Large low-shear-velocity provinces

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Animation showing LLSVPs as inferred using seismic tomography LLSVP.gif
Animation showing LLSVPs as inferred using seismic tomography

Large low-shear-velocity provinces (LLSVPs), also called large low-velocity provinces (LLVPs) or superplumes, are characteristic structures of parts of the lowermost mantle, the region surrounding the outer core deep inside the Earth. [2] These provinces are characterized by slow shear wave velocities and were discovered by seismic tomography of deep Earth. There are two main provinces: the African LLSVP and the Pacific LLSVP, both extending laterally for thousands of kilometers and possibly up to 1,000 kilometres vertically from the core–mantle boundary. These have been named Tuzo and Jason respectively, after Tuzo Wilson and W. Jason Morgan, two geologists acclaimed in the field of plate tectonics. [3] The Pacific LLSVP is 3,000 kilometers (1,900 miles) across and underlies four hotspots on Earth's crust that suggest multiple mantle plumes underneath. [4] These zones represent around 8% of the volume of the mantle, or 6% of the entire Earth. [1]

Contents

Other names for LLSVPs and their superstructures include superswells, superplumes, thermo-chemical piles, or hidden reservoirs, mostly describing their proposed geodynamical or geochemical effects. For example, the name "thermo-chemical pile" interprets LLSVPs as lower-mantle piles of thermally hot and/or chemically distinct material. LLSVPs are still relatively mysterious, and many questions remain about their nature, origin, and geodynamic effects. [5]

Seismological modeling

Directly above the core–mantle boundary is a 200 kilometers (120 miles) thick layer of the lower mantle. This layer is known as the D″ ("D double-prime" or "D prime prime") or degree two structure. [6] LLSVPs were discovered in full mantle seismic tomographic models of shear velocity as slow features at the D″ layer beneath Africa and the Pacific. [7] The global spherical harmonics of the D″ layer are stable throughout most of the mantle but anomalies appear along the two LLSVPs. By using shear wave velocities, the locations of the LLSVPs can be verified, and a stable pattern for mantle convection emerges. This stable configuration is responsible for the geometry of plate motions at the surface. [8]

The LLSVPs lie around the equator, but mostly on the Southern Hemisphere. Global tomography models inherently result in smooth features; local waveform modeling of body waves, however, has shown that the LLSVPs have sharp boundaries. [9] The sharpness of the boundaries makes it difficult to explain the features by temperature alone; the LLSVPs need to be compositionally distinct to explain the velocity jump. Ultra-low velocity zones at smaller scales have been discovered mainly at the edges of these LLSVPs. [10]

By using the solid Earth tide, the density of these regions has been determined. The bottom two thirds are 0.5% denser than the bulk of the mantle. However, tidal tomography cannot determine how the excess mass is distributed; the higher density may be caused by primordial material or subducted ocean slabs. [11] The African LLSVP may be a potential cause for the South Atlantic Anomaly. [12]

Origins

Several hypotheses have been proposed for the origin and persistence of LLSVPs, depending on whether the provinces represent purely thermal unconformities (i.e. are isochemical in nature, of the same chemical composition as the surrounding mantle) or represent chemical unconformities as well (i.e. are thermochemical in nature, of different chemical composition from the surrounding mantle). If LLSVPs represent purely thermal unconformities, then they may have formed as large mantle plumes of hot, upwelling mantle. However, geodynamical studies predict that isochemical upwelling of a hotter, lower viscosity material should produce long, narrow plumes, [13] unlike the large, wide plumes seen in LLSVPs. It is important to remember, however, that the resolution of geodynamical models and seismic images of Earth's mantle are very different. [14]

The current leading hypothesis for the LLSVPs is the accumulation of subducted oceanic slabs. This corresponds with the locations of known slab graveyards surrounding the Pacific LLSVP. These graveyards are thought to be the reason for the high velocity zone anomalies surrounding the Pacific LLSVP and are thought to have formed by subduction zones that were around long before the dispersion—some 750 million years ago—of the supercontinent Rodinia. Aided by the phase transformation, the temperature would partially melt the slabs to form a dense melt that pools and forms the ultra-low velocity zone structures at the bottom of the core-mantle boundary closer to the LLSVP than the slab graveyards. The rest of the material is then carried upwards via chemical-induced buoyancy and contributes to the high levels of basalt found at the mid-ocean ridge. The resulting motion forms small clusters of small plumes right above the core-mantle boundary that combine to form larger plumes and then contribute to superplumes. The Pacific and African LLSVP, in this scenario, are originally created by a discharge of heat from the core (4000 K) to the much colder mantle (2000 K); the recycled lithosphere is fuel that helps drive the superplume convection. Since it would be difficult for the Earth's core to maintain this high heat by itself, it gives support for the existence of radiogenic nuclides in the core, as well as the indication that if fertile subducted lithosphere stops subducting in locations preferable for superplume consumption, it will mark the demise of that superplume. [4]

