（U-Th）/He定年——低温热年代学研究的一种新技术

doi:10.11867/j.issn.1001-8166.2002.01.0126

地球科学进展 ›› 2002, Vol. 17 ›› Issue (1): 126 -131. doi: 10.11867/j.issn.1001-8166.2002.01.0126

吴堑虹，刘厚昌

中国科学院长沙大地构造研究所，湖南长沙 410013

收稿日期:2001-06-23 修回日期:2001-09-25 出版日期:2002-12-20
通讯作者: 吴堑虹(1957-)，女，安徽凤阳人，副研究员，主要从事构造地质及地球化学.E-mail: wqh@ms.csig.ac.cn E-mail:wqh@ms.csig.ac.cn
基金资助:
国家重点基础研究发展规划项目“大陆深俯冲作用”(编号：G199075504)资助.

（U-TH)/HE DATING-A NEW METHOD OF LOW-TEMPERATURE THERMOCHRONOMETRY

WU Qian-hong , LIU Hou-chang

Changsha Istitute of Geotectonics, CAS, Changsha 410013,China

Received:2001-06-23 Revised:2001-09-25 Online:2002-12-20 Published:2002-02-01

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近年来（U-Th）/He定年方法在技术和低温热年代学应用上的优势吸引人们以此开展构造、地形地貌演化等的研究；与裂变径迹、KAr、ArAr等定年方法的比较，该方法是有效的；人们研究了温度及矿物粒径对He扩散的影响，确定了校正方法。（U-Th）/He的封闭温度低于其它体系（磷灰石的仅为75℃±）；有质谱仪只需数十毫克样品就能获得 U、Th、He含量；有较裂变径迹法快速、省样、省力的优点；该技术虽还有待完善，但仍不失为开展低温热年代学和低温热演化研究的潜在有效方法。

关键词: （U-Th）/He, 定年, 低温热年代学

Recent technical and interpretational advances in (U-Th)/He dating have produced an interest in the low-temperature thermochronometric capabilities of He dating. A wide range of applied studies has confirmed the scientifically reasonable of the method in constraining the tectonics and topographic development from many place of the world. Compared with fission track, K-Ar, Ar-Ar dating methods, the age of (U-Th)/He dating is scientifically reasonable. The basic principle of (U-Th)/He dating is that ²³⁸U, ²³⁵U and ²³²Th decay to ⁴He and although these isotopes are seldom in concentrations above a few hundred ppm. A He age is determined by comparing the accumulated ⁴He in a mineral with the concentration of the parent elements. Actually the age are measured by the slope of isochrons which is ⁴He versus ［U+(F×Th)］weighted by the reciprocal of the squared analytical error. After understanding the relative diffusion of He by temperature and size of mineral, the correcting constraint has been carried out. The ³He is much less than ⁴He and ³He is originally from cosmic there is very few ³He in the surface of the earth.. During the measure the ³He is neglect. Laboratory He diffusion experiments suggest that (U-Th)/He system's closure temperature is lower than other isotopic system ( apatite only yields a closure temperature of 75℃). Suppose the earth surface temperature of 10 ℃ and geothermal gradient of 25 ℃/km, it is equivalent to depths of 1 to 3 km beneath the earth's surface. Thus the system can be applied to investigate some geologic processed in the almost uppermost part of the crust, for example, volcanism, large-scale fault slip and landscape development even petroleum exploration. The concentrations of U, Th, He in sample of several mg are measured by mass spectrometer. Compared with fission track technique the (U-Th)/He method have some advantages of efficiency in science and saving samples and labor. There are still some technical difficulties with (U-Th)/He dating such as it is impossible to identify and take out the U and Th rich mineral inclusions usually zircon in an apatite. These mineral inclusions contribute He but do not contribute U and Th during measuring process. Erroneously high He ages thus result. Although the technique is under improving, it is still a potential and efficient technique on low-temperature thermochronometry and thermal developing progress.

Key words: (U-Th)/He, Dating, Low teperature thermochronometry.

中图分类号:

P597

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