Under the earth's crust is the next layer, called the mantle. It surrounds the core of the planet and is almost three thousand kilometers thick. The structure of the Earth's mantle is very complex, and therefore requires a detailed study.

Mantle and its features

The name of this shell (geosphere) comes from the Greek word for a cloak or veil. In fact, the mantle wraps around the core like a veil. It accounts for about 2/3 of the mass of the Earth and approximately 83% of its volume.

It is generally accepted that the temperature of the shell does not exceed 2500 degrees Celsius. Its density in different layers differs significantly: in the upper part it is up to 3.5 t/m3, and in the lower part it is 6 t/m3. The mantle is made up of solid crystalline substances(heavy minerals rich in iron and magnesium). The only exception is the asthenosphere, which is in a semi-molten state.

shell structure

Now consider the structure of the earth's mantle. The geosphere consists of the following parts:

• upper mantle, 800-900 km thick;
• asthenosphere;
• lower mantle, about 2000 km thick.

The upper mantle is the part of the shell that is located below the earth's crust and enters the lithosphere. In turn, it is divided into the asthenosphere and the Golitsyn layer, which is characterized by an intense increase in seismic wave velocities. This part of the Earth's mantle influences processes such as plate tectonic movements, metamorphism and magmatism. It is worth noting that its structure differs depending on which tectonic object it is located under.

Asthenosphere. The very name of the middle layer of the shell with Greek translates as "weak ball". The geosphere, which is attributed to the upper part of the mantle, and sometimes is isolated as a separate layer, is characterized by reduced hardness, strength, and viscosity. The upper boundary of the asthenosphere is always below the extreme line of the earth's crust: under the continents - at a depth of 100 km, under the seabed - 50 km. Its lower line is located at a depth of 250-300 km. The asthenosphere is the main source of magma on the planet, and the movement of amorphous and plastic matter is considered the cause of tectonic movements in the horizontal and vertical planes, magmatism and metamorphism of the earth's crust.

Scientists know little about the lower part of the mantle. It is believed that a special layer D, resembling the asthenosphere, is located at the boundary with the core. It is characterized by high temperature (due to the proximity of the red-hot core) and inhomogeneity of matter. The composition of the mass includes iron and nickel.

Composition of the Earth's mantle

In addition to the structure of the Earth's mantle, its composition is also interesting. The geosphere is formed by olivine and ultramafic rocks (peridotites, perovskites, dunites), but there are also mafic rocks (eclogites). It has been established that the shell contains rare varieties that are not found in the earth's crust (grospidites, phlogopite peridotites, carbonatites).

If we talk about the chemical composition, then the mantle contains in different concentrations: oxygen, magnesium, silicon, iron, aluminum, calcium, sodium and potassium, as well as their oxides.

Mantle and its study - video

The silicate shell of the Earth, its mantle, is located between the sole of the earth's crust and the surface of the earth's core at a depth of about 2,900 km. Usually, according to seismic data, the mantle is divided into the upper (layer B), to a depth of 400 km, the transitional Golitsyn layer (layer C) in the depth interval of 400-1000 km, and the lower mantle (layer D) with a base at a depth of about 2900 km. Under the oceans in the upper mantle, there is also a layer of low seismic wave propagation velocities - the Gutenberg waveguide, usually identified with the Earth's asthenosphere, in which the mantle substance is in a partially molten state. Under the continents, the zone of low velocities, as a rule, is not distinguished or is weakly expressed.

The composition of the upper mantle usually also includes the subcrustal parts of the lithospheric plates, in which the mantle matter is cooled and completely crystallized. Under the oceans, the thickness of the lithosphere varies from zero under the rift zones to 60–70 km under the abyssal basins of the oceans. Under the continents, the thickness of the lithosphere can reach 200-250 km.

Our information about the structure of the mantle and the Earth's core, as well as the state of matter in these geospheres, was obtained mainly from seismological observations, by interpreting the travel time curves of seismic waves, taking into account the known equations of hydrostatics, which relate density gradients and the values ​​of the propagation velocities of longitudinal and transverse waves in the medium . This technique was developed by the well-known geophysicists G. Jeffries, B. Gutenberg, and especially C. Bullen back in the mid-1940s and then significantly improved by C. Bullen and other seismologists. The density distributions in the mantle constructed using this method for several of the most popular models of the Earth are shown in Figs. ten.

Figure 10.
1 - Naimark-Sorokhtin model (1977a); 2 - Bullen model A1 (1966); 3 - Zharkov's model "Earth-2" (Zharkov et al., 1971); 4 - recalculation of the data of Pankov and Kalinin (1975) for the composition of lherzolites with an adiabatic temperature distribution.

As can be seen from the figure, the density of the upper mantle (layer B) increases with depth from 3.3-3.32 to about 3.63-3.70 g/cm 3 at a depth of about 400 km. Further, in the Golitsyn transition layer (layer C), the density gradient sharply increases and the density rises to 4.55-4.65 g/cm 3 at a depth of 1,000 km. The Golitsyn layer gradually passes into the lower mantle, the density of which gradually (according to a linear law) increases to 5.53-5.66 g/cm 3 at a depth of about 2,900 km at its base.

