Stability of Polymeric Carbon Dioxide in the Earth's Mantle

Recent experiments reveal that a polymeric form of CO2 is stable at the pressure-temperature conditions of the Earth's mantle.  This unusual form of CO2 was discovered in 1999 [1] but until now its range of stability and relevance to planetary interiors was uncertain and controversial.  CO2 is also the major product of decarbonation reactions in carbonates subducted into the lower mantle.  Therefore, determini

Recent experiments reveal that a polymeric form of CO2 is stable at the pressure-temperature conditions of the Earth's mantle.  This unusual form of CO2 was discovered in 1999 [1] but until now its range of stability and relevance to planetary interiors was uncertain and controversial.  CO2 is also the major product of decarbonation reactions in carbonates subducted into the lower mantle.  Therefore, determining the high-pressure and temperature phase relations of CO2 is essential to understanding the nature of Earth's deep carbon.

These investigations of the solid-solid phase transitions, melting behavior, and chemical reactivity of CO2 at pressures of 15 to 70 GPa and temperatures up to 2500 K were conducted using in situ Raman spectroscopy in laser-heated diamond anvil cells.  The results [2] reveal that the polymeric is stable at the P-T conditions that correspond to the top of the lower mantle at 900 km.  At more extreme pressures and temperatures (above 33 GPa and temperatures above 1600 K) polymeric CO2 decomposes to molecular oxygen and carbon in the form of diamond.  These findings mean that decarbonation reactions of magnesite deep in Earth proceed directly to form free carbon and fluid oxygen as an intermediate product‚ rather than CO2.  This mechanism is undoubtedly important for diamond formation in the lower mantle. Subsequently, the reactions of free oxygen with lower mantle minerals, such as Mg-perovskite‚ are expected to create significant conductivity anomalies.  Subsequent theoretical and experimental studies are examining ways in which polymeric CO2 can be stabilized at lower pressures and temperatures, with implications for geological carbon sequestration.

Image:  Phase diagram of CO2 pressure-temperature stability of polymeric CO2-V the mantle adiabat.  The new results are shown in bold red lines. 

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