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 (R&rgr;crit) 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, R&rgr;>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. ¿ American Geophysical Union 1989