ma. The results show that the ages of these five references are consistent with the ages of published studies with accuracy for three international references(91500, GJ-1, Plesovice)better than 3% and two young secondary references(FCT, Penglai)lower than 7% at the 2 sigma level, which indicates that our analytical procedure is reliable. For individual laser analysis, the uncertainties are mainly from three sources:Measurement error of isotope ratio, error of correction factors for instrument drift and element fractionation, and error of recommended age of external references. Compared to three international references, there are three extra uncertainties for young reference zircons, including:1)little radioactive isotopes closing to blank level increase the measurement error of isotope ratio; 2) effect of common lead becomes more significant;3) the nonhomogeneous samples couldn't match references well. Therefore, accuracy and precision of measurement depend on absolute age, content of common lead and matching degree between references and samples. In summary, the accuracy and precision obtained using the technique presented in this study are similar to those of other LA-ICP-MS laboratories.

Key words: LA-ICP-MS, zircon, U-Pb dating, geochronology

摘要： 激光剥蚀电感耦合等离子体质谱（LA-ICP-MS）为锆石U-Pb年代学提供了快速、精确的测试技术。文中利用Agilent 7900型四极杆电感耦合等离子体质谱仪（Q-ICP-MS）和Resolution M50-LR型193nm ArF准分子激光剥蚀系统联机，建立了完整的锆石U-Pb年龄测试流程。在激光束斑直径40μm、能量密度3.5J/cm²的条件下，用标准玻璃NIST612对测试系统进行调谐，使²³⁸ U的灵敏度高于30000cps/s。对5个锆石标样（年龄为4~1064Ma）进行了详细的定年研究，所获得的91500、GJ-1、Plesovice、FCT和蓬莱锆石的U-Pb年龄与前人报道的年龄在误差范围内是一致的，3个国际标样（91500、GJ-1、Plesovice）的测试精度优于3%，2个二级标样（FCT和蓬莱）测试精度较低，仅优于15%，结果表明该实验流程是可行的。锆石U-Pb年龄的分析误差主要来自3个方面：同位素比值测定误差、仪器灵敏度漂移和同位素分馏校正系数误差、标样推荐值误差。与国际标样相比，影响FCT和蓬莱锆石的分析误差除了以上3个因素外，还有以下3个方面：放射性成因Pb^*含量过低，测试误差增大；普通Pb对其年龄影响加剧，不易精确地扣除；样品与标样匹配程度降低。因此，样品的测试精度取决于绝对年龄、普通Pb含量和标样与样品匹配程度。

关键词: LA-ICP-MS, 锆石, U-Pb定年, 地质年代学