Rare Gases Geosciences Montpellier

The GRGM platform has two ARGUS VI multi-collection mass spectrometers (Thermofisher) equipped with 5 Faraday cages and a CDD multiplier for precise measurement of 36Ar. The various argon isotopes are measured simultaneously, saving analytical time and improving the accuracy of isotope ratios.

One of the two ARGUS VIs, acquired in 2011, is equipped with 5 faraday cages with 10¹² ohm resistors and a CDD, and is connected to an automated low-volume gas extraction/purification line comprising 2 SAES getters and two sample chambers fitted with ZnSe and UV-treated quartz portholes. Three lasers are used to extract argon from samples: a CO2 laser emitting in the infrared for progressive degassing of classic minerals (feldspar, amphibole, micas, etc.) and less classic minerals (jarosite, cryptomelane, etc.) in monograin or micro-population, a UV laser at 266 nm for in situ analysis of minerals on polished sections and a fibre laser for progressive degassing of clay minerals (celadonite, glauconite, illite, etc.). The entire analytical procedure is controlled using Labview and calculations are made using ArArCALC software. Every day 40 to 50 analyses (heating steps), including the analysis of blanks carried out every 2 or 3 steps, can be carried out on this instrument. The spectrometer’s mass discrimination is monitored daily.

The other Argus VI has just been acquired, in November 2023, and is currently being installed. It is equipped with 5 faraday cages with 10¹³ ohm resistors and a CDD, enabling the 36Ar to be measured either on a cage or on the CDD. The signal-to-noise ratio is greatly improved compared with previous-generation instruments, and will enable very young and very small samples to be analysed with great precision. It is connected to an automated low-volume gas extraction/purification line comprising 4 SAES getters and a cold trap.  It will be used mainly to analyse clay minerals (celadonite, glauconite, illite, etc.).

MAP215-50 mass spectrometer

The MAP215-50 mass spectrometer is equipped with a Nier source and an electron multiplier, and is linked to a line dedicated to the development of simultaneous analysis of K and Ar isotopes by laser ablation, enabling K/Ar dating of potassium samples. The physical coupling of elemental/isotopic analyses is simplified by the use of a single laser emitting in the UV (Nd:YAG), this wavelength making it possible both to produce the elemental plasma for K analysis by Laser-Induced Breakdown Spectroscopy (LIBS) and to extract the Ar contained in the sample. This ablation technique is traditionally used for 40Ar/39Ar dating. Ablation is carried out in a vacuum chamber fitted with an ultra-pure quartz window to ensure optimum beam transmission for LIBS analysis. The LIBS technique is based on the interaction of a pulsed laser with the material to be analysed, resulting in the vaporisation of the material and the formation of a plasma. Analysis of the emission spectrum of the plasma radiation provides access to the elemental atomic composition of the sample. Argon is analysed by expanding the gas in the ultra-high vacuum line to the mass spectrometer.

This spectrometer can also be connected to a low-volume automated line for argon isotope analysis, and the samples are degassed using a CO2 laser emitting in the infrared, the beam of which is focused by a set of lenses through a zinc selenide window. The gas is purified using SAES getters and a liquid nitrogen trap.