Вестник МГТУ. 2018, том 21, № 1.

Сорохтин Н. О. и др. Коромантийная ветвь глобального цикла углерода… 72 It shall be noted that under normal conditions, hydrolysis of alkaline metal carbides proceeds exclusively violently and results in explosion in case of its fast contact with a large amount of water. In the geological system, these processes proceed exclusively slow (hundreds of thousands and million years) in a sub-solidus medium at a relatively high pressure (a few kbar) and in the presence of minor amounts of free water that allows being sure in the possibility of normal proceeding of such chemical reactions. Aluminum and manganese carbide hydration proceeds with the release of methane: Al 4 C 3 + 12H 2 O → 4Al(OH) 3 + 3CH 4 ↑ + T ○ C, (37) Mn 3 C + 3H 2 O → 3Mn(OH) 2 + CH 4 + H 2 ↑ + T ○ C. (38) Beryllium (BeC 2 ) and lithium (Li 2 C 2 ) carbide reactions proceed similarly. The manganese hydroxide resulting from the reaction (38) easily oxidizes in the presence of oxygen, which is abundant in water. The reaction proceeds with the release of heat and yield of pyrolusite in two stages: 2Mn(OH) 2 + O 2 + H 2 O → MnO(OH) 2 → MnO 2 . (39) The iron carbide decomposition is accompanied by the release of ethylene. However, this reaction is most likely not widely common in nature due to the fact that the main iron mass tends to sink to the lower mantle levels and forms no appreciable amount of carbides, which then enter the rift zones of the ocean: 2FeC + 3H 2 O → Fe 2 O 3 + C 2 H 4 ↑ + T ○ C. (40) Hydration of near-surface mantle layers in the rift systems may cause chemical reactions of rock re- crystallization and formation of carbonaceous and siliceous compounds, which are brought from the depths of the oceanic crust and deposited on the seabed. All reactions are irreversible and proceed with the release of energy. Hydration reactions of olivine-bearing rocks in the oceanic crust binding carbon dioxide (CO 2 ) and forming chemogenic carbonates are the main ones: 4Mg 2 SiO 4 + 4H 2 O + 2CO 2 → Mg 6 [Si 4 O 10 ](OH) 8 + 2MgCO 3 + T ○ C, (41) forsterite serpentine magnesite 2CaAl 2 Si 2 O 8 + 4H 2 O + 2CO 2 → Al 4 [Si 4 O 10 ](OH) 8 + 2CaCO 3 + T ○ C. (42) anortite kaoline calcite Due to the processes described for these reactions, source material is continuously added to the ocean for the normal life of skeletal organisms (corals, foraminifer and cocolithofore mollusks), which transform dissolved chemogenic carbonates to the rocks of biogenic origin. During hydration of olivines in the rift zones, the bivalent silicate iron oxidation to trivalent iron in the presence of carbon dioxide forms abiogenic methane: 4Fe 2 SiO 4 + 12Mg 2 SiO 4 + 18H 2 O + CO 2 → 4Mg 6 [Si 4 O 10 ](OH) 8 + 4Fe 2 O 3 + CH 4 + T ○ C. (43) fayalite forsterite serpentine hematite methane A large part of formed methane is oxidized (serves as a nutritive basis) by methane-consuming bacteria, which participate in the generation of organic matter: СН 4 + bacteria → СО 2 + [ОВ]. (44) Some methane is transferred to the atmosphere, and some volatiles may be preserved in the oceanic sediments and form hydrocarbon strata of gas hydrates [34]. Ascending to the surface, mantle substance in insignificant amounts contains metallic iron (Fe), which, being oxidized by water generates abiogenic methane, and then, being bound with carbon dioxide (СО 2 ) forms a highly water soluble iron bicarbonate: 4Fe + 2H 2 O + СО 2 → 4FeO + СH 4 + T ○ C, (45) FeO + 2CO 2 + 2H 2 O → Fe(HCO 3 ) 2 . (46) Carbon isotope geochemistry Considering the laws of geochemical carbon compound transformations, it is worth to recall that, as it was shown above, they initially have exogenic origin, and their isotope composition depends on the compositions of compounds participating in chemical reactions. In rifts and plate underthrust zones, the carbon isotope system has own genetic peculiarities and variability of displacements. In light of this, it is wise to describe carbon isotope geochemical transformations gradually from the convergent to the divergent borders of the oceanic lithospheric plates.