Constitution of the mantle. I. Phase relations in the FeOMgOSiO2 system at 10–30 GPa

Abstract

P - T - x phase diagrams of the FeOMgOSiO 2 (FMS) system are calculated using the ideal solution model at 10–30 GPa and 1000–2000 K. A thermodynamic data base for minerals in the MgOSiO 2 and FeOSiO 2 systems is established by supplementing the calorimetric data for low-pressure phases and equations of state for all phases with data calculated from high-pressure synthesis experiments. The thermodynamic functions of virtual phases in the FeOSiO 2 system are evaluated. The topologies of calculated P - x sections of phase diagrams at 1273 K and 2000 K are in agreement with earlier experimental data. Univariant curves γ + St = Ilm + Mw, γ + Ilm = Pv + Mw + St intersect giving the nondegenerated invariant point (St + Mw + γ + Pv + Ilm) at T ≈ 1650 K, P ≈ 23 GPa and Fe/(Fe + Mg) ratios in the range 0.19–0.30. Univariant curves α + γ = β and Px + γ = β + St do not intersect in the calculated interval of P - T conditions. At temperatures higher than 1650 K, Ilm = Pv + γ + St and γ + St = Pv + Mw equilibria are stable. It is supposed that the disagreement between the earlier experimental phase diagrams is due to the experiments being carried out at temperatures above and below the invariant point.