Thermodynamics of the lowermost mantle

Abstract

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 post-perovskite (CaIrO_3) 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 MgOFeO-CaO-Al_2O_3-SiO_2 system and to define complete mineral physics equations of state, employing 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.

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