E.V. Sharkov
Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry
(IGEM), Russian Academy of Sciences
Moscow, [email protected]
Irrevesible evolution of the Earth and the
terrestrial planets from petrological point of
view
Terrestrial planets (Earth, Venus, Mars, Mercury and Moon) represent hard celestial bodies of the same structure, which are characterized by iron core and silicate envelope. Question arises – if they were always the same, as today, and if no – on what stage of their evolution they stay now?
Our knowledge about formation and evolution of the terrestrial planets based on different physical and geochemical speculations and models. The main disadvantage of such hypotheses is their abstract character and ignoring any data on tectonomagmatic evolution of those planets. At the same time, just this type of data can provide an important information, which is necessary for elaborating of a present-day theory of their formation and evolution.
• The Earth has been much better studied in comparison to the other planets, therefore we will discuss the main questions of planetary tectonomagmatic evolution using the Earth as a reference body.
•
Primordial Earth’s CrustTwo dominating hypotheses exist now:
1. Traditional – implies that the primordial crust had basic composition, whereas the sialic crust resulted from a geosyncline process or, in modern terms, from processes at convergent plate margins. According to this model the Earth’s continental crust has grown at the expense of the oceanic one.
2. The primordial crust was sialic; the plate tectonic mechanisms started in the middle Paleoproterozoic and resulted in oceanic spreading and formation of the secondary oceanic crust; the ancient continental crust has involved into subduction to be piled in the mantle.
Both models require a global melting of a primary chondriticmaterial to form the primordial Earth’s crust. The final result depends on the degree of melt crystallizing differentiation during solidification of a magmatic ocean. Such a solidification, due to differences between adiabatic and melting-points gradients had to proceed in bottom-top direction (Jeffries, 1929) of zone of crystallization, where themost high-T phases precipitated, and resulted in final accumulation of low-temperature derivates at the surface as the primordial crust.
• Geological data, namely granite-dominated Archeancrust, and data from the study of inherited zircon cores (Valley et al., 2002; Harrison et al., 2005) supports the primordial-sialic Earth’s crust hypothesis, as well as data on 176Lu/177Hf ratio in these zircons, which close to the Archean tonalite-trondhjemite-granodiorite (TTG) rocks (Blichert-Toft, Albarede, 2008). All of these make these granitoidsthe most suitable pretender for this role.
• The Moon which is four times smaller than Earth has an basic primordial crust. Such a difference can be explained by different depths of their magmatic oceans (Sharkov, Bogatikov, 2003).
• Formation of the sialic crust was responsible for the depletion of the upper mantle matter, which follows from composition of the early Precambrian mantle-derived magmas and xenoliths of ancient lithosphere in kimberlites.
TECTONO-MAGMATIC PROCESSES IN THE ARCHEAN AND EARLY PALEOPROTEROZOIC
Granite-greenstone terranes (GGTs) and their separating granulite belts were main Archean tectonic structures The GGTs consisting of irregular network of greenstone belts with high-Mg komatiite-basaltic and boninite-like magmatism, “submerged” in TTG granite-gneiss matrix, probably, strongly reworked primordial sialic crust. They were areas of extension, uplifting and denudation, whereas the granulite belts were dominated by compression, sinking and sedimentation. Generally, the Archean geological pattern was drastically different from the modern plate tectonics.
Archean structure of the Fennoscandian Shield1- granite-greenstone terranes; 2 – greenstone belts;3 – reconstructed greenstone belts; 4 –transitional Belomorian Mobile Belt; 5-6 -granulite complex; 7 – middle PaleoproterozoicSvecofennian orogen; 8 – geological boundaries
By the Proterozoic the crust became rigid resulting in formation of rift structures, dike swarms and large platiniferous mafic-ultramafic layered intrusions. In early Paleoproterozoic character of the tectonomagmatic activity remained almost the same: cratons separated by granulite belts appeared on the place of GGTs.
Magmatism was dominated by siliceous high-Mg (boninite-like) series (SHMS), which formed large igneous provinces. SHMS are close in composition to the Phanerozoicsubduction-related magmas, however, instead of them, had intracontinental tectonic settings.
We suggest that origin of the SHMS magmas was linked with floating up of the high-temperature mantle-derived ultramafic magma batch through the crust according to zone refinement principle, i.e. by melting of roof accompanied by crystallization at bottom.
