1. Evgenii V. Sharkov and Oleg A. Bogatikov Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry (IGEM) RAS, Moscow, sharkov@igem.rusharkov@igem.ru 25 th Anniversary Goldschmidt Conference, Prague, CZ. August 2015, Session 21C Problem of the Earth’s core evolution: Evidence from geological-petrological and paleomagnetic data

2. As you know, a lot of ideas about origin of the Earth’s core exist now (VS Safronov, AE Ringwood, AN Halliday, BJ Wood, DC Rubie, and others), however, no consideration has been given to the problem of its evolution. At the same time, according to the modern geophysical data, mantle plumes are generated at the boundary of the outer liquid iron core and overlying silicate mantle. These plumes deliver heat and fluid components from the core to the upper shells of our planet [Artyushkov, 1993; Maruyama, 1994; Dobretsov et al., 2001, etc.]. As it evident from the isotope data, core’s components actually occurred in mantle plume material and may reach the Earth’s surface together with mantle-derived magmas [Walker et al., 1994 and 1997; Brandon et al., 1999; Puchtel et al., 1999]. Accordingly, mantle plumes, systematically “sampling” the material of deep geospheres, are the only carriers of the core-derived material. So, studying of evolution of tectonomagmatic processes throughout the Earth’s history provides an unique possibility for obtaining objective information about core evolution inaccessible by other methods. We plan to discuss the problem of the Earth’s core evolution using petrological and paleomagnetic data, show that the primordial core of the Earth may significantly differ from its modern (secondary ) core, and consider a problem of the core formation as well.

3. TECTONOMAGMATIC EVOLUTION OF THE EARTH The problem of the Earth’s primordial crust. According to modern models, this crust could have basaltic or granitic composition. Both models require global melting of the primary material for the homogeneous primordial crust formation. The strong predomination of TTG-granitoids in the Archean crust, as well as the data on detrital zircons in Archean sediments of Australia with 4.4–4.2 Ga age (Valley et al., 2002), provides evidence for the primordial sialic crust. Very likely, that it was formed as a result of upwards-directed gradual solidification of the global magmatic ocean, which led to accumulation of low-temperature components at surface of the planet as the primordial crust (Sharkov, Bogatikov, 2010), The further geological evolution of the Earth may be subdivided at three stages [Sharkov, Bogatikov, 2010].

4. Nuclearic stage (Archean, 4.0-2.5 Ga) was characteristic by high-Mg mantle-derived magmas. They resulted from ascent of the first generation mantle plumes, composed of depleted ultramafic material. This depletion, very likely, was considered with separation of the primordial sialic crust, how many geochemists think. Irreversible changes occurred in Cratonic (T ransitional) stage, in the Mid-Paleoproterozoic (2.35–2.0 Ga). Then high-Mg magmas of the Early Precambrian were replaced by geochemical-enriched Fe–Ti alkaline and tholeiitic basalts, similar to the Phanerozoic intraplate (plume-related) volcanics. The newly magmas were characterized by high contents of Fe, Ti, Cu, Mn, P, alkalis, and other incompatible elements. We explain it by appearance of mantle plumes of the second generation (thermochemical) which already contain core’s components. Very likely, that mention above elements were released from the core and together with volatiles of the same origin formed specific fluid phase which penetrated to the base of the silicate mantle and provide ascend of thermochemical plumes. Continental–oceanic stage (Neogean) began at ~2 Ga and has lasted till now. Because heads of thermochemical plumes extend at relatively shallow depths, it resulted in breakdown of the ancient lithosphere, appearance of zones of oceanic spreading, forming lithospheric plates, etc., i.e., plate tectonics came.

5. Mantle plumes constantly remove heat from the outer core which led to its gradually solidification which results in release of mention above fluids, dissolved in the iron melt. These fluids impregnated mantle peridotites as interganular phase and jointly form an ascending plume material. The fluids were agents of plume-related metasomatism and participated in processes of adiabatic melting. Isotopic data evidence on presence of the core components in the Mid-Paleoproterozoic picrobasalts of ~2 Ga age in the Pechenga and Onega structures on Fennoscandian Shield [Walker et al., 1997; Puchtel et al., 1999], in Permotriassic Siberian LIP (Walker et al., 1994), as well as in modern Hawaiian plume-related basalts [Brandon et al., 1999]. Thus, composition of mantle-derived magmas and geodynamic processes underwent irreversible changes at 2.35–2.0 Ga because of appearance of new type of the mantle plumes. This transition proceeded 200–350 m.y. and was not controlled by any external reasons.

