Advances in Earth Science ›› 2002, Vol. 17 ›› Issue (1): 126-131.

• Articles •

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

WU Qian-hong ,  LIU Hou-chang

1. Changsha Istitute of Geotectonics, CAS, Changsha  410013,China
• Received:2001-06-23 Revised:2001-09-25 Online:2002-12-20 Published:2002-02-01

WU Qian-hong , LIU Hou-chang. （U-TH)/HE DATING-A NEW METHOD OF LOW-TEMPERATURE THERMOCHRONOMETRY[J]. Advances in Earth Science, 2002, 17(1): 126-131.

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.

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