Вестник МГТУ. 2018, том 21, № 1.

Сорохтин Н. О. и др. Коромантийная ветвь глобального цикла углерода… 64 dehydration processes of oceanic crust and fragments of sedimentary rocks not completely proceed in this zone. Remaining water, carbon, carbon dioxide, and other volatiles may submerge to the convecting mantle. The pulling of carbon-bearing compounds to the subduction zone results in their multi-stage decomposition and transformation with the release of monomineral carbon. At depths of ca. 120–150 km, a phase transition from graphite to diamond is observed, below which there is a diamond stability field. Diamonds crystallize exactly at such depths with distinctive mineral assemblages in eclogites and garnet peridotites of the diamond-pyrope depth facies [12]. On the other hand, it is known [13] that at depths of ca. 350 km orthorhombic olivine shall evolve into the denser cubic modification (spinel phase), or ringwoodite. However, this mineral has not yet been found in kimberlites or diamond inclusions, apparently limiting the maximum depth of diamond-bearing rock generation at 300 km [14] (Fig. 2). Summarizing these data allowed outlining the area of balanced existence of diamond-bearing eclogites and garnet lherzolites in the mantle with some high level of confidence [12; 14; 15]. It turned out quite wide. At a pressure (Р) of ca. 50 kbar, the temperature range is 1120–1380 °С, and 1300–1500 °С at 70 kbar. This area is also wide for garnet lherzolites with a temperature range of 900–1400 °С (Fig. 2). The demonstrated data allow suggesting that at depths exceeding 250–300 km carbon transforms to the graphite phase and forms various compounds fallen to the area of metal carbide stability. There are a few carbide minerals known in the nature, which occur in meteorites, kimberlites, metamorphosed ultramafic rocks and shungites. The most common are cohenite (Fe,Ni,Co) 3 C), moissanite (SiC), tantalum carbide (Ta,Nb)C), niobium carbide (Nb,Ta)C), khamrabaevite (Ti,V,Fe)C), and vanadium (V 8 C 7 and V 2 C) and chromium (Cr 2 C 3 ) compounds. Such a limited amount of metal carbide minerals is accounted for by their deep origin and tendency towards decomposition under low thermobaric conditions and in the presence of water. We suppose that metal carbide minerals are more common in the upper mantle and do not rule out that calcium, aluminum, manganese, iron, etc carbides may exist there. Fig. 3. Drawing of oceanic crust sedimentary units into the plate subduction area at larger depths (down to 250 km) and position of the deep-seated rock melts areas of existence Рис. 3. Затягивание осадочных комплексов океанической коры в зону поддвига плит на большие глубины (до 250 км) и положение областей формирования расплавов глубинных пород Carbon in the subduction zones All chemical reactions in the plate underthrust zones are irreversible and proceed with thermal absorption or release, and under various reducing-oxidizing settings. Geological time plays an important part in the implementation of all the above processes since it ultimately brings the physical and chemical features of the fold system evolution to the equilibrium state. Modern sedimentary strata on the seabed contain up to 20–40 % of water