Earth mantle. Upper mantle of the Earth: composition, temperature, interesting facts. Liquid state and water

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 regard, a special place is occupied by a mineral aggregate from the Lesotho kimberlite pipe (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, 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 of 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

The widely known model of the internal structure of the Earth (its division into the core, mantle and earth's crust) was developed by seismologists G. Jeffreys and B. Gutenberg back in the first half of the 20th century. The decisive factor in this was the discovery of a sharp decrease in the velocity of passage 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 boundary, the velocities of longitudinal waves that have passed 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 popular model 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 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 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 its chemical composition to be much closer to that of the entire deeper mantle. On the contrary, the piclogite model suggests 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 approximately 13%. At the same time, the results of experimental studies of olivine and an olivine-pyroxene mixture at high temperatures and pressures revealed a complete agreement between 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. This model assumes almost complete absence 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 - rhombic 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, 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 have 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), during compression of which pressures are created that are commensurate with the pressures inside the mantle and the earth's core. Nevertheless, there are still many questions regarding this part of the mantle, which accounts for more than half of all the 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. At the same time, 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 boundary 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 orthorhombic 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 over 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 the stoichiometry (that is, the integer ratio of the elements in the 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, justified by N. Ross (1997), indicates possible presence 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. At a slightly higher pressure (~96 GPa) and a temperature of 800 0C, 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 atoms Ni. 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) in the low-spin state (LS) of Fe atoms in the structure of magnesio-wüstite, 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, a similar work by Japanese geophysicists was published. 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 the general features of the 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 The Earth allows us to approach the complex problem of deep geodynamics much less schematically than before. 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

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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.

J. Geol. soc. Japan. 1994 Vol. 100, No. 1. P. VI-VII.

Pushcharovsky Yu.M. Seismic tomography and mantle structure: Tectonic perspective // ​​Doklady AN. 1996. T. 351, N 6. S. 805-809.

The mantle contains most of the Earth's matter. The mantle is also found on other planets. The earth's mantle is in the range from 30 to 2,900 km.

Within its limits, according to seismic data, the following are distinguished: the upper mantle layer AT up to 400 km deep and FROM up to 800-1000 km (some researchers layer FROM called the middle mantle); lower mantle layer D before depth 2700 with transition layer D1 from 2700 to 2900 km.

The boundary between the crust and the mantle is the Mohorovichic boundary, or Moho for short. There is a sharp increase in seismic velocities on it - from 7 to 8-8.2 km / s. This border is located at a depth of 7 (under the oceans) to 70 kilometers (under the fold belts). The Earth's mantle is divided into the upper mantle and the lower mantle. The boundary between these geospheres is the Golitsyn layer, located at a depth of about 670 km.

The structure of the Earth according to various researchers

The difference in the composition of the earth's crust and mantle is a consequence of their origin: the initially homogeneous Earth, as a result of partial melting, was divided into a fusible and light part - the crust and a dense and refractory mantle.

Sources of information about the mantle

The Earth's mantle is inaccessible to direct investigation: it does not reach the earth's surface and has not been reached by deep drilling. Therefore, most of the information about the mantle has been obtained by geochemical and geophysical methods. Data on its geological structure are very limited.

The mantle is studied according to the following data:

• geophysical data. First of all, data on seismic wave velocities, electrical conductivity and gravity.
• Mantle melts - basalts, komatiites, kimberlites, lamproites, carbonatites and some other igneous rocks are formed as a result of partial melting of the mantle. The composition of the melt is a consequence of the composition of the melted rocks, the interanism of melting, and the physicochemical parameters of the melting process. In general, the reconstruction of the source from the melt is a difficult task.
• Fragments of mantle rocks brought to the surface by mantle melts - kimberlites, alkaline basalts, etc. These are xenoliths, xenocrysts and diamonds. Diamonds occupy a special place among the sources of information about the mantle. It is in diamonds that the deepest minerals are found, which may even come from the lower mantle. In this case, these diamonds represent the deepest fragments of the earth available to direct study.
• Mantle rocks in the composition of the earth's crust. Such complexes are most consistent with the mantle, but also differ from it. The most important difference is in the very fact of their being in the composition of the earth's crust, from which it follows that they were formed as a result of not quite ordinary processes and, perhaps, do not reflect the typical mantle. They occur in the following geodynamic settings:

