原位U-Th/He同位素定年技术研究进展及其低温矿床学应用前景

Progress of in situ U-Th/He Isotopic Dating Technique and Its Application to Low Temperature Deposits

应用传统单颗粒方法对目标矿物进行定年具有较高要求(如U、Th等母体同位素均匀分布),需要耗时的酸溶过程,同时还需进行α粒子射出效应校正。原位U-Th/He同位素定年技术是近年发展起来的一种定年技术,其主要原理是采用激光加热目标矿物,并通过与激光系统连接的稀有气体质谱(Alphachron)和电感耦合等离子体质谱(ICP-MS)分别完成 ~4He和U、Th等母体同位素分析,将 ~4He和U、Th分析结果代入年龄公式计算即可获得目标矿物的U-Th/He年龄。本文阐述了原位U-Th/He同位素定年技术的主要原理、实验测试流程、适用矿物等,重点对原位U-Th/He同位素定年的技术难点和低温矿床学应用前景进行了分析。相对于传统单颗粒方法,原位测试方法解决了两个关键问题:1无需进行α粒子射出效应的校正,提高了定年结果的可靠性和准确度;2能完成母体同位素分布不均匀样品的测试,扩展了U-Th/He同位素定年的应用范围。尽管原位U-Th/He同位素定年技术在侧向加热效应、剥蚀坑体积测定以及标准矿物等方面尚存在一些亟待解决的问题,但已在硅酸盐、磷酸盐、钛铁氧化物等矿物的年代学研究方面展示了良好的应用前景。随着原位U-Th/He同位素定年技术的发展和进步,尤其是硫化物的U-Th/He同位素定年的发展,将为解决低温矿床的年代学问题提供一种新的思路。

The traditional single-grain U-Th / He isotopic dating method a uses time-consuming acid dissolution, correction for α-ejection and more requirements on target minerals（euhedral,transparent,no cracks or inclusions）. In situ U-Th /He isotopic dating is a newly developed dating technique, which uses extracted ~4He from target mineral by laser ablation system and analyzes the ~4He and U,Th and other parent isotopes via the Noble Gas Mass Spectrum（Alphachron） coupled with laser system and Inductively Coupled PlasmaMass Spectrometry（ICP-MS）. The U-Th / He age of the deposits can be acquired by an age calculation formula using the analytical results of ~4He,U,and Th isotopes. The principles,analytical processes,and minerals suitable for dating,technique difficulty and application prospect in low-temperature mineral deposits,are presented in this paper. In situ U-Th /He dating technique solved two key problems compared to traditional single-grain method：（1）Correction of α-ejection is unnecessary,improving the reliability and accuracy of dating results,and（2） Overcoming the bias from heterogeneous distribution of parent isotopes（U,Th）,enlarging the range of U-Th /He isotopic dating. Although the in situ U-Th / He isotopic dating technique still needs to address issues such as collateral heating,precise measurement of pit volume and standard materials,it has shown itself to be a promising prospect for silicates,phosphates and Fe-Ti oxides. With the improvement of the in situ U-Th / He isotopic dating technique,particularly the U-Th / He isotopic dating of hydrothermal sulfides,this technique will provide a better way to date low-temperature mineralization.