Saturated decomposition of calcium carbonate at high pressure and implication for deep carbon cycle

The evolution of calcium carbonate at subduction zones can significantly influence the Earth's long-term climate dynamics and planetary habitability. Here we report the effects of high pressure on carbonate decomposition kinetics for the first time. The oxygen fugacity-controlled experiments on CaCO3 were carried out at 1.0-3.0 GPa and 650-1,050°C using a large volume cubic press. CaCO3 is stable at 1.0 GPa, 800°C and relatively reducing conditions, but it decomposes to produce graphite with increasing pressure to 2.5-3.0 GPa at high temperature. The graphite formation via carbonate decomposition was initially controlled by instantaneous nucleation with diffusion-controlled growth according to the classical Avrami model. The nano-sized graphite encapsulated residual samples and then precluded further decomposition of calcium carbonate. This result suggests the unexpected evolution of the graphitization and saturated decomposition of calcium carbonate at 2.5-3.0 GPa under relatively reducing conditions relevant to the Earth's upper mantle.