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Pyrolitic: derived from petrological composition trends from upper mantle peridotite suggesting homogeneity between the upper and lower mantle with a Mg/Si ratio of 1.27. This model implies that the lower mantle is composed of 75% bridgmanite, 17% ferropericlase, and 8% CaSiO 3 -perovskite by volume.
There is debate as to whether the rest of the mantle, especially the lower mantle, has the same bulk composition. [19] The mantle's composition has changed through the Earth's history due to the extraction of magma that solidified to form oceanic crust and continental crust.
The mantle is divided into upper and lower mantle [21] separated by a transition zone. [22] The lowest part of the mantle next to the core-mantle boundary is known as the D″ (D-double-prime) layer. [23] The pressure at the bottom of the mantle is ≈140 GPa (1.4 Matm). [24]
Tectonic plates are pieces of Earth's crust and uppermost mantle, together referred to as the lithosphere. The plates are around 100 km (62 mi) thick and consist of two principal types of material: oceanic crust (also called sima from silicon and magnesium) and continental crust (sial from silicon and aluminium).
The existence of silicate perovskite in the mantle was first suggested in 1962, and both MgSiO 3 and CaSiO 3 had been synthesized experimentally before 1975. By the late 1970s, it had been proposed that the seismic discontinuity at about 660 km in the mantle represented a change from spinel structure minerals with an olivine composition to silicate perovskite with ferropericlase.
Lithospheric mantle is ultramafic and has lost most of its volatile constituents, such as water, calcium, and aluminum. [5] Knowledge of this depletion is based upon the composition of mantle xenoliths. The depth to the base of the CBL can be determined from the amount of forsterite within samples of olivine extracted from the
The silicate mantle of the Earth's moon is approximately 1300–1400 km thick, and is the source of mare basalts. [4] The lunar mantle might be exposed in the South Pole-Aitken basin or the Crisium basin. [4] The lunar mantle contains a seismic discontinuity at ~500 kilometers (310 miles) depth, most likely related to a change in composition. [4]
The transition zone is the part of Earth's mantle that is located between the lower and the upper mantle, most strictly between the seismic-discontinuity depths of about 410 to 660 kilometres (250 to 410 mi), but more broadly defined as the zone encompassing those discontinuities, i.e., between about 300 and 850 kilometres (190 and 530 mi) depth. [1]