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

Abstract

The phase relations in the K 2 CO 3 –FeCO 3 system were studied in multianvil experiments using graphite capsules at 6 GPa and 900–1400°C. Subsolidus assemblages comprise the stability fields of K 2 CO 3 + K 2 Fe(CO 3 ) 2 and K 2 Fe(CO 3 ) 2 + siderite with the transition boundary at X (K 2 CO 3 ) = 50 mol%. The K 2 CO 3 –K 2 Fe(CO 3 ) 2 and K 2 Fe(CO 3 ) 2 –FeCO 3 eutectics are established at 1100°C and 65 mol% and at ~1150°C and 46 mol% K 2 CO 3 , respectively. Siderite is a subliquidus phase at 1400°C at X (K 2 CO 3 ) 2 CO 3 –MgCO 3 system, which has two eutectics at 1200°C and 74 mol% and at 1250°C and 48 mol% K 2 CO 3 , respectively. The natural siderite used in the present study contained 6 mol% MnCO 3 and 7 mol% MgCO 3 . Although the obtained Fe-bearing carbonate phases exhibit uniform Mn/(Fe + Mn + Mg) ratio, magnesium tends to redistribute into the solid phases K 2 Fe(CO 3 ) 2 or siderite. At 1200°C and X (K 2 CO 3 ) = 50 mol%, the K 2 Fe 0.88 Mn 0.06 Mg 0.06 (CO 3 ) 2 melt coexists with the K 2 Fe 0.78 Mn 0.06 Mg 0.16 (CO 3 ) 2 compound. Assuming continuous solid solution between K 2 Fe(CO 3 ) 2 and K 2 Mg(CO 3 ) 2 , the K 2 Fe(CO 3 ) 2 end-member should melt congruently slightly below 1200°C, which is about 50° lower than the melting point of K 2 Mg(CO 3 ) 2 . The siderite–magnesite system was studied at 6 GPa and 900–1700°C. Complete solid solution is recorded between Fe 0.94 Mn 0.06 CO 3 siderite and magnesite. At X (MgCO 3 ) = 7 mol% and 1600°C, the (Fe 0.90 Mn 0.06 Mg 0.04 )CO 3 partial melt coexists with (Fe 0.86 Mn 0.06 Mg 0.08 )CO 3 siderite, whereas at X (MgCO 3 ) = 26 and 35 mol%, the (Fe 0.71 Mn 0.06 Mg 0.23 )CO 3 partial melt coexists with (Fe 0.51 Mn 0.06 Mg 0.43 )CO 3 siderite. Based on these data, Fe 0.94 Mn 0.06 CO 3 siderite should melt slightly below 1600°C, i.e . 300° lower than magnesite. Development of bubbles in the quenched melt at X (MgCO 3 ) = 7 mol% and 1700°C suggests incongruent melting of siderite according to the reaction: siderite = liquid + CO 2 fluid.