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EPSC Abstracts, Vol. 3, EPSC2008-A-00495, 2008 European Planetary Science Congress, © Author(s) 2008 Irreversible evolution of the terrestrial planets (geological and petrological data) E. Sharkov and O. Bogatikov (1) Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry RAS, Moscow, Russia ([email protected] / Fax: +7-495-9511587) Abstract Comparative studying of tectonomagmatic evolution of the Earth and the Moon shows that cardinal irreversible change in character of tectonomagmatic processes occurred at middle stages of their evolution; very likely such changes took place on other terrestrial planets (Venus, Mars and Mercury). As a result, primordial crusts of the planets were in considerable degree replaced by secondary basaltic ones. The established succession of events on the Earth could be provided by a combination of two independent factors: (1) it was originally heterogeneous and 2) its downward heating was followed by the cooling of its outer shells. As a result the primary iron core material was long time remained untouched and was involved into global tectonomagmatic processes at ca. 2.4-2.3 Ga. We concluded about a similar scenario for the evolution of Moon and other terrestrial planets. Tectonomagmatic evolution of the terrestrial planets (Earth, Venus, Mars, Mercury and Moon) was studied. What did major stages of their irreversible evolution occur before they turned into “dead” stone balls? We discuss these problems on examples of the Earth and the Moon, which evolution studied the best. According to modern views, after accretion of these bodies, magma oceans of some hundreds km deep appeared on their surface. According to Jeffries [1], solidification of large molted bodies, because of the difference between adiabatic gradient in silicate melts (0.3oC/km) and gradient of their melting points (3oC/km), could be going only upwards, from the bottom to the surface. As a result a powerful crystallizing differentiation of the oceans’ magmas occurred with accumulation of the most low-melting components to the surface. Due to different deep of the magma oceans on the Earth and the Moon, the primordial crusts on these bodies were rather different: sialic on the Earth and basic (anorthosite) on the Moon. Terrestrial tectonomagmatic activity in Archean and early Paleoproterozoic Geological evolution of the Earth began ~4 Ga ago from appearance of Archean granite-greenstone terranes (GGTs) and divided them granulite belts. The GGTs consisting of greenstone belts “submerged” in tonalite-trondhjemite-granodiorite matrix (TTG) were the areas of extension, uplifting and denudation, whereas the granulite belts were dominated by compression, sinking and sedimentation. They were interspaced by intermediate zones of tectonic flowage. GGTs mantle-derived magmatism of high-Mg komatiite-basaltic series and boninite-like volcanics was located in greenstone belts. It suggests that GGTs appeared above mantle superplumes of the first generation, composed by depleted ultramafic material; granulite belts were formed on places of descending mantle flows. The situation could be described in terms of plume-tectonics. By the Proterozoic the crust became rigid resulting in formation of rifts, dike swarms and large mafic-ultramafic layered intrusions. In the early Proterozoic the character of the tectono-magmatic activity remained almost the same: cratons separated by greenstone belts appeared in the place of GGT. The magmatism was dominated by siliceous high-Mg series (SHMS) forming large igneous provinces (LIPs). Geochemically, they were similar to the Phanerozoic boninites but with higher TiO2 and negative εNd implying an important assimilation of Archean lower crustal rocks. The SHMS and possible Archean geochemical analogues of boninites possibly resulted from the ascent of deep HT ultramafic melts and their percolation through the lithosphere, like zone refinement, implying melting at the top and crystallization at the bottom. This provided large-scale assimilation of upper mantle and lower crust rocks. The appearance of large igneous provinces requires first generation mantle superplumes located beneath them and consisting of depleted mantle material. Such a situation can be described in terms of plume-tectonics typical of the Early Precambrian [2]. Cardinal change in the Earth’s evolution The cardinal change of the magmatic activity with appearance in global scale of geochemical-enriched Fe-Ti picrites and basalts occurred in interval 2.3-2.0 Ga ago. Such melts was typical for Phanerozoic within-plate magmatism and linked with thermochemical mantle superplumes of the second generation, which ascended from the liquid core-mantle boundary (CMB). Change of composition of magmatic melts was followed and, probably, initiated sharp changing in ecological situation on the Earth’s surface; appearance of oxidative atmosphere, global glaciations, positive shift in carbon isotopy in sedimentary carbonates, appearance of phosphorites, hydrocarbons, and, also, to important changing in biosphere – prosperity of cyanobacterias, multicellular organisms, etc.