The low velocity zone (Fig. 2.16) is characterized by low seismic velocities, high seismic attenuation, and a high electrical conductivity. The seismic effects are more pronounced for S waves than for P waves. The low seismic velocities could arise from a number of different mechanisms, including an anomalously high temperature, a phase change, a compositional change, the presence of open cracks or fissures, and partial melting. All but the latter appear to be unlikely, and it is generally accepted that the lower seismic velocities arise because of the presence of molten material. That melting is likely to occur in this region is supported by the fact that it is at this level that mantle material most closely approaches its melting point (Section 2.12, Fig. 2.36).
Only a very small amount of melt is required to lower the seismic velocity of the mantle to the observed values and to provide the observed attenuation properties. A liquid fraction of less than 1% would, if distributed along a network of fissures at grain boundaries, produce these effects (O'Connell & Budiansky, 1977). The melt may also be responsible for the high electrical conductivity of this zone. For the partial melting to occur, it is probable that a small quantity of water is required to lower the silicate melting point, and that this is supplied from the breakdown of hydrous mantle phases. The base of the low velocity zone and even its existence may be controlled by the availability of water in the upper mantle (Hirth & Kohlstedt, 2003).
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