The first 800 million years of Earth's history

Abstract

The Earth grew initially from hot planetesimals of reduced material, and began to differentiate early into a metal-rich core, a silicate-rich mantle and a volcanic surface. Accretion ended with volatile-rich oxidized material, which did not interact with the core because of a mantle barrier. Quenching of magmas at the surface and foundering of cool material was effective in cooling the Earth by 1600 J per gram of magma. Magmas transported radioactive elements and volatiles to the outer 50 km. The lower mantle became stranded below the critical adiabat for melting. The core may have been fully molten. Radiogenic and non-radiogenic heat sources produced a surface heat flow that decreased from ca . 1.3 x 10 14 W at 4.45 Ga B.P . to the present value of 4 x 10 13 W. In spite of uncertainties in the production and transport of heat, it seems safe to conclude that intense volcanism occurred throughout the pre-Archaean era before survival of crust. From phase-equilibrium data for peridotites and calc-alkaline rocks, it is deduced that a range of ultrabasic to basic magmas was produced from a peridotitic upper mantle, and that calc-alkaline magmas were produced during foundering of crust. An anorthositic crust was not formed. From observations of lunar basins, it is concluded that a minimum of ca . 500-1000 impact basins were formed on Earth in ca . 100—200 Ma before 3.95 Ga B.P. Plausible calculations suggest that ca . 20 times as many basins were formed in the preceding ca . 300 Ma. Impact debris would be cooled by water in wet crustal rocks and oceans. It is assumed that after 4.0 Ga B.P. impacts would have only temporary local effects on the crust, and emphasis is placed on upper-mantle convection as the main driving force for geochemical transport. Sedimentation and metamorphism were important factors during foundering of crust. ‘Oceanic' and ‘continental' regions developed above up-welling and down-welling segments of convection cells, and these regions resembled greenstone belts and high-grade regions of the Archaean era. As heat flow declined, polygonal tectonics was replaced by symmetrical linear tectonics during the Archaean era, and then by asymmetrical linear tectonics during the Proterozoic era.