Effect of Iron on the Stability of Rhodochrosite at the Topmost Lower Mantle Conditions

Carbonates are viewed as the principal oxidized carbon carriers during subduction, and thus the stability of subducted carbonates has significant implications for the deep carbon cycle. Here we investigate the high pressure-temperature behaviors of rhodochrosite in the presence of iron up to ∼34 GPa by in-situ X-ray diffraction and ex-situ Raman spectroscopy. At relatively low temperature below ∼1 500 K, MnCO₃ breaks down into MnO and CO₂. Upon heating to ∼1 800 K, however, the MnCO₃-Fe⁰ reactions occur with the formation of Mn₃O₄, FeO and reduced carbon. A ‘three-stage’ reaction mechanism is proposed to understand the kinetics of the carbon-iron-manganese redox coupling. The results suggest that Fe⁰ can serve as a reductant to greatly affect the stability of rhodochrosite, which implies that the effect of Fe-metal should be seriously considered for the high pressure-temperature behaviors of other predominant carbonates at Earth’s mantle conditions, particularly at depths greater than ∼250 km.