Tetracarbonate melts and the fate of primordial carbon in the deep Earth

Abstract Much of Earth’s carbon is thought to have been stripped away from the silicate mantle by dense metallic-iron to form the core1. However, recent studies2,3 suggest that a considerable part of it could have remained stranded in the deep mantle due to a change in its affinity to dissolve into iron metal-alloys at the extreme pressures and temperatures of the deep Earth. The underlying physical phenomena that would render carbon less siderophile at extreme conditions remain elusive. Here we describe the compaction mechanisms and structural evolution of a simple carbonate glass to deep mantle pressures by monitoring the evolution of the electronic state and atomic structure of the glass upon compression. Our new experiments demonstrate a pressure-induced change in hybridization of carbon from sp2 to sp3 starting at 40 GPa, due to the conversion of [3]CO32- groups into [4]CO44- units, which is completed at ~112 GPa. The pressure-induced increase of carbon coordination number from three to four increases possibilities for carbon-oxygen interactions with lower mantle silicates and increased compatibility4,5. Tetracarbonate melts provide a mechanism for changing the presumed siderophile nature of deep carbon and instead imply storage of carbon in the deep mantle as a possible source for carbon-rich emissions registered at the surface in intra-plate and near-ridge hot spots6,7