碳酸盐矿物的高精度U-Pb等时线测年研究

In recent years, with the rapid development of laser-ablation ICP-MS and SIMS zircon U-Pb dating techniques, many traditional isotope dating methods (such as the ID-TIMS method) have been gradually marginalized or abandoned, due to the strict requirements for ultra clean laboratory, sample vetting and pretreatment, and associated intensive time and labor costs, leading to many geochronological problems unresolvable except for limited zircon-bearing geological materials. This project proposes to develop an innovative high-precision U-Pb isochron method for dating relatively young carbonate minerals beyond the dating limit of the U-series dating technique (i.e. >500,000 years) by means of low blank ultraclean laboratory and isotope-dilution MC-ICP-MS technology, aiming to improve measurement precision and accuracy. As a pilot study, the newly-developed method will be used to date cave speleothems and carbonate veins from hydrocarbon-bearing sediment basins that we have already collected from several key locations. The method will be particularly suited for dating samples of Quaternary and Tertiary ages, closing an important geochronological gap beyond the limit of U-series dating method. It will also open new horizons in many other areas, such as dating long-term paleoclimate cycles and paleoenvironment changes, and the associated ancient ecosystem response to environmental changes, as well as the chronologies of hydrocarbon-bearing sedimentary basin evolution and endogenous metal ore genesis. The team hosting the lead applicant has well-established collaborations with two internationally recognised carbonate dating laboratories, and the applicant has the opportunity to visit both laboratories to enhance her career. All of these ensure the success of this project.

近年来，随着激光剥蚀和离子探针锆石U-Pb测年技术的快速发展，许多传统同位素测年方法(如ID-TIMS)，由于对超净实验室的环境要求高，样品选择、预处理和测试流程费时费力，而逐渐被边缘化，进而导致许多无法使用锆石定年的年代学问题得不到有效解决。本课题拟通过低本底超净室创新技术手段，开展对年轻(>50万年)碳酸盐矿物精确定年的U-Pb等时线测年方法研究，提高其测定的精确度和准确性；并将此测年方法应用于洞穴石笋和与油气盆地演化有关的碳酸盐脉的精细年代学研究领域。该定年技术不仅能弥补超出铀系测年范围(>50万年)的第四、第三纪地质测年手段的空缺，还在探究古气候古环境变化周期和古生态系统对环境变迁的响应机制，以及解决油气盆地演化和内生金属成矿的年代学问题等方面，都有广阔的应用前景。申请者团队与两家国际著名的碳酸盐定年实验室有密切合作关系，且有机会去那里学习，这些都为本课题的顺利开展提供了有利保证。

本研究通过激光剥蚀-多接收(离子计数) 等离子体质谱 (Laser Ablation Multi-Ion counter Collection I , LA-MC-ICPMS ) ，首次建立针对低U碳酸盐矿物的U-Pb等时线定年方法，并且对来自油气盆地的古老碳酸盐矿物进行了绝对年代学研究。同时，利用该方法对两个国际认可的碳酸盐矿物标准ASH15和WC-1进行长期测试，其结果与前人数据在误差范围内一致，测试精度达到国际同类实验室先进水平。由此，我们寻找到一个更加合适古老碳酸盐矿物定年的碳酸盐矿物标准AHX-1 (CT10) ，经过与ASH15和WC-1两标准长期交叉对测，最终获得的LA-MC-ICPMS U-Pb加权平均年龄为209.8±1.3 Ma (2S总, n=21, MSWD=2.7)。此年龄结果是在半年内不同时期对样品靶不同部位21 次独立标定的年龄结果的加权平均值, 每一次对测约有50~150 组数据点, 总共2000多组数据点。.塔里木盆地肖尔布拉克组和四川盆地灯影组是中国极为重要的两处油气勘探区域，但绝对年龄数据的缺乏阻碍了人们对成储成藏的认识，进而制约了对有利勘探区带的评价。针对该区域古老海相碳酸盐矿物的低U、Pb的特点，粉末取样难度大的问题，以及基体匹配的碳酸盐矿物的需求，我们推出了这套适用于低U古老海相碳酸盐矿物激光U-Pb同位素定年技术。通过对塔里木盆地肖尔布拉克组和四川盆地灯影组充填孔洞、孔隙和裂隙中白云石和方解石胶结物的定年，所得到的结果与其二元同位素、碳氧同位素、锶同位素、微量和稀土元素地球化学特征相吻合，进一步验证了该定年技术的可靠性。在此基础上，一些合作单位结合构造-埋藏史、盆地热史和烃源岩生烃史，分析碳酸盐岩储集层的孔隙成因和成因-孔隙演化史，为油气运移前有效孔隙评价提供依据。