锆石(U-Th)/He定年技术研究

Study on zircon(U-Th)/He dating technique

锆石(U-Th)/He定年是同位素热年代学体系中重要的定年手段,是记录地质体完整热历史重要的实验方法之一,在火山岩定年、造山带演化、地貌演化、沉积盆地热演化及限定矿床热液活动时代等应用方面发挥着重要的作用。中国地质科学院地质研究所同位素热年代学实验室成功建立了锆石(U-Th)/He定年的实验方法,实验主要包括三个独立的过程:样品前处理、He含量分析和U、Th含量分析。氦同位素质谱仪对锆石样品的氦同位素比值分析精度约为0.1%左右;ICP-MS对锆石的U、Th同位素比值分析精度通常情况下优于1%。对国际上普遍使用的FCT锆石和斯里兰卡锆石标样进行(U-Th)/He年龄测定。测试结果显示27粒FCT锆石(U-Th)/He年龄分布在25.81~30.72Ma之间,加权平均年龄为28.18±0.51Ma(1σ)(参考值为28.3±2.6Ma);20粒斯里兰卡锆石碎片(U-Th)/He年龄分布在445.5~489.5Ma之间,排除异常值后加权平均值为479.0±8.0Ma(1σ)(参考值为470±11Ma)。所测两个标准物质的年龄均与参考值一致,表明本实验室的实验流程准确可靠。本实验方法的建立填补了我国锆石(U-Th)/He定年实验方法的空白,为我国热年代学的发展提供了新的技术支撑。

Abstract Zircon （U-Th）/He dating is an important method of thermochronometry and it is indispensable to recording the complete thermal history of geological bodies. It has been widely used in geological researches such as volcanic rock dating, constraining evolution of orogenic belts, landform, sedimentary basins and hydrothermal activities. Zircon （U-Th）/He dating method, which including sample preparation, He analysis and U, Th analysis, has been established in laboratory of Institute of Geology, Chinese Academy of Geological Sciences （CAGS）. Analytical precision on He isotope ratio measurement of zircon is about 0. 1% , and U, Th isotope ratio measurements are typically 〈 1%. Fish Canyon Tuff （FCT） zircon and Sri Lanka zircon, which are commonly used as standard for zircon （U-Th）/He dating, have been dated by using the newly established experimental procedure. All 27 ages of FCT zircons range from 25.81Ma to 30. 72Ma, with a weighted mean age of 28.18 ± 0. 51Ma （ lσ） （reference value is 28.3 ± 2. 6Ma） ; All 20 ages of Sri Lanka zircons range from 445. 5Ma to 489. 5Ma, with a weighted mean age of 479.0 ±8.0Ma （lσ） （reference value is 470±11Ma）. The ages of two standards are consistent with their reference value, indicating that the experimental procedure we established is accurate and reliable. The establishment of this dating method fills the blank of zircon （U-Th）/He dating method, and provides a new technical support for the development of thermoehronology in China.