Fragments of ancient oceanic crust lie beneath the mantle

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The interior of EarthEarth It still holds many mysteries. Witnesses to the planet’s geological past, some say ContradictionsContradictions At the base of the mantle, at the interface with the outer core, has long remained a mystery. Scientific advances in imaging and modelingmodeling However, the veil may begin to lift on these zones of unusually dense material, named the ULVZ. Very low speed zones (In ultra-low velocity zones). We gave you these odds in the previous article. However, researchers have refined their understanding of their nature and origins.

Bigger odds than previously thought

Previously seen as isolated patches, these areas are weak SpeedSpeed The seismicity actually appears to be quite extensive. These results were obtained following high-level seismic surveys ResolutionResolution It is possible to accurately map and classify these ULVZs.

Remains of old sea shells

Until then, several hypotheses have been proposed to explain the origin of these anomalies at the base of the mantle: remnants of an ocean of ancient magma, results of meteorite impacts or PollutionPollution By Chemical elementsChemical elements of the outer core.

But it was published for the researchers who wrote the study Scientific advancesThe characteristics of the ULVZ are very compatible with the older residues Ocean crustOcean crust Upon entering, he sank into the mantle subjugationsubjugation. This oceanic material was deposited at the level of the core-mantle boundary, lined with a layer of material denser than the surrounding mantle.

Analysis of thousands of seismic records imaging the lower mantle in the Southern Hemisphere shows that the thickness of this low-velocity zone at the base of the mantle varies greatly, up to tens of kilometers. These types of “mountains” at the base of the mantle play an important role in the way heat escapes from the outer core.

Earth’s interior reveals itself a little more every day. Thanks to high-frequency seismic signals, researchers were able to image the bottom of the mantle more precisely than ever before. In particular, they indicate the composition and structure of some very low-velocity zones.

The article Morgan GillardMorgan GillardPublished on May 21, 2022

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

At the interface between the outer core and the crust, seismic waves actually reveal the presence of large structures characterized by very low seismic velocities. These regions contrast with the rest of the deep mantle, which is associated with rather high tidal velocities. PressurePressure Reigns in these depths. Hypotheses to explain this ULVZ (for Ultra-low speed zones, very low speed areas) going well. But their study comes up against the difficulty of obtaining good quality data that would allow them to be accurately classified.

Highly accurate imaging of ULVZs

By some analysis Seismic wavesSeismic waves Contrasted with the core-mantle interface, researchers from the Universities of Cambridge and Oxford were able to obtain new “pictures” of these structures, which lie about 2,800 kilometers below our feet. The images are more accurate than those obtained so far, and they make it possible to publish new hypotheses about their composition and function.

The studied area is located in the foothills Cloak BloomCloak Bloom In Hawaii, just below the island of the same name. This plume is a column of hot mantle that rises from the base of the mantle to the surface, leading to intense volcanism. SharingSharing. However, this plume has its source at a ULVZ level, which is a form of PutuPutu About twenty kilometers high. Until now, this type of “small” structure (compared to the size of the Earth), was almost invisible, because it is located below the resolution limit of tomographic models, which define the structure of the Earth using waves created by large earthquakes.

method DigitalDigital developed by researchers in an article published in the journal Nature Communications This made it possible to analyze the high-frequency seismic signal. A way to image the low-velocity zone at the base of a Hawaiian plume with kilometer precision.

Areas rich in iron

Their results suggest that the internal structure of this ULVZ is very complex, certainly with a heterogeneous structure. Analysis of the high-frequency signal and modeling of the internal structure suggest that ULVZs are particularly concentrated. IronIron, is generally a characteristic feature of the Earth’s core. The amount of iron is also variable, with the bottom of the ULVZ being richer than its top. This compositional anomaly is present in a certain ratio LiquidLiquid The magma explains the almost 40% reduction in the speed of seismic waves relative to the rest of the deep crust.

This iron enrichment can be explained by several hypotheses. It has already been suggested that ULVZs represent remnants of an ancient ocean of magma that predated its formation. Earth’s crustEarth’s crust Early in Earth’s history. In this case StratificationStratification This “sea” of molten rock is observed in the distribution of iron as an indication of the segregation of iron at different stages of crystallization.

Another hypothesis is that this composition is linked to the meteorite impacts that Earth has experienced in its history. Contains iron AsteroidsAsteroids If it hit the earth it would have mixed SilicatesSilicates Accumulated at the interface with the mantle and core.

The final hypothesis assumes that the iron comes directly from the outer core. It actually has the potential to remove some chemical elements, a process suspected to be particularly involved in the geodynamo effect in the origin of the Earth’s magnetic field.

Apart from these hypotheses that need to be verified, these particularly tropical regions participate in significant variations in flux. HeatHeat Recorded at core-mantle transition. In this sense, ULVZs can play an important role ConvectionConvection mantle and therefore participates in the generation of hot spots but also the geodynamo.

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