Structure of the SHMS magmatic system1 - ancient lithospheric mantle; 2 - Archeanmafic lower crust; 3 - Archean sialic uppercrust;4 - volcano-sedimentary rocks; 5 -paths of ascending of magma chamber byzone refinement mechanism; 6 – transition magma chamber (layered mafic-ultramaficintrusion); 7- magma conduits
The appearance of large igneous provinces requires location beneath them the first generation mantle superplumes, consisting of depleted mantle material. The heads of the plumes spreaded at depths of 200-450 km. Such a situation can be described in terms of plume-tectonics typical of the Early Precambrian.
CARDINAL CHANGE IN THE EARTH’S TECTONOMAGMATIC EVOLUTION
Within all Precambrian shields the period of 2.3 to 2.0 Ga was characterized by voluminous eruption of geochemical enriched Fe-Ti picritesand basalts similar to the Phanerozoic intraplatemagmas. Since then they represent a major type of magmatism.
At the beginning, character of tectonic activity was not change: younger lava flows developed in the same riftogenic structures; dike swarms and large layered mafc-ultramafic intrusions, but Ti-bearing, were formed.
A drastic change of the tectonic pattern occurred 200-300 Myr later, at ca. 2 Ga, for form first Phanerozoic-type orogens on all Precambrian shields.
•Since then (~2 Ga), the subduction of the ancient sialic continental crust is a permanent process. The crustal materials has stored in the “slab cemeteries”, revealed in the mantle by seismic tomography. • Systematic consumption of the ancient crust in subduc-tion zones, accompanied by oceanic spreading, led to gradually replacing it by the secondary mafic (oceanic) crust. •System volcanic arc-backarc sea (Bogatikovet al., 2009)•(1) low-density oceanic mantle (“asthenosphere”); (2) lithosphere mantle: beneath (a) continent, (b) ocean; (3) upper mantle beneath discontinuity at 430 km; (4) mantle beneath discontinuity at 680 km; (5) lower crust beneath: (a) continent, (b) ocean; (6) continental crust; (7) mixture of sialic and basic crustal material in subduction zone; (8) magma-generation zones (I – tholeiite, II –boninite, III – calc-alkaline); (9) direction of the oceanic “asthenosphere” moving
Modern pattern of convergent margins according to seismic tomography (Karason, van der Hilst, 2000).
Thus, during the period from 2.3 to 2.0 Ga, the composition of mantle melts and tectonic processes irretrievably changed over the whole Earth. This triggered the processes of plate tectonics which are still active.
Simultaneously, important compositional changes occurred on the surface: atmosphere became oxidative, appeared red beds, global glacials, quick expansion of biosphere, etc. (Melezhik et al., 2005), and situation also has not principally changed till now. We suggest that it was a result of the new kind of magmatic material, arrived to the surface from deep-seated interiors of our planet. These magmas were enriched in Fe-group elements, HFSE, Ca, P, S , alkalis, etc, which provide to processes of metabolism and fermentation, and were trigger for the acceleration of evolution of biosphere.
We believe that the ascending of the second generation mantle plumes (thermochemical plumes), enriched in such elements, were responsible for all those changes. Such plumes were generated at the core-mantle boundary (CMB) in D" layer and this process is active till now, providing practically all present-day tectonomagmatic activity.
The superplumes gradually 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. The thermochemicalplume matter possessed less density and could reach shallower depths. The spread of their heads 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.
Tectonomagmaticevolution of the MoonThe Moon is the second member of our double-planet system. The study of the samples, which became available due to the American and Soviet space mis-sions, the Moon’s oldest magma-tism in lunar highlands is dated by 4.5-4.1 Ga. It was characteri-zed by the low-Ti magnesiansuite analogous to the terrestrial Paleoproterozoic SHMS.
A cardinal change of geological processes, analogous to that on the Earth, happed on the Moon ca. 3.9-3.8 Ga to form lunar mariawith signatures of plume mag-matism (enriched in Ti, Nb, Ta, Hf, etc basalts). We suggest that it was not result of heavy bom-bardment, and they appeared above extended plume heads.
• Probably, the lunar maria were analogues of Earth’s oceans.
