Analysis of Upper and Lower Mantle Structure Using Shear Waves

Coronae are crown-like, tectono-volcanic features found on Venus that typically range in diameter from 100-700 km. Diapirs of warm upwelling material impinging on the lithosphere are often invoked to explain coronae formation. With more than 500 coronae identified on the surface of Venus, if these diapirs are individually linked to a mantle plume, Venus must have a very different mantle structure than Earth. I consider three cases designed to assess the potential relationship between large-scale, long-wavelength lower mantle structure and smaller-scale upper mantle structure that could potentially form diapirs consistent with those that are envisioned to interact with the lithosphere and form coronae. I use the geoid and topography to identify the large-scale pattern of convection because the geoid contains and integral of the temperature anomalies over the depth of the mantle. Plume tails—narrow vertical conduits—integrate to give a positive geoid anomaly while small-scale, time-dependent drips or upwellings are minimized in the depth integration. The first case—the reference case—has a small, stepwise decreases in viscosity between the lower mantle (1022 Pa s), transition zone (1021 Pa s), and upper mantle (5x1020 Pa s) with no phase transformations. This led to 20 ~1000-km diameter mantle plumes that remained stationary for more than 1.5 Gyr. This calculation is consistent with a number of geophysical observations it does not support the formation of coronae by plume-lithosphere interaction. To decouple the lower and upper mantle, I further decrease both the upper mantle and transition zone viscosities to 1020 Pa s while leaving all other parameters unchanged. In this calculation the same 20 ~1000-km diameter mantle plumes formed and remained stationary for more than 1.5 Gyr. The geoid and topography are anti-correlated, inconsistent with the observed values on Venus and, the spatial scale, number, and topographic evolution of the plumes are not consistent with coronae. This calculation does not support the formation of coronae by plume-lithosphere interaction. In an attempt to further decouple the upper and lower mantle I add an endothermic phase transformation at the ringwoodite-bridgmanite boundary in addition to the decreased upper mantle and transition zone viscosities while leaving all other parameters unchanged. Unique to this calculation, a very large number of ~100-km diameter small topographic upwellings form some associated with the large-scale geoid high but never associated with the large-scale geoid low. The inclusion of a phase transformation decoupling the upper and lower mantle has the potential to create diapir-like structures in the upper mantle consistent with coronae formation.

The effect on composition and evolution of the mantle of the recently‐observed strong concentration of iron in (Mg, Fe) O‐magnesiowüstite (mw) at the expense of (Mg, Fe) SiO3‐perovskite (pv) structure is studied by calculating a temperature‐ and pressure‐dependent iron partitioning coefficient for the lower mantle. The value of the standard entropy for MgSiO3‐perovskite is found to be 69.4±10.3 J/mole deg from the recently determined phase diagram of forsterite. Iron remains concentrated in (Mg, Fe) O throughout the entire lower mantle if account is taken of an FeO phase change, with the partitioning coefficient (xpvFe/xpvMg)/(xmwFe/xmwMg) increasing from 0.04 to 0.8 between 670 Km a the core‐mantle boundary. Partitioning has negligible effect on gross density and elastic properties Of the lower mantle. By using recent shock wave and static compression results for FeO and MgSiO3‐perovskite, we find that the lower mantle is more pyroxene‐rich than the upper mantle and as iron‐rich, or somewhat less so, than the upper mantle. Mg/(Mg + Fe) = 0.93–0.95 for the lower mantle compared with 0.85–0.90 for the uppermost mantle. The lower mantle Mg/Si ratio is closer to chondritic values (0.99± 0.03) (≈1.5 for a peridotite with px/ol = 0.4(molar)), thus supporting the idea of a chemically layered mantle with implications for the style of mantle convection. While partitioning of iron has no significant effect on gross lower mantle density, we find that the (Mg, Fe) O and perovskite components of the lower mantle have essentially the same densities. Mantles with higher bulk iron contents have (Mg, Fe) O denser than the perovskite component; for a bulk magnesium mole fraction of, 0.80, the density difference is 0.7–0.8 g/cm3. We investigate the feasibility of the Mao, Bell, and Yagi gravitational separation hypothesis of mantle evolution in which a mantle more iron‐rich than present loses iron through gravitational sinking of the denser (Mg, Fe) O, and we conclude that the process cannot successfully compete with solid state convection unless implausibly large grain sizes or unacceptably low viscosities are invoked. A likely explanation for removal of iron from an initially iron rich lower mantle is upward extraction of FeO‐enriched basalts or picrites and concentration of iron in upper mantle garnets during accretion of the earth or subsequent convection with the entire mantle passing through the partial melt zone. Thus the lower mantle was depleted of iron relative to both the upper mantle and the mantles of the small terrestrial planets and satellites, which do not have mantle pressures sufficient to form perovskite‐structure silicates, or which had lower accretional temperatures and less extensive melting. On this basis, Venus would be expected to have a mantle similarly depleted in iron.

