Вестник МГТУ. 2018, том 21, № 1.

Сорохтин Н. О. и др. Коромантийная ветвь глобального цикла углерода… 68 In general, the diamond formation may be accounted for by reactions between common hydrocarbons and carbon oxides [19; 20] 4C n H 2 n ± k + (2 n ± k )CO 2 → (6 n ± k )C + 2(2 n ± k )H 2 O, (15) 2C n H 2 n ± k + (2 n ± k )CO → (4 n ± k )C + (2 n ± k )H 2 O. (16) Carbon dioxide shall be released due to the thermal dissociation of carbonates in hot parts of the plate underthrust zones by endothermic reactions (1–5). Carbon oxide is possibly also generated by an exothermal reaction during reduction, for example, of wustite to the magnetite stoichiometry 3FeO + CO 2 + T ○ C → Fe 3 O 4 + CO + T ○ C. (17) wustite magnetite In addition to hydrocarbons of purely organic origin, abiogenic methane generated by, for example, reactions No.No. 43 and 45 below may take part in the diamond formation. These reactions become possible due to the multi-stage process of dehydration and hydration in the subduction zone. The ultramafic rock hydration processes are more typical of the rift genesis conditions, and the listed chemical reactions are detailed below. In the course of hydrogen release and its binding with carbon dioxide, the reaction of monomineral carbon generation may be given as follows: 2H 2 + CO 2 → C + 2H 2 O + T ○ C. (18) Reactions like (13–16) and (18) proceed with the energy release, and thus may result in carbon crystallization (to graphite at a moderate pressure, to diamond at a high pressure, and to dispersed graphite at above-critical conditions). For the sake of visualization, it is easy to calculate based on the enthalpy of chemical compounds formation [21] that under normal PT conditions, the reaction (13) of methane combination with carbon dioxide releases energy at 24.6 kcal/mol while the reaction (14) of СН 4 and СО combination produces more energy, at 65.9 kcal/mol. At higher pressure and temperatures, this ratio may be somewhat different, but always formation enthalpy 0 t H ∆ for the reaction products in accordance with the first scheme is lower than that of the second one (since 0 0 t H ∆ < ). It follows that, from the gaseous СО + СО 2 mixture, the carbon crystallization reaction shall initially be fed by СО and only after by СО 2 . Sulphide, especially pyrrhotite inclusions may often be found in diamonds. This makes the following endothermic carbon release reaction possible: 2FeS + CO 2 + T ○ C → 2FeO + S 2 + C, (19) or 2FeS + CH 4 + T ○ C → 2H 2 S + 2Fe + C. (20) An important part in this process is played by nitrogen, which in excess occurs in hydrothermal and pneumatolytic solutions: 3CH 4 + 2N 2 + T ○ C → 3C + 4NH 3 . (21) However, reactions (19–21) are endothermic and may only result in the generation of dispersed carbon. Moreover, at high temperatures, ammonium is instable and most likely falls into nitrogen and hydrogen, which further enter a fluid. Some nitrogen is absorbed by growing diamond crystals and enters it crystalline grid, but its major part probably remains in the fluid. In addition to hydrocarbons of purely organic origin, simplest abiogenic hydrocarbons, and especially methane may arise in kimberlites, eclogites, and garnet peridotites formed from the rocks of the oceanic crust. Being formed from biogenic matter and undergone a series of physical and chemical transformations, these, in the essence of formation and certain acquired features, are abiogenic. Thus, the border between the biogenic and abiogenic nature of hydrocarbons that form under such geodynamic settings vanishes. For the methane generation, significant amount of hydrogen, which may be obtained during the water dissociation in the presence of iron, is necessary. This reaction results in heat absorption and in the presence of water fluid: 4H 2 O + 3Fe + T ○ C → Fe 3 O 4 + 4H 2 . (22) Due to reduction of ferrous (silicate) iron to the magnetite stoichiometry, in contrast, the exothermic reaction proceeds H 2 O + 3FeO → Fe 3 O 4 + H 2 + T ○ C. (23)