The lower mantle

The lower mantle represents approximately 70% of the mass of the solid Earth and almost 50% of the mass of the entire Earth (Schubert et al., 2001). The generally smooth increase in seismic wave velocities with depth in most of this layer led to the assumption that it is relatively homogeneous in its mineralogy, having mostly a perovskite structure. However, more detailed seismo-

Table 2.4 Phase transformations of olivine that are thought to define the upper mantle transition zone (after Helffrich & Wood, 2001).

Depth

Pressure

410 km

13-14 GPa

(Mg,Fe)2SiO4 = (Mg,Fe)2SiO4 Olivine Wadsleyite (P-spinel structure)

520 km

18 GPa

(Mg,Fe)2SiO4 = (Mg.Fe^SO Wadsleyite Ringwoodite (y-spinel structure)

660 km

23 GPa

Ringwoodite Perovskite Magnesiowustite

logical studies have revealed that the lower mantle has thermal and/or compositional heterogeneity, probably as a result of the penetration of subducted oceanic lithosphere through the 660 km discontinuity (Section 2.8.3).

The lowest 200-300 km of the mantle, Layer D" (Section 12.8.4), is often characterized by a decrease in seismic velocity, which is probably related to an increased temperature gradient above the mantle-core boundary. This lower layer shows large lateral changes in seismic velocity, indicating it is very heterogeneous. Ultra-low velocity zones, which show a 10% or greater reduction in both P and S wave velocities relative to the surrounding mantle, have been interpreted to reflect the presence of partially molten material (Williams & Garnero, 1996). These zones are laterally very heterogeneous and quite thin (5-40 km vertical thickness). Laboratory experiments suggest that the liquid iron of the core reacts with mantle silicates in Layer D", with the production of metallic alloys and nonmetallic silicates from perovskite. Layer D" thus is important because it governs core-mantle interactions and also may be the source of deep mantle plumes (Sections 12.8.4, 12.10).

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