Structures and Diamonds in the Earth’s Mantle

The mantle has three to five times more carbon than researchers would expect based on the proportion of elements in stars and other planets. The diamonds found in this layer of Earth might explain the discrepancy. Shim and his team calculated that if even 10% to 20% of the water in oceanic crust makes it to the core-mantle boundary, it could churn out enough diamonds to explain the levels of carbon in the crust.

LVZ [Low velocity zones] studies and lowermost mantle structures.
(A) Maps showing the locations along the CMB of prior ULVZ and seismic wave scattering investigations (1, 6) in both global view (left) and in the Southern Hemisphere (right). Areas denoted in gold indicate regions where ULVZ evidence was found; those outlined in blue indicate regions where no ULVZ was observed, and those outlined in red mark uncertain regions. Pink shading indicates the LLVPs (61), and green dots mark seismic scatters (6). (B) Cartoon highlighting both upwelling (LLVP) and downwelling (subduction) in the lower mantle along with various CMB anomalies. UHVZ, ultrahigh velocity zones.

Researchers from The University of Alabama discovered a dense layer of ancient ocean floor, or ultra-low velocity zone (ULVZ), between Earth’s core and mantle using seismic imaging. These underground “mountains” could play a key role in heat escape from the core and the planet’s magnetic field.

Current knowledge about the deep mantle in terms of structure, chemical, and mineralogical compositions, and dynamics encompasses particular structures in the mantle, such as large low shear-wave velocity provinces (LLSVPs) under the Pacific Ocean and under the southwestern part of Africa and bordering parts of the Atlantic and Indian Ocean, with inferred lateral flow regime in the D”-zone where seismic velocity gradients are anomalously low. Subduction has mostly been confined to this belt with a net divergence and convergence of the plates and two divergence poles at approximately 180° located above the LLSVPs. The global pattern, including upwelling material underneath mid-ocean ridges, suggests a large-scale degree-2 convection.

The study of the Earth’s interior is hampered by a lack of direct observations. One exception is observations from seismology, providing information on the physical state, density structure, and elastic properties of the different layers of the Earth, through travel time, amplitude, and phase measurements of seismic waves. Other key aspects such as flow in the liquid core are very hard to study using seismology. Flow in the liquid core can be derived from other indirect observations that provide constraints on theoretical models, so that the dynamics of the liquid core is deduced from the observed consequences of these flows.

Mantle Structure, Mineralogy and Lithological Density Relations
The dominantly peridotitic mantle is divided into the UM (Upper Mantle), TZ (transition zone), and LM (Lower Mantle) by seismic discontinuities at 410 and 660 km depth, caused by the phase transitions from olivine to wadsleyite and from ringwoodite to bridgmanite + ferropericlase. Less distinct phase transitions at 520–540 km depth may result from the wadsleyite to ringwoodite transition and the stabilization of the minor Ca-perovskite phase. The important TZ mineral, garnet, remains stable in the uppermost part of LM, but dissolves gradually into bridgmanite with increasing pressure in the 660 km to about 800 km depth range, causing a steep seismic velocity gradient in that range.

There are many questions that are still open in that field. Ultra low velocity zones (ULVZs) are patches on the core-mantle boundary that have extremely low seismic velocities. The zones are mapped to be hundreds of kilometers in diameter and tens of kilometers thick. Their shear wave velocities can be up to 30% lower than surrounding material. The composition and origin of the zones remain uncertain. The zones appear to correlate with edges of the African and Pacific large low-shear-velocity provinces (LLSVPs) as well as the location of hotspots

The presence of ULVZ (Ultra low velocity zones) and LLSVPs (large low-shear-velocity provinces) are not questioned but their origin is still under debate. They can have large influences of what is going on at the CMB and vice versa, what is going on at the CMB can influence them. This is certainly a matter of future direction of research. ULVZs are hypothesized to be enriched in iron, be partially molten or a combination of both, or result of the presence of carbon. Different scenarios have been proposed for the iron enrichment: iron could be leaking from the core, have accumulated over past subduction, or be remnants of a basal magma ocean. Both silicate perovskite and periclase (which are thought to be present in the lowermost mantle) show reduced velocities with increasing iron at these pressures and temperatures.

Cartoon of ultra low velocity zones (red structures) and the Pacific large low shear velocity province (red transparent) on the core of the Earth (blue)

Deep within Earth’s mantle, there are giant blobs. The ultra low velocity zones. Some research suggests that these blobs could have large amounts of diamonds as part of the structure. One sits under Africa and the other precisely opposite the first is under the Pacific Ocean. But these two blobs are not evenly matched.

New research finds that the blob under Africa extends far closer to the surface — and is more unstable — than the blob under the Pacific. This difference could ultimately help to explain why the crust under Africa has been lifted upward and why the continent has seen so many large supervolcano eruptions over hundreds of millions of years.