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-Fe-0 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: 6MgCO(3) + 19Fe = 8FeO +10(Mg0.6Fe0.4)O + Fe7C3 + 3C. The presented results suggest that the interaction of carbonates with Fe-0 or Fe-0-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. (C) 2019, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V.