Crossover from melting to dissociation of CO2 under pressure: Implications for the lower mantle

A systematic investigation of the solid-solid phase transitions, melting behavior, and chemical reactivity of CO2 at pressures of 15-70GPa and temperatures up to 2500K has been carried out using in situ Raman spectroscopy in laser-heated diamond anvil cells. We find that molecular CO2 melts to a molecular fluid up to 33 (+/-2)GPa and 1720K (+/-100), where it meets a solid-solid phase line to form a triple point. At higher pressure, non-molecular CO2 phase V does not melt but instead dissociates to carbon and oxygen with a transition line having a negative PT-slope. A comparison with P-T profiles of the Earth's mantle indicates that polymeric CO2-V can be stable near the top of the lower mantle and dissociates at greater depths. Decarbonation reactions of subducted carbonates in the lower mantle would produce diamond and fluid oxygen, which in turn significantly affects redox state, increasing oxygen fugacity by several orders of magnitude. The reaction of free oxygen with lower mantle minerals such as Mg-perovskite can create significant conductivity anomalies.