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

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, Mg-rich Pv makes up ∼70% by mass of the lower mantle. Using results from experimental phase equilibria, first-principles computations and empirical scaling relations for Fe²⁺-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 X_{FeSi O3} = 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 temperature acts to push the mixed layer deeper in the lower mantle perhaps into the D″ thermal-compositional boundary layer resting upon the CMB. For various lower mantle geotherms and CMB temperatures, a single mixed layer of thickness ∼300 km lies within the bottom 40% of the lower mantle. For low iron contents (X_FeSiO3 ∼ 5 mol% or less), two (perched) mixed phase layers are found. This is the divariant analog to the univariant double-crosser of Hernlund et al., 2005 [Hernlund, J., Thomas, C., Tackley, P.J., 2005. A doubling of the post-perovskite phase boundary and structure of the Earth's lowermost mantle. Nature 434, 882-886.]. The hotter the mantle, the deeper the mixed phase layer; the more iron-rich the lower mantle, the shallower the mixed phase layer. In a younger and hotter Hadean Earth with interior temperatures everywhere 200-500 K warmer, pPv is not stable unless the lower mantle bulk composition is Fe-enriched compared to the present-day upper mantle. The interplay of temperature and Fe-content of the lower mantle has important implications for lower mantle dynamics.