6. REASONS FOR IRREVERSIBLE TECTONOMAGMATIC EVOLUTION OF THE EARTH So, composition of the mantle-derived magmas was irreversibly changed from geochemical-depleted to geochemical-enriched in the Mid-Paleoproterozoic, ~2.3 b.y. after the Earth’s accretion. What was this enriched material, wher it was conserved and how it was activated? Very likely that it was the matter, which existed at the early stages of the Solar sysem evolution and can survived only in primordial core, because the mantle get mixed up by convection carried up by mantle plumes. Quite possible that “solar” isotopes 3 Не, 22 Ne и 36 Ar were contained in the primordial core also, not in the mantle. From this follows that the Earth was primarily heterogeneous; i.e. its primordial core proceeded from the material, existing in the early Solar system, and was its embryo. The data available on the geochemistry and isotope composition of iron meteorites (fragments of the cores of destroyed small planets) show that part of their material was not have a chondritic origin [Walker, 2010]. Because access to the material of the primordial core is possible only in case of its melting, the heating of planetary bodies had to proceed downwards, from the surface to the core, accompanied by cooling of the outer shells. Judging from the paleomagnetic data, the highest intensity of the magnetic field on the Earth [Reddy, Evans, 2009] practically coincided with the global change in character of its tectonomagmatic activity and marking this event. Very likely, that it was time, when the primordial core was completely melted.

7. However, according to the paleomagnetic data, the magnetic field on the Earth already existed from the Paleoarchean, ~3.45 Ga [Reedy, Evans, 2009; Tardino et al., 2010]; i.e., a liquid iron core, responsible for this field, existed at that time. Most likely, that this iron was formed at expense of heating of the primary mantle composed of chondrite C1 (assumed primary material of the Earth), where Fe concentration is 18.1 wt %. Liquid iron in the form of heavy Fe+FeS eutectic could sink through the silicate mantle and accumulated on the surface of the cool primordial core. Liquid iron could generate a magnetic field, but did not participate in geodynamical processes. After melting of the primordial core, very likely, that both old and new irons were mixed by convection to form modern secondary core. In other words, the core at the early stages of the Earth’s evolution had different composition and structure compare to modern. Since warming-up of the Earth proceeded over billions years, such heating was gradual and realized by moving of a “heating wave”. Judging from the evolution of mantle-derived magmas composition, such a “wave” should move downwards, heating successively more and more deep mantle horizons and initiating ascent of thermal mantle plumes of the first generation, typical for the early Precambrian. The material of the primordial core was the last involved in this process.

8. The probability of centripetal heating of planets has not been discussed yet, although it has a physical basis. It may be a wave of centripetal deformations, which appear upon acceleration of rotation of the bodies [Belostotskii, 2000]. The energy transport is the most intense at the stages of acceleration of the flywheel and almost absent in the established regime of rotation. Since the mechanical deformations are always accompanied by heat release, such a wave is a “thermal wave” as well. We assume that this “wave” appeared soon after completion of accretion of the Earth as a result of gradual compression of its material, accompanied by appearance of high- density minerals, and corresponding decrease of its radius. According to the law of conservation of the momentum of motion, this should cause acceleration of their rotation around the axis. 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.

9. According to the law of conservation of energy, such a wave of deformations should result in “pumping” of energy into the internal parts of the Earth. This energy was accumulated in the liquid iron core, the temperature of which is higher than that at the lower boundary of the silicate mantle by ~1500°C [McDonough, 2003]. Obviously, the liquid core is the energetic “heart” of the Earth at the second stage of its evolution, which started 2.35–2.3 Ga, when the thermochemical plumes, the main driving force of the modern tectonic processes, appeared. After complete solidification of the core, the tectonomagmatic processes will, probably, stop, similarly to the situation on the Moon, Venus, Mars, and possibly Mercury. These bodies have close inner structure and developed at the same scenario [Sharkov, Bogatikov, 2009]. However, they have no magnetic fields now as well as no signs of modern tectonomagmatic activity. It, probably, evidence for the absence of liquid cores in them; i.e., in contrast to the Earth, they are “dead” bodies now.

10. CONCLUSIONS 1. The mantle thermochemical plumes are the only carriers of the core material reaching the Earth’s surface, and study of evolution of mantle- derived magmas throughout the Earth’s history is a unique source of objective information on the core development over several billion years of its existence that is not accessible by any other method. 2. Appearance of thermochemical plumes about 2.35 Ga we explain by involvement material of the primordial core in geodynamic processes as a result of its melting by the Earth’s centripetal heating. This heating is, probably, considered with the wave of heat-generated deformations caused by acceleration of rotation of the body as a result of a gradually decrease its radius upon compression material of the newly-formed planet. 3. The geological and petrological data available on the Earth evolution are consistent with model of its heterogeneous accretion from material that existed in the early Solar system, and the primordial iron core, probably, was its embryo. 4. Very likely, that the Earth’s core had different composition and structure at the early and late stages of its evolution; material of the modern (secondary) core, very probably, consists on a mixture of the primordial core matter and iron of the chondritic origin.