Isotopic Geochemistry and Geochronology

The objective of this technical platform is the production of geochemical (stable and radiogenic isotopes) and geochronological/thermochronological analyses. This technical platform is an essential part of OSUR’s Analytical Platform and has 6 mass spectrometers, a quadrupole ICP-MS coupled with an Excimer laser, and an automated microscope for fission track analysis.
Système Ablation Laser ICP-MS de Géosciences Rennes

Functions of the technical platform

At the moment, five researchers, lecturers/researchers and one Post-Doc are directly involved in the functioning of the platform:

Philippe Boulvais (Senior Lecturer) - Stable isotopes,
Gilles Ruffet (Research Fellow) - 39Ar-40Ar,
Marc Jolivet (Research Director) - Fission tracks,
Kerry Gallagher (Professor) - Fission tracks,
Nathan Cogné (Post-Doc) - Fission tracks, Track and U-Th-Pb Analyses using LA-ICP-MS,
Marc Poujol (Senior Lecturer) - LA-ICP-MS, U-Pb, Sr, Sm-Nd.

This plateau is currently made up of 5 entities:

Rare gas spectrometry, Stable isotope spectrometry, Solid source spectrometry (TIMS), Laser Ablation ICP-MS and Fission Track Analyses.

David Vilbert, Dominique Bavay and Nathan Cogné are busy developing a helium extraction line for (U-Th)/He mineral dating (see figure below). This involves helium extraction by laser heating, and part of the line will then be used to purify the gas, and ultimately, the analysis will be carried out by a quadrupole mass spectrometer (Prisma). The entire system is controlled in full automatic mode with Labview.

Rare gas spectrometry

The measurement unit

This group was entirely developed by G. Ruffet from late 2000 to late 2003. Before this, there was no “Rare gas” infrastructure at Geosciences Rennes.

A new measurement unit is being developed with the collaboration of D. Bavay. This new unit, comprising two Thermo Helix SFT mass spectrometers, was financed under the CPER by the European Union (ERDF), Rennes Métropole and the CNRS.

Description of the measurement unit

A stainless steel 39Ar-40Ar extraction line with very low background noise, with a differentially pumped viewport, coupled to a powerful CO2 laser (SYNRAD) and an optical display for the step-heating analysis of minerals or individual grains.

A magnetic sector mass spectrometer for the analysis of rare gases (MAP 215 equipped with a Nier source) managed with Labview (mass spectrometer and data acquisition management). The pumping and part of the electronics have been refurbished. A complete infrastructure essential for running this type of laboratory (data processing, pumping systems, baking systems, storage of spare parts, storage of radioactive products, etc.)

Stable isotope spectrometry

Manager P. Boulvais

Since early 2018, the stable isotope geochemistry activity at Geosciences Rennes is focusing on the analysis of carbonates, in all types of geological materials, to be able to address various scientific questions (metallogeny, paleo-environment, tracing fluids).

Solid source spectrometry (TIMS)

This equipment has only worked in service delivery mode up until now. This service is ensured by David Vilbert who works in spectrometry (maintenance and analyses) and the preparation of samples in a clean room.

Equipment available at Geosciences Rennes

The Solid Source Spectrometry group is equipped with a Finnigan Mat 262 multi-collector mass spectrometer acquired in 1990. It is used to determine isotopic ratios (Sm-Nd and Sr) by isotopic dilution (ID-TIMS) and Sr, Sm, Nd and Pb concentrations in rocks and minerals as well as in water (Nd-Sr).

The group is also equipped with a clean room for the preparation of samples. The Finnigan mass spectrometer ion count was upgraded in July 2003, along with an upgrade of mass spectrometer software.

Laser ablation ICP-MS

Manager : M. Poujol

This equipment, financed by the CPER (Brittany Region, Ministry of Higher Education and Research, CNRS, Europe (ERDF)) is dedicated to /*U-Th-Pb in-situ geochronology*/ on separated minerals (zircon, monazite, sphene, baddeleyite, xenotime, rutile, allanite, apatite, calcite, colombo-tantalite, scheelite) or on thin-sections. It is also suitable for the determination of the trace element concentrations in minerals, magmatic inclusions or any other object of interest requiring an in situ analysis at high spatial resolution.

This equipment is comprised of an Agilent 7700x ICP-MS spectrometer equipped with a dual pumping system to enhance the sensitivity. A fully computer-controlled 193 nm ESI (NWR193UC) Excimer laser system equipped with a Coherent laser with an ultra-short pulse duration (<5 ns) and a two-volume ablation cell. Various sample holders (10 x 10 cm) are available depending on the needs. The impact diameters can be adjusted between 1 and 150 μm in diameter at intervals of 1 micron. It is possible to perform ablation using a rectangle-shaped laser beam (max. 150 by 150 micrometers). These ablation rectangles can rotate freely around their center, in order to analyze a given area in a grain.

Apatite fission track analysis

The laboratory has a Zeiss Imager M1m microscope coupled to an automated Zeiss measurement stage to routinely date samples using the apatite fission track method. The uranium assay is performed directly by laser ablation ICP-MS (see above). The advantage of this approach is that the samples do not need to be sent for irradiation and it is possible to simultaneously measure the concentrations of the trace elements (Rare Earth Elements, for example) as well as to date apatite minerals using the U-Pb method.