Isotopic fractionation of argon during stepwise release from shungite

In previous attempts to determine the⁴⁰Ar³⁶Ar ratio in the ancient atmosphere, the only direct measurement yielding a value below the atmospheric value of today is for argon released at low temperatures from a pre-Cambrian shungite, an amorphous carbon mineral [1]. The present work confirms a low value for⁴⁰Ar³⁶Ar in gas released from a type I shungite at low temperatures. But quantitative scrutiny of the accompanying³⁸Ar³⁶Ar ratios and the enhanced ratio of⁴⁰Ar³⁶Ar for the fractions released at high temperatures shows convincingly that the effect seen here is an artifact of the stepwise heating and the argon diffusion mobilized thereby. The low⁴⁰Ar³⁶Ar previously obtained is very likely from the same cause rather than reflecting the isotopic composition of the pre-Cambrian atmosphere. The vitreous character of and the sharp, conchoidal fractures seen in the specimens of type I shungite suggest that the substance may exhibit simple volume diffusion over macroscopic dimensions as glasses do. If so, the diffusion parameters (D_∞ = 3 × 10^?4cm²/s andE = 11kcal/mole) obtained from the data imply rapid exchange with the atmosphere for any argon initially trapped in centimeter-thick veins of the material.