Rare Gases Geosciences Montpellier

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.

Ar/Ar spectrum

Clays encapsulation bench