Material circulation through time: Chemical differentiation within the mantle and secular variation of temperature and composition of the mantle

Abstract

In this chapter, we describe that recent progress of our study of Precambrian geology and geochemistry, and present a synthetic view of the evolution of the solid earth including mantle dynamics and surface geology and environment. The recognition of an accretionary complex and oceanic plate stratigraphy in the 3.8 Ga Isua Supracrustal belt clearly indicates the presence of open sea and the operation of plate tectonics in the early Archean. The change of PT paths of subduction-related metamorphism through time shows that a hotter geothermal gradient at the subduction zone more frequently caused slab melting in the Precambrian, and that the transformation of basaltic crust to a heavy residue after slab melting was a driving force of Precambrian-type plate tectonics. We estimated that 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 based on composition of 3.8 to 1.9 Ga MORB-related greenstones. The temperature decreased not monotonously but episodically. The high temperature increased its Rayleigh number by about 30 times, and decreased its viscosity by about one-third, but it complementarily raised the amount of accumulated slab material in the mantle transition zone to trigger off their frequent flushing to the lower mantle. The estimated FeO content of the Archean upper mantle was higher, ca. 10 wt% , and was constant until the early Proterozoic, and then it decreased. Segregation of iron grains from subducted oceanic crust during slab penetration into the lower mantle is a plausible mechanism to decrease the FeO content in the mantle. The growth curve of the continental crust obtained from U-Pb ages of detrital zircons indicates the sudden increase at 2.8.2.7 and 2.0.1.7 Ga. The sudden growth of continents, the worldwide presence of LIPs and many continental collisions at those times imply mantle overturn or extensive exchange between the upper and lower mantle materials, caused by successive upwelling and dowelling of giant superplumes. The mantle overturn events influenced both the surface environment and solid earth. The segregation of iron grains produced ferric iron as a by-product to make the mantle oxidized. In addition, the upwelling of the lower mantle materials through superplumes to the upper mantle increased the Fe3+/Fe in the upper mantle to oxidize it. The change of redox condition of influx gases from reduced to oxidized weakened the suppression by reduced gases against biological oxygenation of the surface environment,and consequently the surface environment became oxic in the early Proterozoic after a mantle overturn. Moreover, the formation of large continents before and their break-up after the mantle overturn, produced wide habitats for photosynthetic lives, and helped biological oxidation at the surface. © 2007 Springer.