Development of an integrated analytical platform of clay minerals separation, characterization and ⁴⁰K/⁴⁰Ar dating

Isotopic dating is a valuable method to constrain the timing of lithospheric processes: geodynamic episodes, ore deposition and geothermal regimes. The K-Ar dating technique has the main advantage of being applied to ubiquitous K-bearing minerals that crystallize in various temperatures, from magmatic to low temperatures. Clays are of significant interest among all K-bearing minerals, as they crystallize during various hydro-thermo-dynamic processes. Nonetheless, the dating of illites by the K-Ar method is not straightforward. K-Ar dates on illite usually rely on a mixed isotopic signal referring to various illitic populations that might have experienced isotopic resetting or re-crystallization processes. Therefore, reliable K-Ar dates on illite depend on (1) the grain size separation of large amounts of clay fractions, (2) the study of the morphology, mineralogy and crystallography, (3) the determination of precise K-Ar dates on each clay size fraction and (4) the meaningful interpretation of ages using either end-member ages or the Illite-Age-Analysis (IAA) method. This paper describes the instrumentation and methods recently developed at the GeoRessources laboratory of the University of Lorraine to obtain valuable ages on illite mixtures.