The Crust–Mantle Carbon Cycle and Origin of Abiogenic Hydrocarbons

—The paper studies carbon transformation and transfer processes in the crust and mantle. Sediments dragged into subduction zones are dewatered, broken down, and altered by metamorphic processes. Some carbon compounds are immersed in the sublithospheric mantle and transported by convective currents into the rift zones of mid-ocean ridges. There they are transformed again, forming new chemical compounds, and are carried out by hydrothermal fluids to the surface in the form of carbonates, various hydrocarbons, and carbon dioxide dissolved in the fluid. Precipitated from solutions, they are deposited on the seafloor as sediment, forming carbonate and carbon-containing structural–material complexes. As a result of the multistage physicochemical transformation mechanism in the crust–mantle regions of the Earth, carbon compounds acquire features of abiogenic (mantle) origin, although they are initially exogenic formations. The crust–mantle carbon cycle is part of the global cyclic transfer of carbon from the atmosphere to the mantle and vice versa.