Phase relations in the system FeCO₃-CaCO₃ at 6 GPa and 900–1700 °C and its relation to the system CaCO₃-FeCO₃-MgCO₃

The subsolidus and melting phase relations in the CaCO₃-siderite system have been studied in multi-anvil experiments using graphite capsules at pressure of 6 GPa and temperatures of 900–1700 °C. At low temperatures, the presence of ankerite splits the system into two partial binaries: siderite + ankerite at 900 °C and ankerite + aragonite up to 1000 °C. Extrapolated solvus curves intersect near 50 mol% just below 900 °C. At 1100 and 1200 °C, the components appear to form single-phase solid solutions with space group symmetry R3̄c, while CaCO₃ maintains aragonite structure up to 1600 °C and 6 GPa. The FeCO₃ solubility in aragonite does not exceed 1.0 and 3.5 mol% at 900–1000 and 1600 °C, respectively. An increase of FeCO₃ content above the solubility limit at T > 1000 °C, leads to composition-induced phase transition in CaCO₃ from aragonite, Pmcn, to calcite, R3̄c, structure, i.e., the presence of FeCO₃ widens the calcite stability field down to the P-T conditions of sub-cratonic mantle. The siderite-CaCO₃ diagram resembles a minimum type of solid solutions. The melting loop for the FeCO₃-CaCO₃ join extends from 1580 °C (FeCO₃) to 1670 °C (CaCO₃) through a liquidus minimum near 1280 ± 20 °C and 56 ± 3 mol% CaCO₃. At X(Ca) = 0–30 mol%, 6 GPa and 1500–1700 °C, siderite melts and dissolves incongruently according to the reaction: siderite = liquid + fluid. The apparent temperature and X(Ca) range of siderite incongruent dissolution would be determined by the solubility of molecular CO₂ in (Fe,Ca)CO₃ melt.

The compositions of carbonate crystals and melts from the experiments in the low-alkali carbonated eclogite (Hammouda 2003; Yaxley and Brey 2004) and peridotite (Dasgupta and Hirschmann 2007; Brey et al. 2008) systems are broadly consistent with the topology of the melting loop in the CaCO₃-MgCO₃-FeCO₃ system at 6 GPa pressure: a Ca-rich dolomite-ankerite melt coexists with Mg-Fe-calcite in eclogites at CaO/MgO > 1 and Mg-dolomite melt coexists with magnesite in peridotites at CaO/MgO < 1. However, in fact, the compositions of near solidus peridotite-derived melts and carbonates are more magnesian than predicted from the (Ca,Mg,Fe)CO₃ phase relations.