Hydrous K-rich kimberlite-like systems are studied experimentally at 5.5–7.5 GPa and 1200–1450 °C in terms of phase relations and conditions for formation and stability of phlogopite. The starting samples are phlogopite–carbonatite–phlogopite sandwiches and harzburgite–carbonatite mixtures consisting of Ol + Grt + Cpx + L (±Opx), according to the previous experimental results obtained at the same P–T parameters but in water-free systems. Carbonatite is represented by a K- and Ca-rich composition that may form at the top of a slab. In the presence of carbonatitic melt, phlogopite can partly melt in a peritectic reaction at 5.5 GPa and 1200–1350 °C, as well as at 6.3–7.0 GPa and 1200 °C: 2Phl + CaCO3 (L)↔Cpx + Ol + Grt + K2CO3 (L) + 2H2O (L). Synthesis of phlogopite at 5.5 GPa and 1200–1350 °C, with an initial mixture of H2O-bearing harzburgite and carbonatite, demonstrates experimentally that equilibrium in this reaction can be shifted from right to left. Therefore, phlogopite can equilibrate with ultrapotassic carbonate–silicate melts in a ≥ 150 °C region between 1200 and 1350 °C at 5.5 GPa. On the other hand, it can exist but cannot nucleate spontaneously and crystallize in the presence of such melts in quite a large pressure range in experiments at 6.3–7.0 GPa and 1200 °C. Thus, phlogopite can result from metasomatism of peridotite at the base of continental lithospheric mantle (CLM) by ultrapotassic carbonatite agents at depths shallower than 180–195 km, which creates a mechanism of water retaining in CLM. Kimberlite formation can begin at 5.5 GPa and 1350 °C in a phlogopite-bearing peridotite source generating a hydrous carbonate–silicate melt with 10–15 wt% SiO2, Ca# from 45 to 60, and high K enrichment. Upon further heating to 1450 °C due to the effect of a mantle plume at the CLM base, phlogopite disappears and a kimberlite-like melt forms with SiO2 to 20 wt% and Ca# = 35–40.