Another proposed origin for the LLSVPs is that their formation is related to the giant-impact hypothesis, which states that the Moon formed after the Earth collided with a planet-sized body called Theia. [15] The hypothesis suggests that the LLSVPs may represent fragments of Theia's mantle which sank through to Earth's core-mantle boundary. [15] The higher density of the mantle fragments is due to their enrichment in iron(II) oxide with respect to the rest of Earth's mantle. This higher iron(II) oxide composition would also be consistent with the isotope geochemistry of lunar samples, as well as that of the ocean island basalts overlying the LLSVPs. [16] [17]

Dynamics

Geodynamic mantle convection models have included compositional distinctive material. The material tends to get swept up in ridges or piles. [10] When including realistic past plate motions into the modeling, the material gets swept up in locations that are remarkably similar to the present day location of the LLSVPs. [18] These locations also correspond with known slab graveyard locations.

These types of models, as well as the observation that the D″ structure of the LLSVPs is orthogonal to the path of true polar wander, suggest these mantle structures have been stable over large amounts of time. This geometrical relationship is consistent with the position of Pangaea and the formation of the current geoid pattern due to continental break-up from the superswell below. [8]

However, the heat from the core is not enough to sustain the energy needed to fuel the superplumes located at the LLSVPs. There is a phase transition from perovskite to post-perovskite from the down welling slabs that causes an exothermic reaction. This exothermic reaction helps to heat the LLSVP, but it is not sufficient to account for the total energy needed to sustain it. So it is hypothesized that the material from the slab graveyard can become extremely dense and form large pools of melt concentrate enriched in uranium, thorium, and potassium. These concentrated radiogenic elements are thought to provide the high temperatures needed. So, the appearance and disappearance of slab graveyards predicts the birth and death of an LLSVP, potentially changing the dynamics of all plate tectonics. [4]

See also

Related Research Articles

<span class="mw-page-title-main">Asthenosphere</span> Highly viscous, mechanically weak, and ductile region of Earths mantle

The asthenosphere is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~80 and 200 km below the surface, and extends as deep as 700 km (430 mi). However, the lower boundary of the asthenosphere is not well defined.

<span class="mw-page-title-main">Mantle plume</span> Upwelling of abnormally hot rock within Earths mantle

A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hotspots, such as Hawaii or Iceland, and large igneous provinces such as the Deccan and Siberian Traps. Some such volcanic regions lie far from tectonic plate boundaries, while others represent unusually large-volume volcanism near plate boundaries.

Seismic tomography or seismotomography is a technique for imaging the subsurface of the Earth with seismic waves produced by earthquakes or explosions. P-, S-, and surface waves can be used for tomographic models of different resolutions based on seismic wavelength, wave source distance, and the seismograph array coverage. The data received at seismometers are used to solve an inverse problem, wherein the locations of reflection and refraction of the wave paths are determined. This solution can be used to create 3D images of velocity anomalies which may be interpreted as structural, thermal, or compositional variations. Geoscientists use these images to better understand core, mantle, and plate tectonic processes.

<span class="mw-page-title-main">Earth's mantle</span> A layer of silicate rock between Earths crust and its outer core

Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass of 4.01×1024 kg (8.84×1024 lb) and thus makes up 67% of the mass of Earth. It has a thickness of 2,900 kilometers (1,800 mi) making up about 46% of Earth's radius and 84% of Earth's volume. It is predominantly solid but, on geologic time scales, it behaves as a viscous fluid, sometimes described as having the consistency of caramel. Partial melting of the mantle at mid-ocean ridges produces oceanic crust, and partial melting of the mantle at subduction zones produces continental crust.

