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 61 УДК 550.41+551.2/.3+549.21 N. O. Sorokhtin, S. L. Nikiforov, N. Ye. Kozlov Crust-mantle branch of the global carbon cycle and origin of deep-seated hydrocarbons The processes of multi-stage and polycyclic transformation and transfer of carbon in the crust and mantle have been described. The sediments drawn in the plate underthrust zones break down, become transformed and altered by metamorphic events, and part of the newly formed carbon compounds is transferred by the mantle convective currents to rift zones of the mid-oceanic ridges and carried up to the surface as hydrocarbons of various composition and carbon dioxide. This material becomes re-deposited on the sea floor as sediments forming carbonaceous and carbon-bearing units. As a result of multi-stage mechanism of physical and chemical transformations in the crust-mantle areas of the Earth hydrocarbon compounds acquire features of abiogenic origin remaining, in fact, exogenic. The revealed crust-mantle carbon cycle represents part of a global process for the cyclic carbon transfer from the atmosphere to the mantle and back. The scale of its manifestation is likely not so wide, and numerous small (mm and portions of millimeters) particles of exogenic substance and dispersed carbon drawn in the plate underthrust zones form a stable geochemical tail of the crustal direction in the mantle propagating in the plane of convective currents motion. The scale of this process may be indirectly suggested by the volumes of hydrocarbon and carbon dioxide de-gassing and hydrogen in the rift systems of the Earth crust. The amount of generated hydrocarbon gases with deep-seated origin cannot form large gas and oil-and-gas fields since their significant part is transferred to the atmosphere. Just some portion of compounds may be deposited in oceanic sediments and generate gas-hydrate pools. Key words: carbon cycle, subduction zones, rift zones, abiogenic hydrocarbons, crust-mantle transfer, carbon geochemistry. Introduction The evolution of oceans is closely related to the processes of conveyor enrichment of the Earth crust with many chemical elements and compounds, which are accumulated on its surface in the convergent and divergent zones. The investigations of the carbon geochemical cycle between various reservoirs traditionally describe events of its transformation in the crust, hydrosphere, and atmosphere, where living organisms play an important part [1–4]. As well, issues of carbon isotopes behavior in the geochemical cycle are detailed in [5]. Some researches [6–8] proposed an interesting and ambiguous option of the carbon geochemical cycle nature with the involvement of empirical data about possible mantle currents and carbon reservoirs that suggests significant replenishment from the core contact zones and lower mantle of the Earth due to its removal by ascending plumes in the presence of water and oxygen. These studies of the crust-mantle interaction mechanisms composing the geospheres of the rock units allowed justifying the presence of a deep-seated branch in the carbon geochemical cycle without the involvement of mechanisms for its generation in the outer core and lower mantle, and without significant amount of water and oxygen in the latter. Sedimentary units deposited on the seabed are the main source of carbon that forms its crust-mantle branch in the global cycle. These contain a large number of carbon compounds and are represented by carbonaceous sediments of biogenic and chemogenic origin, and by organic substance from pelagic and terrigenous deposits and carbonaceous shales drawn down from the continental margins. These units being drawn in the plate underthrust zones undergo some changes and submerge into the under-lithosphere mantle to almost completely break down, re-melt and mainly rise as magmas and fluid solutions. Part of carbon compounds and its monomineral fraction are encapsulated, reach the level of the under-lithosphere mantle, and are transferred by the upper-mantle convective currents in the area of relief under the rift zones of the lithospheric plates to reenter the Earth's hydrosphere as new compounds (Fig. 1). Materials and methods This paper examines a mechanism of multi-stage carbon compounds transformation and transfer of organic carbon to inorganic form and back. This allows considering the crust-mantle branch of the global carbon cycle as motion and transformation of exogenic carbon in the nature without adding a mantle component. The authors emphasize that writing of corresponding chemical reactions has been based on published theoretical and experimental data. Where such data are unavailable, discussions on the possible chemical reactions proceeding in metamophically loaded conditions shall be deemed as probable. In connection with this, the present work can to some extent be considered as a survey based on a large-volume published material and summarized by the authors, on the basis of which they made original conclusions.