It is generally believed a variation of 3He/4He isotopic ratios in the mantle is due to only the decay of U and Th, which produces 4He as well as heat. Here we show that not only 3He/4He isotopic ratios but also helium contents can be fractionated by thermal diffusion in the lower mantle. The driving force for that fractionation is the adiabatic or convective temperature gradient, which always produces elemental and isotopic fractionation along temperature gradient by thermal diffusion with higher light/heavy isotopic ratio in the hot end. Our theoretical model and calculations indicate that the lower mantle is helium stratified, caused by thermal diffusion due to ~ 400 °C temperature contrast across the lower mantle. The highest 3He/4He isotopic ratios and lowest He contents are in the lowermost mantle, which is a consequence of thermal-diffusion fractionation rather than the lower mantle is a primordial and undegassed reservoir. Therefore, oceanic-island basalts derived from the deepest lower mantle with high 3He/4He isotopic ratios and less He contents—the long-standing helium paradox, is solved by our model. Because vigorous convection in the upper mantle had resulted in disordered or disorganized thermal-diffusion effects in He, Mid-ocean ridge basalts unaffected by mantle plume have a relatively homogenous and lower 3He/4He isotopic compositions. Our model also predicts that 3He/4He isotopic ratios in the deepest lower mantle of early Earth could be even higher than that of Jupiter, the initial He isotopic ratio in our solar system, because the temperature contrast across the lower mantle in the early Earth is the largest and less 4He had been produced by the decay of U and Th. Moreover, the early helium-stratified lower mantle owned the lowest He contents due to over-degassing caused by the largest temperature contrast. Consequently, succeeding evolution of the lower mantle is a He ingassed process due to secular cooling of the deepest mantle. This explains why significant amount of He produced by the decay of U and Th in the lower mantle were not released, another long-standing heat–helium paradox.

SUMMARY The dynamic response of the crust and upper mantle to asthenospheric convective flow related to hot mantle diapiric upwelling is numerically evaluated in order to examine the implications for geological phenomena such as rifting, back-arc spreading, sedimentary basin and mountain building. For island arc areas such as the Japanese Islands and for ridge subducting areas such as the Basin and Range region, where both lower crust and upper mantle may behave as ductile layers in some evolutionary stages, the mechanical coupling between convection in the two layers may play an important role in the mantle dynamics related to these geological phenomena. Significant surface response to mantle diapiric upwelling occurs even when the whole crust effectively works as an elastic layer. The magnitude of surface uplift predicted for this crustal rheological state depends on the thickness of the lithosphere and on the spatial scale of the mantle diapir. However, the effective viscosity of the lower crust may decrease with increasing duration of diapiric upwelling. If the effective viscosities of the lower crust and asthenosphere are 1019-1021 Pa s, the asthenospheric convective flow induced by a mantle diapiric upwelling with a radius about 50 km, a scale of feature which has been seismically observed in the upper mantle beneath the Japanese Islands, may squeeze out the lower crustal material laterally. This would be followed by maximum surface subsidence of about 300–500 m and by upward migration of the Moho discontinuity by up to 3–5 km on a geological time-scale of 5–50 Ma. Thus, the coupling between the convection in the lower crust and upper mantle is mechanical, and the lower crustal material is dragged along by the shear stresses operating at the base of the lower crust. In this process, the space originally occupied by lower crustal material is replaced by mantle material, leading to thinning of the lower crust. Thus, in some geological situations having a hot mantle plume beneath the crust, a change of tectonic style can be expected as a result of a change of the state of convective coupling between the lower crust and upper mantle. Geological phenomena associated with this convective coupling may also depend on the geometry of the region surrounding the mantle diapiric upwelling. For example, the subducted slab in island arc areas will operate as a barrier to the lateral movement of lower crustal material. Thus, accumulation of lower crustal material, referred to as lower crustal megalith, may occur at the edge of the overriding plate. The overpressure caused by the accumulation of this megalith may be a crucial factor in determining the coupling state between the subducted slab and island arc, and may play an important role in and/or work as a trigger in the evolutionary histories of the island arc and related back-arc spreading.

