The study suggests that Earth’s D” layer near the core-mantle boundary formed from a magma ocean created by a massive impact. The iron-magnesium peroxide, formed from the water in this ocean, explains the unique composition and heterogeneity of the D’ layer.
New research suggests that the mysterious D” layer at the boundary of Earth’s core and mantle may have formed from the remnants of an early colossal impact, with iron-rich peroxide playing a key role in its unique and enduring properties.
Deep within the Earth is a mysterious layer called the D” layer. This zone is located roughly 3,000 kilometers below, just above the boundary between the planet’s molten outer core and its solid mantle. Unlike a perfect sphere, the D layer is surprisingly uneven. Its thickness varies greatly from place to place, with some areas even missing the D’ layer – much like the continents rise above the Earth’s oceans. These interesting variations have caught the attention of geophysicists, who describe the D layer as a heterogeneous or patchy region.
A new study led by Dr. Qingyang Hu (High Pressure Science and Technology Advanced Research Center) and Dr. Jie Deng (Princeton University) suggests that layer D may date back to Earth’s earliest days. Their theory hinges on the Giant Impact hypothesis, which proposes a MarsAn object the size of crashed into the proto-Earth and subsequently created a planet-wide magma ocean. They believe that the D layer may be a unique composition left over from this colossal impact that may hold clues to Earth’s formation.
Water in the Magma Ocean
Dr. Jie Deng emphasizes the presence of significant amounts of water in this global magma ocean. The exact origin of this water remains a matter of debate, various theories have been proposed including its formation through reactions between nebular gas and magma or direct delivery by comets. “The prevailing opinion,” continues Dr. Deng, “suggests that water would have concentrated toward the bottom of the magma ocean as it cooled. In the final stages, the magma closest to the core may have contained volumes of water comparable to Earth’s current oceans.
The extreme pressure and temperature conditions at the bottom of the magma ocean would create a unique chemical environment that would promote unexpected reactions between water and minerals. Dr. Qingyang Hu explains, “Our research suggests that this watery magma ocean promoted the formation of an iron-rich phase called iron-magnesium peroxide.” This peroxide with the formula (Fe, Mg)O2 has an even stronger preference for iron compared to other major components expected in the lower mantle. “According to our calculation, its affinity for iron could have led to the accumulation of dominant iron peroxide in layers several to tens of kilometers thick.
The formation of a heterogeneous structure at the interface of the Earth’s core and mantle. Credit: Science China Press
The presence of this iron-rich peroxide phase would change the mineral composition of the D’ layer, which deviates from our current understanding. According to the new model, the minerals in D’ would be dominated by a new composition: iron-poor silicate, iron-rich peroxide (Fe, Mg), and iron-poor oxide (Fe, Mg). This iron-dominated peroxide also has low seismic velocities and high electrical conductivity, making it a potential candidate to explain the unique geophysical properties of the D” layer. These features include ultra-low-velocity zones and high-conductivity layers, both of which contribute to the well-known compositional heterogeneity of the D’ layer.
“Our findings suggest that iron-rich peroxide, formed from ancient water in the magma ocean, played a key role in forming the heterogeneous structures of the D layer,” Qingyang said. The strong affinity of this peroxide for iron creates a sharp density contrast between these iron-rich patches and the surrounding mantle. It essentially acts as an insulator to prevent them from mixing, potentially explaining the long-lived heterogeneity observed at the base of the lower mantle. Jie added, “This model agrees well with recent numerical modeling results, suggesting that the heterogeneity of the lowermost mantle may be long-lived.”
Link: “Earth’s Core-Mantle Boundary Shaped by Crystallization of Earth’s Aqueous Magma Ocean” By Qingyang Hu, Jie Deng, Yukai Zhuang, Zhenzhong Yang, and Rong Huang, 13 May 2024, National Science Review.
DOI: 10.1093/nsr/nwae169