The accretional growth of the Earth necessarily involved a molten zone in the outer layer (magma ocean) of the growing Earth, resulting in downwards extraction of the molten iron alloy, i.e. core formation. The primary stratification resulted from melting in the accretional stage of the Earth and consisted of an Fe-core, a perovskite-rich lower mantle, and a partially molten ultrabasic upper mantle, in which the melt was concentrated in two regions: near the surface of < 300 km depth (shallow magma ocean, SMO) and at depths greater than 400 km of the upper mantle (deep magma ocean, DMO). The cooling, gravitational differentiation and crystallization in the magma ocean should give rise further to a finer stratification; the upper mantle composed of two layers, the upper olivine-rich layer and the lower majorite garnet rich layer, and the lower mantle with a perovskite-rich composition. Both the 400 and 670 km discontinuities of the present mantle may be the chemical boundary. The geophysical constraints of the mantle, i.e. the seismic wave velocity and density profiles favor such a stratified mantle.
ASJC Scopus subject areas
- Earth-Surface Processes