Eye numbers were quantified on the mantles of sea scallops (Placopecten magellanicus) to determine whether they varied with scallop size (correlated with age), and to compare the numbers of eyes present on the upper (left) and lower (right) mantles. We used Poisson regressions to model eye addition with growth and found that larger scallops had significantly more eyes than their smaller conspecifics, although eye addition occurred only on the upper mantle. Consistent with predictions of an adaptive evolutionary response in eye numbers driven by the need for visualizing objects in the water column, we found scallops had significantly more eyes on the upper mantle than on the lower mantle. This may favor detection of predators, food, or habitat. We also documented significantly greater densities of eyes on the mantles of the smallest scallops, which may indicate vision is especially important in juveniles, which are most vulnerable to predation.

Postglacial sealevels on a spherical, self-gravitating viscoelastic earth: effects of lateral viscosity variations in the upper mantle on the inference of viscosity contrasts in the lower mantle

Using seismic body and surface waves, the velocity structure of the Earth's mantle is determined with the emphasis on regions of anomalous variations (so-called ‘discontinuities’). In the upper mantle, the interpretation of Rayleigh and Love wave dispersion curves yields shear velocity profiles with discontinuities at depths 350 km and 700 km, and a low-velocity zone extending to 350km. In the lower mantle P-velocity profile is determined from dt/dΔ measurements using large aperture seismic array and travel times from Long Shot nuclear explosion for the Japan-Kuriles-Aleutian-Montana path. The velocity structure shows anomalous gradients or ‘discontinuities’ at depths 700, 1200 and 1900km, indicating that the lower mantle is not homogeneous. Lateral variations of the velocity structures are investigated. For the upper mantle studies the Earth is divided into three regions: oceanic areas, continental shields, and tectonic zones. Pure path phase velocities of Love waves are extracted from the composite dispersion data. The pure path shear velocity profiles obtained from these data are characterized by lower velocities under the oceans in the uppermost portion of the mantle. Shields have the highest velocities. These velocity differences are interpreted in terms of temperature variations. At a depth of 110 km the temperature of the oceanic mantle is higher (by 100–500° C depending on the temperature coefficient of the velocity) than that of the mantle under the shields. The presence of lateral heterogeneities in the mantle is demonstrated qualitatively by the differences of dt/dΔ vs Δ curves for two separate paths. Undulations of the geoid as determined from satellite observations are investigated for determining the sources of the anomalies. It is concluded that the main sources of lateral density variations must be in the mantle at depths greater than about 100km.

SUMMARY The characteristic broad local maxima exhibited by the long-wavelength geoid over subduction zones are investigated with a numerical model of mantle flow. In a spherical axisymmetric geometry, a synthetic model of buoyancy driven subduction is used to test the effects on the geoid caused by the depth of penetration of the lithosphere into the mantle, by the viscosity stratification and by lateral viscosity variations (LVV) in the lithosphere, upper and lower mantle. The presence of anomalous slab density in the lower mantle guarantees geoid amplitudes comparable with the observations, favouring the picture of slabs that penetrate the transition zone and sink into the deep mantle. The viscosity of the lower mantle controls the long-wavelength geoid to the first order, ensuring a clear positive signal when it is at least 30-times greater than the upper-mantle viscosity. The presence of LVV in the lithosphere, in the form of weak plate margins, helps to increase the contribution of the surface topography, causing a pronounced reduction of the geoid. Localized LVV associated with the cold slab play a secondary role if they are in the upper mantle. On the other hand, highly viscous slabs in the lower mantle exert a large influence on the geoid. They cause its amplitude to increase dramatically, way beyond the values typically observed over subduction zones. Long-wavelength flow becomes less vigorous as the slab viscosity increases. Deformation in the upper mantle becomes more localized and power is transferred to short wavelengths, causing the long-wavelength surface topography to diminish and the total geoid to increase. Slabs may be then weakened in the lower mantle or retain their high viscosity while other mechanisms act to lower the geoid. It is shown that a phase change from perovskite to post-perovskite above the core–mantle boundary can cause the geoid to reduce significantly, thereby helping to reconcile models and observations.

