The system MgO-Al2O3-SiO2 under pressure: A computational study of melting relations and phase diagrams

A computational scheme to predict melting phase relations in multi-component systems at high pressure and temperature is presented and applied to the MgO-Al2O3-SiO2 (MAS) compositional system. A combined approach based on first principles calculations (hybrid DFT and Polarized Continuum Model), polymer chemistry (Hybrid Polymeric Approach, HPA) and equilibrium thermodynamics is developed to compute thermophysical and thermodynamic properties of the solid and liquid phases in the investigated system and infer the liquidus topology of binary and ternary phase diagrams in a broad range of P-T conditions (i.e. up to 25 GPa and 5000 K). The nature of ternary interactions in the liquid is discussed in terms of an excess Gibbs free energy contribution arising from the effect of polarization of charged species in the continuum. The computed phase diagrams show that pressure effects are able to change the nature of melting from congruent to incongruent and drastically reduce the number of solid phases with a primary phase field in the MAS system, thus leading to a remarkable simplication of melting phase relations at HP-HT. At pressures > 2 GPa a primary phase field of pyrope garnet opens and progressively widens from 2 to 8 GPa at the expense of those of enstatite, forsterite and spinel. Anhydrous phase B (AnhB) completely replaces forsterite on the liquidus at 9 GPa, persisting as stable liquidus phase at least up to 16-17 GPa and 2700-2750 K. At P-T conditions compatible with the mantle transition zone, the MAS phase diagram markedly simplifies, with the three pure oxides (i.e. MgO, periclase; Al2O3, corundum; SiO2, stishovite) displaying a primary phase field and majorite-pyrope garnet as the only, and most important, ternary liquidus phase in the system.