Вестник МГТУ. 2018, том 21, № 1.

Сорохтин Н. О. и др. Коромантийная ветвь глобального цикла углерода… 70 Secondly, it is not improbable that diamonds mainly crystallize by the reaction (12), and the amount of carbon oxide in the system is limited. This may possibly explain the extreme slowness of the diamond-generation process that during the lifetime of kimberlite magmas (i. e. over 1–2 Ga), only small crystals could form. Submerging to the convecting mantle, carbon and some encapsulated solid mineral compounds and gas- liquid inclusions of degraded sedimentary units do not form large accumulations. More likely, these are numerous, but small (mm and fractions of mm) dispersed particles of substance forming a stable crust-trending geochemical tail in the mantle spreading in the plane of convective currents motion. At depths of ca. 200–300 km, carbon may contact with hydrogen (see reactions of hydrogen generation 22, 23) by the reaction [26]: 2 n C + m H 2 → 2C n H m , (28) where n and m are factors. Possibly due to this reaction, liquid inclusions of composite hydrocarbons up to alcohol may be found in diamonds. The resultant compounds further react with oxides of various metals with the yield of carbides: (Ме)О + C n H m + T ○ C → (Ме)С + Н 2 + Н 2 О, (29) where (Ме) means metal. The reduction of metal oxides with carbon with the formation of carbides under the conditions of oxygen deficiency may proceed with the absorption of heat by the reaction [27]: (Me)O + C + T ○ C → (Me)C + CO. (30) For example, at temperatures of 700–800 °С, molybdenum carbide forms in the presence of methane and hydrogen by the reaction [28]: Mo + H 2 + CH 4 + T ○ C → MoC + H + CH. (31) In a close temperature range, lithium carbide may form due to the melting with calcite, which is in excess present in the plate underthrust zone (calcite-generation reactions 7 and 8) [29]: Li 2 CO 3 + 4C + T ○ C → Li 2 C 2 + 3C. (32) At temperatures exceeding 900 °С, carbon forms a solid solution with iron with the iron carbide generation (Fe 3 C and Fe 2 C). 3Fe + C + T ○ C = Fe 3 C. (33) In the rocks of mantle composition, in the presence of carbon, iron reduces, and its telluric phase forms. Cohenite also forms under similar conditions (FeNiCo) 3 C). Carbon in rift zones During the spreading of lithospheric plates in oceanic rift systems, open cracks occur, through which basalt melts rise to the surface from the mantle. The oceanic lithosphere overlain by a water bed hydrates, and a serpentinite layer forms in its lower horizons due to the re-crystallization of olivine-bearing ultramafic rocks. The depth of oceanic water penetration is limited by the lithostatic pressure of ca. 2.3 kbar. Below this level, serpentine becomes so plastic to seal all the cracks arising in the lithosphere and prevent water from penetrating deeper [30]. Higher, all the rock units of the oceanic lithosphere become significantly hydrated and contain at least 5 % of bound water of the total weight in hydrosilicates. Serpentinites is composed of bound water for at least 10–11 %. Hydrothermal systems of rift zones, which are common on the sea bottom, remove a large amount of endogenous matter to the hydrosphere [31; 32], which is generated in the oceanic lithosphere and upper mantle. As a result, silica, calcium, magnesium, manganese, metal sulphides, methane, carbonates, sulphates, and many other compounds are removed. In depressive structures, in the north of the Juan de Fuca ridge in the Pacific Ocean, manifestations of methane (CH 4 ), ethane (C 2 H 6 ), propane (C 3 H 8 ), butane (C 4 H 10 ), benzene (C 6 H 6 ), and toluene (C 7 H 8 ), which associate with H 2 O and CO 2 [17], are described. In hydrothermal fields in the Mid- Atlantic ridge, escapes of hydrocarbons are found, which are represented by methane (СН 4 ), ethane (С 2 Н 6 ), ethylene (С 2 Н 4 ), propane (C 3 H 8 ), and butane (C 4 H 10 ) [33]. It is natural to expect that such a variety of hydrocarbons cannot generate in the mantle, and all these are the products of crustal (exogenic) matter decomposition or form due to the processes of near-surface modification of mantle rocks. The generation of carbon compounds in the rift zones may take place by two major methods. The first one lies in the transfer of encapsulated and dispersed fragments of compounds and monomineral phases of crustal substance from the subduction zones by convective mantle currents. The second one is implemented due to the hydration of mantle rocks in the oceanic lithosphere and their serpentinization.