Frequent inclusions of ferropericlase, some coexisting with phases of MgSiO3, CaSiO3 and SiO2 composition, suggest that a large proportion of diamonds from Guinea are derived from the lower mantle. Low aluminium contents in MgSiO3 inclusions indicate derivation from the uppermost lower mantle, where Al solubility in perovskite is low. Trace element analyses (SIMS) of CaSiO3 inclusions reveal extreme degrees of LREE (200–2000 times chondritic) and Sr enrichment (70–1000 times chondritic) together with negative and positive Eu anomalies. This implies a highly enriched lower mantle source, possibly a product of a subducted oceanic slab. A number of phases that are only stable in the upper mantle are found to coexist with lower mantle phases and thereby indicate retrograde equilibration during slow exhumation within a rising plume or convection cell. In one case, however, an inclusion paragenesis of ferropericlase and olivine can be shown to have formed within the upper mantle, indicating that the occurrence of ferropericlase inclusions alone is an unreliable indicator of lower mantle origin.

This review of mantle oxidation state, based on Mossbauer measurements of mantle xenoliths and synthetic high-pressure phases, suggests relatively low Fe 3+ /ΣFe in upper mantle and transition zone phases but high Fe 3+ /ΣFe in lower mantle (Mg,Fe)(Si,Al)O 3 perovskite, even under reducing conditions, with significant implications for physical and chemical properties. Whole-rock Fe 2 O 3 concentrations for pyrolite mantle are calculated to be low in the upper mantle and transition zone (ca. 0.3 wt% Fe 2 O 3 ), but high in the lower mantle (ca. 4 wt% Fe 2 O 3 ). High Fe 2 O 3 concentrations are probably balanced according to the iron disproportionation reaction 3 Fe 2+ = Fe° + 2 Fe 3+ . Oxygen fugacity is relatively high at the top of the upper mantle (near the fayalite-magnetite-quartz buffer) due to the concentration of Fe 3+ in modally minor phases, but probably low in the transition zone and lower mantle (close to metal equilibrium). Minerals in subducting slabs may be more oxidised, however, with higher Fe 2 O 3 concentrations. Coupled redox reactions between iron oxidation.and the reduction of oxidised species such as carbonate may contribute to the genesis of diamonds in the lower mantle. Disproportionation of iron in the lower mantle to produce ca. 1 wt% Fe-rich metal may have occurred in the early Earth, with important consequences for mantle geochemistry. If the metal phase does not segregate appreciably, material can move across the transition zone-lower mantle boundary without a net enrichment or depletion in oxygen.

Mantle dynamics, postglacial rebound and the radial viscosity profile

Peritectic mechanisms for fractional evolution of the upper mantle, transition zone and lower mantle magmatism and petrogenesis of the ultrabasic-basic rock series are justified in theory and experiments. Compositions of the ultrabasic upper mantle peridotite and pyroxenite rocks belong to the multicomponent olivine–orthopyroxene–(jadeite–poor clinopyroxene)–garnet system. On experimental evidence at 4 GPa, evolution of the primary olivine-normative magma, generated at the system, must be accompanied by a disappearance of the orthopyroxene through the invariant peritectic reaction with melt and formation of the univariant cotectic assembly olivine + clinopyroxene + garnet + melt. The problem of a termination of the olivine formation arises in view of the fact that compositions of the basic eclogite and grospydite rocks belong to the silica-oversaturated (jadeite-rich omphacite)–garnet–corundum–coesite system. A further ultrabasic-basic evolution of the system has been made possible with fractional crystallization of the evolved melts which compositions become progressively enriched with jadeitic component. Experimental study of melting relations on the olivine–diopside–jadeite–garnet system at 6 GPa demonstrates a disappearance of the olivine through the invariant peritectic reaction with jadeite-rich melt and formation of the basic univariant cotectic omphacite + garnet + melt. Hence the ultrabasic-basic evolution of the primary upper mantle ultrabasic magma is effected principally by the peritectic reactions of orthopyroxene and olivine when coupled with the regime of fractional crystallization. In the case of the transition zone, compositions of the ultrabasic ringwoodite-bearing and basic stishovite-bearing rocks are rated to the ringwoodite–(majoritic garnet)–magnesiowustite–stishovite system. It has been found experimentally at 20 GPa that the ultrabasic-basic evolution of the primary ringwoodite-normative magma in the polythermal section ringwoodite—(2FeO + SiO2) of the boundary MgO–FeO–SiO2 system is attended with a disappearance of ringwoodite in consequence of its invariant peritectic reaction with melt and formation of the univariant cotectic assembly stishovite + magnesiowustite + melt. For the lower mantle conditions, compositions of the ultrabasic bridgmanite-bearing and basic stishovite-bearing rocks are concerned with the bridgmanite–Ca–perovskite–(periclase ↔ wustite)ss–stishovite system. Experimentally at 24 GPa we find that ultrabasic-basic evolution of the primary bridgmanite-normative magma in the polythermal section bridgmanite—(FeO + SiO2) of the boundary MgO–FeO–SiO2 system has been marked by a disappearance of bridgmanite through its invariant peritectic reaction with melt and formation of the univariant cotectic assembly stishovite + magnesiowustite + melt (effect of “stishovite paradox”). The ultrabasic-basic evolution of the primary transition zone and lower mantle ultrabasic magmas, effected by the peritectic reactions of ringwoodite and bridgmanite, respectively, can result exclusively in the regime of fractional crystallization. The peritectic reactions of the upper mantle orthopyroxene and olivine, transition zone ringwoodite and lower mantle bridgmanite are also operable for genesis of the diamonds and associated minerals in the relevant silicate–(±oxide)–carbonate–carbon systems subjected to the ultrabasic-basic fractional evolution.

