(U-Th)/He thermochronology — methodology and a case study: dating of faulting in the Southern Alps

Abstract

The radiogeic 4He isotope is continuously forming in the lithosphere mainly by the alpha-decay of U, Th and Sm. This decay process was discovered already at the beginning of the 20th century, and the first U/He dating was made by Rutherford (1905). Although his result indicated that the dimensions of the Earth’s history ranges to millions of years, the U/He method was used only scarcely later on because the minerals are usually not closed to the decay product. The helium is extremely mobile and diffuse through the crystal lattices. Thus, the apparent U/He ages were always younger than the radiometric ages determined by other isotope geochronometers like U/Pb, Rb/Sr or K/Ar. The current renaissance of the method has been started at the end of eighties, when H. Lippolt, P. Zeitler, K. Farley and their co-workers have described the parameters of diffusion of some uraniumbearing minerals (e.g. Lippolt & Weigel 1987, Zeitler et al. 1987). It turned out that the (U-Th)/He apparent ages do carry meaningful geological information. The closure temperature of the most widely used apatite-He system is around 70°C. Thus, by the usage of this mineral/method pair it became possible to date low-temperature geological processes, which were undatable by other geochronometers...