Phase relations in the K2CO3-FeCO3 and MgCO3-FeCO3 systems at 6 GPa and 900-1700°C

The phase relations in the K2CO3 FeCO3 system were studied in multianvil experiments using graphite capsules at 6 GPa and 900-1400 degrees 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 degrees C and 65 mol% and at similar to 1150 degrees C and 46 mol% K2CO3, respectively. Siderite is a subliquidus phase at 1400 degrees C at X(K2CO3) <24 mol%. Similar phase relations were established in the K2CO3-MgCO3 system, which has two eutectics at 1200 degrees C and 74 mol% and at similar to 1250 degrees 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 unifoini Mn/(Fe + Mn + Mg) ratio, magnesium tends to redistribute into the solid phases K2Fe(CO3)(2) or siderite. At 1200 degrees 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 degrees C, which is about 50 degrees lower than the melting point of K2Mg(CO3)(2). The siderite-magnesite system was studied at 6 GPa and 900-1700 degrees C. Complete solid solution is recorded between Fe0.94Mn0.06CO3 siderite and magnesite. At X(MgCO3) = 7 mol% and 1600 degrees 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 degrees C, i.e. 300 degrees lower than magnesite. Development of bubbles in the quenched melt at X(MgCO3) = 7 mol% and 1700 degrees C suggests incongruent melting of siderite according to the reaction: siderite = liquid + CO2 fluid.