The increase in the density of the mantle with depth is explained by the compaction of its substance under the influence of the ever-increasing pressure of the overlying mantle layers, which reaches values ​​of 1.35-1.40 Mbar at the base of the mantle. A particularly noticeable compaction of mantle silicates occurs in the depth interval of 400-1000 km. As A. Ringwood showed, it is at these depths that many minerals undergo polymorphic transformations. In particular, the most common mineral in the mantle, olivine, acquires a spinel crystal structure, and pyroxenes acquire an ilmenite, and then the densest perovskite structure. For more great depths most silicates, with the possible exception of only enstatite, decompose into simple oxides with the closest packing of atoms in their corresponding crystallites.

The facts of the movement of lithospheric plates and the drift of continents convincingly testify to the existence of intense convective movements in the mantle, which repeatedly mixed all the substance of this geosphere during the life of the Earth. From this we can conclude that the compositions of both the upper and lower mantles are on average the same. However, the composition of the upper mantle is confidently determined from finds of ultrabasic rocks of the oceanic crust and the compositions of ophiolite complexes. Studying ophiolites of folded belts and basalts of oceanic islands, A. Ringwood, back in 1962, proposed a hypothetical composition of the upper mantle, which he called pyrolite, obtained by mixing three parts of alpine-type peridotite - Habsburgite with one part of Hawaiian basalt. Ringwood pyrolite is close in composition to oceanic lherzolites studied in detail by L.V. Dmitriev (1969, 1973). But in contrast to pyrolite, oceanic lherzolite is not a hypothetical mixture of rocks, but a real mantle rock that has risen from the mantle in the rift zones of the Earth and is exposed in transform faults near these zones. In addition, L.V. Dmitriev showed the complementarity of oceanic basalts and restite (residual after smelting basalts) harzburgites in relation to oceanic lherzolites, thereby proving the primacy of lherzolites, from which, consequently, tholeiitic basalts of mid-ocean ridges are smelted, and the remainder is preserved restite harzburgite. Thus, the closest to the composition of the upper mantle, and consequently, the entire mantle, corresponds to the oceanic lherzolite described by L.V. Dmitriev, the composition of which is given in Table. one.

In addition, the recognition of the existence of convective motions in the mantle makes it possible to determine its temperature regime, since during convection, the temperature distribution in the mantle should be close to adiabatic, i.e. to one in which there is no heat exchange between adjacent volumes of the mantle, associated with the thermal conductivity of the substance. In this case, the heat loss of the mantle occurs only in its upper layer - through the Earth's lithosphere, the temperature distribution in which already differs sharply from the adiabatic one. But the adiabatic temperature distribution is easily calculated from the parameters of the mantle matter.

To test the hypothesis of a single composition of the upper and lower mantle, the density of oceanic lherzolite uplifted in the transform fault of the Carlsberg Ridge in the Indian Ocean was calculated using the method of shock compression of silicates to pressures of about 1.5 Mbar. For such an "experiment" it is not at all necessary to compress the rock sample itself to such high pressures, it is enough to know its chemical composition and the results of previous experiments on shock compression of individual rock-forming oxides. The results of such a calculation, performed for the adiabatic temperature distribution in the mantle, were compared with the known density distributions in the same geosphere, but obtained from seismological data (see Fig. 10). As can be seen from the above comparison, the density distribution of oceanic lherzolite at high pressures and adiabatic temperature approximates well the real density distribution in the mantle, obtained from completely independent data. This testifies in favor of the reality of the assumptions made about the lherzolite composition of the entire mantle (upper and lower) and about the adiabatic temperature distribution in this geosphere. Knowing the distribution of the density of matter in the mantle, one can also calculate its mass: it turns out to be equal to (4.03-4.04) × 10 2 g, which is 67.5% of the total mass of the Earth.

At the base of the lower mantle, another mantle layer with a thickness of about 200 km is distinguished, usually denoted by the symbol D'', in which the gradients of seismic wave propagation velocities decrease and the attenuation of transverse waves increases. Moreover, based on the analysis of the dynamic features of the propagation of waves reflected from the surface of the earth's core, I.S. Berzon and her colleagues (1968, 1972) managed to identify a thin transitional layer between the mantle and the core about 20 km thick, which we called the Berzon layer, in which the velocity of transverse waves in the lower half decreases with depth from 7.3 km/s to almost zero. The decrease in the speed of transverse waves can be explained only by a decrease in the value of the stiffness modulus, and, consequently, by a decrease in the coefficient of effective viscosity of the substance in this layer.

The very boundary of the transition from the mantle to the Earth's core remains quite sharp. Judging by the intensity and spectrum of seismic waves reflected from the core surface, the thickness of such a boundary layer does not exceed 1 km.

