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ORIGIN OF MANTLE PLUMES AND THEIR VARIATION
Abstract
In accordance with the hypothesis of cold homogeneous accretion, it is usually assumed that the Earth?s core and mantle resulted from the gravitational separation of iron and silicate particles in its interior. According to this scenario, the core and mantle temperatures should not differ significantly. Therefore, the reason for mantle convection is difficult to explain. Recent data on the rapid accretion of planets, on the existence of magmatic fractionation trends in mantle xenoliths and in rocks of the crystalline crust, and on the chemical imbalance of mantle rocks with metallic iron all point to hot heterogeneous accretion of Earth and the existence of a global ocean of magma thereupon. Core accretion occurred before the mantle due to the rapid merging of iron particles under the influence of magnetic forces. This was much more powerful than gravitational forces. Therefore, the impact heating of the core was stronger than for the mantle. This caused it temperature to be thousands of degrees higher, as well as resulting in the subsequent heating of the mantle and the occurrence of convection therein. Due to the gradual enlargement of particles in the Protoplanetary disk and the increased impact heat generation during accretion, the mantle?s temperature increased first from the bottom to the top. Therefore, it lacked convection throughout the mantle. It occurred in the Proterozoic eon after the core intensely heated the mantle and a direct geothermal gradient had set therein. Huge super-plumes of a very viscous, predominantly ultrabasic substance began to float in this. Under their influence, the tectonics of lithospheric plates emerged. During the accretion, large bodies crashed at the bottom of the magma ocean, forming impact depressions. The predominantly basic molten that filled them quickly hardened compressed and formed large bodies of main basic rocks among the ultrabasic cumulates. Their subsequent surfacing and decompression melting led to the occurrence of the plumes of basic magmas and enormous volumes of trap rocks, as well as the rocks of large igneous provinces.
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References7
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