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世界の地質図 > グリーンランド > イスア > 説明文6

Implications for geodynamics in the Archean

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.

Figure 22:Cartoons of material circulation in the whole mantle in the Archean (a) and in the Phanerozoic (c). In the Archean, thin oceanic lithosphere with thick oceanic crust subducted beneath small oceanic island arcs in many places, and slab materials were widely distributed over the upper-lower mantle boundary. The high production rate of Archean MORB probably resulted in successive subsidence of slab materials into the lower mantle. The formation of the continental crust at subduction zones by slab melting and segregation of iron blobs during slab penetration into the lower mantle have changed the chemical composition of the mantle through geologic time. Partial melting of the subducted oceanic crust on the core-mantle boundary also possibly plays a significant role in the compositional change of the mantle (Hirose et al., 2004). In the Phanerozoic the thick oceanic lithosphere with thin oceanic crust subducts beneath large continental blocks. A huge amount of slab material accumulates beneath a large continent for a long time, and continually subsides into the lower mantle (Fukao et al., 1992).

 
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