Study identifies superionic iron hydride as key to ultra-low velocity zone at the boundary between Earth’s core and mantle

The core-mantle boundary (CMB) is a key boundary within the Earth, marking the boundary between the outer core and the lower mantle. Over the past two decades, seismological studies have identified anomalously low-velocity regions above the CMB, such as the Large-Scale Low Shear Velocity Provinces (LLSVPs) beneath Africa and the Pacific Ocean. Within these regions, smaller ultra-low-velocity zones (ULVZs) have been detected that are characterized by significantly lower seismic velocities and higher density compared to the surrounding mantle.

Through first-principles calculations and machine learning techniques, the team found that these mysterious regions may be formed by superionic iron hydride. The study was published in the Proceedings of the National Academy of Sciences of the United States of America.

ULVZs have traditionally been attributed to temperature anomalies or compositional differences, and these low-velocity zones have generally been attributed to partial melting due to high temperatures, especially within the LLSVP. However, the discovery of ULVZs in high-velocity regions, such as near subducting slabs, has overturned this view, suggesting that their existence cannot be fully explained by temperature anomalies alone, and that compositional differences must also be taken into account.

The USTC team focused on iron hydride (FeHx), which could be produced by the reaction of the iron-rich outer core with water carried by the subducting plate. Simulations show that under the extreme conditions of the CMB, FeHx can exist in a superionic state, where the iron atoms remain locked in the lattice and the hydrogen atoms diffuse like a fluid. In this state of FeHx, it has extremely low seismic velocities (34% slower P-wave velocities and 63% slower S-wave velocities) and is 50% denser than the surrounding mantle, consistent with the seismic properties of the ULVZ.

This study suggests that superionic FeHx, formed by reaction of slab-derived water with the core, may be an important driver of ULVZ formation, highlighting the important role of water in CMB seismic anomalies.

The research was led by Professor Wang Wenzhong of the University of Science and Technology of China (USTC).