Phase relations in the K2CO3-MgCO3 system have been studied in high-pressure high-temperature (HPHT) multi-anvil experiments using graphite capsules at 6.0 ± 0.5 GPa pressures and 900–1450 °C temperatures. Subsolidus assemblies comprise the fields K2CO3+K2Mg(CO3)2 and K2Mg(CO3)2+MgCO3 with the transition boundary near 50 mol% MgCO3 in the system. The K2CO3-K2Mg(CO3)2 eutectic is established at 1200 °C and 25 mol% MgCO3. Melting of K2CO3 occurs between 1400 and 1450 °C. We propose that K2Mg(CO3)2 disappears between 1200 and 1300 °C via congruent melting. Magnesite is observed as a subliquidus phase to temperatures in excess of 1300 °C.

At 6 GPa, melting of the K2Mg(CO3)2+MgCO3 assemblage can be initiated either by heating to 1300 °C under “dry” conditions or by adding a certain amount of water at 900–1000 °C. Thus, the K2Mg(CO3)2 could control the solidus temperature of the carbonated mantle under “dry” conditions and cause formation of the K- and Mg-rich carbonatite melts similar to those found as microinclusions in “fibrous” diamonds.

The K2Mg(CO3)2 compound was studied using in situ X-ray coupled with a DIA-type multi-anvil apparatus. At 6.5 GPa and 1000 °C, the structure of K2Mg(CO3)2 was found to be orthorhombic with lattice parameters a = 8.8898(7), b = 7.8673(7), and c = 5.0528(5), V = 353.39(4). No structure change was observed during pressure decrease down to 1 GPa. However, recovered K2Mg(CO3)2 exhibited a trigonal Rm structure previously established at ambient conditions.

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