锆石激光剥蚀原位微区(U-Th)/He定年方法研究

Over the past decade, (U-Th)/He chronology of U-Th-rich minerals (apatite, zircon, monazite, and titanite et al.) has become an extremely significant method for the investigation of low-temperature processes. However, procedure of tradition solution (U-Th)/He dating is both complex and time consuming. Moreover, different crystals of the same sample often get over dispersed single ages due to the fact that they always contain some inclusions and complicated grain shapes, which cause grate trouble not only for data acquisition but also for data interpretation. The development of technique of laser microprobe ablation makes laser ablation in-situ microprobe (U-Th)/He dating of zircon a highly promising method. This project is intended to carry out laser ablation in-situ microprobe (U-Th)/He dating of zircon utilizing the available facility in (U-Th)/He chronology laboratory, which was established in 2013, of institute of geology and geophysics, Chinese academy of sciences. Based on previous work, we attempt to use 193nm ArF excimer laser ablation system to simultaneously analyze daughter isotope (4He) on Alphachron MK II Quadrupole mass spectrometer and parent isotopes (235U，238U，230Th，147Sm) on X-Series II ICP-MS, which could overcome the above-mentioned difficulties encountered in traditional zircon (U-Th)/He dating.

近十多年来，富含U，Th矿物（锆石、磷灰石、独居石、榍石等）的 (U-Th)/He年代学已经成为研究地质体低温地质过程的重要手段。然而锆石的常规溶液法 (U-Th)/He定年溶样的过程十分繁琐耗时，而且常因样品富含包裹体及本身自形程度的限制，同一样品的不同颗粒常得到分散的单颗粒年龄，这数据的获取及解释带来了极大的困难。激光微区剥蚀技术的发展使锆石激光剥蚀原位微区 (U-Th)/He定年法具有很好的前景。本项目拟利用中国科学院地质与地球物理研究所 2013年建立的(U-Th)/He定年实验室中现有的实验设备，开展锆石原位微区 (U-Th)/He定年研究。本研究在前人实验方案的基础上，试图用193nm ArF准分子激光剥蚀装置，在同一个剥蚀斑上同时分析子体同位素（4He）和母体同位素（235U，238U，230Th，147Sm），克服锆石常规 (U-Th)/He定年法的技术限制。

本项目旨在通过准分子激光和4He提取系统和ICP-MS联用，在锆石晶体的几十微米区域上进行原位(U-Th)/He定年。据已有文献报道，东北穆棱地区的第三纪玄武岩中产出巨晶的锆石晶体，可以作为原位定年的研究对象。我们团队于2017年赴黑龙江穆棱地区采集了样品，但遗憾的是没有分选出文献中报道的锆石晶体。于是我们在产自缅甸抹谷地区的伟晶磷灰石进行了原位微区定年的测试，并获得了满意的结果。我们本次的工作，是国内首次对于副矿物进行的原位U-Th)/He定年研究。原位微区(U-Th)/He定年方法的建立，相比常规的溶液法可以大大提高工作效率，在今后碎屑矿物源区示踪以及盆地热史恢复等研究方向中有着重要的应用。