束斑直径和能量密度对锆石U-Pb定年准确度的影响研究

Discussion on Spot Size and Energy Density Effects on Zircon U-Pb Dating Precision

激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)是锆石U-Pb定年常用的方法之一。应用LA-ICP-MS进行锆石U-Pb定年的过程中,束斑直径的大小和能量密度的高低是影响定年准确度的两个关键因素。本文开展了应用LA-ICP-MS在不同束斑直径和能量密度条件下对锆石标样91500和GJ-1 U-Pb定年结果准确度的对比研究,旨在优化LA-ICP-MS锆石U-Pb定年的方法,提高分析准确度,也为其他实验室开展类似的研究工作提供一定的借鉴。实验结果表明:在剥蚀频率、扫描速度、载气流速保持不变的前提下,当能量密度固定,随着剥蚀束斑直径从25μm增大至65μm的过程中,锆石91500和GJ-1在互为标样和盲样的条件下实测年龄结果与推荐值之间的相对误差(RE)不断减小(RE分别为0.61%、0.41%、0.13%和0.08%),即增加束斑直径降低了相对误差,提高了定年准确度。而当束斑直径固定时,能量密度≤3.5 J/cm^(2)时,随着能量密度不断增大,实测年龄结果与推荐值之间的相对误差逐渐减小(RE分别为8.69%、3.48%、3.95%、4.16%和0.41%),定年准确度逐渐增加;但是当能量密度大于3.5 J/cm^(2)时,能量密度增大导致剥蚀速率显著增大,在深度方向上的分馏效应明显增加,对比3.5 J/cm^(2)条件下,此时的实测年龄结果与推荐值之间的相对误差增大(RE分别为2.40%和0.83%),定年准确度降低。因此,在实际测试中,考虑到样品大小和尽可能少地对样品进行破坏,合理的能量密度和束斑大小可以有效提高定年的准确度。

LA-ICP-MS,an effective method for zircon U-Pb dating,is the most commonly used technology in geochronological studies.In the LA-ICP-MS zircon U-Pb dating process,the spot diameter and energy density are two important factors that can affect the dating precision.Zircon standard 91500 and GJ-1 are selected as both standards and unknown samples,respectively,and the effects of spot size and energy densities on the LA-ICP-MS U-Pb dating were determined.We aim to optimize the LA-ICP-MS zircon U-Pb dating method,improve the analysis accuracy,and provide reference for other laboratories to carry out similar research work.The results show that with fixed energy density when the ablation beam spot increases from 25 to 65μm,the relative error(RE)between the measured age results and the recommended values of zircon 91500 and GJ-1 decreases continuously(RE=0.61%,0.41%,0.13%,and 0.08%,respectively),i.e.,increasing the beam spot diameter can reduce RE and improve the dating precision.Under fixed beam spot diameter(35μm)with energy density below or equal to 3.5 J/cm^(2),as the energy density increases,the relative error between the measured age results and the recommended values gradually decreases(RE=8.69%,3.48%,3.95%,4.16%,and 0.41%,respectively),and the dating precision gradually increases.However,when the energy density is over 3.5 J/cm^(2),the energy density increase can lead to a significant increase in the denudation rate,and the fractionation effect in the depth direction increases greatly.The relative error between the measured ages and recommended values increases(RE=2.40%and 0.83%,respectively),and the dating precision decreases.Therefore,in the actual test,considering the sample size and the need to preserve the integrity of the sample,choosing reasonable energy density and beam spot size can effectively improve the dating precision.