Reca Project

Dating methodologies

For this project, we are using two mains geochronological methods: (U-Th)/He and Electron Paramagnetic Resonance (EPR), that allow to go deep in time, i.e. millions of years.
(U-Th)/He and EPR are absolute dating methods, in opposition with relative method, that are based on the production of He and damage in the crystal structure due to the radioactivity.

The (U-Th)/He method

The (U-Th-Sm)/He method (short (U-Th)/He) is based in the radioactive decay of uranium, thorium and samarium. Small amounts of these elements are nearly always contained in most minerals of the Earths’ crust. The four isotopes 238U, 235U, 232Th and 147Sm are radioactive (=unstable) and have half-live times in the order of magnitude of geological processes. During the decay chain to their stable forms, they emit alpha particles (alpha particle = helium core).

Helium as a noble gas has a high diffusivity in solid phases, and for the purpose of geochronological use, only mineral phase that retain He within the crystal structure are used. In this project, we used this method on hematite and goethite iron oxides and hydroxides and work on He retention in those minerals have been performed (Balout et al., 2017; Balout et al., submitted). This is done on the one hand by performing theoretical calculations on the diffusion behavior of He in goethite (Post-doc Fadel Bassal, Physics department Paris Saclay) and on the other hand by performing diffusion experiments on natural goethite samples (GEOPS, Paris Saclay). We demonstrated that He is well retained in hematite and goethite crystal lattice and minor diffusion correction should be applied.
By measuring the amounts of He, U, Th and Sm by mass spectrometry and knowing the decay constants of the different isotopes it is possible to calculate since when He is accumulating in the mineral. An (U-Th)/He age gives thus information on the duration of He accumulation.
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He lab (GEOPS, University Paris Saclay)

The electron paramagnetic resonance spectroscopy (EPR) method

The electron paramagnetic resonance spectroscopy was discovered in 1944. It uses property (the spin) of certain electrons associated to ions in matter. It is a very sensitive method that is specific and allows one to detect, characterize and quantify unpaired electrons mainly from transition elements, electron defects, free radicals or rare earth elements. It operates under the action of a fixed frequency of microwaves (that allow spin energy transitions) in a variable magnetic field imposed by an electromagnet (that allows splitting of energy levels). Each paramagnetic species can be assigned to one or several g spectrocopic factors derived from the resonance condition that binds magnetic field and microwave hyperfrequency :
hv = gßH
where h is the Planck constant, v the microwave hyperfrequency, ß the Bohr magneton constant and H the imposed magnetic field.
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EPR spectrometer
The spectrum is the derivative intensity as a function of the magnetic field. The g factor depends on the number and environment of concerned electrons. For minerals of the environment such as clay minerals, we work on powders typically of a few tens milligrams and we obtain an average spectrum corresponding to the absorption of microwaves by the species present in all orientations in the material. EPR spectroscopy is used in the RECA program to characterize kaolinites i.e. major clay minerals in most tropical weathering covers, and measure their ages.

The electron paramagnetic resonance spectroscopy (EPR) method

A typical EPR spectrum of kaolinite exhibits several signals from a few species:
- trivalent iron substituting from aluminium ions in the structure (structural iron)
- trivalent iron occurring as clusters or associated small iron oxides sensu lato.
- radiation-induced defects associated to oxygen ions of the structure
- sometimes tetravalent vanadium as vanadyle complex VO2+

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typical X-band spectrum of kaolinite
All these species can be quantified to reveal different conditions of formation of the kaolinite populations along the profile. In particular, the radiation-induced defects are used for dating kaolinite formation, through a calibration provided by artificial irradiations on a particle accelerator which allows one to determine the so-called paleodose, i.e. the natural cumulative dose experienced by the clay mineral. The knowledge of the radiation sources from geochemical and petrographic data, combined to the paleodose yields the age of the kaolinite. A profile near Manaus developped on the Alter do Chao formation was previously studied, where dating was interpreted as the replacement of an old saprolite generation by a younger one toward surface (Balan et al., 2005). In RECA, a detailed sampling was performed on a closely related profile containing a Fe duricrust: EPR confirms that the degree of crystalline disorder of kaolinite strongly increases from the saprolite to the topsoil. Dating is in progress.
Balan et al. (2005) Formation and evolution of lateritic profiles in the middle Amazon basin: Insights from radiation-induced defects in kaolinite. Geochimica et Cosmochimica Acta. 69, 9, 2193-2204.