Вестник МГТУ. 2018, том 21, № 1.

Сорохтин Н. О. и др. Коромантийная ветвь глобального цикла углерода… 62 Fig. 1. Crust-mantle carbon cycle in the ocean: 1: oceanic lithosphere; 2: continental crust; 3: undercrustal continental lithosphere; 4: transition zone of the undercrustal continental lithosphere to the oceanic-type lithosphere; 5: trend of convective currents in the upper mantle; 6: trend of the carbon compounds migration Рис. 1. Коромантийный цикл углерода в океане: 1 – океаническая литосфера; 2 – континентальная кора; 3 – подкоровая литосфера континента; 4 – зона перехода подкоровой литосферы континента к литосфере океанического типа; 5 – направление конвективных течений в верхней мантии; 6 – направление миграции соединений углерода Results and discussion Thermodynamic setting in the plate underthrust zones The oceanic crust dehydration and anatexis processes taking place in the plate underthrust zones develop in accordance with quite a complicated multi-stage scenario under the conditions of reducing environment. All transformation stages are still debatable; however, it is possible to see the common trend of these processes. The spatial-temporal variability of metamorphic transformations lies in the fact that rock assemblages of the subducting oceanic lithospheric plate undergo progressive metamorphism in the contact zone with the overthrusting continent. These rocks gradually pass transformation stages from lower to higher steps. The mineralized and gas-saturated fluid formed under these conditions rises up through cracks and becoming cooled causes retrograde contact-metasomatic alterations of surrounding rocks. Numerous ultramafic protrusions and ophiolites having passed the peak of changes also undergo retrograde processes. Alongside, sedimentary rocks of the continental margins drift to the ocean become mixed with pelagic sediments and drawn together in the plate underthrust zone. Terrigenous sediments significantly increase the inflow of carbon to the total balance and undergo progressive metamorphism, being washed by saturated hydrothermal solutions. In the course of metamorphic oceanic crust rock transformations, olivine, enstatite, magnetite, and other high-melt minerals, as well as garnets formed at the depths of eclogite transition are mainly removed from the system together with the lithospheric plate submerging to the mantle. Aqueous fluids, silica and lithophile compounds are assimilated by silicate melts generated in the plate underthrust zones and predominantly pressed out upwards. Melting of sediments and sedimentary rocks in the subduction zones mainly takes place due to the energy dissipation of viscous friction inside the strata and friction at the contact of lithospheric plates. This is added by the value of deep thermal flow permeating the continental lithospheric plates and water saturation of strata that reduces the melting temperature. This all allows suggesting that the temperature in the gap between the plates approximately corresponds to the continental plate geotherm or slightly above. Therefore, substance fallen down to the subduction zone begin to melt only at those depths, where the continental plate geotherm intersects with the sediments melting temperature (Fig. 2). The melting temperature for the majority of silicates in the presence of water and pressure increased up to 5–10 kbar abruptly falls down to 600–700 °С [9]. Water- saturated carbonates [10] and many other compounds behave similarly. The indicated laws allow concluding that aluminosilicate water-saturated sediments start melting at depths of ca. 50–70 km, while carbonate ones at a depth of ca. 80 km.