Вестник МГТУ. 2018, том 21, № 1.

Сорохтин Н. О. и др. Коромантийная ветвь глобального цикла углерода… 66 oceanic crust submerged to the mantle together with the residues of the sedimentary cover becomes sandwiched in between the homogenous units with the mantle composition. This process unavoidably causes the separation and isolation of large and small volumes of substance, encapsulation of crustal melts, metamorphized crustal rocks, dispersed matter, fluid solutions, and gas-liquid inclusions. In the under-lithosphere mantle, gas-liquid inclusions become a supercritical fluid with the absent differences between these two phases. Under the conditions of viscous flow, capsules are transferred to the convecting mantle and spread for long distances, being detached from the lithospheric plate submerging towards the core, or jointly moving to the area of ascending convective current in case of its flattening (Fig. 4). Fig. 5. Distribution diagram for metamorphic facies in the PT coordinates, after [16] Рис. 5. Схема распределения метаморфических фаций в координатах РТ, по [16] The formation of hydrothermal solutions is tightly related to the processes of progressive stress- metamorphism and overheating of the water-saturated rock system. Carbonates entering the plate underthrust area become transformed and decomposed with the release of CO 2 . This results in the binding of some bases in silicate phases, and other in carbonates. Thus, the siderite heat of formation from oxide components is equal to 22.3 kcal/mol, magnesite – 23 kcal/mol, and calcite – 42.6 kcal/mol [11]. Hence it appears that the direct carbonate dissociation could only take place in hot areas of the plate underthrust zones: for siderite and magnesite at depths of ca. 80–100 km, and for calcite deeper than 150 km. This means that it only occurs in the melting areas of water-saturated sediments entered the subduction zones. All the below reactions are endothermic: CaCO 3 + SiO 2 + T ○ C → CaSiO 3 + CO 2 ↑, (1) calcite wollastonite MgCO 3 + SiO 2 + T ○ C → MgSiO 3 + CO 2 ↑, (2) magnesite enstatite 2MgCO 3 + SiO 2 + T ○ C → Mg 2 SiO 4 + 2CO 2 ↑, (3) magnesite forsterite 2FeCO 3 + SiO 2 + T ○ C → Fe 2 SiO 4 + 2CO 2 ↑, (4) siderite fayalite CaCO 3 + MgCO 3 + 2SiO 2 + T ○ C → CaMg[Si 2 O 6 ] + 2CO 2 ↑. (5) calcite magnesite diopside