Mantle subdivision :
- In Geodynamic model the mantle is divided into two parts : The upper and Lower with boundary at 660 km
- In a chemical-mineralogical model, the mantle is subdivided into four parts:
- The upper part down to a depth of 400km
- The transition part within 400-700 km
- The lower part from 700-1650 km
- The D” layer at the bottom of the mantle, approx. 250 km thick
In the PREM seismic model, the boundary between the upper mantle and the lower mantle is accepted at a depth of 670km, where seismic velocities and density increases. This discontinuity is related to the transition of olivine from the ringwoodite phase into the mixture of perovskite and mangnesiowustite.
In the Pyrolite model, the transition zone of the upper mantle is consists of olivine (60%) and garnet (40%). The latest data of high pressure garnet show phase transition being converted into Mg-Perovskite with impurity of Ca-Perovskite. This Garnet to perovskite transition occurs at 660-710 km depth.
The top part of mantle has Olivine, Opx, Cpx and Garnet.
- At the upper boundary of transition zone phase transition occurs in Olivine ; The Hexagonal lattice changes into Face centered lattice. The initial α-phase olivine converted into β-phase called Wadsleyite. Further in the zone of 500- 560 km range β-phase changes into γ-phase called ringwoodite or γ-Spinal (change in Si distribution).
- Within the range of 100-300 km Opx passes into Cpx, which at 100-450 km is transformed into Garnet Majorite.
Therefore the transition zone is consists of mostly Olivine (60%) in wadsleyite and ringwoodite phase and of Garnet (40%).
- Within the depth range of 660-710 Km, the Garnet segregates into Mg-Perovskite (Mg coordi. =8,12), Mg-Wustite, Ca-Perovskite and Corundom.
Therefore the Lower mantle is mostly consists of Mg-Perovskite, Ca-Perovskite and Mg-Wustite [(Mg,Fe)O]
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