A simple model for melt‐solid two‐phase flow involving diffusion‐controlled chemical reaction is presented. The governing equations describing mass conservation were numerically integrated to clarify chemical evolution of the flow. The calculations show that the conditions for local equilibrium are violated concerning elements with small diffusion coefficient less than about 10−19 m2/s in the solid. Behavior of these elements at melting and segregation stages in the mantle should be investigated in terms of diffusivity in solid as well as partitioning between melt and solid.