U—Th／He热年代学原理、测试及应用

FUNDAMENTALS, DATING AND APPLICATION OF U-Th/He THERMOCHRONOLOGY

U—Th／He年代学方法已经被广泛地应用于地质学研究。特别是由于磷灰石中氦的低封闭温度，该方法对于晚新生代构造、河流和冰川等地表过程非常敏感，具有其他同位素年代学方法无法比拟的优越性。本文回顾了U-Th／He法的发展历史，介绍了U—Th／He法的基本原理、测试方法、α-校正以及U—Th／He法面临的问题，总结了U—Th／He法在新生代构造变形、河流地貌演化、气候变迁和源区示踪4个方面的应用，最后探讨了U-Th／He法未来的研究方向。

U-Th/He method has been widely applied to geological study. Especially the apatite U-Th/He chronometry is sensitive to late Cenozoic structure deformation, and to the geomorphic evolution drove by river incision and glacial erosion because of the low closure temperature of Helium. Therefore the apatite U-Th/He is more suitable than other isotopic chronometry for near surface process of crust. Here we will firstly review the meandering evolution history of U-Th/He method, then introduce the principle, analysis method, c^-correction and the facing problems of U-Th/He method, following summarize the four aspects of application of U-Th/He on Cenozoic deformation, on river geomorphic evolution, on climate change and on source tracing, finally look into the future of the method. The fundamentals of the U-Th/He dating are similar to U-Th/Pb dating method, and the difference between these two methods is the radioactive decay daughter. The decay daughter of U-Th/He method is helium, however the decay daughter of the U-Th/Pb is lead. Commonly the dilution method is used to analysize the uranium, Thorium and helium content and then calculate the age of mineral grain. The uncertainty of U-Th/He age by dilution method usually is 5 ~ 10 percent. The large uncertainty generally attribute to four factors, i.e. long α-stopping distance effect, unhomogeneous uranium and thorium distribution, helium implanting and helium closure temperature changing resulted from the radioactive damage. Therefore the boring grain-picking has been asked to make sure the mineral grain quality is good enough. In site helium dating by excimer laser is one potential mothed to overcome long α-stopping distance effect and reduce grain-picking work. However larger uncertainty （ 15 percent or more） of the in site dating method impede the widely application of the helium dating method. The helium has now been applied to a range of tectonic, geologic, geomorphologic, weathering, rive incision, erosion and source tracing problem. Four kinds of application of U-Th/He was presented to illustrate using U-Th/He age. The first one is dating the onset time of northern Qilian Shan exhumation drove by the Tibetan Plateau outgrowth ; the second one is dating the evolution of Colorado River Big Canyon; the third one is dating the goethite and studying the climate of Axel Heiberg Island; the last one is tracing the sediment source by U-Th/He and U-Th/Pb double dating. The future of the U-Th/he method was looked forward. Three potential researching aspects were proposed. The three aspects include improving analysis precision, U-Th/He and U-Th/Pb double dating and modeling the geomorphic evolution by combining the U-Th/He age, digital elevation model （DEM） and deformation data.