—Experimental modeling of decarbonation reactions with the formation of Mg,Fe-garnets and CO2 fluid during mantle–crust interactions was carried out in a wide range of the upper-mantle pressures and temperatures. Experimental studies were performed in the MgCO3–Al2O3–SiO2 and (Mg,Fe)CO3–Al2O3–SiO2 systems in the pressure range 3.0–7.5 GPa and temperature range 950–1450 °C (t = 10– 60 h), using a multianvil high-pressure apparatus of the “split-sphere” type (BARS). Experiments were carried out with a specially designed high-pressure buffered cell with a hematite container that prevents the diffusion of hydrogen into a Pt-capsule with a sample. It has been experimentally established that in the MgCO3–Al2O3–SiO2 system decarbonation occurs by the schematic reaction MgCO3 + SiO2 + Al2O3 → Mg3Al2Si3O12 + CO2 at 1100 ± 20 °C (3.0 GPa), 1150 ± 20 °C (6.3 GPa), and 1400 ± 20 °C (7.5 GPa) and in the (Mg,Fe)CO3–Al2O3– SiO2 system, by the reaction (Mg,Fe)CO3 + SiO2 + Al2O3 → (Mg,Fe)3Al2Si3O12 + CO2 at 1000 ± 20 °C (3.0 GPa), 1150 ± 20 °C (6.3 GPa), and 1400 ± 20 °C (7.5 GPa). Based on Raman spectroscopic characterization of the synthesized garnets, the position of the main modes R, υ2, and υ1 in the pyrope has been determined to be 364, 562, and 924–925 cm-1, respectively, and that in pyrope-almandine, 350–351, 556–558, and 918–919 cm-1. The effectiveness of the hematite container was demonstrated by means of mass spectrometry analysis. It has been found that the fluid composition corresponded to pure CO2 in all experiments. The P,T-positions of decarbonation curves leading to the formation of a CO2 fluid in assemblage with pyrope and pyrope-almandine have been experimentally reconstructed and compared with the previous calculation and experimental data. It has been established that the experimentally reproduced reaction lines with the formation of pyrope + CO2 or pyrope-almandine + CO2 assemblages are shifted to lower temperatures by 50–150 °C relative to the calculated ones. When considering the obtained results with regard to the stability of natural carbonates of various compositions in subduction settings, it has been found that at depths of ~90–190 km Mg,Fe-carbonates react with oxides in the temperature range 1000–1250 °C, and at depths of ~225 km, at 1400 °C.

An experimental study aimed at the modeling of dolomite- and ankerite-involving decarbonation reactions, resulting in the CO2 fluid release and crystallization of Ca, Mg, Fe garnets, was carried out at a wide range of pressures and temperatures of the upper mantle. Experiments were performed using a multi-anvil high-pressure apparatus of a “split-sphere” type, in CaMg(CO3)2-Al2O3-SiO2 and Ca(Mg,Fe)(CO3)2-Al2O3-SiO2 systems (pressures of 3.0, 6.3 and 7.5 GPa, temperature range of 950–1550 °C, hematite buffered high-pressure cell). It was experimentally shown that decarbonation in the dolomite-bearing system occurred at 1100 ± 20 °C (3.0 GPa), 1320 ± 20 °C (6.3 GPa), and 1450 ± 20 °C (7.5 GPa). As demonstrated by mass spectrometry, the fluid composition was pure CO2. Composition of synthesized garnet was Prp83Grs17, with main Raman spectroscopic modes at 368–369, 559–562, and 912–920 cm−1. Decarbonation reactions in the ankerite-bearing system were realized at 1000 ± 20 °C (3.0 GPa), 1250 ± 20 °C (6.3 GPa), and 1400 ± 20 °C (7.5 GPa). As a result, the garnet of Grs25Alm40Prp35 composition with main Raman peaks at 349–350, 552, and 906–907 cm−1 was crystallized. It has been experimentally shown that, in the Earth’s mantle, dolomite and ankerite enter decarbonation reactions to form Ca, Mg, Fe garnet + CO2 assemblage at temperatures ~175–500 °C lower than CaCO3 does at constant pressures.

—Modeling the compositions, properties, and conditions of generation of natural agents of mantle metasomatism is one of the most topical subjects in experimental petrology. Particular attention is paid to the study of C- and S-bearing metasomatic agents and their role in the global carbon and sulfur cycles and in the processes of natural diamond formation. Experimental studies aimed at the estimation of sulfur solubility in carbonate melts under lithospheric mantle conditions were carried out on a multianvil high-pressure apparatus of the “split-sphere” type (BARS) in the carbonate–sulfur ((Mg,Ca)CO3–S) and carbonate–pyrite ((Mg,Ca)CO3–FeS2) systems at 6.3 GPa and 1050–1550 °C for 20–60 h. It has been experimentally established that the main processes occurring in the carbonate–sulfur system are the recrystallization of carbonate in a sulfur melt (1050–1350 °C) and the generation of a high-calcium carbonate melt with dissolved sulfur (5.0–6.5 wt.%) (1450–1550 °C) as well as graphite recrystallization and the initial stage of diamond growth (1550 °C) in this melt. The work demonstrates that the carbonate–pyrite interaction is accompanied by the recrystallization of carbonates and pyrite (1050–1250 °C) and the generation of two immiscible melts (sulfide one with dissolved oxygen and carbonate one with dissolved sulfur (1.7–2.5 wt.%) (1350–1550 °C)) along with the formation of graphite and the growth of diamond on seed crystals (1550 °C). It has been found that the solubility of sulfur in carbonate melts increases with temperature, which goes together with a decrease in CaO (±FeO) concentrations and an increase in MgO concentrations. The present study has shown for the first time that melts of alkaline-earth carbonates are capable of dissolving up to 6.5 wt.% sulfur and they are probable sulfur concentrators under the conditions of the lithospheric mantle.