摘要
单颗粒锆石化学溶蚀同位素稀释热电离质谱(CA-ID-TIMS)U-Pb法是目前精度最高的同位素地质年代学方法,由于存在^(236)U ^(16)O ^(18)O^(+)和^(236)U ^(18)O ^(16)O^(+)对^(238)U ^(16)O_(2)^(+)的干扰问题,目前国际上通用的U-Pb定年稀释剂是^(205)Pb-^(233)U-235 U而不采用^(205)Pb-^(233)U-^(236)U。针对该问题,本文详细探讨了热电离质谱法(TIMS)测定U同位素过程中,^(236)U ^(16)O ^(18)O^(+)和^(236)U ^(18)O ^(16)O^(+)对^(238)U ^(16)O_(2)^(+)的干扰对U-Pb年龄测定结果的影响程度,指出ID-TIMS U-Pb分析过程中控制^(233)U-^(236)U稀释剂的加入量,保证样品-稀释剂^(238)U/^(236)U混合比>0.5,^(236)U ^(16)O ^(18)O^(+)和^(236)U ^(18)O ^(16)O^(+)对^(238)U ^(16)O_(2)^(+)的干扰对定年结果的影响并不显著。此外,详细探讨了热电离质谱(TIMS)U、Pb同位素测定过程中二次电子倍增器(SEM)死时间的准确设定对高精度CA-ID-TIMS U-Pb定年的重要性。在此基础上,建立了在线氧校正^(205)Pb-^(233)U-^(236)U稀释剂CA-ID-TIMS单颗粒锆石U-Pb年龄高精度测定方法。最后,采用所建立方法对标准锆石ZS进行了测定,获得的^(206)Pb/^(238)U年龄加权平均值为559.91±0.25Ma(2σ,仅分析误差),与文献报道值在误差范围内一致。由于采用了^(233)U-^(236)U双稀释剂内部校正U同位素分馏效应,该标样年龄测定精度达到0.05%,优于其^(205)Pb-^(235)U单稀释剂法年龄测定结果的精度。
Due to the interferences of ^(236)U ^(16)O ^(18)O^(+)and ^(236)U ^(18)O ^(16)O^(+)on ^(238)U ^(16)O_(2)^(+),a ^(205)Pb-^(233)U-^(235)U spike rather than a ^(205)Pb-^(233)U-^(236)U spike is usually used for the single grain zircon CA-ID-TIMS U-Pb dating worldwide.In order to use the ^(205)Pb-^(233)U-^(236)U spike for correcting the U isotope fractionation effect by the ^(233)U-^(236)U double spike,in this study,the impact of isobaric interferences of ^(236)U ^(16)O ^(18)O^(+)and ^(236)U ^(18)O ^(16)O^(+)on ^(238)U ^(16)O_(2)^(+) for the TIMS U isotope analysis is thoroughly addressed.We found that it is critical to ensure the sample is not over-spiked using the ^(205)Pb-^(233)U-^(236)U spike by controlling the amount of the spike to be added into the sample during the CA-ID-TIMS U-Pb analysis.Actually,when the ^(238)U/^(236)U ratio in the sample-spike mixture is not<0.5,the effect of the interferences of ^(236)U ^(16)O ^(18)O^(+)and ^(236)U ^(18)O ^(16)O^(+)on ^(238)U ^(16)O_(2)^(+) on the final U-Pb dating result is not significant.In addition,the importance of the correct setting of the dead-time of SEM(secondary electron multiplier)for the U and Pb isotope analyses for the CA-ID-TIMS U-Pb dating has also been fully addressed.Accordingly,a high precision CA-ID-TIMS U-Pb geochronology method for single zircons using ^(205)Pb-^(233)U-^(236)U tracers with in-run measured ^(18)O/^(16)O for UO_(2)^(+) isobaric oxide corrections has been developed.Finally,a standard zircon-ZS was analyzed using the method,yielding a weighted mean ^(206)Pb/^(238)U age of 559.91±0.25Ma(2σ,analytical error),which is consistent with the previously reported date within analytical error.Due to the use of ^(233)U-^(236)U double spike for internal correction of U isotope fractionation effect,the analytical precision for this standard zircon is better than 0.05%(2RSE),which is significantly better than that obtained by using the single ^(205)Pb-^(235)U tracer.
作者
储著银
许俊杰
凌潇潇
张普
程海
郭敬辉
周新华
CHU ZhuYin;XU JunJie;LING XiaoXiao;ZHANG Pu;CHENG Hai;GUO JingHui;ZHOU XinHua(State Key Laboratory of Lithospheric Evolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;Innovation Academy for Earth Science,Chinese Academy of Sciences,Beijing 100029,China;Institute of Global Environmental Change,Xi an Jiaotong University,Xi an 712000,China)
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2022年第12期3695-3702,共8页
Acta Petrologica Sinica
基金
国家自然科学基金项目(42073050、41673061)
国家重点研发计划项目(2020YFA0714803)联合资助.
