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