Mantle Samples Included in Volcanic Rocks
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Xenolith
Xenolith
Metasomatism
Primitive mantle
Asthenosphere
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Geophysical observations suggest that the transition zone is wet locally. Continental and oceanic sediment components together with the basaltic and peridotitic components might be transported and accumulated in the transition zone. Low-velocity anomalies at the upper mantle-transition zone boundary might be caused by the existence of dense hydrous magmas. Water can be carried farther into the lower mantle by the slabs. The anomalous Q and shear wave regions locating at the uppermost part of the lower mantle could be caused by the existence of fluid or wet magmas in this region because of the water-solubility contrast between the minerals in the transition zone and those in the lower mantle. δ-H solid solution AlO2H-MgSiO4H2 carries water into the lower mantle. Hydrogen-bond symmetrization exists in high-pressure hydrous phases and thus they are stable at the high pressures of the lower mantle. Thus, the δ-H solid solution in subducting slabs carries water farther into the bottom of the lower mantle. Pyrite FeO2H x is formed due to a reaction between the core and hydrated slabs. This phase could be a candidate for the anomalous regions at the core-mantle boundary.
Core–mantle boundary
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The mantle’s compositional structure reflects the thermochemical evolution of Earth. Yet, even the radial average composition of the mantle remains debated. Here, we analyze a global dataset of shear and compressional waves reflecting off the 410- and 660-km discontinuities that is 10 times larger than any previous studies. Our array analysis retrieves globally averaged amplitude-distance trends in SS and PP precursor reflectivity from which we infer relative wavespeed and density contrasts and associated mantle composition. Our results are best matched by a basalt-enriched mantle transition zone, with higher basalt fractions near 660 (~40%) than 410 (~18–31%). These are consistent with mantle-convection/plate-recycling simulations, which predict that basaltic crust accumulates in the mantle transition zone, with basalt fractions peaking near the 660. Basalt segregation in the mantle transition zone also implies that the overall mantle is more silica enriched than the often-assumed pyrolitic mantle reference composition.
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Classification of discontinuities
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Abstract The recycling of oceanic crust, with distinct isotopic and chemical signature from the pyrolite mantle, plays a critical role in the chemical evolution of the Earth with insights into mantle circulation. However, the role of the mantle transition zone during this recycling remains ambiguous. We here combine the unique resolution reflected body waves (P410P and P660P) retrieved from ambient noise interferometry with mineral physics modeling, to shed new light on transition zone physics. Our joint analysis reveals a generally sharp 660-km discontinuity and the existence of a localized accumulation of oceanic crust at the bottom mantle transition zone just ahead of the stagnant Pacific slab. The basalt accumulation is plausibly derived from the segregation of oceanic crust and depleted mantle of the adjacent stagnant slab. Our findings provide direct evidence of segregated oceanic crust trapped within the mantle transition zone and new insights into imperfect whole mantle circulation.
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Crustal recycling
Core–mantle boundary
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We report ab initio atomistic simulations of hydrous silicate melts under deep upper mantle to shallow lower mantle conditions and use them to parameterise density and viscosity across the ternary system MgO-SiO2-H2O (MSH). On the basis of phase relations in the MSH system, primary hydrous partial melts of the mantle have 40-50 mol% H2O. Our results show that these melts will be positively buoyant at the upper and lower boundaries of the mantle transition zone except in very iron-rich compositions, where ≳ 75% Mg is substituted by Fe. Hydrous partial melts will also be highly inviscid. Our results indicate that if melting occurs when wadsleyite transforms to olivine at 410 km, melts will be buoyant and ponding of melts is unexpected. Box models of mantle circulation incorporating the upward mobility of partial melts above and below the transition zone suggest that the upper mantle becomes efficiently hydrated at the expense of the transition zone such that large differences in H2O concentration between the upper mantle, transition zone and lower mantle are difficult to maintain on timescales of mantle recycling. The MORB source mantle with ∼0.02-0.04 wt% H2O may be indicative of the H2O content of the transition zone and lower mantle, resulting in a bulk mantle H2O content of the order 0.5 to 1 ocean mass, which is consistent with geochemical constraints and estimates of subduction ingassing.
Ringwoodite
Stishovite
Post-perovskite
Peridotite
Mantle plume
Primitive mantle
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Core–mantle boundary
Post-perovskite
Crustal recycling
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The areas of low-velocity middle mantle shift into the transition zone are distinguished according to 3-d P-velocity model of the Fennoscandian shield mantle and its surroundings using Taylor approximation method. Within the Kola-Karelia megablock there are the White Sea and Varanger mantle domains along with the Kostomuksha and Lapland upper mantle domains. Within the Svecofennian megablock there is the Shellefteo domain, and within the Sveconorwegian megablock - subvertical mantle column of altering high and low-velocity P-waves corresponding to superdeep mantle of the Oslo graben. Based on the conducted analysis the following general velocity features of the distinguished domains are defined: the areas of low-velocity middle mantle shift into the transition zone of the upper mantle, stratification of the upper mantle and its transition zone.
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Core–mantle boundary
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Xenolith
Peridotite
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Study on REE in mantle fluids has significance for understanding of regional geochemical difference, mantle enrichment and depletion. Recently, REE study on mantle fluids is indirectly achieved through the comparison of CO2rich and CO2poor fluid inclusions in mantle minerals. REE in fluidmelt inclusions of mantle xenolith from Changbaishan are measured directly by ICPMS. The primary study shows that fluidmelt inclusions enrich REE, particularly with higher LREE. The REE distribution pattern curve is decline to the right with a slight positive Eu anomaly and is similar to that of the mantle xenolith host basalt. It indicates that the source of mantle xenolith may be experienced a metasomatic process.
Xenolith
Metasomatism
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