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Subduction carries atmospheric and crustal carbon hosted in the altered oceanic crystalline basement and in pelagic sediments back into the mantle. Reactions involving complex carbonate solid solutions(s) lead to the transfer of carbon into the mantle, where it may be stored as graphite/diamond, in fluids or melts, or in carbonates. To constrain the thermodynamics and thus reactions of the ternary Ca-Mg-Fe carbonate solid solution, piston cylinder experiments have been performed in the system CaCO 3 -MgCO 3 -FeCO 3 at a pressure of 3.5 GPa and temperatures of 900-1,100°C. At 900°C, the system has two miscibility gaps: the solvus dolomite-calcite, which closes at X MgCO3 *0.7, and the solvus dolomite-magnesite, which ranges from the Mg to the Fe side of the ternary. With increasing temperature, the two miscibility gaps become narrower until complete solid solutions between CaCO 3 -Ca 0.5 Mg 0.5 CO 3 is reached at 1,100°C and between CaCO 3 -FeCO 3 at 1,000°C. The solvi are characterized by strong compositional asymmetry and by an order-disorder mechanism. To deal with these features, a solid solution model based on the van Laar macroscopic formalism has been calculated for ternary carbonates. This thermodynamic solid solution model is able to reproduce the experimentally constrained phase relations in the system CaCO 3 -MgCO 3 -FeCO 3 in a broad P-T range. To test our model, calculated phase equilibria were compared with experiments performed in carbonated mafic protolithes, demonstrating the reliability of our solid solution model at pressures up to 6 GPa in complex systems.

To constrain the thermodynamics and melting relations of the siderite-magnesite (FeCO 3-MgCO 3) system, 27 piston cylinder experiments were conducted at 3.5 GPa and 1170-1575 °C. Fe-rich compositions were also investigated with 13 multi anvil experiments at 10, 13.6 and 20 GPa, 1500-1890 °C. At 3.5 GPa, the solid solution siderite-magnesite coexisting with melt over a compositional range of X Mg (=Mg/(Mg+Fe tot)) = 0.38-1.0, while at ≥10 GPa solid solution appears to be complete. At 3.5 GPa the system is pseudo-binary because of the limited stability of siderite or liquid FeCO 3 , Fe-rich carbonates decomposing at subsolidus conditions to magnetite-magnesioferrite solid solution, graphite and CO 2. Similar reactions also occur with liquid FeCO 3 resulting in melt species with ferric iron components, but the decomposition of the liquid decreases in importance with pressure. 2 At 3.5 GPa the metastable melting temperature of pure siderite is located at 1264 °C whereas pure magnesite melts at 1629 °C. The melting loop is non-ideal on the Fe-side where the dissociation reaction resulting in Fe 3+ in the melt depresses melting temperatures and causes a minimum. Over the pressure range of 3.5-20 GPa, this minimum is 20-35 °C lower than the (metastable) siderite melting temperature. By merging all present and previous experimental data, standard state (298.15 K, 1 bar) thermoydynamic properties of the magnesite melt (MgCO 3 L) end-member are calculated and the properties of (Fe,Mg)CO 3-melt fit by a regular solution model with an interaction parameter of-7600 J/mol. The solution model reproduces the asymmetric melting loop and predicts the thermal minimum at 1240 °C near the siderite side at X Mg =0.2 (3.5 GPa). The solution model is applicable to pressures reaching to the bottom of the upper mantle and allows calculation of phase relations in the FeO-MgO-O 2-C system.

Dolomite occurs in a wide range of rock compositions, from peridotites to mafic eclogites and metasediments, up to mantle depths of more than 200 km. At low-temperatures dolomite is ordered (R3), but transforms with increasing temperature into a disordered higher symmetry structure (R3c).To understand the thermodynamics of dolomite, we have investigated temperature, pressure, kinetics, and compositional dependence of the disordering process in Fe-bearing dolomites. To avoid quench effects, in situ X-ray powder diffraction experiments were performed at 300-1350 K and 2.6-4.2 GPa. The long-range order parameter s, quantifying the degree of ordering, has been determined using structural parameters from Rietveld refinement and the normalized peak area variation of superstructure Bragg peaks characterizing structural ordering/disordering. Time-series experiments show that disordering occurs in 20-30 min at 858 K and in a few minutes at temperatures ≥999 K. The order parameter decreases with increasing temperature and X Fe . Complete disorder is attained in dolomite at ∼1240 K, 100-220 K lower than previously thought, and in an ankeritic-dolomite s.s. with an X Fe of 0.43 at temperatures as low as ∼900 K. The temperature-composition dependence of the disorder process was fitted with a phenomenological approach intermediate between the Landau theory and the Bragg-Williams model and predicts complete disorder in pure ankerite to occur already at ∼470 K.The relatively low-temperature experiments of this study also constrain the breakdown of dolomite to aragonite+Fe-bearing magnesite at 4.2 GPa to temperature lower than ∼800 K favoring an almost straight Clapeyron-slope for this disputed reaction.

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