Implications of Nb/U, Th/U and Sm/Nd in plume magmas for the relationship between continental and oceanic crust formation and the development of the depleted mantle

Recent experimental investigations of grain size evolution in bridgmanite-ferropericlase assemblages have suggested very slow growth for these bimodal phases. Despite numerous speculations on grain size-dependent viscosity, a comprehensive test with realistic grain size evolution parameters compatible with the lower mantle has been lacking. In this study, we develop self-consistent 2-D spherical half-annulus geodynamic models of Earth’s evolution using the finite volume code StagYY to assess the role of grain size on lower mantle viscosity. We explore several models with and without grain size evolution to compare their effects on mantle viscosity. In models with grain size evolution, we consider three scenarios: (1) uniform grain growth throughout the entire mantle with a composite rheology, (2) different grain growth in the upper and lower mantle with a composite rheology, and (3) different grain growth in the upper and lower mantle with purely diffusion creep rheology. In the case of different grain size evolution, the upper mantle’s grain size evolution law is controlled by forsterite-enstatite grain growth, while the lower mantle’s grain size evolution law is controlled by bridgmanite-ferropericlase grain growth. Our results suggest that mantle viscosity is primarily controlled by temperature, whereas grain size has a minor effect compared to the effect of temperature. We attribute two primary reasons for this: First, the bridgmanite-ferropericlase growth is very slow in the lower mantle and the grain size variation is too small to significantly alter the mantle viscosity. Secondly, if grains grow too fast, thus the mantle deforms in the dislocation creep regime, making viscosity grain size-independent. To establish the robustness of this finding we vary several other model parameters, such as surface yield strength, phase transition grain size reset, different transitional stresses for creep mechanisms, pressure dependence on grain growth, and different grain damage parameters. For all our models, we consistently find that grain size has a very limited effect on controlling lower mantle viscosity in the present-day Earth. However, large grain size may have affected the lower mantle viscosity in the early Earth as larger grains of single phase bridgmanite could increase the viscosity of the early mantle delaying the onset of global convection.

<p>East Asia is a tectonically active area on earth and has a complicated lithospheric deformation due to the western continental collision from the cratonic Indian plate and the eastern oceanic subduction mainly from Pacific plate. Studies have suggested that the Indo–Asian continental collision may have driven significant lateral mantle flow, but the velocity, range and effect of the mantle flow remain uncertain. Hence, a series of 3-D numerical models are conducted in this study to reveal the impacts of the Indo–Asian collision on mantle dynamics beneath the East Asia, especially on the asthenospheric mantle. Global model domain encompasses the lithosphere, upper mantle and the lower mantle with different viscosity for each layer. A global temperature structure built from seismic tomography and absolute plate field are applied subsequently to get a better constraint of the initial temperature condition and surficial velocity boundary condition. Thus, the reasonable velocity and temperature distributions of upper mantle beneath East Asia at different depths are retrieved based on our 3-D global mantle flow simulations, and the key controlling parameters in shaping the present-day observed mantle structure are investigated. The results show different scales of convection beneath East Asia.</p><p>Our results suggest that Indo–Asian collision may have induced mantle flow beneath the Indian plate and the different velocity structures between the asthenosphere and lithosphere indicate the shear drag of asthenospheric mantle. That may explain the reason that Indo–Asian collision has occurred since 50 Ma, and this collision can still continue to accelerate in the Tibetan Plateau. The simulation results also show the lithospheric delamination and the induced mantle upwelling, which is consistent with the general understanding from previous observations. The Indian lithosphere and its asthenosphere move northward, while the Yunnan lithosphere and its asthenosphere move southward, that may reflect the differences in deep mantle dynamics between the eastern and western Himalayan Syntaxis.</p>