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Earth’s mysterious D” layer

Context (Phys): Deep within Earth, there lies a mysterious layer called the D” layer.

Credits: Spring8
Located roughly 3,000 kilometres down, this zone sits just above the boundary between the planet’s molten outer core and its solid mantle.
Theory relying on the Giant Impact hypothesis believes the D” layer may be a unique composition leftover from this colossal impact, potentially holding clues to Earth’s formation.

The giant Impact hypothesis proposes a Mars-sized object slammed into the proto-Earth, creating a planet-wide magma ocean in the aftermath.

The D” layer is not perfectly sphere and is surprisingly patchy. Its thickness varies greatly from place to place, with some regions even lacking a D” layer altogether.
The D” layer may also be where deep mantle plumes originate and where subducting slabs terminate.
The new research 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.
Some suggest that water would have concentrated towards the bottom of the magma ocean as it cooled, forming hydrous oceans. This hydrous magma ocean favoured the formation of an iron-rich phase called iron-magnesium peroxide.
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, minerals in D” would be dominated by a new assemblage: the iron-poor silicate, iron-rich (Fe, Mg) peroxide, and iron-poor (Fe, Mg) oxide.
This iron-dominant peroxide also possesses low seismic velocities and high electrical conductivity, making it a potential candidate to explain the D” layer’s unique geophysical features.

The D” layer consists of ultra-low velocity zones (ULVZ) and layers of high conductance, both of which contribute to its well-known compositional heterogeneity.

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