Water-induced Diamond Formation at the Earth's Core-Mantle Boundary

A recent seismic study reported thin layered structures with unusually high shear wave velocities at the core-mantle boundary (CMB) beneath Central America and Caribbean (Yu et al., in review). The ultra-high velocity zones (UHVZs) are characterized by a shear velocity increase up to 18% relative to PREM. Among a few possibilities, a diamond-bearing structure can account for such high Vs of UHVZs because of the exceptionally higher Vs of diamond (~13 km/s at the CMB) than the main minerals of the mantle. Previous experiments showed that paraffin reacts with metallic iron and form FeH_x and diamond at 58-66 GPa and 3220-3710 K, suggesting that hydrogen can limit the carbon solubility in metallic iron (Hirose et al., 2019). However, whether diamond can form directly from the iron-carbon alloy at the CMB conditions has never been studied . We report experimental results of Fe₃C + H₂O reaction in the laser-heated diamond anvil cells (LHDACs) combined with the in-situ X-ray diffraction at 120 GPa and 3000-4000 K. We observed diffraction lines of diamond, together with pyrite-type FeOOH and fcc-FeH_x. The X-ray diffraction analysis suggests the reaction of Fe₃C + yH₂O -> y/2FeOOH + (3-y/2)FeH_x + C. The newly discovered UHVZs may be explained by a diamond-bearing structure that forms and accumulates in some regions at the CMB where water is supplied by subducted materials. Furthermore, this reaction could be a mechanism for carbon loss from the outer core into the mantle over a significant part of Earth's history (e.g., due to deep subduction).