EPR dating of clay minerals formation through geological times

Abstract

Revealing the environment and timing of clay formation in the geosphere is of major importance to understand and model the evolution of geological systems at the surface or near-surface of the continents, such as weathering covers, sedimentary basins or hydrothermal systems. Dating clay minerals by electron paramagnetic resonance spectroscopy (EPR) is a promising method that relies on the measurement of stable radiation-induced defects (RIDs) accumulating in their structure over time due to natural radioactivity. This approach has not yet been challenged by the inter-comparison with other geochronological methods, mostly because clay minerals accurately dated with methods independent from the EPR approach and also suitable for the EPR dating remain scarce in the geological record. Herein, an up-to-date protocol for the EPR dating and benchmarking are provided and developed by analyzing selected clay samples. The series includes a Mesoproterozoic illite (Thelon Basin, Canada), two paleosol kaolinites (Ukraine, Estonia) from at least late Ediacaran period, an Ypresian sedimentary kaolinite from the Aquitan Basin (France) and two Miocene and Pliocene kaolinites from lateritic duricrusts (Amazonia, Brazil). Despite some discussed uncertainties mainly related to the Th distribution in the samples, the time variation of dose rate and the thermal history of some clay samples, the EPR ages show a trend close to the 1/1 line with ages determined by other dating methods. These results bring promising support to the EPR dating methodology of clay minerals and extend its potential application field over a time-range spanning from Quaternary to Proterozoic.