Читать книгу Earth Materials - John O'Brien - Страница 24

The lower mantle, also called the mesosphere, extends from depths of 660 km to the core–mantle boundary at approximately 2900 km. Based on high pressure, high temperature laboratory studies, bridgmanite [(Mg,Fe,Al)SiO₃], ferropericlase [(Mg,Fe)O], magnesiowustite [(Mg,Fe)O], stishovite (SiO₂), and calcium‐rich ferrite (Ca,Na,Al)Fe₂O₄ are thought to be the major minerals in the lower mantle. Our knowledge of the deep mantle continues to expand, largely based on high temperature, high pressure laboratory studies and on anomalous seismic signals deep within the Earth. A deeper layer has been proposed at about 1600 km depth where the rigidity of the mantle may increase considerably (Miyagi and Marquardt 2015). Anomalous seismic velocities are particularly common in a complex zone, of variable thickness, near the core–mantle boundary called the D″ layer. The D″ discontinuity ranges from ~130 to 340 km above the core–mantle boundary. Williams and Garnero (1996) proposed an ultra‐low velocity zone (ULVZ) in the lowermost mantle on seismic evidence. These sporadic ultra‐low velocity zones may be related to the formation of deep mantle plumes within the lower mantle. Other areas near the core–mantle boundary are characterized by anomalously fast velocities. Hutko et al. (2006) detected subducted lithosphere which had sunk all the way to the D″ layer and may be responsible for the anomalously fast velocities. Deep subduction and deeply rooted mantle plumes support some type of whole mantle convection and may play a significant role in the evolution of a highly heterogeneous mantle, but these concepts are still controversial (Foulger et al. 2005).