实时在线氧校正205Pb-233U-236U稀释剂单颗粒锆石ID-TIMS U-Pb高精度年龄测定方法

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²³⁶U¹⁶O¹⁸O⁺和²³⁶U¹⁸O¹⁶O⁺对²³⁸U¹⁶O₂⁺的干扰问题，目前国际上通用的U-Pb定年稀释剂是²⁰⁵Pb-²³³U-²³⁵U而不采用²⁰⁵Pb-²³³U-²³⁶U。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，²³⁶U¹⁶O¹⁸O⁺和²³⁶U¹⁸O¹⁶O⁺对²³⁸U¹⁶O₂⁺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²³³U-²³⁶U稀释剂的加入量，保证样品-稀释剂²³⁸U/²³⁶U混合比>0.5，²³⁶U¹⁶O¹⁸O⁺和²³⁶U¹⁸O¹⁶O⁺对²³⁸U¹⁶O₂⁺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²⁰⁵Pb-²³³U-²³⁶U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²⁰⁶Pb/²³⁸U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²³³U-²³⁶U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²⁰⁵Pb-²³⁵U单稀释剂法年龄测定结果的精度。

Precise ID-TIMS U-Pb dating of single zircons using 205Pb-233U-236U tracers: Isobaric oxide corrections for UO2+ with in-run measured 18O/16O

Due to the interferences of 236U16O18O+ and 236U18O16O+ on 238U16O2+, a 205Pb-233U-235U spike rather than a 205Pb-233U-236U spike is usually used for the single grain zircon CA-ID-TIMS U-Pb dating worldwide. In order to use the 205Pb-233U-236U spike for correcting the U isotope fractionation effect by the 233U-236U double spike, in this study, the impact of isobaric interferences of 236U16O18O+ and 236U18O16O+ on 238U16O2+ 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 205Pb-233U-236U 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 238U/236U ratio in the sample-spike mixture is not < 0.5, the effect of the interferences of 236U16O18O+ and 236U18O16O+ on 238U16O2+ 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 205Pb-233U-236U tracers with in-run measured 18O/16O for UO2+ isobaric oxide corrections has been developed. Finally, a standard zircon-ZS was analyzed using the method, yielding a weighted mean 206Pb/238U 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 233U-236U 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 205Pb-235U tracer.