Kiel diagram of stars with [Fe/H]_i and (see description in Appendix A). The sub-panels are labelled as follows: g for models that include atomic diffusion without radiative accelerations; g + g_rad for models that include atomic diffusion with radiative accelerations; g + D_T, Fe for models that include atomic diffusion without radiative accelerations and the turbulent diffusion coefficient parametrised in Section 4.2 (see Table 1 for more details about the input physics).