An isotopic study of surficial alunite in Australia 2. Potassium-argon geochronology

Abstract

The potential for dating weathering and diagenetic events in Australia by the K Ar dating of alunite-group minerals is assessed. Twenty-five samples, collected from a wide range of surficial environments, yielded K Ar ages ranging from <1 to 62 Ma. K Ar dating of an alunite sample from a central Western Australian laterite profile has provided the first numerical evidence of early- or pre-Tertiary deep-weathering in Western Australia. Dates from alunite in the South Australian opal fields indicate precious opal formation during several episodes during the Miocene epoch. Interpretation of the data is complicated by the presence of small (<1%) quantities of detrital K-bearing mica in some alunite samples. A correction for the presence of the mica is made to the apparent age of the alunite, based on the age of the bedrock mica source. This correction, coupled with independent evidence that alunite is a late-stage product of weathering, means that a K Ar age obtained from alunite should be considered a minimum age for the development of the regolith profile in which it is found. It is concluded that K Ar dating of alunite provides a useful quantitative estimation of age of weathering where, in the majority of cases, independent indicators of age are lacking.