Experimental Modeling of the Mantle–Slab Interaction in the Metal–Carbonate System, Conditions of Crystallization and Indicator Characteristics of Diamond

—Experimental studies aimed at determining the conditions for the formation of diamond and graphite as a result of the redox interaction of reduced mantle rocks and oxidized rocks of the slab in a wide temperature range, including the conditions of both “cold” and “hot” subduction, were carried out on a “split-sphere” multianvil high-pressure apparatus (BARS) in the (Fe,Ni)–(Mg,Ca)CO3 system, at 6.3 GPa and 800–1550 °C for 35–105 h, using the “sandwich” assembly. We have established that the interaction of Fe,Ni metal and carbonate is due to the creation and propagation of a redox front, at rates from 1.3 (800 °C) to 118 μm/h (1550 °C). At T < 1200 °С, this interaction leads to the formation of alternating reaction zones (from the reduced center to the oxidized periphery): metal → metal + wüstite/magnesiowüstite → magnesiowüstite + graphite ± Mg,Fe,Ca carbonates → magnesite + aragonite. In this case, in the reduced part of the samples, the formation of a Ni,Fe metal phase strongly enriched in Ni (up to 65–70 wt.% vs. the initial 10 wt.%) was recorded. At higher temperatures, the formation of Fe,Ni metal–carbon (≥1200 °C) and carbonate (≥1330 °C) melts was observed. We have found that the presence of nickel precludes the formation of carbides in the reduced part of the sample and ensures stable diamond crystallization at 1400–1550 °C both in metal–carbon and carbonate melts. Our experiments demonstrate that diamonds from the metal–carbon melt are characterized by inclusions of taenite and magnesiowüstite. The morphology of these diamonds is determined by the {111} layer-by-layer grown faces, and their indicator characteristics are nitrogen–vacancy and nickel-related (884 nm) centers at 1400 °C or nickel–nitrogen centers (S3, 598 nm, 727 nm, 746 nm, etc.) at 1550 °C. For diamonds formed in the carbonate melt, the morphology is determined by the {100} and {111} (vicinal-growth) faces; carbonates are identified as inclusions; and nitrogen–vacancy centers H3, NV0, and NV– are fixed in the photoluminescence spectra. Experiments show that the indicator of the metal–carbonate interaction temperature is the degree of structural perfection of graphite, which increases in the range of 800–1550 °C.