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MODERN DATA ON THE EARTH-S HOT HETEROGENEOUS ACCRETION AS A BASIS FOR SCIENTIFIC REVOLUTION IN GEOLOGY

V. S. Shkodzinskiy

Abstract

Major genetic concepts currently existing in geology are based on the hypothesis of the Earth?s cold homogeneous accretion put forth in the mid-20th century. The data obtained in the last decades are in conflict with these concepts. It has been found that compositions, isotope ages and formation temperatures of the crustal and mantle rocks are interrelated according to the laws of magmatic differentiation. This indicates that the rocks formed as a result of fractionation of the global magma ocean. Chemical disequilibrium between the mantle rocks and metal iron suggests that the Earth?s core formed earlier than the mantle, as a result of aggregation of iron particles mainly under the effect of magnetic forces. Therefore, the accretion was heterogeneous. In the light of these data, many of the debatable genetic problems receive quite a new solution. Due to rapid accretion, the temperature of the core was very high. This caused heating of the mantle by the core, which provided the existence of convective flows in it. Crystallization of the near-bottom parts of the silicate magma ocean as a result of impact melting occurred in the conditions of the growing pressure of its newly formed upper parts. The accumulated cumulates formed the lower mantle, and the residual melts of varying composition formed a magma ocean. The arrangement of the residual melts in correspondence with their density defined a layered nature of the ocean. A shallow depth of the early magma ocean and low gravity on the as yet small Earth provided low-pressure conditions for fractionation of the near-bottom parts of the ocean. The fractionation produced large volumes of acid residual melts, and may explain early formation of the acid crystalline crust of continents. Due to the growing density of the layered magma ocean with depth, no extensive convective flows existed in it, and the ocean was long cooling and crystallizing from top to bottom due to conductive heat losses. A successive ascent of the residual melts from different layers of the magma ocean may account for the trend in the evolution of magmatism in ancient cratons from acid through subalkaline and alkali-basic to ultrabasic. Decompression melting of eclogites at the upwelling of the lower mantle plumes led to massive formation of the tholeiitic magma chambers in the asthenosphere. Fractionation of the chambers was accompanied by the formation of acid magmas in low-pressure conditions and of alkaline magmas under high pressures.

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DOI resolver: doi.org/10.5593/sgem2018/1.1/s01.038

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