Speciation in carbonatitic brine-melts and fluids from molecular simulations

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Carbonatites are among the most important rocks that contain economically important ore minerals. Their formation is usually assumed to arise from mantle sources. Despite intensive research, primary mantle melt compositions and ultimate carbonatite genesis are still controversial. Fluid and melt inclusions hosted in different carbonatite minerals provide unique information about fluid and melt compositions during entrapment but their interpretation is complicated due to post-entrapment modifications and insufficient knowledge of the thermodynamic phase relations of the usually complex multicomponent chemical systems. In this project, we will use molecular simulation methods to study the molecular structure and thermodynamic properties of evolved carbonatitic brine-melts and related hydrothermal fluids at relevant pressure and temperature conditions. Based on existing reactive force fields, the particle interaction potential will be parameterized for the model system H2O-CO2-Na2CO3-NaCl, which allows simulation in a composition range from high-salinity, high-density, alkali carbonatite fluids to low density, aqueous CO2-rich fluids, which are observed in associated fluid inclusions. The speciation will be derived directly from molecular dynamics simulations and/or from advanced free energy sampling methods such as thermodynamic integration. Results will be benchmarked by reference to observations and ab initio simulations. To connect to results from the ongoing project of the first funding period of the DOME program, we will add the rare earth elements (REE) La and Y to the carbonatitic system to investigate possible REE speciation with carbonate ligands. With this project, we expect to shed new light on the evolution of late-stage carbonatitic melts, during which significant enrichment of incompatible and economically important elements such as the REEs is expected.