Carbonatites are rare igneous rocks formed predominantly of carbonates, which have unambiguous origins in the mantle and the limit to their depth is not known. Their presence in the lower mantle needs to be considered since they are direct and important participants in the mobility and long-term storage of deep carbon in the Earth. The lower polymerization found in carbonate melts is explained by the electronic structure of the CO₃^2- groups, which unlike the SiO₄^4- tetrahedra, do not have unpaired orbitals available for covalent bonding, and hence are unable to polymerize. Recently, the discovery of carbonate’s high-pressure structures characterized by the presence of CO₄^4- units, has motivated us to investigate the carbonate glass structure at pressures relevant for the whole Earth’s mantle, to verify if a similar behaviour could be expected in C-rich melts. Indeed as an example, changes in the density and viscosity in silicate melts occur in correspondence to phase changes in the equivalent crystalline phase due to increases in coordination.

At present, the use of glasses as analogues for melts provides the only experimental means of studying carbonate melt structures, due to technical difficulties to preserve melting in diamond anvil cell for long periods of time. Here we investigate the behaviour of a simple C-rich glass with composition K₂Mg(CO₃)₂ by means of X-Ray Diffraction and X-Ray Raman Spectroscopy up to 115 GPa and ambient temperature. The combination of these two techniques allowed us to probe the long-range structure of the glass and the local structure around the oxygen atoms.

Our results show a continue evolution of the glass structure upon compression. Our data evidence a rapid structural compression from 1 bar to ~33 GPa, i.e. filling of the voids, and change in the intermediate-range order. At higher pressures a dramatic variation in the local environment around the carbon atoms reflects a different structural rearrangement. The average distances between C-O (its first neighbour) becomes larger in the pressure interval between ~46 to 85 GPa, to then decrease again until the last pressure point investigated at 104 GPa. Also the local environment around the oxygen atoms reflect this structural transition, where the π* resonance in X-Ray Raman spectra becomes progressively less intense and disappears above ~112 GPa. This transition is accompanied by the appearance of a new oscillation in higher energy part of the X-Ray Raman spectra. These variations reflect a change in the electrons hybridization around the oxygen atoms likely caused by the change in polymerization between C and O, as observed in crystalline carbonates above ~70 GPa. Fully sp³ polymerized carbonate melts at P > 100 GPa could imply the presence of viscous and stagnant primitive C-rich fluids at the bottom of the Earth’s lower mantle.