40Ar/39Ar analysis of supergene yavapaiite and preliminary investigation on Ar closure temperature

Abstract

Yavapaiite (KFe(SO4)2) is a new sulfate mineral with high potassium content, discovered in the oxidation zone of non-ferrous metal mining in the Tu-Ha Basin, Xinjiang Uygur Autonomous Region, northwestern China. Dating of this mineral by the K-Ar and 40Ar/39Ar method allows us to study the time and process of the region’s aridity, and further we can provide some insight into the influence of not only the Tibetan Plateau uplift but also formation and evolution of polar ice caps on climate of this region. However, as the temperature of this region is up to 60°C in summer, it is still questionable whether this high temperature causes the diffusive loss of 40Ar from yavapaiite in a long time, or whether the fine grain of the mineral can lead to diffusive loss of 40Ar from yavapaiite. These are important for interpreting the mineral age data. According to diffusion theory, we use 40Ar/39Ar step heating experiment to determine the apparent Ar diffusivity. Then we present a simple model of diffusive loss and radiogenic in-growth to evaluate the effect of extreme high ambient temperature and the grain size upon the Ar age. The experimental results show that the diffusion parameters as follows: the activation energy Ea is 71.30 kcal/mol and the frequency factor logD0/a2 is 13.71/s, the corresponding closure temperature Tc is 294°C (assuming a cooling rate of 10°C/Ma), and the activation energy and closure temperature are very high. The simulation results show that the high ambient temperature and the grain size have no effect on the Ar age after yavapaiite was precipitated. Furthermore, the reproducibility of the age attests to the suitability of supergene yavapaiite for K-Ar and 40Ar/39Ar dating.