Isotope effects in the thermodiffusion of dilute atomic liquids are examined using a non-equilibrium thermodynamic model, where the thermodynamic parameters are calculated using equations rooted in statistical mechanics. In this approach, isotope effects in thermodiffusion are quantified through the variation in chemical potential and its temperature dependence with isotope mass.  The model is applied to silicate melts, in order to compare our results to recent approaches that incorporate quantum mechanics and kinematic concepts. We show that the previous theories either require unrealistic values of physical parameters or are based on invalid assumptions. The model provides an adequate description of isotope effects in thermodiffusion in silicate melts, with reasonable values of the Soret coefficient.