Superplumes – Material Circulation Over Time

Australia: The Land Where Time Began

Komiya¹ describes the recent progress of his team’s study of Precambrian geology and geochemistry, presenting a synthetic view of the solid Earth that includes mantle dynamics and surface geology and environment. In the Isua Supracrustal Belt from 3.8 Ga the recognition of an accretionary complex and oceanic plate stratigraphy indicates clearly the presence of open sea and plate tectonics that was operating in the early Archaean. A hotter geothermal gradient at the subduction zone that more frequently caused the melting of slabs in the Precambrian, and that the transformation of basaltic crust to a heavy residue following slab melting was a driving force for plate tectonics of Precambrian type, is shown by the change of PT paths of metamorphism that was subduction-related. Komiya et al.¹ estimated that the potential temperature of the upper mantle was about 1,480^oC in the Archaean and by about 150-200^oC than the modern mantle based on the composition of MORB-related greenstones dating from 3.8-1.9 Ga. The temperature decreased episodically. The Rayleigh number of the temperature increased by about 30 times, with its viscosity decreasing by about ¹/₃, though it complementarily raised the accumulated amount of slab material in the mantle transition zone to trigger their frequent flushing to the lower mantle. The FeO content estimated for the upper mantle in the Archaean was higher, about 10 wt%, and remained constant until the early Proterozoic, following which it decreased. A plausible mechanism for the decreasing the FeO content of the mantle is segregation of iron grains from oceanic crust that had been subducted during slab penetration into the lower mantle. The sudden increase at 2.8 and 2.7 Ga and 2.0-1.7 Ga is indicated by the growth curve of the continental crust that was obtained from U-Pb ages of detrital zircons. Komiya says that overturn of mantle or extensive exchange between the upper and lower mantle materials that resulted from successive upwelling and downwelling of giant mantle plumes is implied by the sudden growth of continents, the presence of LIPs around the world and many collisions between continents at those times. The surface environment and the solid Earth were both influenced by the mantle overturn. Ferric iron as a by-product resulted from the segregation of iron grains to make the mantle oxidised. Additionally, the upwelling of the lower mantle materials by superplumes to the upper mantle increased the Fe³⁺/_∑Fe in the upper mantle to oxidise it. The suppression by reduced gases against biological oxidation of the surface environment resulted from the change of redox condition of influx gases from reduced to oxidised, leading to the surface environment becoming oxic in the early Proterozoic after mantle overturn. Moreover, the production of wide habitats for photosynthetic lives, and help to biological oxidation at the surface, resulted from the formation of large continents before they were fragmented after the mantle overturn.