D.Yu. Pushcharovsky, Yu.M. Pushcharovsky (Moscow State University named after M.V. Lomonosov)

The composition and structure of the deep shells of the Earth in recent decades continue to be one of the most intriguing problems of modern geology. The number of direct data on the matter of deep zones is very limited. In this respect, a mineral aggregate from the Lesotho kimberlite pipe occupies a special place ( South Africa), which is considered as a representative of mantle rocks occurring at a depth of ~250 km. The core recovered from the world's deepest well, drilled on the Kola Peninsula and reaching 12,262 m, has significantly expanded scientific understanding of the deep horizons of the earth's crust - a thin near-surface film of the globe. At the same time, the latest data of geophysics and experiments related to the study of structural transformations of minerals already now allow modeling many features of the structure, composition and processes occurring in the depths of the Earth, the knowledge of which contributes to the solution of such key problems of modern natural science as the formation and evolution of the planet, dynamics the earth's crust and mantle, sources mineral resources, risk assessment of hazardous waste disposal at great depths, energy resources of the Earth, etc.

Seismic model of the structure of the Earth

well-known model internal structure Earth (its division into the core, mantle and earth's crust) was developed by seismologists G. Jeffreys and B. Gutenberg in the first half of the 20th century. The decisive factor in this was the discovery of a sharp decrease in the velocity of seismic waves inside the globe at a depth of 2900 km with a radius of the planet of 6371 km. The velocity of propagation of longitudinal seismic waves directly above the specified border is 13.6 km/s, and below it - 8.1 km/s. That's what it is mantle-core boundary.

Accordingly, the core radius is 3471 km. The upper boundary of the mantle is the seismic section of Mohorovic ( Moho, M), identified by the Yugoslav seismologist A. Mohorovichich (1857-1936) back in 1909. It separates the earth's crust from the mantle. At this speed limit longitudinal waves passing through the earth's crust increase abruptly from 6.7-7.6 to 7.9-8.2 km/s, but this happens at different depth levels. Under the continents, the depth of the section M (that is, the soles of the earth's crust) is a few tens of kilometers, and under some mountain structures (Pamir, Andes) it can reach 60 km, while under the ocean basins, including the water column, the depth is only 10-12 km . In general, the earth's crust in this scheme appears as a thin shell, while the mantle extends in depth to 45% of the earth's radius.

But in the middle of the 20th century, ideas about a more fractional deep structure of the Earth entered science. Based on new seismological data, it turned out to be possible to divide the core into inner and outer, and the mantle into lower and upper (Fig. 1). This model, which received wide use, is still in use today. It was started by the Australian seismologist K.E. Bullen, who proposed in the early 40s a scheme for dividing the Earth into zones, which he designated with letters: A - the earth's crust, B - a zone in the depth interval of 33-413 km, C - a zone of 413-984 km, D - a zone of 984-2898 km , D - 2898-4982 km, F - 4982-5121 km, G - 5121-6371 km (center of the Earth). These zones differ in seismic characteristics. Later, he divided zone D into zones D "(984-2700 km) and D" (2700-2900 km). At present, this scheme has been significantly modified, and only the D "layer is widely used in the literature. Its main characteristic is a decrease in seismic velocity gradients compared to the overlying mantle region.

Rice. 1. Diagram of the deep structure of the Earth

The more seismological studies are carried out, the more seismic boundaries appear. The global boundaries are considered to be 410, 520, 670, 2900 km, where the increase in seismic wave velocities is especially noticeable. Along with them, intermediate boundaries are distinguished: 60, 80, 220, 330, 710, 900, 1050, 2640 km. Additionally, there are indications of geophysicists on the existence of boundaries 800, 1200-1300, 1700, 1900-2000 km. N.I. Pavlenkova recently singled out boundary 100 as a global one, which corresponds to the lower level of the division of the upper mantle into blocks. Intermediate boundaries have a different spatial distribution, which indicates the lateral variability of the physical properties of the mantle, on which they depend. Global boundaries represent a different category of phenomena. They correspond to global changes in the mantle environment along the radius of the Earth.

The marked global seismic boundaries are used in the construction of geological and geodynamic models, while intermediate ones in this sense have so far attracted almost no attention. Meanwhile, differences in the scale and intensity of their manifestations create an empirical basis for hypotheses concerning phenomena and processes in the depths of the planet.

Below we will consider how the geophysical boundaries correlate with the recent results of structural changes in minerals under the influence of high pressures and temperatures, the values ​​of which correspond to the conditions of the earth's depths.

The problem of the composition, structure, and mineral associations of deep earth shells or geospheres, of course, is still far from a final solution, but new experimental results and ideas significantly expand and detail the corresponding ideas.

According to modern views, the composition of the mantle is dominated by a relatively small group chemical elements: Si, Mg, Fe, Al, Ca and O. Suggested geosphere composition models are primarily based on the difference in the ratios of these elements (variations Mg / (Mg + Fe) = 0.8-0.9; (Mg + Fe) / Si = 1.2Р1.9), as well as differences in the content of Al and some other rarer elements for deep rocks. In accordance with the chemical and mineralogical composition, these models received their names: pyrolitic(the main minerals are olivine, pyroxenes and garnet in a ratio of 4:2:1), piklogitic(the main minerals are pyroxene and garnet, while the proportion of olivine decreases to 40%) and eclogitic, which, along with the pyroxene-garnet association characteristic of eclogites, also contains some rarer minerals, in particular Al-bearing kyanite Al2SiO5 (up to 10 wt. % ). However, all these petrological models refer primarily to upper mantle rocks extending to depths of ~670 km. With regard to the bulk composition of deeper geospheres, it is only assumed that the ratio of oxides of divalent elements (MO) to silica (MO/SiO2) ~ 2, being closer to olivine (Mg, Fe)2SiO4 than to pyroxene (Mg, Fe)SiO3, and The minerals are dominated by perovskite phases (Mg, Fe)SiO3 with various structural distortions, magnesiouustite (Mg, Fe)O with a structure of the NaCl type, and some other phases in much smaller amounts.