1. Alpine-type hyperbasites are parts of the mantle embedded in the earth's crust as a result of mountain building. Most common in the Alps, from which the name comes.
2. Ophiolitic hyperbasites - peredotites in the composition of ophiolite complexes - parts of the ancient oceanic crust.
3. Abyssal peridotites are projections of mantle rock at the bottom of oceans or rifts.

These complexes have the advantage that geological relationships between different rocks can be observed in them.

It was recently announced that Japanese explorers are planning to attempt to drill into the oceanic crust down to the mantle. For this, the ship Chikyu was built. The start of drilling is planned for 2007.

The main drawback of the information obtained from these fragments is the impossibility of establishing geological relationships between different types of rocks. These are puzzle pieces. As the classic said, “determining the composition of the mantle from xenoliths is reminiscent of attempts to determine geological structure mountains on the pebbles that the river carried out of them.

Composition of the mantle

The mantle is composed mainly of ultrabasic rocks: peridotites, (lerzolites, harzburgites, wehrlites, pyroxenites), dunites and, to a lesser extent, basic rocks - eclogites.

Also, among the mantle rocks, rare varieties of rocks that are not found in the earth's crust have been identified. These are various phlogopite peridotites, grospidites, and carbonatites.

The structure of the mantle

The processes taking place in the mantle have the most direct impact on the earth's crust and surface of the earth, are the cause of the movement of continents, volcanism, earthquakes, mountain building and the formation of ore deposits. There is growing evidence that the mantle itself is actively influenced by the metallic core of the planet.

Convection and plumes

Bibliography

• Pushcharovsky D.Yu., Pushcharovsky Yu.M. Composition and structure of the Earth's mantle // Soros Educational Journal, 1998, No 11, p. 111–119.
• Kovtun A.A. Electrical conductivity of the Earth // Soros Educational Journal, 1997, No 10, p. 111–117

Source: Koronovsky N.V., Yakushova A.F. "Fundamentals of Geology", M., 1991

Links

• Images of the Earth's Crust & Upper Mantle // International Geological Correlation Program (IGCP), Project 474

> What is the Earth made of?

internal structure of the earth. Study the structure of the planet: crust, core, mantle, what chemical elements the Earth consists of, the history of research, geology.

The earth is more than we can see from our vantage point. If it were possible to cut it in half, then you would be very surprised. We rush in search of new worlds, but we still do not know much about ours.

But seismology has managed to open the structure of the Earth and show the layers. Each is endowed with its own properties, characteristics and composition. And all this affects the earth processes. What is the earth made of?

Modern theory

The inner space of the planet is differentiated. That is, the structure (like the rest of the planets) is represented by layers. Remove one and you'll be taken to the next. And each will have its own temperature and chemical composition.

Our understanding of the layers of the planet is based on the results of seismological monitoring. It contains an examination of the sound waves created by an earthquake, as well as an analysis of how passing through different layers slows down their pace. Changes in seismic velocity lead to refraction.

They are used in conjunction with transformations in gravitational and magnetic fields and experiments with crystalline solids that simulate pressure and temperature in the interior of the planet.

Research

Even in ancient times, mankind tried to figure out the composition of the Earth. The first attempts were not even related to science. These were rather legends and myths associated with divine intervention. However, several theories have circulated among the population.

You may have heard of the flat earth. This opinion was common in Mesopotamian culture. The planet was depicted as a flat disk plowing the ocean. The Maya also considered it flat, but at the corners there were four jaguars that held the sky. The Persians saw the cosmic mountain, while the Chinese saw it as a four-sided cube.

In the 6th century BC e. the Greeks tended towards a rounded shape, and in the 3rd century BC. e. the idea of ​​a spherical Earth was gaining ground underfoot and the first evidence base. At the same moment, scientists begin to come into contact with geological research, and philosophers begin to consider minerals and metals.

