We investigate the effect of an endothermic phase transition on the average petrology of the mantle as function of depth. We study chemical differentiation using a numerical model in which the oceanic lithosphere, comprised of a crustal layer overlying depleted harzburgite, is injected at the ridges. Three families of tracers are used, each one with its own buoyancy, to distinguish oceanic crust, harzburgite and peridotite. The depth of the phase transition and the Clapeyron slope are assumed to be the same whatever the petrological component of the mantle. Velocities are imposed at the surface to mimic plate tectonics. The viscosity is depth and temperature dependent. The negative Clapeyron slope tends to induce a segregation of the heavy material below the phase transition and of the light material above the phase transition with a compositional jump across the transition zone.