Tectonomagmatic evolution of the Earth and Moon |
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Authors: | E V Sharkov O A Bogatikov |
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Institution: | (1) Department of Earth and Environmental Sciences, Florida International University, Miami, FL 33155, USA |
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Abstract: | The Earth and Moon evolved following a similar scenario. The formation of their protocrusts started with upward crystallization
of global magmatic oceans. As a result of this process, easily fusible components accumulated in the course of fractional
crystallization of melt migrating toward the surface. The protocrusts (granitic in the Earth and anorthositic in the Moon)
are retained in ancient continents. The tectonomagmatic activity at the early stage of planet evolution was related to the
ascent of mantle plume of the first generation composed of mantle material depleted due to the formation of protocrusts. The
regions of extension, rise, and denudation were formed in the Earth above the diffluent heads of such superplumes (Archean
granite-greenstone domains and Paleoproterozoic cratons), whereas granulite belts as regions of compression, subsidence, and
sedimentation arose above descending mantle flows. The situation may be described in terms of plume tectonics. Gentle uplifts
and basins (thalassoids) in lunar continents are probable analogues of these structural elements in the Moon. The period of 2.3–2.0 Ga ago was a turning point in the
tectonomagmatic evolution of the Earth, when geochemically enriched Fe-Ti picrites and basalts typical of Phanerozoic within-plate
magmatism became widespread. The environmental setting on the Earth’s surface changed at that time, as well. Plate tectonics,
currently operating on a global scale, started to develop about ∼2 Ga ago. This turn was related to the origination of thermochemical
mantle plumes of the second generation at the interface of the liquid Fe-Ni core and silicate mantle. A similar turning point
in the lunar evolution probably occurred 4.2–3.9 Ga ago and completed with the formation of large depressions (seas) with thinned crust and vigorous basaltic magmatism. Such a sequence of events suggests that qualitatively new material previously
retained in the planets’ cores was involved in tectonomagmatic processes at the middle stage of planetary evolution. This
implies that the considered bodies initially were heterogeneous and were then heated from above to the bottom by propagation
of a thermal wave accompanied by cooling of outer shells. Going through the depleted mantle, this wave generated thermal superplumes
of the first generation. Cores close to the Fe + FeS eutectics in composition were affected by this wave in the last turn.
The melting of the cores resulted in the appearance of thermochemical superplumes and corresponding irreversible rearrangement
of geotectonic processes. |
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