[en] ⁴⁰Ar-³⁹Ar step heating analyses of clay concentrates from the Gortdrum and Tynagh orebodies (Ireland) previously dated by conventional K-Ar, indicate major losses of ³⁹Ar (32 to 45%) and rad. ⁴⁰Ar (25 to 35%) during the irradiation. The proportion of rad. ⁴⁰Ar loss, unlike that of ³⁹Ar, increases with J-value. The difference between ³⁹Ar and rad. ⁴⁰Ar proportion losses is related to the mineralogy and grain intimacy. These also affect the stepwise release patterns - the Gortdrum concentrates yield age spectra very consistent with ³⁹Ar recoil predictions, whereas the Tynagh concentrates in which the grains are intimately intergrown, show no clear evidence for ³⁹Ar recoil depletion in the K-rich phases. The difference is resolvable if illite argon release is not a simple volume diffusion type process under vacuum conditions. (author)