But the real shift took place only in the 16th and 17th centuries. Edmund Halley proposed the "Void Earth" theory in 1692. He believed that there is a cavity inside, that is, a certain core, whose thickness is 800 km.

Between these spheres there is an air gap. In order to avoid the effect of friction, the inner sphere must be held in place by gravity. The model displayed two concentric shells around the nucleus. The diameter corresponded to Mercury, Venus and Mars.

Halley was based on the densities of the Moon and Earth put forward by Isaac Newton in 1687. Next, scientists decided to consider the reliability of the Bible. It was important for researchers to calculate the real age of the planet and find evidence of a flood. Here they began to consider fossils and develop a system for classifying the dating of layers.

In 1774, Abraham Werner presented in his writings detailed system identification of certain minerals based on their external characteristics.

In 1741, the first position in geology appeared at the National Museum of Natural History of France. After 10 years, the term "geology" came into use.

In the 1770s chemical analyzes come to the fore in research. One of the important tasks was to study places for the presence of liquid flooding in the past (flood). In the 1780s there were those who believed that the layers were created not because of water, but due to fire. The followers were called plutonists. They believed that the planet was formed due to the solidification of molten masses. And all this happened very slowly. This implied that the planet was much older than the Bible said.

In the 19th century, geology was greatly influenced by the industrial revolution, as well as the concept of the stratigraphic column - rock formations are arranged in the order of their appearance in time. Scientists began to realize that the age of fossils can be calculated geologically (the deeper they are found, the older).

The researchers got the opportunity to go on voyages to broaden their horizons and compare finds in different places. Among these lucky ones was Charles Darwin, recruited by the captain of the Beagle ship.

The giant fossils he found made him a geologist, and his theories about the causes of extinction led to the most important work, On the Origin of Species, written in 1859.

Scientists increased their knowledge and created geological maps of the Earth. They already calculated the earth's age in terms of millions, not thousandths. But the development of technology has helped to shift the remnants of dogmatic ideas.

In the 20th century, radiometric dating appeared. Then they thought that the planetary age reaches 2 billion years. In 1912, Alfred Wegener proposed the theory of continental drift. That is, once all the continents were one. This was later confirmed by geological analysis of the samples.

The theory of plate tectonics originated from the study of the ocean floor. Geophysical data show the lateral movement of the continents, and the oceanic crust is younger than the continental one.

In the 20th century, seismology, the study of earthquakes and the passage of waves through the Earth, was actively developed. This is what helped to understand the composition and get to the core.

In 1926, Harold Jeffis stated that the earth's core was liquid, and in 1937, Inge Lehmann expanded on this theory by adding that there was a solid solid inside the liquid core.

Earth layers

Earth can be divided mechanically or chemically. The first method studies liquid states. Here appears the lithosphere, asthenosphere and mesosphere, the outer and inner core. But the chemical method, which discovered the crust, mantle and core, gained great popularity.

The inner core is solid and the outer is liquid. The lower mantle is under strong pressure, and therefore has a lower viscosity than the upper one. All differences are caused by the processes accompanying the planetary development during 4.5 billion years. Let's take a closer look internal structure Earth.

Bark

This is the outer, cooled and frozen layer. It extends for 570 km and represents only 1% of the planetary volume.

The narrower parts are the oceanic crust underlying the ocean basins (5-10 km), and the denser part is the continental crust. The upper part of the mantle and the earth's crust is the lithosphere, covering 200 km. Most of the rocks were formed 100 million years ago.

Upper mantle

It occupies 84% ​​of the volume and appears mostly solid, but sometimes behaves like a viscous liquid. It starts from the "Mohorovicic Surface" - 7-35 km and deepens to 410 km.

Movement in the mantle is reflected in the movement of tectonic plates. The process is driven by heat from the depths. This is what leads to earthquakes and the formation of mountain ranges.

