Development of an integrated analytical platform for clay mineral separation, characterization and K–Ar dating

Abstract

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 at various temperatures, from magmatic to low temperatures. Clays are of significant interest among all K-bearing minerals, as they crystallize during various hydrothermodynamic 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 recrystallization processes. Therefore, reliable K–Ar dates on illite depend on (1) the grain size separation of large numbers 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.