THE INVERSE PROBLEM OF ARGON DIFFUSION FROM MINERALS - DETERMINATION OF KINETIC-PARAMETERS FROM STEPWISE-HEATING EXPERIMENTS

Results of two routine Ar-40/Ar-39 stepwise heating experiments on a b iotite and a basanite are interpreted in terms of Fick's and Arrhenius ' laws. Both patterns represent a saddle-shaped Ar-39 release. Argon i sotope spectra are suggested to be controlled by the activation energy of diffusion E and the frequency factor D-0. The activation energy of Ar-39 in lower than the one of Ar-40. This results in a preferable re lease of Ar-40 relatively to Ar-39 at high-temperature steps and an in creasing high-temperature wing in the saddle-shaped age spectrum. At l ow temperatures, considerable losses and irregularities in release of mainly Ar-39 are observed, which cause the decreasing low-temperature wing in the ''saddle''. The suggestion of argon losses (mainly of Ar-3 9) from a loose, ''unstable'' zone of the mineral structures becomes j ustified. The n-irradiation of the samples and the shift of E of Ar-39 towards lower values seems to explain the saddle-shaped age-spectra o ften encountered in Ar-40/Ar-39-geochronometry.