Page 45 - Global Tectonics

32 CHAPTER 2

most closely approaches its melting point (Section pressure studies have shown that olivine, the dominant
2.12, Fig. 2.36). mineral in mantle peridotite, undergoes transforma-
Only a very small amount of melt is required to tions to the spinel structure at the pressure/tempera-
lower the seismic velocity of the mantle to the observed ture conditions at 410 km depth and then to perovskite
values and to provide the observed attenuation proper- plus magnesiowüstite at 660 km (Table 2.4) (Helffrich &
ties. A liquid fraction of less than 1% would, if distrib- Wood, 2001). Within subducting lithosphere, where the

uted along a network of fissures at grain boundaries, temperature at these depths is colder than in normal
produce these effects (O’Connell & Budiansky, 1977). mantle, the depths at which these discontinuities occur
The melt may also be responsible for the high electrical are displaced exactly as predicted by thermal modeling
conductivity of this zone. For the partial melting to and high-pressure experiments (Section 9.5). This lends
occur, it is probable that a small quantity of water is excellent support to the hypothesis that the upper and
required to lower the silicate melting point, and that this lower bounds of the transition zone are defi ned by
is supplied from the breakdown of hydrous mantle phase transformations. The other components of
phases. The base of the low velocity zone and even its mantle peridotite, pyroxene and garnet, also undergo
existence may be controlled by the availability of water phase changes in this depth range but they are gradual
in the upper mantle (Hirth & Kohlstedt, 2003). and do not produce discontinuities in the variation of
The mantle low velocity zone is of major impor- seismic velocity with depth. Pyroxene transforms into
tance to plate tectonics as it represents a low viscosity the garnet structure at pressures corresponding to 350–
layer along which relative movements of the lithosphere 500 km depth; at about 580 km depth Ca-perovskite
and asthenosphere can be accommodated. 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
2.8.5 The mantle lower mantle mostly consists of phases with perovskite
structure.
transition zone

There are two major velocity discontinuities in the
mantle at depths of 410 km and 660 km. The former 2.8.6 The lower mantle
marks the top of the transition zone and the latter its
base. The discontinuities are rarely sharp and occur The lower mantle represents approximately 70% of the

over a finite range in depth, so it is generally believed mass of the solid Earth and almost 50% of the mass of
that they represent phase changes rather than changes the entire Earth (Schubert et al., 2001). The generally
in chemistry. Although these discontinuities could be smooth increase in seismic wave velocities with depth
due to changes in the chemical composition of the in most of this layer led to the assumption that it is
mantle at these depths, pressure induced phase changes relatively homogeneous in its mineralogy, having mostly
are considered to be the more likely explanation. High- 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) 2 SiO 4 = (Mg,Fe) 2 SiO 4
Olivine Wadsleyite (β-spinel structure)
520 km 18 GPa (Mg,Fe) 2SiO 4 = (Mg,Fe) 2SiO 4
Wadsleyite Ringwoodite (γ-spinel structure)
660 km 23 GPa (Mg,Fe) 2 SiO 4 = (Mg,Fe)SiO 3 + (Mg,Fe)O
Ringwoodite Perovskite Magnesiowüstite