The study suggests that Earth’s D” layer, near the core-mantle boundary, was formed by a magma ocean created by a giant impact. 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 features.
Deep inside the Earth is a mysterious layer called the D” layer. Located approximately 3,000 kilometers below, this zone lies just above the boundary between the planet’s molten outer core and its solid mantle. Unlike a perfect sphere, the D” layer is surprisingly curvaceous. Its thickness varies widely from place to place, and some regions even lack the D” layer—much like continents rise above Earth’s oceans. These intriguing variations have attracted the attention of geophysicists, who describe the D” layer as a heterogeneous or non-uniform region.
A new study led by Dr. Qingyang Hu (Center for Advanced Research in High Pressure Science and Technology) and Dr. Jie Deng (Princeton University) suggests that layer D” could date back to the earliest days of the Earth. Their theory is based on the Giant Impact hypothesis, which proposes a MarsThe -200 meter object struck proto-Earth, creating a planet-wide ocean of magma. They believe that layer D” could be a unique composition left behind from this colossal impact, potentially preserving clues about Earth’s formation.
Water in the ocean of magma
dr. Jie Deng points out the presence of a significant amount of water within this global magma ocean. The exact origin of this water remains a matter of debate, with various theories being proposed, including its formation by reactions between nebular gas and magma or direct delivery by a comet. “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 could contain amounts of water comparable to today’s Earth’s oceans.”
The extreme pressure and temperature conditions within the magma ocean floor would create a unique chemical environment, promoting unexpected reactions between water and minerals. dr. Qingyang Hu explains, “Our research suggests that this watery magma ocean favored the formation of an iron-rich phase called iron-magnesium peroxide.” This peroxide, with the formula (Fe, Mg)O2, prefers iron even more compared to the other major components expected in the lower mantle. “According to our calculation, its affinity for iron could have led to the accumulation of iron-dominated peroxide in layers from several to tens of kilometers thick.
Creation of a heterogeneous structure at the boundary 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, deviating from our current understanding. According to the new model, minerals in D” would be dominated by a new group: 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 features of the D” layer. These features include ultra-low-velocity zones and high-conductivity layers, contributing to the well-known compositional heterogeneity of the D” layer.
“Our findings suggest that iron-rich peroxide, formed from ancient water within the magma ocean, played a key role in shaping the heterogeneous structures of the D layer,” Qingyang said. The strong affinity of this peroxide for iron creates a strong density contrast between these iron-rich spots and the surrounding mantle. In essence, it acts as an insulator, preventing them from mixing and potentially explaining the long-term 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 a long-lasting feature.”
Reference: “Earth’s Core-Mantle Boundary Formed by the Crystallization of the Earth’s Aqueous Magma Ocean” Qingyang Hu, Jie Deng, Yukai Zhuang, Zhenzhong Yang, and Rong Huang, May 13, 2024. National Science Review.
DOI: 10.1093/nsr/nwae169