A numerical model for isotopic patterns from thermal-extraction experiments

Abstract— A model based on volume diffusion is proposed to describe thermal extraction (progressive or stepped vacuum-pyrolysis or combustion) experimental results (yield and isotopic composition), reported against time or temperature. It accounts for the common asymmetrical shape of the release patterns, with peak-tails due to the extraction kinetics. By analogy to this model, a simulation function is introduced which allows calculation of the concentration and isotopic patterns resulting from component mixings during such experiments. In this calculation, each component's isotopic composition is assumed to be constant throughout its extraction. The simulation readily explains the shift between the isotopic composition peaks and the concentration peaks in the temperature release pattern commonly observed during pyrolysis or combustion of meteorite samples. Because of its simplicity, the principles of the simulation can be easily applied by the readers to real data. One example is taken from the meteoritic stable isotope literature: the progressive pyrolysis extraction of hydrogen from the Parnallee LL3 meteorite (Robert et al., 1987a). The simulation permits calculation of a range for the isotopic composition of a D-depleted reservoir in this meteorite: ?460 < δD < ?320‰ rel. SMOW.