S.V. Rasskazov I.S. Brandt A.V. Ivanov Michael J. Kunk Stephen B. Brandt 1997 <p><span>Results of two routine&nbsp;</span>⁴⁰<span>Ar/</span>³⁹<span>Ar 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&nbsp;</span>³⁹<span>Ar release. Argon isotope spectra are suggested to be controlled by the activation energy of diffusion&nbsp;</span><i>E</i><span>&nbsp;and the frequency factor&nbsp;</span><i>D_o<span>&nbsp;</span></i><span>. The activation energy of&nbsp;</span>³⁹<span>Ar is lower than the one of&nbsp;</span>⁴⁰<span>Ar. This results in a preferable release of&nbsp;</span>⁴⁰<span>Ar relatively to&nbsp;</span>³⁹<span>Ar 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&nbsp;</span>³⁹<span>Ar are observed, which cause the decreasing low-temperature wing in the “saddle”. The suggestion of argon losses (mainly of&nbsp;</span>³⁹<span>Ar) from a loose, “unstable” zone of the mineral structures becomes justified. The&nbsp;</span><i>n</i><span>-irradiation of the samples and the shift of&nbsp;</span><i>E</i><span>&nbsp;of&nbsp;</span>³⁹<span>Ar towards lower values seems to explain the saddle-shaped age-spectra often encountered in&nbsp;</span>⁴⁰<span>Ar/</span>³⁹<span>Ar-geochronometry.</span></p> application/pdf 10.1080/10256019708234052 en Taylor & Francis The inverse problem of argon diffusion from minerals: Determination of kinetic parameters from stepwise-heating experiments article