Tradeoffs in chemical and thermal variations in the post-perovskite phase transition: Mixed phase regions in the deep lower mantle?

Abstract

The discovery of a phase transition in Mg-rich perovskite (Pv) to a post-perovskite (pPv) phase at lower mantle depths and its relationship to D �� , lower mantle heterogeneity and iron content prompted an investigation of the relative importance of lower mantle compositional and temperature fluctuations in creating topographic undulations on mixed phase regions. Above the transition, Mgrich Pv makes up ∼70% by mass of the lower mantle. Using results from experimental phase equilibria, first-principles computations and empirical scaling relations for Fe 2+ –Mg mixing in silicates, a preliminary thermodynamic model for the Pv to pPv phase transition in the divariant system MgSiO3–FeSiO3 is developed. Complexities associated with components Fe2O3 and Al2O3 and other phases (Ca-Pv, magnesiowustite) are neglected. The model predicts phase transition pressures are sensitive to the FeSiO3 content of perovskite ( ∼− 1.5 GPa per 1 mol% FeSiO3). This leads to considerable topography along the top boundary of the mixed phase region. The Clapeyron slope for the Pv → pPv transition at XFeSiO3 = 0.1 is +11 MPa/K about 20% higher than for pure Mg-Pv. Increasing bulk concentration of iron elevates the mixed (two-phase) layer above the core–mantle boundary (CMB); increasing