The phase relations in the K2CO3–FeCO3 system were studied in multianvil experiments using graphite capsules at 6 GPa and 900–1400°C. Subsolidus assemblages comprise the stability fields of K2CO3 + K2Fe(CO3)2 and K2Fe(CO3)2 + siderite with the transition boundary at X(K2CO3) = 50 mol%. The K2CO3–K2Fe(CO3)2 and K2Fe(CO3)2–FeCO3 eutectics are established at 1100°C and 65 mol% and at ~1150°C and 46 mol% K2CO3, respectively. Siderite is a subliquidus phase at 1400°C at X(K2CO3) < 24 mol%. Similar phase relations were established in the K2CO3–MgCO3 system, which has two eutectics at 1200°C and 74 mol% and at ~1250°C and 48 mol% K2CO3, respectively. The natural siderite used in the present study contained 6 mol% MnCO3 and 7 mol% MgCO3. Although the obtained Fe-bearing carbonate phases exhibit uniform Mn/(Fe + Mn + Mg) ratio, magnesium tends to redistribute into the solid phases K2Fe(CO3)2 or siderite. At 1200°C and X(K2CO3) = 50 mol%, the K2Fe0.88Mn0.06Mg0.06(CO3)2 melt coexists with the K2Fe0.78Mn0.06Mg0.16(CO3)2 compound. Assuming continuous solid solution between K2Fe(CO3)2 and K2Mg(CO3)2, the K2Fe(CO3)2 end-member should melt congruently slightly below 1200°C, which is about 50° lower than the melting point of K2Mg(CO3)2.
The siderite–magnesite system was studied at 6 GPa and 900–1700°C. Complete solid solution is recorded between Fe0.94Mn0.06CO3 siderite and magnesite. At X(MgCO3) = 7 mol% and 1600°C, the (Fe0.90Mn0.06Mg0.04)CO3 partial melt coexists with (Fe0.86Mn0.06Mg0.08)CO3 siderite, whereas at X(MgCO3) = 26 and 35 mol%, the (Fe0.71Mn0.06Mg0.23)CO3 partial melt coexists with (Fe0.51Mn0.06Mg0.43)CO3 siderite. Based on these data, Fe0.94Mn0.06CO3 siderite should melt slightly below 1600°C, i.e. 300° lower than magnesite. Development of bubbles in the quenched melt at X(MgCO3) = 7 mol% and 1700 °C suggests incongruent melting of siderite according to the reaction: siderite = liquid + CO2 fluid.