Monocrystalline type Ia diamonds with octahedral growth morphology prevail among lithospheric diamonds, including precious stones. Unlike less common ‘fibrous’ diamonds that grew from alkali-rich carbonate-bearing melts and fluids, the growth medium of ‘monocrystalline’ type Ia diamonds remains debatable. Here we report the first finding of an optically visible (~30 μm in size) carbonate inclusion in the center of a gem type Ia octahedral diamond from the Sytykanskaya kimberlite pipe, Yakutia. We found that the inclusion consists of submicron size carbonate phases represented by K2Ca(CO3)2 bütschliite (~15 vol%), Na2Mg(CO3)2 eitelite (~5 vol%), and dolomite (~80 vol%). Although neither bütschliite nor eitelite can coexist with dolomite under mantle P-T conditions, these phases readily appear all together in the quenched products of carbonatite melt under mantle pressures. Thus, at the moment of capture, the inclusion material was a carbonatite melt with the following composition 10(K0.75Na0.25)2CO3∙90(Ca0.57Mg0.43)CO3. The content of alkali carbonates at the level of 10 mol% indicates that the melt was formed at a temperature of ≥1300 °C. The high K/Na and Ca/(Ca + Mg) ratios in this melt indicate its derivation by partial melting of recycled marine sediments (pelites). Considering an age of the last subduction event beneath the Siberian craton, our new finding implies that subducting slabs drag carbonated material of the continental crust beneath ancient cratons, where it experiences partial melting to form a potassic dolomitic melt responsible for the formation of most diamonds, since the Late Archean.