The temperature rises by 500-900°C. The layer at a depth of 410-660 km is considered a transition zone.

lower mantle

The temperature at a depth of 660-2891 km can reach 4000°C. But the pressure here is too strong, so the viscosity and melting are limited. Little is known about this layer, but it is believed to be seismically homogeneous.

outer core

This is a liquid shell with a thickness of 2300 km, and in a radius it covers 3400 km. Here the density is much higher - 9900-12200 kg / m 3. It is believed that the core is represented by 80% iron, as well as nickel and other light elements. There is no strong pressure, so it does not harden, although the composition resembles the inner core. Temperature - 4030 ° С.

In the liquid core, due to temperature and turbulence, a dynamo is created that affects the magnetic field.

inner core

What elements make up the Earth's core? It is represented by iron and nickel, and in a radius it covers 1220 km. Density - 12600-13000 kg / m 3, which hints at the presence of heavy elements (platinum, gold, palladium, tungsten and silver).

The temperature here rises to 5400°C. Why do solid metals remain liquid? Because the melting point is extremely high, as is the pressure. Internally, it is not strongly connected to the solid mantle, so it is believed that it rotates faster than the planet itself.

There is also an opinion that the inner core also has layers separated by a transition zone with a thickness of 250-400 km. The lowest layer is capable of extending 1180 km in diameter. Scientists testify to the dynamics, due to which the core expands by 1 mm per year.

As you can see, our planet is an amazing and full of mysteries place. It still lurks the heat accumulated billions of years ago. And this is not a dead body, but a dynamic object that is constantly changing.

There are practically no direct data on the material composition of deep zones. The conclusions are based on geophysical data supplemented by the results of experiments and mathematical modeling. Significant information is provided by meteorites and fragments of upper mantle rocks carried from the depths by deep magmatic melts.

The gross chemical composition of the Earth is very close to the composition of carbonaceous chondrites - meteorites, similar in composition to the primary cosmic substance from which the Earth and other cosmic bodies of the Solar System were formed. In terms of gross composition, 92% of the Earth consists of only five elements (in descending order of content): oxygen, iron, silicon, magnesium and sulfur. All other elements account for about 8%.

However, in the composition of the Earth's geospheres, the listed elements are distributed unevenly - the composition of any shell differs sharply from the gross chemical composition of the planet. This is due to the processes of differentiation of the primary chondrite substance in the process of the formation and evolution of the Earth.

The main part of iron in the process of differentiation was concentrated in the nucleus. This is in good agreement with the data on the density of the substance of the nucleus, and with the presence of a magnetic field, with the data on the nature of the differentiation of the chondrite substance, and with other facts. Experiments at ultrahigh pressures have shown that at pressures reached at the boundary of the core and mantle, the density of pure iron is close to 11 g/cm 3 , which is higher than the actual density of this part of the planet. Therefore, there are some light components in the outer core. Hydrogen or sulfur are considered as the most probable components. So calculations show that a mixture of 86% iron + 12% sulfur + 2% nickel corresponds to the density of the outer core and should be in a molten state under the P-T conditions of this part of the planet. The solid inner core is represented by nickel iron, probably in the ratio of 80% Fe + 20% Ni, which corresponds to the composition of iron meteorites.

To date, several models have been proposed to describe the chemical composition of the mantle (Table). Despite the differences between them, all authors accept that approximately 90% of the mantle consists of oxides of silicon, magnesium, and ferrous iron; another 5 - 10% are oxides of calcium, aluminum and sodium. Thus, 98% of the mantle consists of only six listed oxides.

The form of finding these elements is debatable: in the form of what minerals and rocks are they found?

Down to a depth of 410 km, according to the lherzolite model, the mantle consists of 57% olivine, 27% pyroxene, and 14% garnet; its density is about 3.38 g/cm3. At the boundary of 410 km, olivine passes into spinel, and pyroxene into garnet. Accordingly, the lower mantle consists of a garnet-spinel association: 57% spinel + 39% garnet + 4% pyroxene. The transformation of minerals into denser modifications at the turn of 410 km leads to an increase in density up to 3.66 g/cm3, which is reflected in an increase in the velocity of seismic waves passing through this substance.

