A relic of ancient oceans occupies the Earth’s interior

May 24. () –

A layer 3,000 kilometers deep, above the boundary between the molten outer core and the solid mantle of the Earth, It may have its origin in the early days of our planet.

Unlike a perfect sphere, the D” (double prime D) layer is surprisingly irregular. Its thickness varies greatly from place to place, and some regions even lack this layer entirely, just as continents rise above the Earth’s oceans. These intriguing variations have caught the attention of geophysicists, who describe the D” layer as a heterogeneous, or non-uniform, region.

A new study led by Dr. Qingyang Hu (Advanced Research Center in High Pressure Science and Technology) and Dr. Jie Deng (Princeton University) suggests that this layer formed according to the Giant Impact hypothesis, which proposes that a Mars-sized object crashed into proto-Earth, creating a planet-wide magma ocean as a consequence. They believe that the double prime D layer may be a unique composition left over from this colossal impact, which potentially contains clues about the formation of the Earth.

In the research, published in National Science Review, Dr. Jie Deng highlights the presence of a substantial amount of water within this global magma ocean. The exact origin of this water remains a topic of debate, and several theories have been proposed, including its formation through reactions between the nebula’s gas and magma, or direct supply from 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 could have contained volumes of water comparable to the present oceans on the Earth’s surface.”

HYDRATED MAGMA OCEAN

The extreme pressure and temperature conditions within the magma ocean floor would have created a unique chemical environment, fostering unexpected reactions between water and minerals. Dr. Qingyang Hu explains: “Our research suggests that this hydrated magma ocean favored the formation of an iron-rich phase called magnesium iron peroxide.” This peroxide, with the formula (Fe,Mg)O2, has an even greater preference for iron compared to other important components expected in the lower mantle.

“According to our calculations, its affinity for iron could have caused the accumulation of iron-dominated peroxide in layers several to tens of kilometers thick.

The presence of this iron-rich peroxide phase would alter the mineral composition of the D” layer, deviating from our current understanding. According to the new model, the minerals in D” would be dominated by a new set: the iron-poor silicate, peroxide rich in iron (Fe, Mg) and poor oxide in iron (Fe, Mg). This iron-dominated peroxide also possesses low seismic velocities and high electrical conductivity, making it a potential candidate to explain the unique geophysical characteristics of the D layer. These features include ultra-low velocity zones and high-conductance layers, and both contribute to the known compositional heterogeneity of the D layer.”

“Our findings suggest that iron-rich peroxide, formed from ancient water within the magma ocean, has played a crucial role in shaping the heterogeneous structures of the D layer.” Qingyang said. The strong affinity of this peroxide for iron creates a marked density contrast between these iron-rich patches and the surrounding mantle. Essentially, it acts as an insulator, preventing mixing and potentially explaining the long-lasting heterogeneity observed in the base. Jie added: “This model aligns well with the results of recent numerical models, suggesting that lower mantle heterogeneity may be a long-lived feature.”