超低本底高精度单颗粒锆石CA-ID-TIMS U-Pb年代学方法及应用研究

Sing grain zircon CA-ID-TIMS U-Pb radiometric method is very important in high-precision geochronology. The method has been widely used in research fields such as high-precision chronostratigraphy, high-precision dating of important geological events, and calibration of the age of standard zircons used for SIMS and LA-ICP-MS. At current, the main technical bottlenecks for CA-ID-TIMS U-Pb method in China are the relatively high Pb laboratory blank (2-3 pg), and the lack of well-calibrated U-Pb spikes. In this project, we plan to cooperate with famous international ID-TIMS U-Pb geochronological laboratory, to further reduce the U-Pb blank level of the laboratory at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS) (to less than 0.5 pg), and produce well-calibrated U-Pb spikes. We will share the spikes with other ID-TIMS U-Pb labs in China. Through these important works, we will establish the high-precision CA-ID-TIMS U-Pb dating method at IGGCAS thoroughly. Then, we will try to further calibrate the age of E-C boundary in south China with the method. The implementation of this project will greatly improve the high-precision geochronology research in China, and contribute to the Earthtime-CN project.

单颗粒锆石CA-ID-TIMS U-Pb法是目前最重要的高精度同位素地质年代学方法。我国CA-ID-TIMS U-Pb方法目前仍存在Pb流程本底相对较高(2-3 pg)以及缺乏高精度U-Pb同位素稀释剂等主要技术瓶颈。本项目拟借鉴国外先进实验室经验，进一步探索降低实验室流程本底的措施，力争将我们CA-ID-TIMS U-Pb实验室Pb流程本底降低至0.5 pg以下，并且精确配制U-Pb同位素稀释剂。通过上述工作，在我国建立达到国际先进实验室水平的CA-ID-TIMS U-Pb高精度年代学方法。配制的稀释剂将供国内ID-TIMS U-Pb年代学实验室共享使用。在此基础上，拟开展华南寒武纪底界地层剖面高精度CA-ID-TIMS U-Pb年代学示范性应用研究工作。本项目对推动我国高精度地质年代学实验技术及应用研究的深入发展，提高我国在同位素地质年代学研究领域的国际地位具有重要意义。

通过本项目的实施，基本成功建立了单颗粒锆石CA-ID-TIMS高精度年代学方法。包括：（1）配制和标定了205Pb-235U及205Pb-233U-236U稀释剂；（2）进一步降低了实验室流程本底，Pb全流程本底已降低至并基本稳定在1-5 pg，U本底< 0.5 pg，基本满足单颗粒锆石CA-ID-TIMS U-Pb高精度分析要求；（3）通过长期NIST981和U500标样测定，确定了实验室TRITON Plus仪器的Pb、U同位素分馏系数；（4）通过长期本底Pb同位素测定，基本确定了实验室本底Pb同位素组成（用于本底扣除计算）；（5）全面制定和编写了ID-TIMS U-Pb年龄及误差传递计算程序。此外，我们还利用TRITON Plus仪器多离子计数器系统，建立了多离子计数器动态跳扫高精度锆石ID-TIMS Pb同位素测定方法，相对传统的单离子计数器跳扫测定方法，该方法可以提高单位时间内锆石ID-TIMS Pb同位素测定精度。.采用所建立方法对贵州平引剖面留茶坡组顶部寒武纪底界两层火山灰锆石进行了高精度CA-ID-TIMS U-Pb年龄测定工作，并结合硅质岩有机碳同位素分析，对华南前寒武纪底界年龄进行了进一步限定（~540.58 ± 0.66 Ma）。同时，对新疆塔里木盆地阿克苏地区羊吉坎剖面晚奥陶世火山灰锆石开展了高精度CA-ID-TIMS U-Pb同位素年代学研究工作。特别地，对羊吉坎剖面同一层位火山岩锆石分别在我们实验室和美国MIT U-Pb年代学实验室进行了测定，两实验室年龄结果在误差范围内一致，进一步表明了本项目所建立CA-ID-TIMS U-Pb方法的可靠性。此外，所建立方法已开始服务于我国年代地层学及标准锆石定值等高精度定年研究工作。