All proposed models are very generalized and hypothetical. The pyrolitic model of the olivine-dominated upper mantle suggests that it is much closer in chemical composition with all the deeper mantle. On the contrary, the piclogitic model assumes the existence of a certain chemical contrast between the upper and the rest of the mantle. A more particular eclogitic model allows for the presence of separate eclogitic lenses and blocks in the upper mantle.

Of great interest is the attempt to harmonize the structural-mineralogical and geophysical data related to the upper mantle. It has been assumed for about 20 years that the increase in seismic wave velocities at a depth of ~410 km is mainly associated with the structural rearrangement of olivine a-(Mg, Fe)2SiO4 into wadsleyite b-(Mg, Fe)2SiO4, accompanied by the formation of a denser phase with large values ​​of the coefficients elasticity. According to geophysical data, at such depths in the Earth's interior, seismic wave velocities increase by 3–5%, while the structural rearrangement of olivine into wadsleyite (in accordance with the values ​​of their elastic moduli) should be accompanied by an increase in seismic wave velocities by about 13%. However, the results experimental studies olivine and olivine-pyroxene mixture at high temperatures and pressures revealed a complete coincidence of the calculated and experimental increase in seismic wave velocities in the depth interval of 200-400 km. Since olivine has approximately the same elasticity as high-density monoclinic pyroxenes, these data should indicate the absence of a highly elastic garnet in the underlying zone, the presence of which in the mantle would inevitably cause a more significant increase in seismic wave velocities. However, these ideas about the garnetless mantle came into conflict with the petrological models of its composition.

Table 1. Mineral composition of pyrolite (according to L. Liu, 1979)

Thus, the idea arose that the jump in seismic wave velocities at a depth of 410 km is associated mainly with the structural rearrangement of pyroxene garnets inside Na-enriched parts of the upper mantle. Such a model assumes an almost complete absence of convection in the upper mantle, which contradicts modern geodynamic concepts. Overcoming these contradictions can be associated with the recently proposed more complete model of the upper mantle, which allows the incorporation of iron and hydrogen atoms into the wadsleyite structure.

Rice. 2. Change in volume proportions of pyrolite minerals with increasing pressure (depth), according to M. Akaogi (1997). Symbols of minerals: Ol - olivine, Gar - garnet, Cpx - monoclinic pyroxenes, Opx - orthorhombic pyroxenes, MS - "modified spinel", or wadsleyite (b-(Mg, Fe)2SiO4), Sp - spinel, Mj - mejorite Mg3(Fe, Al, Si)2(SiO4)3, Mw - magnesiowustite (Mg, Fe)O, Mg-Pv -Mg-perovskite, Ca-Pv-Ca-perovskite, X - putative Al- containing phases with structures like ilmenite, Ca-ferrite and/or hollandite

While the polymorphic transition of olivine to wadsleyite is not accompanied by a change in the chemical composition, in the presence of garnet, a reaction occurs that leads to the formation of wadsleyite enriched in Fe compared to the initial olivine. Moreover, wadsleyite can contain significantly more hydrogen atoms than olivine. The participation of Fe and H atoms in the wadsleyite structure leads to a decrease in its rigidity and, accordingly, to a decrease in the propagation velocities of seismic waves passing through this mineral.

In addition, the formation of Fe-enriched wadsleyite suggests the involvement of a larger amount of olivine in the corresponding reaction, which should be accompanied by a change in the chemical composition of rocks near section 410. Ideas about these transformations are confirmed by modern global seismic data. On the whole, the mineralogical composition of this part of the upper mantle seems to be more or less clear. If we talk about the pyrolitic mineral association (Table 1), then its transformation down to depths of ~800 km has been studied in sufficient detail and is summarized in Fig. 1. 2. In this case, the global seismic boundary at a depth of 520 km corresponds to the rearrangement of wadsleyite b-(Mg, Fe)2SiO4 into ringwoodite - g-modification of (Mg, Fe)2SiO4 with a spinel structure. The transformation of pyroxene (Mg, Fe)SiO3 garnet Mg3(Fe, Al, Si)2Si3O12 occurs in the upper mantle over a wider depth range. Thus, the entire relatively homogeneous shell in the interval of 400-600 km of the upper mantle mainly contains phases with garnet and spinel structural types.

