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 69 Since the latter reaction releases heat, it is thus more preferable, and free hydrogen in the diamond- formation areas most likely is generated by reaction (23). It shall be remembered that magnetite is the spinel phase of iron oxides, and thus more stable under the conditions of increased pressure. It is thus improbable that part of magnetite coronas around olivine crystals and other ferro-silicates forms exactly in this way. The methane synthesis takes place through exothermic reactions of simple binding of СО and СО 2 with hydrogen or water. In the presence of catalysts, for example, nickel, nickel carbonate, or native iron, these reactions significantly accelerate and begin to proceed from 250–400 °С (though at normal pressure). All these reactions are accompanied by heat release, and there are reasons to expect that under higher PT conditions typical of plate underthrust zones, these may proceed with no catalyst added CO + 3H 2 → CH 4 + H 2 O + T ○ C, CO 2 + 4H 2 → CH 4 + 2H 2 O + T ○ C. (24) At comparably low temperatures of green-schist and epidote-amphibolite metamorphic facies (up to 400–500 °С), abiogenic synthesis of methane may take place through serpentinization reaction for iron- containing olivines in the presence of carbon dioxide: 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. (25) olivine forsterite serpentine hematite At higher temperatures (over 660–700 °С), this reaction apparently proceeds as a side one at the formation of metasomatic pyroxene (clinoenstatite) crystalls 4Fe 2 SiO 4 + 4Mg 2 SiO 4 + 2H 2 O + CO 2 → 8MgSiO 3 + 4Fe 2 O 3 + CH 4 + T ○ C. (26) Moreover, abiogenic methane may be generated at direct reduction of native iron in the presence of carbon dioxide: 4Fe + 2H 2 O + CO 2 → 4FeO + CH 4 + T ○ C. (27) Studying the shown methane release reactions, it shall be remembered that carbon isotopes easily fractionate between СН 4 and СО 2 . Abiogenic methane like organic one always predominantly concentrate light carbon isotope, 12 С. In addition to the above, it may be added that in [22; 23], possible formation of complex hydrocarbons up to C 10 H 22 with the use of solid iron oxide, marble, and water was experimentally proven. This reaction becomes possible at a temperature of 1500 °C and a pressure of above 30 kbar that corresponds to the depths of over 100 km. These and some other exchange reactions between carbon and hydrogen-containing compounds shall lead to the formation of composite kimberlite fluid phase. Gas-liquid inclusions in diamonds preserved sealed compositions of those sulphides, from which they had crystallized, are of special interest. The executed [24] studies of the inclusion compositions have shown the following concentrations: Н 2 О, 10 to 60 %; Н 2 , 2 to 50 %; СН 4 , 1 to 12 %; СО 2 , 2 to 20 %; CO, 0 to 45 %; N 2 , 2 to 38 %; Ar, ca. 0.5–1.2 %. Moreover, it turned out that these inclusions contain ca. 0.5 % of ethylene (С 2 Н 4 ) and 0.05 to 3 % of ethyl alcohol (С 2 Н 5 ОН). Free oxygen in such inclusions was not found witnessing the reducing conditions of the diamond formation. This specific set of gases, in our opinion, almost unambiguously indicates predominantly exogenous origin of fluid phase, from which diamonds in kimberlites crystallized. Thus, hydrocarbons necessary for the formation of diamonds could enter kimberlites both due to the thermolysis of organic matter drawn together with carbonate sediments and terrigenous rocks into the plate underthrust zones and reduction of carbon dioxide at oxidation of iron and iron-containing silicates. It follows that all carbon in diamonds is of exogenic origin. For many diamond grains, noticeable (up to 0.25 %) nitrogen (up to 0.25 %) entered directly into the crystal lattice of this mineral [14; 25] is typical. As carbon, this element falls within diamonds from the fluid phase of deep subduction zone segments formed due to the melting of drawn-in pelagic sediments. In addition to gas-liquid inclusions, diamond crystals often contain solid inclusions of deep mineral assemblages, among which sulphides predominate, but also olivine, serpentine, phlogopite, omphacite, pyrope, almandine, magnetite, wustite, native iron, chromite, and some other minerals. Almost all the solid inclusions in diamonds represent high-pressure mineral phases of eclogite or peridotite paragenesis. Examining the diamond generation conditions, a consistent question arises, why this uniquely rare accessory mineral does not become a rock-forming one under the conditions of excess of initial carbon- containing compounds (СО 2 and СН 4 )? The explanation may be ambivalent. Firstly, in the overheated and apparently exclusively liquid kimberlite melt, diamonds as a heavier fraction (with a density of ca. 3.51 g/cm 3 ) shall sink in the hearth of kimberlite magma and enter the under-lithosphere mantle levels. In these areas, diamonds again transforms into graphite, becomes bound with metals by endothermic reactions, and forms a series of metal carbide compounds, which further spread by convective currents through the whole mantle.