<span class="mw-page-title-main">Core–mantle boundary</span> Discontinuity where the bottom of the planets mantle meets the outer layer of the core

The core–mantle boundary (CMB) of Earth lies between the planet's silicate mantle and its liquid iron–nickel outer core, at a depth of 2,891 km (1,796 mi) below Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth due to the differences between the acoustic impedances of the solid mantle and the molten outer core. P-wave velocities are much slower in the outer core than in the deep mantle while S-waves do not exist at all in the liquid portion of the core. Recent evidence suggests a distinct boundary layer directly above the CMB possibly made of a novel phase of the basic perovskite mineralogy of the deep mantle named post-perovskite. Seismic tomography studies have shown significant irregularities within the boundary zone and appear to be dominated by the African and Pacific Large low-shear-velocity provinces (LLSVP).

<span class="mw-page-title-main">Large igneous province</span> Huge regional accumulation of igneous rocks

A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive and extrusive, arising when magma travels through the crust towards the surface. The formation of LIPs is variously attributed to mantle plumes or to processes associated with divergent plate tectonics. The formation of some of the LIPs in the past 500 million years coincide in time with mass extinctions and rapid climatic changes, which has led to numerous hypotheses about causal relationships. LIPs are fundamentally different from any other currently active volcanoes or volcanic systems.

<span class="mw-page-title-main">Iceland hotspot</span> Hotspot partly responsible for volcanic activity forming the Iceland Plateau and island

The Iceland hotspot is a hotspot which is partly responsible for the high volcanic activity which has formed the Iceland Plateau and the island of Iceland.

<span class="mw-page-title-main">East African Rift</span> Active continental rift zone in East Africa

The East African Rift (EAR) or East African Rift System (EARS) is an active continental rift zone in East Africa. The EAR began developing around the onset of the Miocene, 22–25 million years ago. It was formerly considered to be part of a larger Great Rift Valley that extended north to Asia Minor.

<span class="mw-page-title-main">Mantle convection</span> Gradual movement of the planets mantle

Mantle convection is the very slow creep of Earth's solid silicate mantle as convection currents carry heat from the interior to the planet's surface. Mantle convection causes tectonic plates to move around the Earth's surface.

Pyrolite is a term used to characterize a model composition of the Earth's mantle. This model is based on that a pyrolite source can produce the Mid-Ocean Ridge Basalt by partial melting. It was first proposed by Ted Ringwood (1962) as being 1 part basalt and 4 parts harzburgite, but later was revised to being 1 part tholeiitic basalt and 3 parts dunite. The term is derived from the mineral names PYR-oxene and OL-ivine. However, whether pyrolite is representative of the Earth's mantle remains debated.

<span class="mw-page-title-main">Marquesas hotspot</span> Volcanic hotspot in the Pacific Ocean

The Marquesas hotspot is a volcanic hotspot in the southern Pacific Ocean. It is responsible for the creation of the Marquesas Islands – a group of eight main islands and several smaller ones – and a few seamounts. The islands and seamounts formed between 5.5 and 0.4 million years ago and constitute the northernmost volcanic chain in French Polynesia.

<span class="mw-page-title-main">Society hotspot</span> Pacific volcanic hotspot

The Society hotspot is a volcanic hotspot in the south Pacific Ocean which is responsible for the formation of the Society Islands, an archipelago of fourteen volcanic islands and atolls spanning around 720 kilometres (450 mi) of the ocean which formed between 4.5 and <1 Ma.

<span class="mw-page-title-main">Low-velocity zone</span>

The low-velocity zone (LVZ) occurs close to the boundary between the lithosphere and the asthenosphere in the upper mantle. It is characterized by unusually low seismic shear wave velocity compared to the surrounding depth intervals. This range of depths also corresponds to anomalously high electrical conductivity. It is present between about 80 and 300 km depth. This appears to be universally present for S waves, but may be absent in certain regions for P waves. A second low-velocity zone has been detected in a thin ≈50 km layer at the core-mantle boundary. These LVZs may have important implications for plate tectonics and the origin of the Earth's crust.

<span class="mw-page-title-main">Crustal recycling</span> Tectonic recycling process

Crustal recycling is a tectonic process by which surface material from the lithosphere is recycled into the mantle by subduction erosion or delamination. The subducting slabs carry volatile compounds and water into the mantle, as well as crustal material with an isotopic signature different from that of primitive mantle. Identification of this crustal signature in mantle-derived rocks is proof of crustal recycling.

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.

