Time Between 3 and 2 Ga: Transitional Events in the Earth’s History

Abstract

—The time span between 3 and 2 Ga in the geologic history encompassed a number of key events on the cooling Earth. The cooling interrupted heat transfer within and across the mantle, which caused changes in Earth’s major spheres and in the mechanisms of their interaction. The great thermal divergence at 2.5 Ga and differentiation into the depleted upper asthenospheric and primitive lower mantle affected the compositions of oceanic basalts. The lower mantle cooling recorded by a systematic decrease in the temperature of komatiite magma generation at the respective depths began at 2.5 Ga and was accompanied by increasing abundance of arc basalts and by changes in the behavior of the Sr, Nd, and O isotope systems. It was the time when the continental lithosphere consisting of subcontinental lithospheric mantle and crust began its rapid growth, while the crust became enriched in felsic material with high contents of lithophile elements. Magmatism of the 3–2 Ga time span acquired more diverse major-element chemistry, with calc-alkaline and alkaline lithologies like carbonatite and kimberlite. The dramatic changes were driven by subduction processes, whereby the crust became recycled in the mantle and the double layer (D”) formed at the core–mantle boundary. The events of the 3–2 Ga interval created prerequisites for redox changes on the surface and release of free oxygen into the atmosphere. In terms of global geodynamics, it was transition from stagnantlid tectonics to plate tectonic regime, which approached the present-day style about 2.0–1.8 Ga.