Deep Earth Structure: Lower Mantle and D″

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The thick layer of rock extending from the base of the transition zone near 650–800 km depth down to the core–mantle boundary (CMB) at a depth of 2891 km comprises most of Earth's volume and is called the lower mantle. In general, the physical properties of this vast region inferred from seismology are indicative of relatively uniform composition material, with increasing pressure and temperature with depth accounting for systematic increases in density, incompressibility, rigidity, and associated seismic wave velocities. The bulk mineralogy is believed to be relatively simple as well, with the predominant mineral form being (Mg0.85,Fe0.15)SiO3 magnesium silicate perovskite, with perhaps 20% (Mg0.85,Fe0.15)O ferropericlase and several percent calcium perovskite. In detail, however, the structure of the lower mantle is found to have both large-scale complexity and small-scale complexity indicative of thermal and chemical heterogeneities that are manifestations of a complex dynamic regime. There are also complexities expected due to changes from high spin to low spin for iron in both perovskite and ferropericlase minerals. Large-scale regions of anomalously high or anomalously low seismic velocities have been detected, as well as spatially concentrated structures with distinctive seismic velocities, likely related to upwelling and downwelling regions of the mantle thermal convection system. The lowermost few hundred kilometers of the lower mantle are particularly anomalous in its seismological properties and are identified as the D″ region. Abrupt seismic velocity increases are observed at the top of the D″ region in many places, probably due to a phase transition in perovskite. Large volumes of apparently chemically distinct material have been characterized in the lowermost mantle; these may be relics of Earth's early chemical stratification or possibly accumulations of subducted oceanic slab materials. Dramatic drops in seismic velocities are observed in thin layers at the bottom of D″ adjacent to the CMB, with partial melting probably being involved. Seismic wave anisotropy has been detected in this lowermost layer of the mantle, providing a possible probe of boundary layer deformation. The lower mantle appears to host complex thermochemical dynamic processes that likely have played a key role in mantle evolution throughout the history of planet Earth.