Anomalous structure of MgCO3 liquid and the buoyancy of carbonatite melts

MgCO3 is one of the most important components of mantle-derived carbonatite melts, and yet also one of the most difficult to study experimentally. Attempts to constrain its thermodynamic properties are hampered by decarbonation, which occurs at only similar to 500 degrees C, far below its metastable 1 bar melting temperature. Molecular dynamic simulations, however, can predict the thermodynamic properties of the MgCO3 liquid component in spite of experimental challenges. Using the recently developed empirical potential model for high-pressure alkaline-earth carbonate liquids (Hurt and Wolf, 2018), we simulate melts in the MgCO3-CaCO3-SrCO3-BaCO3 system from 773 to 2373 K up to 20 GPa. At 1 bar, MgCO3 liquid assumes a novel topology characterized by a 4-fold coordination of the metal cation (Mg) with both the carbonate molecule and oxygen ion; this is distinct from the other alkaline-earth carbonate liquids in which the metal cation is in similar to 6- and similar to 8-fold coordination with carbonate and oxygen. With increasing pressure, MgCO3 liquid structure becomes progressively more like that of (Ca, Sr, Ba)CO3 liquids with Mg2+ approaching 6-fold coordination with carbonate groups. The novel network topology of MgCO3 liquid results in a melt that is significantly more buoyant and compressible than other alkaline-earth carbonate liquids. Simulations of mixed MgCO3-bearing melts show that metal cation coordination with O and C is independent of bulk composition. Mixed simulation also reveal that molar volume, compressibility, enthalpy and heat capacity do not mix ideally with (Ca, Sr, Ba)CO3 liquids at 1 bar, a consequence of preferential metal-cation ordering in MgCO3-bearing mixtures. As pressure increases, however, mixing progressively approaches ideality with respect to molar volume, becoming nearly ideal by 12 GPa. The model is further applied to mantle-derived primary carbonatite melts with compositions, temperatures and pressures determined by published phase equilibrium experiments. The voluminous structure of liquid MgCO3 results in a buoyant melt that inhibits a density crossover with the surrounding mantle. Assuming FeCO3 liquid also adopts the same anomalous high-volume structure as MgCO3, we predict that even the most Fe-rich ferrocarbonatites would remain buoyant and be barred from sinking or stagnating in the mantle. (C) 2019 Elsevier B.V. All rights reserved.