All currently proposed models for the composition of mantle rocks allow the content of Al2O3 in them in an amount of ~4 wt. %, which also affects the specifics of structural transformations. At the same time, it is noted that in some areas of the upper mantle with a heterogeneous composition, Al can be concentrated in such minerals as corundum Al2O3 or kyanite Al2SiO5, which, at pressures and temperatures corresponding to depths of ~450 km, transforms into corundum and stishovite - a modification of SiO2, structure which contains a framework of SiO6 octahedra. Both of these minerals are preserved not only in the lower mantle, but also deeper.

The most important component of the chemical composition of the 400-670 km zone is water, the content of which, according to some estimates, is ~0.1 wt. % and the presence of which is primarily associated with Mg-silicates. The amount of water stored in this shell is so significant that on the surface of the Earth it would make up a layer with a thickness of 800 m.

Composition of the mantle below the boundary of 670 km

The studies of structural transitions of minerals carried out in the last two or three decades using high-pressure X-ray chambers made it possible to model some features of the composition and structure of the geospheres deeper than the 670 km boundary. In these experiments, the crystal under study is placed between two diamond pyramids (anvils), when compressed, pressures are created that are commensurate with the pressures inside the mantle and the earth's core. Nevertheless, there are still many questions about this part of the mantle, which accounts for more than half of the entire interior of the Earth. Currently, most researchers agree with the idea that all this deep (lower in the traditional sense) mantle mainly consists of a perovskite-like phase (Mg,Fe)SiO3, which accounts for about 70% of its volume (40% of the volume of the entire Earth). ), and magnesiowiustite (Mg, Fe)O (~20%). The remaining 10% are stishovite and oxide phases containing Ca, Na, K, Al and Fe, the crystallization of which is allowed in the structural types of ilmenite-corundum (solid solution (Mg, Fe)SiO3-Al2O3), cubic perovskite (CaSiO3) and Ca- ferrite (NaAlSiO4). The formation of these compounds is associated with various structural transformations upper mantle minerals. In this case, one of the main mineral phases of a relatively homogeneous shell lying in the depth interval of 410–670 km, spinel-like ringwoodite, transforms into an association of (Mg, Fe)-perovskite and Mg-wustite at the turn of 670 km, where the pressure is ~24 GPa. Another important component of the transition zone, a member of the garnet family, pyrope Mg3Al2Si3O12, undergoes a transformation with the formation of rhombic perovskite (Mg, Fe)SiO3 and a solid solution of corundum-ilmenite (Mg, Fe)SiO3 - Al2O3 at somewhat higher pressures. This transition is associated with a change in the velocities of seismic waves at the turn of 850-900 km, corresponding to one of the intermediate seismic boundaries. The transformation of andradite Ca-garnet at lower pressures of ~21 GPa leads to the formation of another important component of the lower mantle mentioned above, cubic Ca-perovskite CaSiO3 . The polar ratio between the main minerals of this zone (Mg,Fe) - perovskite (Mg,Fe)SiO3 and Mg-wustite (Mg, Fe)O varies within a fairly wide range and at a depth of ~1170 km at a pressure of ~29 GPa and temperatures of 2000- 2800 0C changes from 2:1 to 3:1.

The exceptional stability of MgSiO3 with a rhombic perovskite structure in a wide range of pressures corresponding to the depths of the lower mantle allows us to consider it one of the main components of this geosphere. The basis for this conclusion was the experiments, during which samples of Mg-perovskite MgSiO3 were subjected to a pressure 1.3 million times higher than atmospheric pressure, and simultaneously a laser beam with a temperature of about 2000 0C was applied to the sample placed between diamond anvils.

Thus, the conditions that exist at depths of ~2800 km, that is, near the lower boundary of the lower mantle, were modeled. It turned out that neither during nor after the experiment did the mineral change its structure and composition. Thus, L. Liu, as well as E. Nittle and E. Zhanloz came to the conclusion that the stability of Mg-perovskite allows us to consider it as the most common mineral on Earth, constituting, apparently, almost half of its mass.

FexO wustite is no less stable, the composition of which under conditions of the lower mantle is characterized by the value of the stoichiometric coefficient x< 0,98, что означает одновременное присутствие в его составе Fe2+ и Fe3+. При этом, согласно экспериментальным данным, температура плавления вюстита на границе нижней мантии и слоя D", по данным Р. Болера (1996), оценивается в ~5000 K, что намного выше 3800 0С, предполагаемой для этого уровня (при средних температурах мантии ~2500 0С в основании нижней мантии допускается повышение температуры приблизительно на 1300 0С). Таким образом, вюстит должен сохраниться на этом рубеже в твердом состоянии, а признание фазового контраста между твердой нижней мантией и жидким внешним ядром требует более гибкого подхода и уж во всяком случае не означает четко очерченной границы между ними.

