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 73 Defining carbon isotope shifts in the subduction zones formed by the reactions (13 and 14) from abiogenic methane (С meth ) and carbonate carbon (С carb ) may be made as follows: 13 13 13 meth carb dia , 2 С С С δ + δ δ = (47) 13 13 13 meth carb dia 2 . 3 С С С δ + ⋅ δ δ = (48) Carbon isotope shifts in diamonds, which formed by the reactions (15) and (16) with the participation of organic carbon (С org ) from the wide-range hydrocarbons C n H 2 n ± k may be determined as follows: 13 13 org carb 13 dia 4 (2 ) 6 n C n k С C n k ⋅ δ + ± ⋅ δ δ = ± . (49) In case of diamond generation by the reaction (18), the following shall be accepted as follows: δ 13 C dia ≈ δ 13 C carb . (50) A carbon isotope shift in abiogenic methane, which formed in mid-oceanic ridges, is approximately equal to –3–14 ‰ [35]. The deviation of the organic substance isotope signature is most often δ 13 C org ≈ –15 to –50 ‰, being in average –25 ‰ [36]. At the age of accumulating iron-bearing sediments, ca. 2.3 to 2 Ga, carbonate carbon demonstrated a positive isotope anomaly with a shift of up to +13 ‰ [37]. Thus, using the equation (47) at an average value of δ 13 C meth ≈ –25 ‰, it is possible to identify that δ 13 C dia ≈ –6 ‰. Such distributions of isotope shifts by the expression (48) vary from +0.3 ‰ to –6.3 ‰ [30]. The theoretical estimations of isotope ratios in diamonds generally correspond to the available experimental data. Thus, the paper [38] provides results of a layer-to-layer analysis for the variations of carbon isotope ratios in individual diamond crystals and shows that, in majority cases, a regular trend of varying carbon isotope ratios is observed from the center of crystals towards the periphery. Cores of crystals are always enriched with a light isotope, 12 С, and outer shells are weighted by the isotope 13 С. The general shift of isotope compositions reaches 4 ‰ and varies in average from –11.01 ‰ in the center of diamonds to –7.32 ‰ at the surface. The revealed changes of the carbon isotope composition in diamonds most likely indicate the initial growth of crystals due to the biogenic carbon. As the lithospheric plates sink to the subduction zone, their further growth took place with the participation of deep chemogenic carbon forming a relatively weighted shell. In light of this, we may conclude that the majority of diamond crystals form from the mixture of biogenic and abiogenic methane, and decomposition products of carbonates various in origin. In [39], an equally interesting feature of isotope distribution in diamonds of various parageneses was revealed. When studying the diamond grains from eclogite and peridotite xenolyths, the authors found that the above-described distributions of δ 13 С dia values belong only to crystals formed in the kimberlite matrix and eclogites. Diamonds of peridotite paragenesis show a relatively narrow distribution of δ 13 С dia (–2 to –8 ‰) values, being –6 ‰ on the average. This feature is apparently related with the fact that diamonds could form from exogenic carbon containing organogenic carbonates and hydrocarbons in kimberlites and eclogites. Exactly this may account for the wide scatter of δ 13 С dia variations in these rock units. By contrast, carbon could enter the diamonds of peridotite paragenesis only from chemogenic carbonates formed at the hydration stage of rocks from the former oceanic crust by the reactions like (41 and 42) and from chemogenic methane generated by the reactions (43). As it was noticed earlier, all the carbon generated in rift zones forms by two main ways. The first one suggests its transfer from the subduciton zones by convective currents in the upper mantle. The second one means hydration of mafic and ultramafic rocks of the oceanic lithosphere. During the transfer of dispersed carbon, metal carbides, and encapsulated particles of the crustal substance from the plate underthrust zones, its isotope shifts shall comply with those indicators, which formed in the parental geodynamic setting. Consequently, spectra of carbon isotope shifts typical of the subduciton zones and formed in-situ due to the hydration of the oceanic lithosphere rocks are superimposed in the rift zone. Examining main laws of carbon isotope shifts in the black smokers system of the rift zones, it may be concluded that the negative values of δ 13 C ≈ –13… –14 ‰ in methane do not correspond to the average values of the 3 HCO − and СО 2 isotope composition in the oceanic waters, δ 13 C = –5.5 ‰. This fact may be accounted for by carbon isotope fractionation at the generation of methane from carbon dioxide. Based on the Le Chatelier's principle, it may be inferred that a chemical reaction proceeding with the release of heat always evolves on the path of highest reduction of internal energy (enthalpy). Thus, predominantly atoms of light isotope, 12 С, shall participate in the methane (СН 4 ) generation reaction from carbon dioxide with