Fragments of ancient ocean crust lie at the bottom of the mantle

The inner part of LandLand It still holds many mysteries. Eyewitnesses to the planet’s geological past are some anomalyanomaly At the base of the mantle, at the interface with the outer core, it has long been obscure. Scientific advances in photography f designdesign However, it can start to lift a veil over these areas of an abnormally dense material called ULVZ Very low speed zones (Continued extremely low speed zones). We introduced you to these anomalies in a previous article. However, researchers have just refined their understanding of their nature and origin.

The anomaly is larger than previously thought

Previously seen as isolated patches, these areas are weak spots SpeedSpeed It will actually look more spacious. These results were obtained after high level seismic investigations AccuracyAccuracy making it possible to accurately map and characterize these ULVZs.

remnants of ancient ocean crusts

Until then, several hypotheses had been put forward to explain the origin of these anomalies at the bottom of the mantle: ocean remnants of primordial magma, the results of meteorite impacts or even radioactive contaminationradioactive contamination by chemical elementschemical elements from the outer core.

But for the researchers who wrote the study published in Science advancesthe characteristics of the ULVZ will be more or less consistent with the old remnants ocean crustsocean crusts Having sunk into the mantle after entering subductionsubduction. Thus, this oceanic material was deposited at the level of the core-mantle boundary, lining it with a layer of material denser than the surrounding mantle.

Analysis of thousands of seismic records depicting the lower mantle in the Southern Hemisphere has shown that the thickness of this low-velocity region at the base of the mantle can vary widely, up to ten kilometres. These kinds of “mountains” in the lower part of the mantle could also play an important role in the way heat escapes from the outer core.

The Earth’s interior reveals itself a little bit each day. Thanks to high-frequency seismic signals, researchers have been able to image the mantle base more accurately than before. In particular, it defines the configuration and geometry of some very low-speed regions.

Article from Morgan GillardMorgan Gillardpublished on May 21, 2022

The depths of the Earth still hold many mysteries. The difficulty of imaging the inner layers of our planet means that many questions remain to this day, particularly about composition, but also about what drives Earth’s mantleEarth’s mantle deep. However, for some time now, some mysterious structures have been particularly attracting the attention of scientists.

At the interface between the outer core and the mantle, seismic waves are already revealing large structures with very low seismic velocities. These regions contrast with the rest of the deep mantle, which is conversely associated with high wave velocities, and associated with massive waves. pressurepressure that prevails in these depths. hypotheses to explain these ULVZ (l Extremely low speed zones, areas with very low speeds) it runs fine. But their study struggles with the difficulty of obtaining good quality data that would allow them to accurately characterize.

More accurate imaging of ULVZs

By analyzing some seismic wavesseismic waves Researchers from the Universities of Cambridge and Oxford have been able to obtain new “images” of these structures, which lie about 2,800 kilometers below our feet, by diffraction along the mantle-primary front. The images are much more accurate than those obtained so far, which makes it possible to make new assumptions about their formation and operation.

The studied area is located at the base mantle column;mantle column; Hawaii, far from the island of the same name. This plume is a plume of hot mantle rising from the base of the mantle to the surface, generating intense volcanic activity. HotspotHotspot. However, the source of this column appears to be at the ULVZ level, which is in fig Viscous liquidViscous liquid About twenty kilometers. Until now, this kind of “small” (compared to Earth’s scale) structure was almost invisible, because it falls below the resolution limit of tomographic models, which map the Earth’s structure using seismic waves generated by large earthquakes.

method DigitalDigital developed by the researchers in the article published in the journal Nature Communications It made it possible to analyze a high-frequency seismic signal. A method for imaging the low velocity region at the base of the Hawaiian plume with kilometer resolution.

iron enriched areas

Their results indicate that the internal structure of the ULVZ is very complex, with a decidedly heterogeneous composition. Particularly high-frequency signal analysis and internal structure modeling indicate that ULVZs will be enriched in ironiron, which is a feature of the Earth’s core core. The amount of iron will also be variable, with the base of the ULVZ being richer than the top of it. This compositional anomaly, with a certain proportion of liquidliquid Magma, explains the nearly 40% reduction in the velocity of seismic waves compared to the rest of the deep mantle.

This iron enrichment can be explained by several hypotheses. It has already been suggested that ULVZs represent remnants of an ancient magma ocean, which predated the formation of CrustalCrustal At the beginning of Earth’s history. In this case stratificationstratification The observed distribution of iron would be an indicator of the fragmentation of iron during the different stages of crystallization of this “ocean” of molten rock.

Another hypothesis is that this formation is related to meteorite impacts that the Earth has witnessed during its history. iron in asteroidsasteroids Hit the ground to mingle with silicatessilicates of the mantle and accumulated at the interface with the core.

The final hypothesis is that the iron comes directly from the outer core. This would in fact have the possibility of decomposing some chemical elements, as this process is suspected to be particularly involved in the geodynamo effect which is at the origin of the Earth’s magnetic field.

Apart from these hypotheses that remain to be verified, these particularly hot regions appear to be involved in important variations in flow heatheat Recorded in the core-to-mantle transition. In this sense, ULVZs could have a major role in pregnancypregnancy of the mantle and thus involved in the generation of hotspots but also in the geodynamo.