It should be noted that the perovskite-like phases prevailing at great depths can contain a very limited amount of Fe, and elevated concentrations of Fe among the minerals of the deep association are characteristic only of magnesiowustite. At the same time, for magnesiowiustite, the possibility of the transition under the influence of high pressures of a part of the ferrous iron contained in it into ferric iron, which remains in the structure of the mineral, with the simultaneous release of the corresponding amount of neutral iron, has been proved. Based on these data, H. Mao, P. Bell, and T. Yagi, employees of the geophysical laboratory of the Carnegie Institute, put forward new ideas about the differentiation of matter in the depths of the Earth. At the first stage, due to the gravitational instability, magnesiowustite sinks to a depth where, under the influence of pressure, some of the iron in a neutral form is released from it. Residual magnesiowustite, which is characterized by a lower density, rises to the upper layers, where it mixes again with perovskite-like phases. Contact with them is accompanied by the restoration of stoichiometry (that is, the integer ratio of elements in chemical formula) of magnesiowiustite and leads to the possibility of repeating the described process. The new data make it possible to somewhat expand the set of chemical elements probable for the deep mantle. For example, the stability of magnesite at pressures corresponding to depths of ~900 km, substantiated by N. Ross (1997), indicates the possible presence of carbon in its composition.

Identification of individual intermediate seismic boundaries located below the 670 line correlates with data on structural transformations mantle minerals, which can take a wide variety of forms. An illustration of the change in many properties of various crystals at high values ​​of physicochemical parameters corresponding to the deep mantle can be, according to R. Jeanlose and R. Hazen, the restructuring of the ion-covalent bonds of wuestite recorded during experiments at pressures of 70 gigapascals (GPa) (~1700 km). in connection with the metallic type of interatomic interactions. The 1200 milestone may correspond to the rearrangement of SiO2 with the stishovite structure into the structural type CaCl2 (rhombic analogue of rutile TiO2), and 2000 km to its subsequent transformation into the phase with a structure intermediate between a-PbO2 and ZrO2 , characterized by a denser packing of silicon-oxygen octahedra (data from L.S. Dubrovinsky et al.). Also, starting from these depths (~2000 km), at pressures of 80–90 GPa, decomposition of perovskite-like MgSiO3 is allowed, accompanied by an increase in the content of periclase MgO and free silica. With several more pressure(~96 GPa) and a temperature of 800 0С, a manifestation of polytypy in FeO was established, associated with the formation of structural fragments of the nickeline NiAs type, alternating with anti-nickel domains, in which Fe atoms are located in the positions of As atoms, and O atoms - in the positions of Ni atoms. Near the D" boundary, the transformation of Al2O3 with the corundum structure into a phase with the Rh2O3 structure takes place, which is experimentally modeled at pressures of ~100 GPa, i.e., at a depth of ~2200–2300 km. "The transition from high-spin (HS) into the low-spin state (LS) of Fe atoms in the magnesiowustite structure, that is, a change in their electronic structure. In this regard, it should be emphasized that the structure of wuestite FeO at high pressure is characterized by compositional nonstoichiometry, atomic packing defects, polytype, and a change in magnetic ordering associated with a change in the electronic structure (HS => LS - transition) of Fe atoms. The noted features allow us to consider wustite as one of the most complex minerals with unusual properties that determine the specifics of the deep zones of the Earth enriched with it near the D boundary.

Rice. 3. Tetragonal structure of the Fe7S-possible component of the inner (solid) core, according to D.M. Sherman (1997)

Seismological measurements indicate that both the inner (solid) and outer (liquid) cores of the Earth are characterized by a lower density compared to the value obtained on the basis of a core model consisting only of metallic iron with the same physicochemical parameters. Most researchers attribute this decrease in density to the presence in the core of elements such as Si, O, S, and even O, which form alloys with iron. Among the phases that are probable for such "Faustian" physicochemical conditions (pressure ~250 GPa and temperatures 4000-6500 0С), Fe3S with the well-known structural type Cu3Au and Fe7S, the structure of which is shown in Fig. 3. Another phase supposed to be in the core is b-Fe, the structure of which is characterized by a four-layer close packing of Fe atoms. The melting temperature of this phase is estimated at 5000 0C at a pressure of 360 GPa. The presence of hydrogen in the core has been controversial for a long time due to its low solubility in iron at atmospheric pressure. However, recent experiments (data from J. Badding, H. Mao and R. Hamley (1992)) made it possible to establish that iron hydride FeH can form at high temperatures and pressures and is stable at pressures in excess of 62 GPa, which corresponds to depths of ~1600 km. In this regard, the presence of significant amounts (up to 40 mol.%) hydrogen in the core is quite acceptable and reduces its density to values ​​consistent with seismological data.

It can be predicted that new data on structural changes in mineral phases at great depths will make it possible to find an adequate interpretation of other important geophysical boundaries fixed in the bowels of the Earth. The general conclusion is that at such global seismic boundaries as 410 and 670 km, there are significant changes in the mineral composition. mantle rocks. Mineral transformations are also noted at depths of ~850, 1200, 1700, 2000 and 2200-2300 km, that is, within the lower mantle. This is a very important circumstance that makes it possible to abandon the idea of ​​its homogeneous structure.

By the 80s of the 20th century, seismological studies using the methods of longitudinal and transverse seismic waves, capable of penetrating through the entire volume of the Earth, and therefore called volumetric, in contrast to surface ones, which are distributed only over its surface, turned out to be so significant that they made it possible to draw up maps of seismic anomalies for different levels of the planet. Fundamental work in this area was carried out by the American seismologist A. Dzevonsky and his colleagues.