The next phase transition is confined to the boundary of 670 km. At this level, the pressure determines the decomposition of minerals typical of the upper mantle to form denser minerals. As a result of this rearrangement of mineral associations, the density of the lower mantle at the boundary of 670 km becomes about 3.99 g/cm3 and gradually increases with depth under pressure. This is fixed by an abrupt increase in the speed of seismic waves and a further gradual increase in the speed of the 2900 km boundary. At the boundary between the mantle and the core, silicate minerals are probably decomposed into metallic and nonmetallic phases. This the process of differentiation of the mantle substance is accompanied by the growth of the metallic core of the planet and the release of thermal energy.

Summarizing the above data, it should be noted that separation of the mantle is due to the rearrangement of the crystal structure of minerals without a significant change in its chemical composition. Seismic interfaces are confined to areas of phase transformations and are associated with a change in the density of matter.

The core/mantle section is, as noted earlier, very sharp. Here, the speed and nature of the passage of waves, density, temperature and other physical parameters change dramatically. Such radical changes cannot be explained by the rearrangement of the crystal structure of minerals and are undoubtedly associated with a change in the chemical composition of the substance.

More detailed information is available in the material composition of the earth's crust, the upper horizons of which are available for direct study.

The chemical composition of the earth's crust differs from the deeper geospheres primarily by its enrichment in relatively light elements - silicon and aluminum.

Reliable information is available only on the chemical composition of the uppermost part of the earth's crust. The first data on its composition were published in 1889 by the American scientist F. Clark, as the arithmetic mean of 6000 chemical analyzes of rocks. Later, on the basis of numerous analyzes of minerals and rocks, these data were repeatedly refined, but even now the percentage of a chemical element in the earth's crust is called clarke. About 99% of the composition of the earth's crust is occupied by only 8 elements, that is, they have the largest clarks (data on their content are given in the table). In addition, several more elements can be named that have relatively high clarks: hydrogen (0.15%), titanium (0.45%), carbon (0.02%), chlorine (0.02%), which in total make up 0.64%. For all other elements contained in the earth's crust in thousandths and millionths, 0.33% remains. Thus, in terms of oxides, the Earth's crust mainly consists of SiO2 and Al2O3 (has a "sialic" composition, SIAL), which significantly distinguishes it from the mantle, enriched in magnesium and iron.

At the same time, it should be borne in mind that the above data on the average composition of the earth's crust reflect only the general geochemical specificity of this geosphere. Within the limits of the earth's crust, oceanic and continental types of crust differ significantly in composition. The oceanic crust is formed due to magmatic melts coming from the mantle, therefore it is much more enriched in iron, magnesium and calcium than the continental one.

The average content of chemical elements in the earth's crust
(according to Vinogradov)

Chemical composition of continental and oceanic crust

No less significant differences are found between the upper and lower parts of the continental crust. This is largely due to the formation of crustal magmas arising from the melting of crustal rocks. During the melting of rocks of different composition, magmas are smelted, largely consisting of silica and aluminum oxide (they usually contain more than 64% SiO 2), and oxides of iron and magnesium remain in the deep horizons in the form of an unmelted "residue". Melts with low density intrude into higher horizons of the earth's crust, enriching them with SiO 2 and Al 2 O 3 .

Chemical composition of the upper and lower continental crust
(according to Taylor and McLennan)

Chemical elements and compounds in the earth's crust can form their own minerals or be in a dispersed state, entering in the form of impurities in any minerals and rocks.

It has a special composition, differing from the composition of the earth's crust covering it. Data on the chemical composition of the mantle were obtained from analyzes of the deepest igneous rocks that entered the upper horizons of the Earth as a result of powerful tectonic uplifts with the removal of mantle material. These rocks include ultrabasic rocks - dunites, peridotites occurring in mountain systems. The rocks of the St. Paul Islands in the middle part Atlantic Ocean, according to all geological data, refer to the mantle material. The mantle material also includes rock fragments collected by Soviet oceanographic expeditions from the bottom indian ocean in the Indian Ocean Ridge. As regards the mineralogical composition of the mantle, significant changes can be expected here, starting from the upper horizons and ending with the base of the mantle, due to an increase in pressure. The upper mantle is composed mainly of silicates (olivines, pyroxenes, garnets), which are stable and within relatively low pressures. The lower mantle is composed of high-density minerals.

