Seismic Discontinuities in the Transition Zone
The transition zone (TZ) is the mantle layer bounded by the 410- and 660-km seismic boundaries. The high P-wave and S-wave velocity gradients within the TZ are caused by a series of polymorphic phase transitions, the depths (pressures) of which are controlled by temperature and composition. Structure of the TZ plays an important role in the heat/mass transfer between the upper and the lower mantle.
Mineral Physics Data on the Phase Transitions in the TZ
The most frequently used model of mantle composition is pyrolite which contains ∼60% of olivine (Mg,Fe)2SiO4. At a depth of ∼410 km olivine (α) transforms to wadsleyite (β, modified spinel). The Clapeyron slope of this transition is positive (4.0 MPa/K, Katsura et al. 2004); the increase of the S-wave velocity is ~12%. At a depth of ∼550 km wadsleyite transforms to ringwoodite (γ, silicate spinel). At a depth of ∼660 km ringwoodite transforms to a mixture of perovskite (Mg,Fe)SiO3and magnesiowüstite (Mg,Fe)O. The S velocity...
- Litasov KD, Ohtani E, Sano A, Suzuki A, Funakoshi K (2005) Wet subduction versus cold subduction. Geophys Res Lett 32(13). https://doi.org/10.1029/2005GL022921