Melting of siderite to 20GPa and thermodynamic properties of FeCO3-melt

The siderite (FeCO3) melting curve is determined through multi-anvil experiments at 6-20GPa, and 1300-1870°C. The experiments define a melting curve with a Clapeyron slope steepening from 45 to 18°C/GPa but without backbend at upper mantle conditions, i.e., siderite is denser than FeCO3-melt (FeCO3L). The melting curve fits Tm=1037(44)+70.0(88)*P-1.43(37)*P2 (valid from 5 to 20GPa) where pressure is in GPa and temperature in °C. Siderite melting is not stoichiometric, minor quench magnetite was always observed and is interpreted as the result of partial redox dissociation of FeCO3L leading to dissolved Fe3+ and CO2 in the carbonate melt. At pressures below ~6.8GPa, siderite does not melt but decomposes through an auto redox dissociation reaction to magnetite, a carbon polymorph and CO2. From the experimental determination of the pure siderite melting curve, we calculate thermodynamic properties of the FeCO3L end-member, which reproduce the siderite melting curve in P-T space better than 10°C. The metastable 1atm melting temperature is calculated to 1012°C. Siderite has the lowest melting temperature of the Ca-Mg-Fe carbonates, its melting curve may cross the mantle geotherm at transition zone pressures. The stability of siderite is not only dependent on pressure and temperature but also strongly on oxygen fugacity (fO2). Model calculations in P-T-fO2 space in a Fe-C-O2 system show that the siderite or FeCO3-melt maximum stability is always reached at conditions of the CCO buffer. Our experimental and thermodynamic data constitute a cornerstone to model carbonate melting in the deep Earth, necessary to understanding the deep carbon cycle.