There are two major velocity discontinuities in the mantle at depths of 410 km and 660 km. The former marks the top of the transition zone and the latter its base. The discontinuities are rarely sharp and occur over a finite range in depth, so it is generally believed that they represent phase changes rather than changes in chemistry. Although these discontinuities could be due to changes in the chemical composition of the mantle at these depths, pressure induced phase changes are considered to be the more likely explanation. High-
pressure studies have shown that olivine, the dominant mineral in mantle peridotite, undergoes transformations to the spinel structure at the pressure/temperature conditions at 410 km depth and then to perovskite plus magnesiowustite at 660 km (Table 2.4) (Helffrich & Wood, 2001). Within subducting lithosphere, where the temperature at these depths is colder than in normal mantle, the depths at which these discontinuities occur are displaced exactly as predicted by thermal modeling and high-pressure experiments (Section 9.5). This lends excellent support to the hypothesis that the upper and lower bounds of the transition zone are defined by phase transformations. The other components of mantle peridotite, pyroxene and garnet, also undergo phase changes in this depth range but they are gradual and do not produce discontinuities in the variation of seismic velocity with depth. Pyroxene transforms into the garnet structure at pressures corresponding to 350500 km depth; at about 580 km depth Ca-perovskite begins to exsolve from the garnet, and at 660-750 km the remaining garnet dissolves in the perovskite phase derived from the transformation of olivine. Thus the lower mantle mostly consists of phases with perovskite structure.
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