Abstract: Zircon is one of the most commonly used accessory minerals rich in U and Th for(U-Th)/He dating system. Compared with apatite, zircon has a higher He closure temperature (~190℃), which gives it more advantages in solving the problem of source material and thermal history reconstruction in sedimentary basins. However, the crystals of zircons often have U and Th zoning development, with obvious differences in concentration. Even the standard sample of FCT(Fish Canyon Tuff)zircon which is widely used in (U-Th)/He dating has an average age dispersion of about 10%. In this study, the Alphachron He isotope mass spectrometer is used for laser melting of a batch of single grains of FCT zircon(11 grains)to determine their ⁴He content. The contents of U and Th of parent isotopes are accurately determined by automatic injection of Agilent 7900 ICP-MS and isotope diluent method. The Th/U ratios of the 10 FCT zircons calculated with (U-Th)/He average age in this paper range from 0.52 to 0.67, which are consistent with the Th/U ratios of 186 reported so far. According to the Th/U ratios of 189 FCT zircons published in the statistical literature, we found that only three of them had high Th/U ratios, namely, 1.12, 1.16 and 1.5, the other 186 FCT zircons(occupy>98%) had a Th/U ratio less than 1. Based on previous results and the 10 Th/U ratios measured in this paper, 196 FCT zircons have a normal Th/U ratio ranging from 0.27 to 1.00, with an average ratio of 0.56(n=196). Excluding one abnormally old age, the(U-Th)/He ages of the remaining FCT zircons in this study range from 26.61 to 31.91Ma, with a weighted mean age of (28.8±3.1)Ma (2SD, n=10), which is consistent with the mean age ((28.3±3.1)Ma, 2σ, n=127) or (28.29±2.6)Ma(2σ external error, 9.3%, n=114)obtained by several other international laboratories. This indicates that the zircon single particle(U-Th)/He dating process established by our laboratory is reliable. For the zircon samples with U, Th banding and concentration differences prevailing, determining the distribution of U, Th elements in the crystal prior to the (U-Th)/He experiment is essential for understanding effects of geometry and elemental zoning on nuclear recoil and diffusion and the interpretation of (U-Th)/He age data.

Key words: (U-Th)/He dating, FCT zircon, laser melting, isotope dilution method

摘要： 锆石是（U-Th）/He测年体系中最常用的富含U、Th的副矿物之一。相对于磷灰石而言，锆石的He封闭温度较高（约190℃），在解决沉积盆地物源和热史恢复方面更具优势。但锆石晶体内的U、Th分带普遍发育，浓度差异明显，即使是国际普遍使用的（U-Th）/He测年标样FCT（Fish Canyon Tuff）的年龄值分散度也可约达10%。文中依托中国地震局地质研究所新建立的（U-Th）/He年代学实验室，利用配备的Alphachron He同位素质谱仪对一批FCT锆石单颗粒采用激光加热萃取后进行⁴He含量测定，并应用自动进样的安捷伦7900 ICP-MS和同位素稀释剂法测定母体同位素U、Th的含量，得到FCT锆石的年龄范围为26.61~31.91Ma，加权平均年龄为（28.8±3.1）Ma（2SD，n=10），该年龄值与国际上多个实验室所获得的平均年龄（（28.3±3.1）Ma，2σ，n=127；（28.29±2.6）Ma，2σ外部误差，9.3%，n=114）在误差范围内一致；Th/U比值范围是0.52~0.67，与国际报道值一致，说明本实验室所建立的锆石单颗粒（U-Th）/He测年实验流程可靠。

关键词: (U-Th)/He定年, FCT锆石, 激光熔融法, 同位素稀释剂法