The Mg-carbonate-Fe interaction:Implication for the fate of subducted carbonates and formation of diamond in the lower mantle

The fate of subducted carbonates in the lower mantle and at the core-mantle boundary was modelled via experiments in the MgCO₃-Fe⁰ system at 70-150 GPa and 800-2600 K in a laser-heated diamond anvil cell. Using in situ synchrotron X-ray diffraction and ex situ transmission electron microscopy we show that the reduction of Mg-carbonate can be exemplified by:6MgCO₃ + 19Fe=8FeO +10(Mg_0.6Fe_0.4)O + Fe₇C₃ + 3C. The presented results suggest that the interaction of carbonates with Fe⁰ or Fe⁰-bearing rocks can produce Fe-carbide and diamond, which can accumulate in the D"region, depending on its carbon to Fe ratio. Due to the sluggish kinetics of the transformation, diamond can remain metastable at the core-mantle boundary (CMB) unless it is in a direct contact with Fe-metal. In addition, it can be remobilized by redox melting accompanying the generation of mantle plumes.