GEOLOGICAL SIGNS OF HOT HETEROGENEOUS ACCRETION OF THE EARTH AND THE CONSEQUENCES OF ITJournal: Science and world (Vol.2, No. 50)
Publication Date: 2017-10-30
Authors : Shkodzinsky V.S.;
Page : 70-83
Keywords : Earth accretion; magma ocean; origin of geospheres and magmas; early planetary evolution;
It is established that structures of rocks of early Precambrian crystal complexes and mantle xenoliths of kimberlites form trends of global magmatic fractionation. Their isotope age and crystallization temperature decrease according to the formation sequence at fractionation. Temperature at forming early Precambrian crystal complexes and xenocrysts in kimberlites increased with depth respectively by 3,5 and 2 °/km t hat is close to the size of an adiabatic gradient for fluxes. The earliest geothermal gradients are projected to the area of very high temperature (900 – 1000 °C) on the land surface. All this demonstrates formation of the earth's crystal crust and mantle as a result of fractiona- tion of the global magma ocean. Signs of a sharp chemical disequilibrium of mantle rocks with metallic iron indicate accretion of the Earth core before a silicate mantle under the influence of magnetic forces. In the silicate magmat ic ocean formed by impact melting there were processes of compression crystallization and fractionation of near - bottom parts. Owing to rather small pressure in the arising magmatic ocean early residual liquors varied on structure from granites to tholeiite s depending on crystallization degree. It has caused very early formation of substance of acid crys- talline crust. Temperature increase during accretion owing to particle coarsening in a protoplanetary disk has resulted in more high temperature of the upper mantle in comparison with lower. Therefore in the early Precambrian there was no rise of lower mantle plumes and on ancient magma platforms they were formed mainly from residual liquors of the magmatic ocean layers, various on structure. In the Phanerozoi c under heating influence initially the lower mantle has gained more high temperature by very hot core than the upper. The lower mantle plumes and oceanic areas resulted. In rising plumes the decompressive remelting of eclogites' bodies led to formation of large volume of tholeitic magmas. Fractionation of the arising magma pockets was followed by formation of acid residual liquors under the conditions of the low pressure and various alkaline – under the conditions of high pressure. As the analysis has show n, existence of layered magma oceans on inferior planets and their early hardening from top to down is the reason of flooding by lavas of arising early impact basins. The emission at their crystallization of a large number of volatile components explains s igns of existence of water basins at the early stage of Mars evolution.
Other Latest Articles
Last modified: 2018-08-10 16:43:50