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.