On fig. 4 shows samples of similar maps from a series published in 1994, although the first publications appeared 10 years earlier. The paper presents 12 maps for deep sections of the Earth in the range from 50 to 2850 km, that is, almost covering the entire mantle. On these most interesting maps it is easy to see that the seismic pattern is different at different depth levels. This can be seen from the areas and contours of distribution. seismic anomalous areas, the features of the transitions between them and, in general, the general appearance of the cards. Some of them are distinguished by great diversity and contrast in the distribution of areas with different seismic wave velocities (Fig. 5), while others show smoother and simpler relationships between them.

In the same year, 1994, it was published similar work Japanese geophysicists. It contains 14 maps for levels from 78 to 2900 km. On both series of maps, the Pacific heterogeneity is clearly visible, which, although changing in outline, can be traced right down to the earth's core. Beyond this large inhomogeneity, the seismic pattern becomes more complex, changing significantly when moving from one level to another. But, no matter how significant the difference between these maps, there are similarities between some of them. They are expressed in some similarity in the placement of positive and negative seismic anomalies in space and, ultimately, in common features deep seismic structure. This makes it possible to group such maps, which makes it possible to distinguish intramantle shells of different seismic patterns. And this work has been done. Based on the analysis of maps by Japanese geophysicists, it turned out to be possible to propose a much more fractional the structure of the earth's mantle shown in fig. 5 compared to the conventional earth shell model.

There are two fundamentally new provisions:

How do the proposed boundaries of the deep geospheres correlate with the seismic boundaries previously isolated by seismologists? The comparison shows that the lower boundary of the middle mantle correlates with the boundary of 1700, the global significance of which is emphasized in the work. Its upper limit approximately corresponds to the lines of 800-900. As regards the upper mantle, there are no discrepancies here: its lower boundary is represented by the 670 boundary, and the upper one by the Mohorovichic boundary. Let us pay special attention to the uncertainty of the upper boundary of the lower mantle. In the process of further research, it may turn out that the recently outlined seismic boundaries of 1900 and 2000 will make it possible to make adjustments to its thickness. Thus, the results of the comparison testify to the validity of the proposed new model of the mantle structure.

Conclusion

The study of the deep structure of the Earth is one of the largest and most important areas of geological sciences. New mantle stratification Earth allows a much less schematic than before, to approach difficult problem deep geodynamics. The difference in the seismic characteristics of the earth's shells ( geospheres), reflecting the difference in their physical properties and mineral composition, creates opportunities for modeling geodynamic processes in each of them separately. Geospheres in this sense, as is now quite clear, have a certain autonomy. However, this extremely important topic is beyond the scope of this article. The further development of seismic tomography, as well as some other geophysical studies, as well as the study of the mineral and chemical composition of the depths, will depend on much more substantiated constructions regarding the composition, structure, geodynamics and evolution of the Earth as a whole.

Bibliography

geotimes. 1994 Vol. 39, No. 6. P. 13-15.

Ross A. The Earths Mantle Remodeled // Nature. 1997 Vol. 385, No. 6616. P. 490.

Thompson A.B. Water in the EarthXs Upper Mantle // Nature. 1992 Vol. 358, No. 6384. P. 295-302.

Pushcharovsky D.Yu. Deep minerals of the Earth // Priroda. 1980. N 11. S. 119-120.

Su W., Woodward R.L., Dziewonski A.M. Degree 12 Model of Shear Velocity Heterogeneity in the Mantle // J. Geophys. Res. 1994 Vol. 99, N B4. P. 6945-6980.

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And a core of molten iron. It occupies the bulk of the Earth, making up two-thirds of the planet's mass. The mantle begins at a depth of about 30 kilometers and reaches 2900 kilometers.

Earth structure

Earth has the same composition of elements as (not including hydrogen and helium, which escaped due to Earth's gravity). Leaving aside the iron in the core, we can calculate that the mantle is a mixture of magnesium, silicon, iron, and oxygen, which roughly corresponds to the composition of minerals.

But exactly what a mixture of minerals is present at a given depth is a complex issue that is not well substantiated. We can get samples from the mantle, pieces of rock brought up by certain volcanic eruptions, from a depth of about 300 kilometers, and sometimes much deeper. They show that the uppermost part of the mantle is composed of peridotite and eclogite. The most interesting thing we get from the mantle is diamonds.

Activities in the mantle

The upper part of the mantle is slowly stirred by the movements of the plates passing over it. This is caused by two activities. First, there is a downward movement of the movable plates, which slide under each other. Secondly, there is an upward movement of the mantle rock, when two tectonic plates diverge and move apart. However, all this action does not completely mix the upper mantle, and geochemists consider the upper mantle to be a rocky version of the marble cake.

World patterns of volcanism reflect the action of plate tectonics, except for a few areas of the planet called hot spots. Hotspots may hold the key to the rise and fall of materials much deeper in the mantle, perhaps from its very foundation. Today there is an energetic scientific discussion about the hot spots of the planet.

