Postdepositional evolution of platform claystones based on a simulation of thermally induced diffusion of radiogenic 40Ar from diagenetic illite

The Lower Cambrian Lontova claystones contain well crystallized authigenic 1M illite mixed with detrital 2M illite. Part of the authigenic illite probably precipitated during sedimentation and subsequently experienced a temperature up to 140degreesC, despite a limited burial to 500-1000 m. The difference between the Rb-Sr and K-Ar dates of this authigenic illite are attributed to preferential loss of radiogeni Ar-40 induced by a short-lasting thermal pulse. This interpretation is based on an Ar-diffusion model suggesting that the Lontova blue clays were heated to around 140degreesC for 2-5 Myr. This thermal pulse could have occurred in relation with hot-fluid migration induced by either tectonic dislocations in the basement rocks during the Late Silurian-Early Devonian, or rifting conditions during the Permian. By comparing Rb-Sr and K-Ar data, evaluation of radiogenic Ar-40 losses by small authigenic illite particles at low temperatures could be a powerful tool for reconstruction of evolutionary histories of sedimentary successions that were either shallowly buried, or percolated by migrating fluids released during tectono-thermal activities.