The structure of carbonate melts and implications for the petrogenesis of carbonatite magmas

The structure and properties of carbonate melts are fundamental to their physical and chemical behaviour in the evolution of carbonatites and in the upper mantle. The structure of carbonate melts was investigated by vibrational spectroscopy of carbonate glasses in the systems La(OH)₃-Ca(OH)₂-CaCO₃-CaF₂-BaSO₄ and MgCO₃-K2CO₃ and by computer simulation. The vibrational spectroscopy of glasses and molecular dynamics simulations (MDS) of molten CaCO₃ both suggest the presence of molecular metal-carbonate complexes in the melt, MDS additionally suggesting octahedral Ca²⁺ sites up to 11.5GPa. The decarbonation of carbonate melts was studied using quantum mechanical simulations of calcite and magnesite and suggests the dissociation of CO₃²⁻ by electron delocalisation related to the field strength of the coordinated metal cation. The physical properties of carbonate melts were investigated through molecular dynamics simulations of molten CACO₃ and allowed a volumetric equation of state and expressions for expansivity and compressibility to be derived to high pressure. Estimates for the constant pressure heat capacity of 1.93 KJ⁻¹ g⁻¹ and cation diffusivities of 3.98×10⁻⁹ m² s⁻¹ were also derived. A two liquid field was discovered in the system La(OH)₃-Ca(OH)₂-CaCO₃-CaF₂-BaSO₄ at 1kbar between two conjugate carbonate melts preserved as glasses in run products and represents the first texturally unequivocal evidence for such phenomenon. A model is proposed both for the mechanisms of immiscibility based on complexed and uncomplexed conjugate melts and for the evolution of late-stage REE-carbonatites. The Ca-rich carbonate glass is a previously unreported glass forming composition. The physical behaviour of carbonate melts in the mantle was also reinvestigated and suggests ascent rates of 0.2-0.7 m s⁻¹ in magma driven cracks. Ascent rates for compaction driven porous flow of 3.6×10⁻¹⁰ m s⁻¹ suggest carbonate melts may only separate from their source regions at low lithospheric thinning rates