Exploring the mantle with seismic waves

Our most powerful method of studying the mantle is monitoring seismic waves from earthquakes around the world. Two different types seismic waves: P waves (similar sound waves) and waves S (for example, waves from a shaken rope) correspond to physical properties the rocks through which they pass. Seismic waves reflect some types of surfaces and refract (bend) other types of surfaces when they are struck. Scientists use these effects to determine the interior surfaces of the Earth.

Our instruments are good enough to view the Earth's mantle the way doctors take ultrasounds of their patients. After a century of collecting earthquake data, we can make some impressive maps of the mantle.

Simulation of the mantle in the laboratory

Minerals and rocks change under high pressure. For example, a common mantle mineral, olivine, transforms into various crystalline forms at depths of about 410 kilometers and again at 660 kilometers.

The study of the behavior of minerals in the mantle occurs in two ways: computer simulation based on the equations of physics of minerals and laboratory experiments. In this way, modern research mantles are conducted by seismologists, programmers and laboratory researchers who can now reproduce conditions anywhere in the mantle using high-pressure laboratory equipment such as a diamond anvil cell.

Layers of the mantle and internal boundaries

A century of research has filled in some of the gaps in knowledge about the mantle. It has three main layers. The upper mantle extends from the base of the crust (Mohorovic) to a depth of 660 kilometers. The transition zone is located between 410 and 660 kilometers, where there are significant physical changes in minerals.

The lower mantle extends from 660 to about 2700 kilometers. Here, seismic waves are strongly muted, and most researchers believe that the rocks beneath them are different in chemical composition, and not just in crystallography. And the last disputed layer at the bottom of the mantle has a thickness of about 200 kilometers and is the boundary between the core and the mantle.

Why is the Earth's mantle special?

Since the mantle is the main part of the Earth, its history is of fundamental importance for. The mantle formed during the birth of the Earth, like an ocean of liquid magma on an iron core. As it hardened, elements that didn't fit in with the base minerals accumulated as scale on top of the crust. Then, the mantle began a slow circulation that has continued for the last 4 billion years. The upper mantle began to cool because it was stirred and hydrated by the tectonic movements of the surface plates.

At the same time, we learned a lot about the structure of others (Mercury, Venus and Mars). Compared to them, the Earth has an active, lubricated mantle that is special because of the same element that makes its surface different: water.

Many people know that the planet Earth in the seismic (tectonic) sense consists of a core, mantle and lithosphere (crust). We will consider what a mantle is. This is a layer or intermediate shell that is located between the core and the bark. The mantle makes up 83% of the Earth's volume. If we take the weight, then 67% of the Earth is the mantle.

Two layers of mantle

Even at the beginning of the twentieth century, it was generally accepted that the mantle is homogeneous, but by the middle of the century, scientists came to the conclusion that it consists of two layers. The layer closest to the core is the lower mantle. The layer that borders the lithosphere is the upper mantle. The upper mantle goes deep into the Earth for about 600 kilometers. The lower boundary of the lower mantle is located at a depth of up to 2900 kilometers.

What is the mantle made of?

Scientists have not yet been able to get close to the mantle. No drilling has yet made it possible to approach it. Therefore, all research is carried out not experimentally, but theoretically and indirectly. Scientists draw their conclusions about the earth's mantle primarily on the basis of geophysical studies. Electrical conductivity, seismic waves, their propagation speed, and strength are taken into account.

Japanese scientists have announced their intentions to approach the earth's mantle by drilling through oceanic rocks, but so far their plans have not yet been put into practice. At the bottom of the ocean, some places have already been found where the layer of the earth's crust is the thinnest, that is, only some 3,000 km can be drilled to the upper part of the mantle. The difficulty lies in the fact that drilling should be carried out at the bottom of the ocean, and at the same time, the drill will have to go through areas of heavy-duty rocks, and this can be compared with an attempt by a tail of a thread to break through the walls of a thimble. Undoubtedly, the opportunity to study rock samples taken directly from the mantle would give a more accurate idea of ​​its structure and composition.

Diamonds and peridots

Informative are the mantle rocks, which as a result of various geophysical and seismic processes appear on the surface of the earth. For example, diamonds belong to the mantle rocks. Some of them, the researchers suggest, rise from the lower mantle. The most common breeds are peridots. They are often ejected in lava by volcanic eruptions. The study of mantle rocks allows scientists to speak with a certain accuracy about the composition and main features of the mantle.

Liquid state and water

The mantle is made up of silicate rocks rich in magnesium and iron. All substances that make up the mantle are incandescent. molten, liquid state, because the temperature of this layer is quite high - up to two and a half thousand degrees. Water is also part of the Earth's mantle. In quantitative terms, there is 12 times more of it than in the world's oceans. The supply of water in the mantle is such that if it were splashed onto the surface of the earth, the water would rise above the surface by 800 meters.

Processes in the mantle

The mantle boundary is not a straight line. On the contrary, in some places, for example, in the region of the Alps, at the bottom of the oceans, mantle, that is, rocks related to the mantle, rise quite close to the surface of the Earth. It is the physical and chemical processes that flow in the mantle affect what happens in the earth's crust and on the earth's surface. It's about about the formation of mountains, oceans, the movement of continents.