—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.