激光剥蚀系统气体流速变化对LA-ICP-MS锆石U-Pb定年精度的影响

Effect of Gas Flow Rates in Laser Ablation System on Accuracy and Precision of Zircon U-Pb Dating Analysis by LA-ICP-MS

锆石U-Pb定年精度一直是激光剥蚀-电感耦合等离子体质谱(LA-ICP-MS)分析方法的研究重点,激光剥蚀系统气体流速变化影响ICP-MS信号稳定性而影响锆石U-Pb定年精度,但影响程度和机制尚不清楚。本文以锆石标样91500及Plesovice为研究对象,采用LA-ICP-MS开展了载气和补偿气流速变化对锆石U-Pb定年结果准确度和精密度影响的研究工作。实验结果表明:固定补偿气Ar流速为1.0 L/min,而增大载气He流速(0.2~1.2L/min),锆石标样91500的^(206)Pb/^(238)U加权平均年龄增大(1002.0±10.4Ma~1083.0±6.8Ma,1σ),即样品气溶胶运输效率影响锆石U-Pb定年分析准确度,但He流速高于0.8L/min时由于大颗粒气溶胶引入使ICP-MS信号波动性和氧化物增加,导致锆石U-Pb定年分析精度降低。进一步以Plesovice锆石为例分析发现,Ar/He流速组合为0.95/0.8、0.8/0.8和0.8/0.6L/min时^(206)Pb/^(238)U加权平均年龄无显著性差异,但Ar/He流速均为0.8L/min时1σ单点分析相对偏差最小(1.4%),即通过控制载气和补偿气流速组合,优化样品气溶胶运输效率可提高LA-ICP-MS锆石U-Pb定年精度。在本实验条件下,0.8L/min为载气和补偿气流速最佳取值。

BACKGROUND:Despite zircon U-Pb dating analysis by LA-ICP-MS receiving wide acceptance,it remains a challenge to obtain results with high accuracy and precision.It is known that gas flow rates of LA system can affect the signal stability of ICP-MS and thus result in impacts on analytical uncertainty of zircon U-Pb dating.However,the exact effects and mechanism of gas flow rates on zircon U-Pb dating analysis are still unclear.OBJECTIVES:To thoroughly understand the influence of gas flow rates on the analytical uncertainty of zircon U-Pb dating,and to provide valuable information to propose a reliable and robust LA-ICP-MS approach for zircon U-Pb dating analysis.METHODS:By applying zircon standard samples of Harvard 91500 and Plesovice as researching subjects,ICP-MS connected to a 193nm nanosecond laser ablation system was used to investigate the influence of gas flow rate settings on accuracy and precision of U-Pb dating analysis.RESULTS:With fixed make-up gas(Ar)of 1.0L/min,the average ^(206)Pb/^(238)U ages of Harvard 91500 were found to increase from 1002.0±10.4Ma(1σ)to 1083.0±6.8Ma(1σ)with increasing carrier gas(He)from 0.2 to 1.2L/min.Thus,it was clear that the sample aerosol transportation efficiency can greatly affect the analytical accuracy of zircon U-Pb dating.Furthermore,when the He flow rate was higher than 0.8L/min,the analytical accuracy and precision of zircon U-Pb dating decreased due to the increased signal intensity oscillations and formation of oxides from the introduction of large particles of sample aerosols.The comparison of the data of Plesovice obtained under 0.95/0.8,0.80/0.8 and 0.8/0.6L/min for He/Ar gas flow rate patterns indicated that there were no significant differences in U/Pb weighted average age.However,the relative deviation of 1σsingle-point analysis was the smallest(1.4%)when the Ar and He flow rates were both 0.8L/min.CONCLUSIONS:The analytical accuracy and precision of zircon U-Pb dating by LA-ICP-MS can be improved by optimizing the gas flow rate setting of carrier gas and make-up gas,and highly recommending 0.8L/min of both Ar and He.