Iron upper mantle composition

Densities and bulk sound velocities in the lower mantle are also consistent with the high-pressure mineralogy of a bulk composition approximating an upper-mantle peridotite, such as pyrolite. Seismic constraints provide no support for iron enrichment of the lower mantle relative to such an upper mantle. Silica enrichment of the lower... 

Temperature and pressure both increase with depth in the Earth and control the composition and properties of the material present at various depths. The Earth comprises a number of layers, the boundaries between which are marked by relatively abrupt compositional and density changes (Fig. 1.1). The inner core is an iron-nickel alloy, which is solid under the prevailing pressure and temperature ranges. In contrast, the outer core is molten and comprises an iron alloy, the convection currents within which are believed to drive the Earth s magnetic field. The core-mantle boundary lies at c. 2900km depth and marks the transition to rocky material above. The mantle can be divided into upper and lower parts, although the boundary is quite a broad transitional zone (c.lOOCM-OOkm depth). It behaves in a plastic, ductile fashion and supports convection cells. The upper mantle layer from c.100 to 400 km depth is called the asthenosphere, and its convection system carries the drifting continental plates. 

In this model the mantle is differentiated, into an upper layer (1,600-2,000 km thick), and a compositionally distinct lower layer, enriched in iron and heat producing elements. This lower layer, making up 20-30% of the mantle, is thought to have originated either during the early differentiation of the Earth or by the burial of subducted oceanic crust. 

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