Rare Gases Geosciences Montpellier

Platform of Rare Gases subnet of REGEF French network

Methods

Argon/Argon datings

The 40Ar/39Ar dating method is derived from the K-Ar method and is used to date both very old rocks of the order of a billion years and very recent rocks of the order of a thousand years. It can be applied to individual minerals (micas, hornblende, feldspar, feldspathoids, cryptomelane, glass….), to whole rocks or to sections of rock polished by laser ablation. This method requires samples to be irradiated with a stream of fast neutrons in a nuclear reactor in order to transform part of the 39K into 39Ar. The measurement of this artificial argon isotope replaces that of 40K, so the age of the sample becomes a function of the 40Ar/39Ar ratio. To determine the yield of the nuclear reaction that produced 39Ar, standard samples calibrated by different laboratories are irradiated together with samples of unknown age. The 40Ar/39Ar method has a wide range of applications, including orogenic, volcanic, diagenetic and hydrothermal processes. The significant improvement in the sensitivity of gas source mass spectrometers and the use of high spatial resolution lasers now make it possible to work at crystal scale, either by progressive degassing of an isolated grain or by in situ fusion on a polished section. A technique for encapsulation in quartz tubes has been developed in the laboratory for dating small mineral phases (clays in particular) likely to lose argon during neutron irradiation.

fig. ArAr

Ar/Ar spectrum

banc encapsulation

Clays encapsulation bench

Uranium/Thorium/Samarium/Hélium datings

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.