The K-Ar and 40Ar/39Ar methods revisited for dating fine-grained K-bearing clay minerals

The strengths and weaknesses of the two Ar isotopic methods (K–Ar and 40Ar/39Ar) were evaluated on the basis of respective recent applications mainly on low-temperature K-bearing illite-type clay minerals. This review includes a presentation of basic, analytical and technical aspects for both methods, as well as a discussion of varied claims on the two methods and of requests about sample preparation and characterization. Whenever possible, the advantages and weaknesses of each method were compared on coeval results obtained by both methods on the same mineral separates. The comparative review examines stratigraphic dating of glauconites, indirect dating of low-temperature ore deposits, dating of burial-related illitization, and dating of polyphased tectono-thermal activity, more specifically of fault gouges. Some pending questions such as the necessary encapsulation due to 39Ar recoil and its restoration into step-heating patterns are also raised, together with the new potential of Ar-dating of nanometric illite crystals. ss of the K–Ar method is in its pioneering status that makes many believe that it is no longer accurate, because of its traditional analytical aspects, and of the K determinations leading to somewhat large uncertainties. However, precise evaluation of varied applications points to a K–Ar method having probably larger applicability in sedimentary to diagenetic environments than the 40Ar/39Ar method. The drawbacks become less important if the method is applied to nanometer-sized clay minerals in diagenetic to low-grade metamorphic environments. In this instance, the extracted size fractions are generally homogeneous and the relative uncertainty given by the age calculations, if mathematically justified, can be reduced by duplicate analyses. Weakness of the 40Ar/39Ar method is in its basics such as the 39Ar recoil, the necessary encapsulation, the reintegration of the 39Ar into the step-heating patterns, and the meaning of the step-heating patterns that are more suggestive of variable 39Ar “reservoirs” created among the clay particles by irradiation than of meaningful geologic ages. If the K–Ar method is the preferred method for dating diagenetic clay processes such as glauconitization, illite crystal nucleation and growth, or low-temperature hydrothermal activities, then the 40Ar/39Ar method has more potential in dating low-temperature tectono-thermal activities, and in detailing mixtures of multi-generation illite.