• Therefore, this stage of the Moon evolution can be correlated with the continental-oceanic stage of the Earth’s evolution.
• 1 – granite-greenstone terranes of the Earth; 2 –terrestrial SHMS and lunar magnesian suite; 3 –terrestrial Fe-Ti picrites and basalts of transitional stage and lunar KREEP basalts; 4 – Phanerozoic type of tectonomagmatic activity on the Earth; 5 – lunar mariamagmatism
On Venus and Mars, two major types of morphostructures, which are vast fields of flood basalts, and older lightweight uplifted segments with a complicated topography (tesserason the Venus and earths (terras) on the Mars) occurs. Like on the Earth, red beds and global glacials appeared on the Mars at the middle stage of it’s evolution.
So, it possible suggest a two-stage evolution of these planets. During the first stage the primordial lithospheres formed due to solidification of global magmatic oceans. During the second stage the secondary basaltic crust formed due to ascent of thermochemical plumes from their then existing CMBs. Smaller Mercury is less studied, however, its relief also contains morhostructures resembling lunar highlandsand maria.
CAUSES OF THE TERRESTRIAL PLANETS’EVOLUTION
So, the tectonomagmatic processes on the Earth at ca. 2.3 Ga and ca. 3.9 Ga on the Moon started to involve previously absent geochemical-enriched material.
Where this material was stored and how it was activated?
From our point of view, the established succession of events could be provided only by a combination of two independent factors: (1) Earth originally was heterogeneous, with silicate mantle and iron core, i.e. formed due to the heterogeneous accretion, and (2) the downward heating of the Earth (from the surface to the core) occurred and accompanied by cooling of it’s outer shells.
Very likely that formation of other terrestrial planets took place on the same scenario.
The most probable cause of the centripetal heating of the Earth and other terrestrial planets was a zone/wave of heat-generating deformation directed inside the planets. According to experimental data, such waves appear at stage of acceleration of rotated bodies. We suggest, that those zone of deformation appeared after the planets were formed (accretion finished) and their rotation around axes accelerated due to law of conservation of momentum as a result of materials compaction and shortening their radii. That wave could reach the interiors of the planets thus heating deep mantle material and generating first superplumes. Finally, it reached the iron core, melted it and produced secondary thermochemical plumes, which are still active on the Earth, but already absent on the other terrestrial planets, which iron cores hardened by this time. Accordingly, these planets have no magnetic fields and present-day tectonomagmatic activity, and so, from my point of view, they are “dead” bodies now.
• Scheme, illustrated the major stages of the Earth’s inner evolution
• 1 – primordial core; 2 – primordial crust; 3 – magma ocean; 4 – primordial sialic crust кора; 5 – depleted mantle; 6 – core: а – liquid, b – solid; 7 – wave of heating; 8 – mantle plumes.
CONCLUSIONS1. The Earth’s primordial crust the most likely was sialic,
close in composition to TTG complexes, dominated in the Archean crust.
2. The Early Precambrian (Archean, Early Paleoproterozoic) tectono-magmatic activity on Earth was different from the Phanerozoic one: the major structures were granite-greenstone terranes and their separating granulite belts; high-Mg mantle melts were derived from a depleted source.
3. A drastic change of the tectonomagamtic and ecological processes on it’s surface occurred at ca. 2.3-2.0 Ga: high-Mg magmas were changed by geochemical-enriched Fe-Ti basalts, and plume tectonic was changed by plate tectonics, which is still active now, as well as ecologic situation. Since that time the primordial sialic continental crust has been gradually replaced by the secondary basaltic oceanic crust.
4. The established succession of events was not link with external action and could be provided by combination of two independent factors: 1) Earth originally was heterogeneous and 2) the downward heating of Earth was followed by the cooling of its outer shells. As a result the primary core material was long time remained untouched.
5. The Earth is independent self-propagating systems which evolved in two phases: (1) inwards heating and (2) upwards general cooling; liquid iron core is an “energetic heart” of our planet now and determines all present-day tectonomagmatic activity by producing of superplumes of the second generation.
6. The data available on tectonomagmatic evolution of the terrestrial planets (Moon, Venus, Mars and Mercury) suggest that they were formed and developed at the same scenario. Their evolution had already completed and they are “dead” bodies now.