<span class="mw-page-title-main">Superswell</span> Large area of anomalously high topography and shallow ocean regions

A superswell is a large area of anomalously high topography and shallow ocean regions. These areas of anomalous topography are byproducts of large upwelling of mantle material from the core–mantle boundary, referred to as superplumes. Two present day superswells have been identified: the African superswell and the South Pacific superswell. In addition to these, the Darwin Rise in the south central Pacific Ocean is thought to be a paleosuperswell, showing evidence of being uplifted compared to surrounding ancient ocean topography.

The Skagerrak-Centered Large Igneous Province (SCLIP), also known as the European-Northwest African Large Igneous Province (EUNWA), and Jutland LIP, is a 300 million year old (Ma) large igneous province (LIP) centered on what is today the Skagerrak strait in north-western Europe. It was named by Torsvik et al. 2008.

<span class="mw-page-title-main">Lower mantle</span> The region from 660 to 2900 km below Earths surface

The lower mantle, historically also known as the mesosphere, represents approximately 56% of Earth's total volume, and is the region from 660 to 2900 km below Earth's surface; between the transition zone and the outer core. The preliminary reference Earth model (PREM) separates the lower mantle into three sections, the uppermost (660–770 km), mid-lower mantle (770–2700 km), and the D layer (2700–2900 km). Pressure and temperature in the lower mantle range from 24–127 GPa and 1900–2600 K. It has been proposed that the composition of the lower mantle is pyrolitic, containing three major phases of bridgmanite, ferropericlase, and calcium-silicate perovskite. The high pressure in the lower mantle has been shown to induce a spin transition of iron-bearing bridgmanite and ferropericlase, which may affect both mantle plume dynamics and lower mantle chemistry.

<span class="mw-page-title-main">Kevin C. A. Burke</span> British geologist (1929–2018)

Kevin C. A. Burke was a geologist known for his contributions in the theory of plate tectonics. In the course of his life, Burke held multiple professorships, most recent of which (1983-2018) was the position of professor of geology and tectonics at the Department of Earth and Atmospheric Science, University of Houston. His studies on plate tectonics, deep mantle processes, sedimentology, erosion, soil formation and other topics extended over several decades and influenced multiple generations of geologists and geophysicists around the world.

Intraplate volcanism is volcanism that takes place away from the margins of tectonic plates. Most volcanic activity takes place on plate margins, and there is broad consensus among geologists that this activity is explained well by the theory of plate tectonics. However, the origins of volcanic activity within plates remains controversial.

