CO2 CYCLING IN THE DEEP EARTH

We have used laser-heated Diamond Anvil Cells (LDAC's) coupled with synchrotron X-ray diffraction to make in situ observations of magnesite formation from CO₂ plus simple oxide (i.e., MgO) and silicate (e.g., Mg₂SiO₄) components at mantle pressures and temperatures. Previously we demonstrated that the reaction MgO + CO₂ = MgCO₃ proceeds readily at pressures up to 40 GPa and temperatures around 1,800 K, despite the existence of relatively dense high-pressure CO₂ polymorphs. For our most recent study, we have incorporated the magnesium end-member of olivine to consider the reaction CO₂ + Mg₂SiO₄ = MgSiO₃ + MgCO₃, as studied extensively at low pressures (i.e., less than 4 GPa) by previous workers (e.g., Newton and Sharp, 1975). As expected, higher temperatures favor the forsterite side of the reaction, especially at low pressures. In this presentation, we will discuss our extension of this system to pressures greater than 20 GPa and the role played by high-pressure polymorphism in CO₂ and the magnesium silicates.