Subsolidus and melting phase relationships in the system Na2CO3–CaCO3–MgCO3 have been studied at 6 GPa
and 900–1250 °C using a Kawai-type multianvil press. At 900 and 1000 °C, the system has four intermediate
compounds: Na2Ca4(CO3)5 burbankite, Na2Ca3(CO3)4, Na4Ca(CO3)3, and Na2Mg(CO3)2 eitelite. The Na-Ca
compounds dissolve noticeable amounts of Mg component, whereas eitelite dissolves a few percents of Ca
component: Na2(Ca≥0.91Mg≤0.09)4(CO3)5, Na2(Ca≥0.94Mg≤0.06)3(CO3)4, Na4(Ca≥0.67Mg≤0.33)(CO3)3, and
Na2(Mg≥.93Ca≤0.07)(CO3)2. At 1050 °C, the system is complicated by an appearance of dolomite. Na-Ca burbankite
decomposes at 1075 ± 25 °C to aragonite plus Na2Ca3(CO3)4. Na4Ca(CO3)3 and eitelite disappear via
congruent melting between 1200 and 1250 °C. Na2Ca3(CO3)4 remains stable through the whole studied temperature
range. The liquidus projection of the studied ternary system has eight primary solidification phase
regions for magnesite, dolomite, calcite-dolomite solid solutions, aragonite, Na2Ca3(CO3)4, Na4Ca(CO3)3, and
Na2CO3 solid solutions. The system has five ternary peritectic reaction points and one minimum on the liquidus
at 1050 °C and 48Na2CO3·52(Ca0.75Mg0.25)CO3. The minimum point resembles a eutectic controlled by a four-phase
reaction, by which a liquid transforms into three solid phases upon cooling: Na2(Ca0.94Mg0.06)3(CO3)4,
Na4(Ca0.67Mg0.33)(CO3)3, and Na2(Mg0.93Ca0.07)(CO3)2 eitelite. Since at 6 GPa, the system has a single eutectic,
there is no thermal barrier preventing continuous liquid fractionation from alkali-poor toward Na-rich dolomitic
compositions. Cooling of the Na-Ca-Mg carbonatite melt from 1400 to 1100 °C within the lherzolite substrate
will be accompanied by magnesite crystallization and wehrlitization keeping calcium number of the melt at 40
and shifting the Na2CO3 content to ≥40 mol%. In the case of the eclogitic wall rock, the cooling will be accompanied
by dolomite crystallization keeping calcium number of the melt at 60–65 and shifting the Na2CO3
content to ≥30 mol%.