Deep within Earth's interior, at the boundary between the planet's mantle and core, scientists have discovered a manganese oxide compound that may finally explain one of geology's lingering mysteries: why seismic waves mysteriously slow down in certain ultra-deep zones.
The compound, Mn4O, was identified by researchers using computational models and swarm intelligence algorithms to predict stable manganese oxides under the crushing pressures of Earth's interior. The discovery, published in Physical Review B, reveals that this manganese-rich compound remains stable at pressures up to 150 GPa—conditions found in Earth's deepest zones, particularly near the core-mantle boundary where temperatures soar above 4,000 kelvin.
This breakthrough matters because Earth's deep interior remains one of the least understood frontiers in planetary science. While the mantle is predominantly oxygen, magnesium, silicon, and iron, manganese and its oxides play crucial roles in the planet's geochemical cycles. The four known manganese oxide compounds—MnO, Mn3O4, Mn2O3, and MnO2—have been studied for decades, but researchers suspected something was missing. Mn4O appears to be that missing piece.
The newly discovered compound has a remarkable property: its low sound velocity, combined with that of another high-pressure phase of Mn3O4 (stable above 72 GPa), explains why seismic waves travel more slowly in ultralow velocity zones, or ULVZs. These mysterious regions, scattered throughout Earth's lower mantle, have puzzled seismologists for years. "The density of Mn4O allowed for it to float above the iron there and the low sound velocity of both Mn4O and Mn3O4 can explain why seismic waves travel more slowly in certain zones," the research team noted. Because manganese is relatively rare in the mantle, these effects are likely regional rather than global.
The implications extend far into Earth's past. Manganese's variable oxidation states make it a powerful, pressure-sensitive agent in deep-Earth chemistry, actively participating in geochemical cycling by reacting with and oxidizing subducted iron-bearing minerals. The team suggests that Mn4O may have played a critical role in one of Earth's most dramatic moments: the Great Oxidation Event around 2.4 billion years ago, when atmospheric oxygen levels surged and manganese ore deposits suddenly accumulated.
According to the researchers' model, Mn4O forms from redox reactions involving subducted, oxidized materials near the core-mantle boundary. As Earth's internal plumes carry this material toward the surface, Mn4O reacts with oxygen to form common manganese ores. "During this process, Mn4O is carried by the plume to the surface, reacting with O2 to form common manganese ores. This mechanism provides a plausible explanation for the rapid and extensive precipitation of manganese ores during the GOE," the study authors explained.
The research represents a significant theoretical advance, though the team emphasizes that experimental confirmation in laboratories remains essential. For now, their computational predictions offer a tantalizing window into how Earth's deepest chemistry may influence the planet's surface and its ancient history. Understanding manganese's journey from the core-mantle boundary to the surface—and its role in shaping Earth's atmosphere billions of years ago—illuminates how the planet's interior and exterior are profoundly connected.