Crossover from melting to dissociation of CO 2 under pressure: Implications for the lower mantle

Abstract

A systematic investigation of the solid–solid phase transitions, melting behavior, and chemical reactivity of CO 2 at pressures of 15–70 GPa and temperatures up to 2500 K has been carried out using in situ Raman spectroscopy in laser-heated diamond anvil cells. We find that molecular CO 2 melts to a molecular fluid up to 33 (± 2) GPa and 1720 K (± 100), where it meets a solid–solid phase line to form a triple point. At higher pressure, non-molecular CO 2 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 CO 2-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.