—The phase relations and melting of subducting sediment were studied in two series of high-pressure experiments at 750– 900 °C and 2.9 GPa and by thermodynamic modeling. In the runs we used a chemical mixture corresponding in composition to global subducting sediment (GLOSS) but enriched in water (H2O = 15.52 wt.% as compared with 7.29 wt.% in GLOSS). The first series of runs was carried out in open capsules, and the second series was performed by the same procedure and at the same P–T–t parameters but in closed (welded) capsules. The products of the runs of both series showed agreement on the P–T conditions of melting and the presence of garnet, carbonate, kyanite, SiO2 phase, and phengite in the parageneses. However, the products of all runs in open capsules contain omphacite, including magmatic one, whereas the products of the runs in welded capsules lack it, except for those obtained in the subsolidus run (at 750 °C). The results of thermodynamic modeling for the composition of the experimental mixture (H2O–GLOSS) in the closed system are in agreement with the experimental data on the hydrous solidus and the stability of most minerals, showing a decrease in the content of omphacite as melt appears. This specific effect caused by the increased pressure of aqueous fluid in the closed system is observed in the welded capsules. Thermodynamic modeling for the H2O–GLOSS composition also shows that a complete decomposition of slab carbonates under “hot” subduction conditions is possible, but this result is not confirmed by our experimental data. Since the melting and mineral growth processes in subduction zones are controlled by migrating fluids and melts, it seems correct to rely on the results of runs in open capsules.