Material circulation model including chemical differentiation within the mantle and secular variation of temperature and composition of the mantle

It is 20 years since Allègre [Tectonophys 81 (1982) 109] proposed chemical geodynamics as an integrated study of the chemical and physical structure and evolution of the solid Earth. Accumulation of geochemical data of modern magmatic rocks (e.g. [Annu. Rev. Earth Planet. Sci. 14 (1986) 493] and whole mantle tomography (e.g. [J. Geophys. Res. 97 (1992) 4809]) allow us to understand present-day mantle dynamics, and consider more recent geodynamics models (e.g. [Earth Planet. Sci. Lett. 90 (1988) 297]). However, complete investigation of geodynamics requires addressing not only the present-day structure of the earth and its elemental distributions, but also the historical evolution of the earth [Earth Planet. Sci. Lett. 86 (1987) 175], but no current tectonic models include the latter object. Recent remarkable progress of geology, petrology and geochemistry of Precambrian orogenic belts brings about decoding of the Archean tectonics and evolution of the solid earth through geologic time. Especially, understanding the thermal and compositional evolution of the mantle is essential for decoding the whole history of the Earth, because the mantle constitutes more than 85% of the earth. However, there are two inherent problems that require explanation; post-magmatic alteration, and interpretation of the tectonic setting of the magmatism. We eliminated elemental movements during alteration by comparison with whole rock compositions and with major and rare earth element compositions of relict igneous clinopyroxenes (Cpx). The tectonic setting of mafic magmatism was estimated by an independent method of the composition of greenstones; application of the concept of accretionary geology to the Archean greenstone belts. And we found MORB-affinity greenstones from five different-aged greenstone belts. tential temperature and FeO content of the upper mantle were estimated by comparison of the most primitive MORB from 3.8 to 1.9 Ga with recent melting experiments. The result indicates that the upper mantle had higher FeO content (10 wt.%), and that the FeO content was constant until early Proterozoic, and then decreased. Segregation of iron grains from subducted oceanic crust during slab penetration into the lower mantle is plausible to decrease the FeO content in the mantle. If the produced metallic iron sinks and accumulates on the core, the metallic iron layer would be about 57 km thick. The potential mantle temperature of the upper mantle was about 1480 °C in the Archean and was hotter by ca. 150–200 °C than the modern mantle. The temperature decreased not monotonously but episodically. In addition, recent ultra-high pressure experiments presumed chemical differentiation within the mantle, dehydration or slab melting of subducted oceanic crust beneath a subduction zone, segregation of iron grains from slab materials during slab penetration [Science 273 (1996) 1522], and partial melting of subducted oceanic crust on the core-mantle boundary [Phys. Earth Planet. Inter. (2002)]. This work proposes a global material circulation model, which includes three chemical differentiations within the mantle and the secular change of temperature and composition of the mantle.