Résumé
Stellar evolution models taking into account the atomic diffusion of 28 species, have been calculated for Pop II stars of 0.5 to 1.2 M⊙ with [Fe/H] from -4.31 to -0.71. Overabundances are expected in some turnoff stars with Teff > 5900 K. They depend strongly on the metallicity of the cluster. At the metallicity of M 92, they reach a factor of 10 for many species, at 12 Gyr, but a factor of at most 2, at 13.5 Gyr. Series of models were also calculated with turbulence to determine to what extent it reduces predicted abundance anomalies. The level of abundance anomalies observed in turnoff stars may then determine a level of turbulence. Even in the presence of turbulence however, allowance for diffusive processes leads to a 10%-12% reduction in age at a given turnoff luminosity. For M 92 an age of 13.5 Gyr is determined.