The most common component of the mantle is silicon oxide in the composition of silicates. But at high pressures, silica can go into a denser polymorphic modification - stishovite. This mineral was obtained by the Soviet researcher Stishov and named after him. If ordinary quartz has a density of 2.533 r/cm 3 , then stishovite, formed from quartz at a pressure of 150,000 bar, has a density of 4.25 g/cm 3 .

In addition, denser mineral modifications of other compounds are also probable in the lower mantle. Based on the foregoing, it can be reasonably assumed that with increasing pressure, the usual iron-magnesian silicates of olivines and pyroxenes decompose into oxides, which individually have a higher density than silicates, which turn out to be stable in the upper mantle.

The upper mantle consists mainly of ferruginous-magnesian silicates (olivines, pyroxenes). Some aluminosilicates can transform here into denser minerals such as garnets. Beneath the continents and oceans, the upper mantle has different properties and probably a different composition. One can only assume that in the area of ​​continents the mantle is more differentiated and has less SiO 2 due to the concentration of this component in the aluminosilicate crust. Beneath the oceans, the mantle is less differentiated. In the upper mantle, denser polymorphic modifications of olivine with a spinel structure, etc., can occur.

The transitional layer of the mantle is characterized by a constant increase in seismic wave velocities with depth, which indicates the appearance of denser polymorphic modifications of matter. Here, obviously, FeO, MgO, GaO, SiO 2 oxides appear in the form of wustite, periclase, lime, and stishovite. Their number increases with depth, while the amount of ordinary silicates decreases, and below 1000 km they make up an insignificant fraction.

The lower mantle within the depths of 1000-2900 km almost completely consists of dense varieties of minerals - oxides, as evidenced by its high density in the range of 4.08-5.7 g/cm 3 . Under the influence of increased pressure, dense oxides are compressed, further increasing their density. The content of iron also probably increases in the lower mantle.

Earth's core. The question of the composition and physical nature of the core of our planet is one of the most exciting and mysterious problems of geophysics and geochemistry. Only recently there has been a little enlightenment in solving this problem.

The vast central core of the Earth, which occupies the inner region deeper than 2900 km, consists of a large outer core and a small inner one. According to seismic data, the outer core has the properties of a liquid. It does not transmit transverse seismic waves. The absence of cohesive forces between the core and the lower mantle, the nature of the tides in the mantle and crust, the peculiarities of the movement of the Earth's rotation axis in space, the nature of the passage of seismic waves deeper than 2900 km indicate that the outer core of the Earth is liquid.

Some authors assumed that the composition of the core for a chemically homogeneous model of the Earth was silicate, and under the influence of high pressure, the silicates passed into a “metallized” state, acquiring an atomic structure in which the outer electrons are common. However, the geophysical data listed above contradict the assumption of a "metallized" state of the silicate material in the Earth's core. In particular, the absence of cohesion between the core and the mantle cannot be compatible with a "metallized" solid core, which was assumed in the Lodochnikov-Ramsay hypothesis. Very important indirect data on the core of the Earth were obtained during experiments with silicates under high pressure. In this case, the pressure reached 5 million atm. Meanwhile, in the center of the Earth, the pressure is 3 million atm., and at the boundary of the core - approximately 1 million atm. Thus, experimentally, it was possible to block the pressures that exist in the very depths of the Earth. In this case, for silicates, only linear compression was observed without a jump and transition to a “metallized” state. In addition, at high pressures and within the depths of 2900-6370 km, silicates cannot be in a liquid state, like oxides. Their melting point increases with increasing pressure.

