【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²³⁶U¹⁶O¹⁸O⁺和²³⁶U¹⁸O¹⁶O⁺对²³⁸U¹⁶O⁺₂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²³⁸U¹⁶O⁺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²³⁸U¹⁶O⁺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。更多还原

【Abstract】 Due to the interferences of ²³⁶U¹⁶O¹⁸O⁺ and ²³⁶U¹⁸O¹⁶O⁺ on ²³⁸U¹⁶O⁺₂, 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 ²³⁸U¹⁶O⁺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 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 ²³⁸U¹⁶O⁺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 ²⁰⁵Pb-²³³U-²³⁶U tracers with in-run measured ¹⁸O/¹⁶O for UO⁺₂ 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±0.25 Ma（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%（2 RSE）, which is significantly better than that obtained by using the single ²⁰⁵Pb-²³⁵U tracer.更多还原