Вестник МГТУ. 2018, том 21, № 1.

Вестник МГТУ. 2018. Т. 21, № 1. С. 61–79. DOI: 10.21443/1560-9278-2018-21-1-61-79 67 Carbon dioxide released as a result of reactions (1–5) is dissolved in the originating melts and contained in a water-carbon dioxide fluid. Under the high-pressure conditions (40–50 kbar) typical of the lower continental plate parts, the decomposition of carbonatites shall apparently be accompanied by the reduction of ferrous iron with the formation of compacted crystalline magnetite structures and reduction of СО 2 to carbon oxide, see the reaction below (6): 3CaCO 3 + 3FeTiO 3 + T ○ C → 3CaTiO 3 + Fe 3 O 4 + 2CO 2 + CO↑. (6) calcite ilmenite perovskite magnetite At larger depths, garnet, corundum, and calcite form with the release of carbon dioxide: 3Ca[Al 2 Si 2 O 8 ] + 6MgCO 3 + T ○ C → 2Mg 3 Al 2 [SiO 4 ] + 3CaCO 3 + Al 2 O 3 + 3CO 2 ↑, (7) anorthite magnesite pyrope calcite corundum 3Ca[Al 2 Si 2 O 8 ] + 6FeCO 3 + T ○ C → 2Fe 3 Al 2 [SiO 4 ] + 3CaCO 3 + Al 2 O 3 + 3CO 2 ↑. (8) anorthite siderite almandine calcite corundum In addition to exchange reactions (6–8), at higher temperatures and pressure, decomposition of carbonates possibly takes place in the presence of olivine and pyroxene with the release of free carbon dioxide, generation of monticellite and periclase: CaCO 3 + Mg 2 SiO 4 + T ○ C → CaMgSiO 4 + MgO + CO 2 ↑, (9) calcite forsterite monticellite periclase CaCO 3 + MgSiO 3 + T ○ C → CaMgSiO 4 + CO 2 ↑. (10) calcite enstatite monticellite As per [17] in the presence of carbon dioxide (CO 2 ) and hydrogen sulphide (H 2 S), olivine (fayalite) forms marcasite (FeS 2 ), magnetite (Fe 3 O 4 ), quartz, water, and abiogenic methane: 4Fe 2 SiO4 + 4H 2 S + CO 2 + T ○ C = 2FeS 2 + 2Fe 3 O 4 + 4SiO 2 + 2H 2 O + CH 4 ↑, (11) and even heavier hydrocarbons (ethane): 3.5Fe 2 SiO 4 + 14H 2 S + 2CO 2 + T ○ C = 7FeS 2 + 3.5SiO 2 + 11Н 2 O + С 2 Н 6 ↑. (12) At depths below 120–150 km, diamonds are generated through the reduction of carbon by reactions between carbon oxide and carbon dioxide with methane or other organic and abiogenic hydrocarbons drawn in through the subduction zones together with sediments at larger depths. It is known that oceanic sediments and sedimentary rocks and those taken away from the continental margins often contain increased concentrations of organic matter, which, when entered to the plate underthrust zones, is subject to the thermolysis and hydrolysis, and promptly passes all stages of transformation to hydrocarbons, nitrates, and ammonium compounds. Part of these movable compounds together with porous waters is squeezed from the plate underthrust zones at the uppermost horizons. However, some portion together with the terrigenous rocks continues moving to the mantle depths. In modern subduction zones, where water-saturated silicate melts are easily removed from the plate friction zones, the magma-generation temperature quickly rises to a basalt melting level. That is why hydrocarbons cannot enter deeply into the mantle through such zones since far before this, they have to almost completely dissociate with the generation of disseminated graphite. It is known [18] that the stability of all the hydrocarbons significantly decreases with the increase of temperature and pressure. It occurs due to the rupture of carbon bonds in long chains of composite hydrocarbon molecules. As a result of this cracking process, the content of composite hydrocarbons becomes gradually reduced in the system with the increase of simple hydrocarbon concentration. The highest stability is shown by methane sustaining heating (at normal pressure) up to 1200 °С. At a quite long reaction under the conditions of high temperature and pressure, all the organic matter may ultimately transform into methane, hydrogen, and free carbon. However, the thermal decomposition of hydrocarbons, or an endothermic process, cannot lead to the formation of crystalline carbon phases. The releasing carbon remains dispersed. To form crystalline phases of carbon, it has to be released due to the exothermal reaction resulting in the reduction of the system's internal energy. Such conditions are met by the reactions that bind hydrocarbons with carbon oxide and carbon dioxide and proceed with the release of energy, for example [19]: СН 4 + СО 2 → 2С + 2Н 2 О + T ○ C, (13) СН 4 + 2СО → 3С + 2Н 2 О + T ○ C. (14)