Laser ablation K-Ar dating of illite on oriented aggregate mounts

Abstract

Illite, a clay mineral formed during diagenesis and low-grade metamorphism, can be dated using the K-Ar method to reconstruct the chronological history of sedimentary basins, fault and fold formation, and hydrothermal alterations. However, conventional K-Ar and Ar-Ar dating of illite faces challenges due to the clay's small grain size, hygroscopicity, limited availability in finer fractions, and other analytical issues. This study proposes a modified K-Ar dating approach that uses a laser ablation technique to date small amounts of oriented clay aggregate mounts separated by grain size fractions and prepared on glass discs.

Four groups of illite-bearing samples from various geological settings were analyzed, ranging in age from 350 Ma to 40 Ma. X-ray diffraction of the aggregate mounts revealed different mineral compositions, with illite proportions between 20 % and 90 %, along with kaolinite, chlorite, quartz, and feldspars in some samples. The K-Ar dating setup includes ultrahigh vacuum laser ablation of oriented clay mounts combined with LIBS spectroscopy for potassium measurement and noble gas mass spectrometry for argon quantification within the same experiment. Ages were obtained through various methods (single craters, lines, and multiple craters), making the isochron method utilizing multiple craters the most precise and accurate.

This innovative K-Ar method simplifies the dating of clay fractions by avoiding problematic issues such as drying, weighing, homogeneity, and Ar recoil during irradiation while enabling mineralogical characterization through X-ray diffraction on the same sample preparation. This approach offers new insights into the geological history recorded by clay minerals. During this work, we identified unknown issues concerning sample preparation and geochronological data, including differential clay sedimentation during drying and the associated age dispersion in clays from detrital samples.