Thermodynamics Of The Lowermost Mantle : Example Of SiO2-MgO System

Seismic observations of the region above the core mantle boundary (CMB), including Ultra-Low Velocity Zones (ULVZ) and D'' discontinuities, and recent mineral physics data suggest the bottom of the mantle may be chemically heterogeneous, possibly, on account of the occurrence of the recently discovered, CaIrO₃ structure. Moreover, the large decrease in shear wave velocity in selected areas (ULVZ) in the lowermost 40 km of the mantle indicates the likely presence of partially molten material. To provide a complete petrologic and mineral physics model of this region, it is important to define both constituent material properties, and phase equilibria including melting behavior. We expect to describe models that encompass the entire MgO-FeO-CaO-Al_{2O_3-SiO_2} system and to define complete mineral physics equations of state, and employ thermochemical data to obtain complete phase diagrams. Our mineral physics-based seismic models are to be obtained using global inversion methods. Initial model results are reported for the SiO_2-MgO system. For the SiO₂ system, we modeled the high-pressure phase diagram over the TiO₂-, CaCl₂-, α PbO₂- structure and liquid phase stability range. The high-pressure liquid SiO₂ has an EOS with: ρ = 4.05 g/cm³, K = 180 GPa, K' = 6, γ = 1.15, q = 0.8, Cv = 1.55 J/K/g. The melting of CaCl₂ structure occurs at ∼5000 K at 135 GPa. Also the α PbO₂-structured SiO₂ EOS is described by : ρ = 4.18 g/cm³, K = 270 GPa, K' = 3.6, γ = 1.35, q = 2.6, Cvm = 1.215 J/K/g, θ ₀ = 1222 K. Examination of the diamond anvil melting data for MgO yields the liquid EOS and a model melting curve. The melting point is ∼5200 K at 135 GPa. We also calculate the melting behavior of binary SiO_2-MgO system. For MgSiO3 composition the melting point at 135 GPa is ∼5300 K, which combined with the above MgO data indicates melting temperatures of mantle composition at 135 GPa is ∼4400±400 K.