One study suggests that Earth’s D” layer, near the core-mantle boundary, was formed by a magma ocean created by a massive impact. The iron-magnesium peroxide, formed by the water in this ocean, explains the unique composition and heterogeneity of the D layer.”
New research suggests that the mysterious D’ layer at Earth’s core-mantle boundary may have formed from the remnants of an early colossal impact, with iron-rich peroxide playing a key role in its unique and stable characteristics.
Deep inside the Earth, there is a mysterious layer called the D layer”. Located roughly 3,000 kilometers down, this zone sits just above the boundary between the planet’s molten outer core and its solid mantle. Unlike a perfect sphere, the D” layer is surprisingly fragmented. Its thickness varies greatly from place to place, with some regions lacking a “D” layer altogether – much like the continents that 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 of High Pressure Science and Technology) and Dr. Jie Deng (Princeton University) suggests that the D” layer may have originated from Earth’s earliest days. Their theory hinges on the Giant Impact hypothesis, which proposes a March-sized object crashed into proto-Earth, creating a planet-wide magma ocean as a result. They believe the D’ layer may be a unique compound left over from this colossal impact, potentially holding clues to Earth’s formation.
Water in the Magma Ocean
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 subject of debate, with various theories being proposed including its formation through reactions between nebular gas and magma, or direct delivery by comets. “The prevailing view,” continues Dr. Deng, “suggests that water would have concentrated toward the bottom of the magma ocean as it cooled. In late stages, the magma closer to the core could have contained volumes of water comparable to Earth’s oceans today.”
The extreme pressure and temperature conditions within the terminal magma ocean would have created a unique chemical environment, driving 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, has an even stronger preference for iron than the other major components expected in the lower mantle. “According to our calculations, its affinity for iron could have led to the accumulation of the dominant iron peroxide in layers ranging from a few to tens of kilometers thick.
Formation of heterogeneous structure at the Earth’s core-mantle boundary. 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” will be dominated by a new group: iron-poor silicate, iron-rich peroxide (Fe, Mg) and iron-poor oxide (Fe, Mg). This dominant iron peroxide also possesses low seismic velocities and high electrical conductivity, making it a likely candidate to explain the unique geophysical features of the D layer.” These features include ultra-low-velocity regions and high-conductivity layers, both of which contribute to the well-known heterogeneity of the D-layer composition.
“Our findings suggest that iron-rich peroxide, formed from ancient water within the magma ocean, played a crucial role in the formation of 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. Essentially, 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 pattern matches well with recent numerical modeling results, suggesting that lower mantle heterogeneity may be a long-lived feature.”
Reference: “Earth’s core-mantle boundary formed by crystallizing a terrestrial magma ocean with water” by Qingyang Hu, Jie Deng, Yukai Zhuang, Zhenzhong Yang and Rong Huang, 13 May 2024, National Science Review.
DOI: 10.1093/nsr/nwae169