Genesis of crystalline crust and lithosphere of ancient platforms according to data on the hot heterogeneous accretion of the Earth

Abstract

Compositions of gneisses from the Early Precambrian crystalline complexes of the Aldan shield and of mantle xenoliths from kimberlites plot along the magmatic fractionation trends. Their isotope age and crystallization temperature decrease in according to the sequence of their formation during fractionation. This indicates that the crystalline crust and lithosphere mantle formed as a result of crystallization and fractionation of a layered magma ocean. Evidence of chemical disequilibrium between the mantle rocks and metallic iron suggests that accretion of the Earth’s core occurred before that of the silicate mantle under the effect of magnetic forces. In the silicate magma ocean, which originated through impact melting, the processes of compressional crystallization and fractionation of its near-bottom parts occurred. Due to a very low pressure in the incipient magma ocean, the early formed residual melts varied in composition from granites to tholeiites. This provided very early formation of the acid crystalline crust. An increase in temperature during the accretion process resulted in the higher temperature of the upper mantle compared to the lower one. For this reason the lower mantle plumes did not ascend in the Early Precambrian, and magmas in ancient platforms were forming mainly from residual melts of compositionally varying layers of the magma ocean. In the Phanerozoic, the temperature of the lower mantle became higher than that of the upper one. As a result, lower mantle plumes, oceans and lithosphere of platforms came into existence. In the ascending mantle plumes, basic eclogites were subject to decompression melting. Fractionation of the formed magma chambers led to the formation of acid magmas under low pressure conditions and of various alkali-basic magmas under high pressures.