EXCESS ARGON IN MINERALS - RESULTS OF HYDROTHERMAL EXPERIMENTS

Excess Ar-40 of diverse geneses (entrapped and inherited) was detected in minerals (feldspars and glauconite) after hydrothermal treatment f or three days at T = 200-600 degrees C and P = 1 kbar. The orthoclase contains significant amounts of air-derived argon (10(-4) cm(3)/g) at a low partial pressure, which is constrained by the solubility of argo n in water. By contrast with argon in plagioclase, this element in ort hoclase occurs in an adsorbed mode and has high energy in the crystal structure (in anion vacancies, which easily form in the presence of wa ter). Excess radiogenic argon was detected in the products of complete glauconite decomposition: biotite and feldspar. As indicated by isoto pe data, excess argon of this type was not entrapped but inherited fro m the precursor mineral.