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

Abstract

The subsolidus and melting phase relations in the CaCO 3 -siderite system have been studied in multianvil 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 R 3 c , while CaCO 3 maintains aragonite structure up to 1600 °C and 6 GPa. The FeCO 3 solubility in aragonite does not exceed 1.0 and 3.5 mol% at 900-1000 and 1600 °C, respectively. An increase of FeCO 3 content above the solubility limit at T > 1000 °C, leads to composition-induced phase transition in CaCO 3 from aragonite, Pmcn , to calcite, R 3 c , structure, i.e., the presence of FeCO 3 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 FeCO 3 -CaCO 3 join extends from 1580 °C (FeCO 3 ) to 1670 °C (CaCO 3 ) through a liquidus minimum near 1280 ± 20 °C and 56 ± 3 mol% CaCO 3 . 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 CO 2 in (Fe,Ca)CO 3 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 CaCO 3 - MgCO 3 -FeCO 3 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.