The estimate of compositions of the upper mantle indicates the Archean
upper mantle was enriched in FeO content, compared with the modern equivalent
based on the composition of the ancient MORB (Komiya et al., 2002; 2004;
Komiya, 2004). Iron segregation from the subducted oceanic crust at slab
penetration into the lower mantle is plausible mechanisms for the decrease
of FeO content through geologic time (Komiya, 2004; Komiya, 2007). We will
discuss below the model of iron segregation from the subducted oceanic
crust at slab penetration into the lower mantle. Recent ultra-high-pressure
experiments of natural peridotite and aluminous Mg-perovskite under lower
mantle conditions showed that the iron content in Mg-perovskite increases
through the coupled substitution of Al3++Fe3+=Mg2++Si4+ with Al2O3 (Wood & Rubie, 1996; McCammon, 1997). Moreover, some ferrous ions
were transformed into metallic iron by an electron exchange reaction of
3Fe2+=Fe+2Fe3+ accompanying the substitution reaction of Al3++Fe3+=Mg2++Si4+ (McCammon et al., 1997; Frost et al., 2004). The reactions occur during
slab penetration into the lower mantle because the subducted oceanic crust
contains high Al2O3 and FeO content. The estimate of the amount of metallic iron precipitated
from subducted oceanic crust during slab penetration into the lower mantle
shows that if the produced metallic iron sinks and is accumulated on the
core, the thickness of the layer of metallic iron would be about 57 km.
Figure 22 shows the view for global material cycling through time, including
chemical differentiation of the subducted materials within the mantle.
In the Archean, oceanic lithosphere was thinner, and had a thicker oceanic
crust and a shorter life span because of the higher potential mantle temperature.
In addition, there were many subduction zones, namely plate boundaries,
because the size of plates was smaller. A high geothermal gradient at subduction
zones resulted in partial melting of subducted oceanic crust, and changed
the oceanic crust into a denser garnet-bearing residue. As a result, large
amounts of oceanic crust subducted into the deep mantle and accumulated
on the upper-lower mantle boundary. The accumulated materials delaminated
at the bottom, and subsided into the lower mantle. Iron grains were segregated
from the slab materials during the slab penetration into the lower mantle,
and subsided onto the core-mantle boundary. On the other hand, some of
the residue was mixed with the surrounding mantle, and other materials
sank to the core-mantle boundary. On the core-mantle boundary, the subducted
oceanic crust was partially molten, and differentiated into a dense FeO-rich
picritic melt and light Ca-perovskite-bearing residue. The FeO-rich melt
accumulated on the core-mantle boundary, whereas the light Ca-perovskite-bearing
residue rose to the upper mantle. The upper-lower mantle boundary intermittently
opened during subsidence of slab materials into the lower mantle and enabled
injection of superplumes from the lower to the upper mantle. In the Phanerozoic,
oceanic lithospheres have become thick and wide, whereas the oceanic crusts
are thinner. As a result, the subducted oceanic crust in the Phanerozoic
plays a less significant role in the chemical differentiation than in the
Archean. Most oceanic crust undergoes dehydration instead of slab melting
at a subduction zone, except for very young oceanic plates. Subducted materials
accumulate on the upper-mantle boundary, and produce megaliths, which continually
sink into the lower mantle, whereas superplumes rise up to the upper mantle
from the core-mantle boundary.
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