摘要
Formulating the original composition of weathered tephra can be a chaUenging task. A previous study by Wadia （1998 and 2007） demonstrated the enrichment of silica according to the hypothesis that the percentage of aluminum oxide remains constant within the unweathered parent material and its weathered counterpart （Faure, 1991）, which indicated the average enrichment of silica to be 6.5%. This study demonstrates the enrichment of microcrystalline-cryptocrystalline quartz in the Reid＇s Mistake Formation of the Newcastle Coal measures of Sydney Basin, Australia, incorporating petrographic analyses and MBC （mass balance calculations） using trace element zircon as the immobile element. For MBC, zircon is more favorable, as it is a trace element that tends to be immobile during the process of chemical weathering. The composition of the unweathered reference samples was collected from the Tertiary volcanic complexes of Northeastern New South Wales and Southeastern Queensland. ICP-AES （inductively coupled plasma-atomic emission spectroscopy） was used to determine the chemical composition of the weathered samples. The quantitative enrichment of silica was provided by performing MBC using zircon as the immobile trace element. The petrographic analyses results indicated that the tephra was subjected to chemical weathering in a meteoric regime, as represented by minerals observed in the weathered tephra. The silica enrichment by MBC involving zircon resulted in 40.11%, which is indicative of chemical weathering, and was additionally supported by the presence of chert lenses in the volcanic horizon, indicating the enrichment of silica as a result of chemical weathering.
