The inverse problem of argon diffusion from minerals: Determination of kinetic parameters from stepwise-heating experiments

Results of two routine ⁴⁰Ar/³⁹Ar stepwise heating experiments on a biotite and a basanite are interpreted in terms of Fick's and Arrhenius' laws. Both patterns represent a saddle-shaped ³⁹Ar release. Argon isotope spectra are suggested to be controlled by the activation energy of diffusion E and the frequency factor D_o . The activation energy of ³⁹Ar is lower than the one of ⁴⁰Ar. This results in a preferable release of ⁴⁰Ar relatively to ³⁹Ar at high-temperature steps and an increasing high-temperature wing in the saddle-shaped age spectrum. At low temperatures, considerable losses and irregularities in release of mainly ³⁹Ar are observed, which cause the decreasing low-temperature wing in the “saddle”. The suggestion of argon losses (mainly of ³⁹Ar) from a loose, “unstable” zone of the mineral structures becomes justified. The n-irradiation of the samples and the shift of E of ³⁹Ar towards lower values seems to explain the saddle-shaped age-spectra often encountered in ⁴⁰Ar/³⁹Ar-geochronometry.