A possible origin of the ultralow-velocity zones (ULVZs) near the core mantle boundary

Figure caption: Water released from the slab (blue) reacts with the iron-rich outer core (orange) to form FeO<sub>2</sub>H<sub>x</sub>-bearing domains (red patches) at the CMB. The inset shows the sound velocities of FeO<sub>2</sub>H<sub>x</sub> at high pressure and high temperature. The solid and dashed curves are the theoretical calculations for FeO<sub>2</sub>H and FeO<sub>2</sub> at 130 GPa.
Figure caption: Water released from the slab (blue) reacts with the iron-rich outer core (orange) to form FeO2Hx-bearing domains (red patches) at the CMB. The inset shows the sound velocities of FeO2Hx at high pressure and high temperature. The solid and dashed curves are the theoretical calculations for FeO2H and FeO2 at 130 GPa.

The core–mantle boundary (CMB) represents a region with the largest compositional and rheological contrast within the Earth. Seismological studies have revealed a number of thin domains, with variable thicknesses of 5–40 km, lying directly above the CMB, called ULVZs. However, their origin has long been debated. Recently at HPCAT, hydrogen-bearing iron peroxide (FeO2Hx) in the pyrite-type crystal structure was found to be stable under the conditions of the lowermost mantle [Hu et al, Nature 534, 241, 2016; Hu et al, PNAS, 114, 1498, 2017]. Using these observations together with the new experiments and theoretical calculations, a research team found that the iron peroxide with a varying amount of hydrogen has a high density and high Poisson ratio as well as extremely low sound velocities consistent with ULVZs, providing a possible origin of the observed ULVZs. Unlike other candidates for the composition of ULVZs, FeO2Hx synthesized from the superoxidation of iron by water would not require an extra transportation mechanism to migrate to the CMB. These dense FeO2Hx-rich domains would be expected to form directly through the reaction of about one-tenth the mass of Earth’s ocean water in subducted hydrous minerals with the effectively unlimited reservoir of iron in Earth’s core in the CMB region. Their properties would provide an explanation for the many enigmatic seismic features that are observed in ULVZs. More in J. Liu et al, Nature, 551, 494–497, 2017.