Comparative 40Ar/39Ar and K-Ar dating of illite-type clay minerals: a tentative explanation for age identities and differences
Résumé
The 40 K/ 40 Ar (K-Ar) and 40 Ar/ 39 Ar dating methods are applied here to the same, very small, micrometric illite-type particles that crystallized under low-temperature (b175 °C) hydrothermal conditions in deeply buried Rotliegend (Permian) gas-bearing sandstones of NWGermany. Four samples with a total offifteen size fractions fromb2to 20-40μm yield K-Ar ages that range from 166.0±3.4 to 214.0±5.9 Ma.Thesamesizefractionsdatedby the 40 Ar/ 39 Ar method give total-gas ages ranging from 173.3±2.0 to 228.8±1.6 Ma. Nearly all 40 Ar/ 39 Ar total-gas ages are slightly older, which cannot be explained by the recoil effect only, the impact of which being amplified by the inhomogeneous shapeof the clayminerals and their crystallographic characteristics,withvariedcrystallinity indices, and a particlewidth about10 times large than thickness. The 40 Ar/ 39 Ar data outline some advantages, such as theplateaus obtainedby incremental stepheatingof the various size fractions, even if not translatable straight as ages of the illite populations; they allow identification of two generations of authigenic illite that formed at about 200 and 175 Ma, and one detrital generation. However, 40 Ar/ 39 Ar dating of clay minerals remains challenging as technical factors, such as the non-standardized encapsulation, may have potential unexpected effects. Both dating methods have their limitations: (1) K-Ar dating requires relatively large samples (ca. 10-20 mg) incurring potential sample homogeneity problems, with two aliquots required for K and Ar analysis for an age determination, also inducing a higher analytical uncertainty; (2) an identified drawback of 40 Ar/ 39 Ar dating is Ar recoil and therefore potential loss that occurs during neutronic creation of 39 Ar from 39 K, mostly in thefiner mineral particles. If all the recoiled39 Ar is redistributed into adjacent grains/minerals, thefinal 40 Ar/ 39 Ar age of the analyzed size fraction remains theoretically identical, but it is not systematic in clay-type material. The finest grain sizes (e.g.,b0.2μm) are usually least contaminated with detrital components and can be dated by the K-Ar method without special preparation. Alternatively, suchfine fractions are most susceptible to 39 Ar recoil, and are, therefore, only datable by the 40 Ar/ 39 Ar method using an encapsulation technique that still needs to be technically evaluated. In this study, 39 Ar recoil during irradiation was quantified by encapsulation; it ranged from about 20% of the total 39 Ar for theb0.2μm fractions to ca. 10% for the 2-6μm fractions. Basic interpolation of this 39 Ar recoil confirms that grain size increasing minimizes the recoil effect, but how it pro-ceeds has still to be explained. Despite recent developments in 40 Ar/ 39 Ar dating, the conventional K-Ar method is still a valuable tool for clay dating due to a convenient and straightforward use supported by a standardized and well-controlled technical approach. The present comparison of the two Ar-dating methods as applied to clay material shows that neither method is presently outdated, and that they are even of reciprocal use. Both methods have distinct application fields in clay geochronology and complementary application fields in clay crystallography.