Properties of molten CaCO3 at high pressure

International audienceWe report here the structure of molten CaCO 3 studied by in situ X-ray diffrac-tion using a Paris-Edinburgh press up to 8.7 GPa. Variations are observed in the medium range order with shrinkage of the intermolecular contributions, reflecting higher packing efficiency of the carbonate molecules. Density of the melt is obtained from the radial distribution functions, assuming a constant coordination number of 3 for the CO contribution. Bulk modulus values increase by a factor of 2 over the experimental pressure-temperature range, reaching a value similar to that of underlying crystalline phases at the highest pressure investigated. Those are the first direct density measurements of compressed CaCO 3 melt; they agree well with recent ab initio predictions (Li et al., 2017), which implies that the reported flattening of the melting curve followed by a slightly negative curve above 8 GPa cannot be due to crystal-melt density inversion. Instead, the enthalpy of fusion is likely responsible, and we note the peculiar high diffusivity of oxygen in crystalline calcite-V in this regard, with further implications for the mobility of carbonate melts at depth