Rare Gases Geosciences Montpellier

The (U-Th-Sm)/He method, discovered at the very beginning of the 20th century by Ernest Rutherford (Nobel Prize in Chemistry, 1908), was abandoned for a time, but gained new momentum when the loss of daughter elements (helium nucleus or alpha particle) could be quantified from the 1990s onwards, thanks in particular to its applications in low-temperature thermochronology. In the decay chains of U, Th and Sm, one or more helium nuclei are produced, and it was this phenomenon that Rutherford studied and highlighted. However, depending on the kinetic energy of the alpha particle, a certain number of daughter elements may be ejected from the host mineral. It is therefore necessary to estimate the ejection loss and apply a correction factor to the measured age. Furthermore, the daughter elements (alpha particle or helium nucleus) diffuse out of the mineral as a function of temperature. The mineral is an open system up to low temperature and the age measured corresponds to the age at which the closing temperature was reached (apatite: 60-80°C and zircon: 150-180°C) and not to an age of crystallisation. As a result, geochronological applications of Rutherford’s discoveries were abandoned. The (U-Th-Sm)/He ages correspond to thermochronological ages that incorporate both the production and diffusion of the daughter element in the mineral. By analysing the ages and their dispersion in one or more samples, it is possible to reconstruct the thermal history of the samples and therefore their exhumation history.