References

  1. 1 2 Cottaar; Lekic (2016). "Morphology of lower mantle structures". Geophysical Journal International . 207 (2): 1122–1136. Bibcode:2016GeoJI.207.1122C. doi: 10.1093/gji/ggw324 .
  2. ↑ Garnero, Edward J.; McNamara, Allen K.; Shim, Sang-Heon (2016). "Continent-sized anomalous zones with low seismic velocity at the base of Earth's mantle". Nature Geoscience . 9 (7): 481–489. Bibcode:2016NatGe...9..481G. doi:10.1038/ngeo2733.
  3. ↑ Lau, Harriet; Al-Attar, David (2021-12-01). "Weighing TUZO and JASON individually". 2021: DI13A–05.{{cite journal}}: Cite journal requires |journal= (help)
  4. 1 2 3 Maruyama; Santosh; Zhao (January 2007). "Superplume, supercontinent, and post-perovskite: Mantle dynamis and anti-plate tectonics on the Core-Mantle Boundary". Gondwana Research . 11 (1–2): 7–37. Bibcode:2007GondR..11....7M. doi:10.1016/j.gr.2006.06.003.
  5. ↑ Davies, D. R.; Goes, S.; Lau, H. C. P. (2015), Khan, Amir; Deschamps, Frédéric (eds.), "Thermally Dominated Deep Mantle LLSVPs: A Review", The Earth's Heterogeneous Mantle: A Geophysical, Geodynamical, and Geochemical Perspective, Cham: Springer International Publishing, pp. 441–477, doi:10.1007/978-3-319-15627-9_14, ISBN   978-3-319-15627-9 , retrieved 2024-04-09
  6. ↑ Peltier, W.R. (2007). "Mantle dynamics and the D″ layer implications of the post-perovskite phase" (PDF). In Kei Hirose; John Brodholt; Thome Lay; David Yuen (eds.). Post-Perovskite: The Last Mantle Phase Transition. AGU Geophysical Monographs. Vol. 174. American Geophysical Union. pp. 217–227. ISBN   978-0-87590-439-9. Archived (PDF) from the original on 2015-09-23. Retrieved 2015-05-05.
  7. ↑ Lekic, V.; Cottaar, S.; Dziewonski, A. & Romanowicz, B. (2012). "Cluster analysis of global lower mantle". Earth and Planetary Science Letters . 357–358. EPSL: 68–77. Bibcode:2012E&PSL.357...68L. doi:10.1016/j.epsl.2012.09.014.
  8. 1 2 Dziewonski, A.M.; Lekic, V.; Romanowicz, B. (2010). "Mantle Anchor Structure: An argument for bottom up tectonics" (PDF). EPSL.[ permanent dead link ]
  9. ↑ To, A.; Romanowicz, B.; Capdeville, Y.; Takeuchi, N. (2005). "3D effects of sharp boundaries at the borders of the African and Pacific Superplumes: Observation and modeling". Earth and Planetary Science Letters. 233 (1–2). EPSL: 137–153. Bibcode:2005E&PSL.233..137T. doi:10.1016/j.epsl.2005.01.037.
  10. 1 2 McNamara, A.M.; Garnero, E.J.; Rost, S. (2010). "Tracking deep mantle reservoirs with ultra-low velocity zones" (PDF). EPSL. Archived from the original (PDF) on 2021-05-18. Retrieved 2013-06-22.
  11. ↑ Lau, Harriet C. P.; Mitrovica, Jerry X.; Davis, James L.; Tromp, Jeroen; Yang, Hsin-Ying; Al-Attar, David (15 November 2017). "Tidal tomography constrains Earth's deep-mantle buoyancy". Nature. 551 (7680): 321–326. Bibcode:2017Natur.551..321L. doi:10.1038/nature24452. PMID   29144451. S2CID   4147594. Archived from the original on 11 May 2021. Retrieved 19 July 2019.
  12. ↑ Jackie Appel (March 31, 2023). "Scientists Are Getting Kinda Anxious About a Pothole in Space". Archived from the original on 2023-04-01. Retrieved 2023-04-01.
  13. ↑ Campbell, Ian H.; Griffiths, Ross W. (1990). "Implications of mantle plume structure for the evolution of flood basalts". Earth and Planetary Science Letters . 99 (1–2): 79–93. Bibcode:1990E&PSL..99...79C. doi:10.1016/0012-821X(90)90072-6.
  14. ↑ Davies, D. Rhodri; Goes, S.; Davies, J.H.; Schuberth, B.S.A.; Bunge, H.-P.; Ritsema, J. (November 2012). "Reconciling dynamic and seismic models of Earth's lower mantle: The dominant role of thermal heterogeneity". Earth and Planetary Science Letters. 353–354: 253–269. doi:10.1016/j.epsl.2012.08.016.
  15. 1 2 Yuan, Qian; Li, Mingming; Desch, Steven J.; Ko, Byeongkwan; Deng, Hongping; Garnero, Edward J.; Gabriel, Travis S. J.; Kegerreis, Jacob A.; Miyazaki, Yoshinori; Eke, Vincent; Asimow, Paul D. (November 2023). "Moon-forming impactor as a source of Earth's basal mantle anomalies". Nature. 623 (7985): 95–99. Bibcode:2023Natur.623...95Y. doi:10.1038/s41586-023-06589-1. ISSN   1476-4687. PMID   37914947. S2CID   264869152.
  16. ↑ Yuan, Qian; Li, Mingming; Desch, Steven J.; Ko, Byeongkwan (2021). "Giant impact origin for the large low shear velocity provinces" (PDF). 52nd Lunar and Planetary Science Conference. Archived (PDF) from the original on 24 March 2021. Retrieved 27 March 2021.
  17. ↑ Zaria Gorvett (12 May 2022). "Why are there continent-sized 'blobs' in the deep Earth?". BBC Future . Archived from the original on 21 May 2022. Retrieved 21 May 2022.
  18. ↑ Steinberger, B.; Torsvik, T.H. (2012). "A geodynamic model of plumes from the margins of Large Low Shear Velocity Provinces" (PDF). G^3. Archived (PDF) from the original on 2014-08-15. Retrieved 2013-06-22.
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