Per last years Very interesting research results have been obtained on the effect of very high pressures on the melting point of metals. It turned out that a number of metals at high pressures (300,000 atm. and above) go into a liquid state at relatively low temperatures. According to some calculations, an alloy of iron with an admixture of nickel and silicon (76% Fe, 10% Ni, 14% Si) at a depth of 2900 km under the influence of high pressure should be in a liquid state already at a temperature of 1000 ° C. But the temperature at these depths, according to the most conservative estimates of geophysicists, it should be much higher.

Therefore, in the light of modern geophysics and high-pressure physics data, as well as cosmochemistry data indicating the leading role of iron as the most abundant metal in space, it should be assumed that the Earth's core is mainly composed of liquid iron with an admixture of nickel. However, the calculations of the American geophysicist F. Birch showed that the density of the earth's core is 10% lower than that of an iron-nickel alloy at temperatures and pressures prevailing in the core. It follows that the metallic core of the Earth must contain a significant amount (10-20%) of some kind of lung. Of all the lightest and most common elements, silicon (Si) and sulfur (S) are the most probable | The presence of one or the other can explain the observed physical properties of the earth's core. Therefore, the question of what is an admixture of the earth's core - silicon or sulfur, turns out to be debatable and is connected with the way our planet is formed in practice.

A. Ridgwood in 1958 assumed that the earth's core contains silicon as a light element, arguing this assumption by the fact that elemental silicon in an amount of several weight percent is found in the metal phase of some reduced chondrite meteorites (enstatite). However, there are no other arguments in favor of the presence of silicon in the earth's core.

The assumption that there is sulfur in the Earth's core follows from a comparison of its distribution in the chondrite material of meteorites and the Earth's mantle. Thus, a comparison of the elementary atomic ratios of some volatile elements in a mixture of the crust and mantle and in chondrites shows a sharp lack of sulfur. In the material of the mantle and crust, the concentration of sulfur is three orders of magnitude lower than in the average material of the solar system, which is taken as chondrites.

The possibility of loss of sulfur at the high temperatures of the primitive Earth is eliminated, since other more volatile elements than sulfur (for example, H2 in the form of H2O), found to be much less deficient, would be lost to a much greater extent. In addition, when solar gas cools, sulfur chemically bonds with iron and ceases to be a volatile element.

In this regard, it is quite possible that large amounts of sulfur enter the earth's core. It should be noted that, other things being equal, the melting point of the Fe-FeS system is much lower than the melting point of iron or mantle silicate. So, at a pressure of 60 kbar, the melting point of the system (eutectic) Fe-FeS will be 990 ° C, while pure iron - 1610 °, and mantle pyrolite - 1310. Therefore, with an increase in temperature in the bowels of the initially homogeneous Earth, an iron melt enriched with sulfur , will form first and, due to its low viscosity and high density, will easily drain into the central parts of the planet, forming a ferruginous-sulphurous core. Thus, the presence of sulfur in the nickel-iron environment acts as a flux, lowering its melting point as a whole. The hypothesis of the presence of significant amounts of sulfur in the earth's core is very attractive and does not contradict all the known data of geochemistry and cosmochemistry.

Thus, modern ideas about the nature of the interior of our planet correspond to a chemically differentiated the globe, which turned out to be divided into two different parts: a powerful solid silicate-oxide mantle and a liquid, mostly metallic core. The earth's crust is the lightest upper solid shell, consisting of aluminosilicates and having the most complex structure.

Summarizing the above, we can draw the following conclusions.

1. The earth has a layered zonal structure. It consists of two-thirds of a solid silicate-oxide shell - the mantle and one-third of a metallic liquid core.
2. The main properties of the Earth indicate that the core is in a liquid state and only iron from the most common metals with an admixture of some light elements (most likely sulfur) is able to provide these properties.
3. In its upper horizons, the Earth has an asymmetric structure, covering the crust and upper mantle. The oceanic hemisphere within the upper mantle is less differentiated than the opposite continental hemisphere.

The task of any cosmogonic theory of the origin of the Earth is to explain these basic features of its internal nature and composition.