Irreversible evolution of tectono-magmatic processes at the Earth and Moon: Petrological data

Abstract

The tectono-magmatic evolution of the Earth and Moon started after the solidification of their magmatic “oceans”, whose in-situ crystallization produced the primordial crusts of the planets, with the composition of these crusts depending on the depths of the “oceans”. A principally important feature of the irreversible evolution of the planetary bodies, regardless of their sizes and proportions of their metallic cores and silicate shells, was a fundamental change in the course of their tectono-magmatic processes during intermediate evolutionary stages. Early in the geological evolution of the Earth and Moon, their magmatic melts were highly magnesian and were derived from mantle sources depleted during the solidification of the magmatic “oceans”; this situation can be described in terms of plume tectonics. Later, geochemically enriched basalts with high concentrations of Fe, Ti, and incompatible elements became widespread. These rocks were typical of Phanerozoic within-plate magmatism. The style of tectonic activity has also changed: plate tectonics became widespread at the Earth, and large depressions (maria) started to develop at the Moon. The latter were characterized by a significantly thinned crust and basaltic magmatism. These events are thought to have been related to mantle superplumes of the second generation (thermochemical), which are produced (Dobretsov et al., 2001) at the boundary between the liquid core and silicate mantle owing to the accumulation of fluid at this interface. Because of their lower density, these superplumes ascended higher than their precursors did, and the spreading of their head parts resulted in active interaction with the superjacent thinned lithosphere and a change in the tectonic regime, with the replacement of the primordial crust by the secondary basaltic one. This change took place at 2.3–2.0 Ga on the Earth and at 4.2–3.9 Ga on the Moon. Analogous scenarios (with small differences) were also likely typical of Mars and Venus, whose vast basaltic plains developed during their second evolutionary stages. The change in the style of tectonic-magmatic activity was associated with important environmental changes on the surfaces of the planets, which gave rise to their secondary atmospheres. The occurrence of a fundamental change in the tectono-magmatic evolution of the planetary bodies with the transition from depleted to geochemically enriched melts implies that these planets were originally heterogeneous and had metal cores and silicate shells enriched in the material of carbonaceous chondrites. The involvement of principally different material (that had never before participated in these processes) in tectono-magmatic processes was possible only if these bodies were heated from their outer to inner levels via the passage of a heating wave (zone) with the associated cooling of the outermost shells. The early evolutionary stages of the planets, when the waves passed through their silicate mantles, were characterized by the of development of super-plumes of the first generation. The metallic cores were the last to melt, and this processes brought about the development of thermochemical super-plumes.