Numerical simulations and scale analysis are used to assess the viability of the marginal box‐filling mechanism for producing compositional zonation in magma bodies. Scale analysis and two‐dimensional numerical experiments both show that box‐filling occurs provided a critical ratio of compositional to thermal buoyancy (Rρcrit) is exceeded. This critical ratio depends on the ratio of thermal to chemical diffusivity (Le). For box‐filling to occur in the simplified dike‐like model system, Rρ > 2/9 Le2/3. Application of this result to magma bodies suggests that box‐filling may occur for components with relatively high chemical diffusivities such as water. However, box‐filling will not produce significant zonation for components with small chemical diffusivities such as silica, unless diffusive coupling increases silica diffusivity.