Layered convection induced by phase transitions.

We report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection. The 2- D numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the 'extended Boussinesq approximation', i.e., considering the effects of adiabatic gradient, latent heat, and frictional heating in the energy equation. We find that the critical value of the negative Clapeyron slope, which must be surpassed in order to induce layered convection, decreases in magnitude with increasing Rayleigh number Ra in the range 10⁴= or <Ra= or <2X10⁶. A possible superplastic rheology within the transition zone would tend to favor layered convection. Scaling the model results to the 670-km discontinuity in the Earth's mantle as a possible endothermic phase boundary, we estimate the critical Clapeyron slope to be in the range of -4 to -8 MPa/K (-40 to -80 bar/K). The possibility that the spinelrt arrow perovskite + periclase transition is within this range appears to be remote but certainly cannot be neglected.-from Authors