[3]. According to Jeffries model, the superplumes draw away the heat from the liquid core resulting in its solidification, which goes upwards and thus provide the growth of the inner (solid) core. Such a process relieves big amounts of the fluids dissolved in the melt and initiates the ascent of the thermochemical plumes. deformation directed inside the planets after they accretion finished as a result of materials compaction and shortening of their sizes which led to acceleration of their rotation around axes. That wave could reach the interior of the planets thus heating deep mantle material and generating first superplumes. Finally, it reached the metallic core, melted it and produced secondary thermochemical plumes, which are still active on the Earth. We suggest that the terrestrial planets were developed at the same, but shortened scenario, and more quick. At the Moon the earliest magmatism of highlands were close to terrestrial early Paleoproterozoic SHMS and at the boundary 3.9-3.8 Ga was changed by maria magmatism, close in composition to MORB and OIB. By analogy with the Earth, we suggest that maria magmatism was linked with ascending of thermochemical superplumes, generated Tectonomagmatic evolution of the Moon and other at the lunar CMB, when it’s liquid iron core was yet existed. Ancient planums on Mars and tesseras at the terrestrial planets The study of the samples which became available Venus among vast planides, composed by basaltic flows due to the American and Russian space missions the can also evidence about two stages of their development. Moon’s oldest magmatism in lunar highlands is dated Probably, drastic changing of environmental situations on by 4.4-4.0 Ga. It was characterized by the low-Ti their surfaces, like on the Earth, were also depend on magnesium suite analogous to the terrestrial change in tectononagmatic activity. Judging on absence of Paleoproterozoic SHMS [4]. A geological catastrophe magnetic field, their liquid metallic cores (“energetic analogous to that on the Earth happed on the Moon ca. hearts”) are of no consequence and they are “dead” bodies 3.9 Ga to form lunar maria with signatures of plume now. magmatism (high-Ti melts, etc). Probably, the lunar maria are analogues of Earth’s oceans to some extent, CONCLUSIONS therefore this stage of the Moon evolution can be 1.The Early Precambrian (Archean, early Paleoproterozoic) correlated with the continental-oceanic stage of the tectonomagmatic activity on the Earth was different from Earth’s evolution. In the Venus and Mars, two main the Phanerozoic one: the mean features were granitetypes of morphostructures, which are vast fields of greenstone terranes and their separating granulite belts; flood basalts, and older uplifted segments with a mantle melt were derived from a depleted source. complicated topography (tesseras in the Venus and 2. A drastic change of the tectonomagamtic processes terras in the Mars) possible suggest a two-stage occurred at ca. 2.3-2.0 Ga: the plume tectonic was evolution of these planets. During the first stage the changed by the plate tectonics, which is still active. Since primordial lithospheres formed due to the solidification that time the primordial sialic crust has been replaced by of global magmatic “oceans”. During the second stage the secondary basaltic crust. the secondary basaltic crust formed due to the ascent of 3. The established succession of events could be provided thermochemical plume from the CMB. The Mercury is by a combination of two independent factors: (1) the Earth less studied, however, its relief also suggests two originally was heterogeneous and 2) the downward groups of morhostructures resembling lunar highlands heating of the Earth was followed by the cooling of its and maria. outer shells. As a result the primary iron core material was long time remained untouched and was involved into Causes of irreversible tectonomagmatic evolution of global tectonomagmatic processes at ca. 2.4-2.3 Ga. 4. We concluded about a similar scenario for the evolution the terrestrial planets Our data show that at ca. 2.5 Ga on the Earth and of Moon and other terrestrial planets. Work was supported by grant RFBR 07-05-00496. 1.5 Ga on the Moon the tectonomagmatic processes started to involve previously absent geochemically enriched material. Where the enriched matter was References stored, how it was activated and why its participation [1] Jeffries H. (1929) The Earth, 2nd Ed. London: resulted in such drastic consequences? The established Cambridge Univ. Press,. succession of events could be provided by a [2] Bogatikov O.A., Kovalenko V.I., Sharkov E.V., combination of two independent factors: 1) The Earth Yarmolyuk V.V. (2000) Magmatism and Geodynamics. originally was heterogeneous, i.e. formed due to the Terrestrial Magmatism Throughout the Earth's History. heterogeneous accretion and 2) the downward heating Amsterdam: Gordon and Breach Science Publ. of the Earth – from the surface to the iron core – was [3] Melezhik V.A., Fallik A.E., Hanski E. et al. (2005) followed by the cooling of its outer shells. GSA Today, 15 (11), 4-10. The most probable cause of the centripetal heating of[4] Sharkov E.V., Bogatikov O.A. (2001) Petrology, 9. the Earth was appearance of zone/wave of heat-generating97-118. The thermochemical plume matter possessed less density and could reach shallower depths. The spread of the head parts led to their active interaction with the upper part of the ancient lithosphere including the crust. This, in turn, resulted in crust fracturing, oceanic spreading, formation and movement of plates, subduction, etc., i.e. plate tectonics, existed till now. From this particular time, ancient Earth’s continental crust began to involved in subduction processes and interchange by secondary oceanic crust which forms about 70% of the present-day crust. Thus, during the period from 2.3 to 2.0 Ga, the tectonic processes and the composition of mantle melts irretrievably changed over the whole Earth. This triggered the processes of plate tectonics which are still active.