K-Ar and 40Ar/39Ar geochronology of weathering processes

Recent developments in the application of K-Ar and 40Ar/39Ar, dating of continental weathering process demonstrate the method's suitability for dating minerals present in weathering profiles. Alunite-group sulfates and hollandite-group manganese oxides, which often precipitate through weathering reactions, were first analyzed by the K-Ar method 30 years ago. Recently these minerals were shown to be suitable to 40Ar/39Ar, geochronology, despite their fine-grained habits. The bulk nature of the K-Ar technique and the complex mineral assemblages in weathering profiles restrict K-Ar dating of weathering processes. The single-crystal approach possible with the 40Ar/39Ar, method allows the study of weathering profiles where alunite- and hollandite-group minerals occur as minor phases. Step-heating analysis possible with the 40Ar/39Ar, method provides information about the Ar and K retention histories, the presence of hypogene contaminants, and possible 39Ar recoil during sample irradiation. Fully automated, modern 40Ar/39Ar systems enable analysis of several samples, providing a comprehensive weathering database. These results are useful in the study of continental paleoclimates and the geochemical, geomorphological, and tectonic histories of an area.