K-Ar AND ⁴⁰Ar/³⁹Ar GEOCHRONOLOGY OF WEATHERING PROCESSES

Abstract

▪ Abstract 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.

Geochronology of the Baye Mn oxide deposit, southern Yunnan Plateau: Implications for the late Miocene to Pleistocene paleoclimatic conditions and topographic evolution

“…7e, f). Such spectra may represent a mixture of multiple Mn oxide generations that had contrasting thermal stability and released their radiogenic argon gas in distinct temperature ranges (Vasconcelos, 1999a). This interpretation is partly confirmed by the shapes of the Rad.…”

“…Figure 7a represents the most common (>80%) age spectra, in which several continuous heating steps, accounting for 60-95 % of the total amount of 39 Ar released from the sample, have apparent ages that are indistinguishable at the 95% confidence level (2σ). The shapes of the spectra indicate that these samples host their radiogenic and nucleogenic gas fractions in a tight crystallographic reservoir (Vasconcelos, 1999a). Thus, the plateau ages can be interpreted as the precipitation age of the Mn A second type of spectra shows anomalously high apparent ages in the initial step, followed by a series of heating steps that form an age plateau (Fig.…”

“…7b). This type of spectra is indicative of 39 Ar loss by recoil during neutron irradiation, with the recoiled 39 Ar having lost completely to the surrounding atmosphere (Vasconcelos, 1999a).…”

“…EA1c, d), with the intermediate steps having the oldest apparent ages of 8.0 ± 0.4 and 9.3 ± 0.3 Ma, respectively. These two 14 ages can be considered as the minimum age of the Mn oxide grains (Vasconcelos, 1999a;Beauvais et al, 2008). The age probability diagrams of individual profiles reveal four major peaks at ca.…”

“…Lateritic weathering, and associated supergene mineralization, is favored by abundant rainfall, relatively high temperature, and widespread vegetation cover, characteristics of humid tropical and subtropical zones (Nahon, 1986;Pracejus and 15 Bolton, 1992;Vasconcelos, 1999a). As such, lateritic weathering profiles and supergene ores therein serve as good proxies for past climates (Li and Vasconcelos, 2002;Carmo and Vasconcelos, 2004;Colin et al, 2005;Beauvais et al, 2008).…”