Deep Mantle Heterogeneities Research Articles

Light oxygen isotopic composition in deep mantle reveals oceanic crust subduction before 3.3 billion years ago

Compositional heterogeneity exists in Earth’s deep mantle, which can be caused by the subduction of oceanic slabs. How early this process started on Earth remains highly debated due to the scarcity of early Archean materials with pristine mantle compositional signatures. Here, using the oxygen isotope and elemental compositions of fresh olivine grains in the 3.27-Ga komatiites of the Weltevreden Formation in the Barberton Greenstone Belt in Southern Africa, we discovered two groups of samples with primitive olivine grains. Group I exhibits normal mantle-like δ18O values and high Fo contents (δ18O = 4.9–5.4‰; Fo = 93–95); Group II is characterized by lower δ18O values with slightly lower Fo contents (δ18O = 3.6–4.7‰; Fo = 91–93). These δ18O values correlate with other geochemical proxies of olivine-poor iron-rich pyroxenite sources, indicating that the Weltevreden komatiites were derived from two distinct mantle sources. The existence of the low-δ18O magmas can be best explained by recycling of the altered oceanic crust into deep mantle arguably by subduction, which started 3.3 billion years ago and is responsible for the deep mantle heterogeneity in early Earth.

Deciphering the deep Earth heterogeneities from the temperature fluctuation of mantle plumes

Distinguishing deep lower mantle heterogeneities and resolving their physical and chemical properties are challenging due to the difficulty in achieving severe high-temperature and high-pressure conditions simultaneously in mineral experiments. Deep-seated mantle plumes bring these heterogeneities to the Earth's surface, therefore providing a unique insight into the Earth's deep interior. Here, we link the surface temperature fluctuation of mantle plume to the properties of deep-mantle heterogeneities via three-dimensional geodynamic modeling. Our results show that high-viscosity primordial mantle materials significantly increase the surface plume temperature due to their excessive viscous heating, whereas high-density oceanic crust slightly reduces it. Give some estimates for the maximum plume fluctuation temperature through time, the survival of scattered primordial blobs with a high viscosity contrast of ∼10-50 times in the deep-mantle reservoir is required. The temperature variations of the thermochemical mantle plume likely control the observed multiple volcanic episodes of large igneous provinces and periodic changes of volcanic flux along hotspot tracks.

Global mantle flow retrodictions for the early Cenozoic using an adjoint method: evolving dynamic topographies, deep mantle structures, flow trajectories and sublithospheric stresses

SUMMARY During the Cenozoic, the Earth experienced multiple first-order geological events that are likely mantle flow related. These include the termination of large-scale marine inundation in North America in the Palaeocene, the late Tertiary rise of Africa relative to other continents and the long-wavelength tilting of Australia since the late Cretaceous, which occurred when the continent approached the southeast Asia subduction systems on its northward passage from Antartica. Here we explore a suite of eight high-resolution, compressible, global mantle flow retrodictions going back to 50 Ma, using an adoint method with $\approx$670 million finite elements. These retrodictions show for the first time that these events emerge jointly as part of global Cenozoic mantle flow histories. Our retrodictions involve the dynamic effects from an upper mantle low-viscosity zone, assimilate a past plate-motion model for the tangential surface velocity field, probe the influence of two different present-day mantle state estimates derived from seismic tomography, and acknowledge the rheological uncertainties of dynamic Earth models by taking in four different realizations for the radial mantle viscosity profile, two of which were published previously. We find the retrodicted mantle flow histories are sensitive to the present-day mantle state estimate and the rheological properties of the Earth model, meaning that this input information is testable with inferences gleaned from the geological record. For a deep mantle viscosity of $1.7\times 10^{22}$ Pa s and a purely thermal interpretation of seismic structure, lower mantle flow velocities exceed 7 cm yr–1 in some regions, meaning they are difficult to reconcile with the existence of a hotspot reference frame. Conversely, a deep mantle viscosity of $10^{23}$ Pa s yields modest flow velocities (< 3 cm yr–1 ) and stability of deep mantle heterogeneity for much of the retrodiction time, albeit at the expense that African uplift is delayed into the latest Neogene. Retrodictions allow one to track material back in time from any given sampling location, making them potentially useful, for example, to geochemical studies. Our results call for improved estimates on non-isostatic vertical motion of the Earth’s surface—provided, for instance, by basin analysis, seismic stratigraphy, landform studies, thermochronological data or the sedimentation record—to constrain the recent mantle flow history and suggest that mantle flow retrodictions may yield synergies across different Earth science disciplines.

Palaeoarchaean deep mantle heterogeneity recorded by enriched plume remnants

The thermal and chemical state of the early Archaean deep mantle is poorly resolved due to the rare occurrences of early Archaean magnesium-rich volcanic rocks. In particular, it is not clear whether compositional heterogeneity existed in the early Archaean deep mantle and, if it did, how deep mantle heterogeneity formed. Here we present a geochronological and geochemical study on a Palaeoarchaean ultramafic–mafic suite (3.45-Gyr-old) with mantle plume signatures in Longwan, Eastern Hebei, the North China Craton. This suite consists of metamorphosed cumulates and basalts. The meta-basalts are iron rich and show the geochemical characteristics of present-day oceanic island basalt and unusually high mantle potential temperatures (1,675 °C), which suggests a deep mantle source enriched in iron and incompatible elements. The Longwan ultramafic–mafic suite is best interpreted as the remnants of a 3.45-Gyr-old enriched mantle plume. The first emergence of mantle-plume-related rocks on the Earth 3.5–3.45 billion years ago indicates that a global mantle plume event occurred with the onset of large-scale deep mantle convection in the Palaeoarchaean. Various deep mantle sources of these Palaeoarchaean mantle-plume-related rocks imply that significant compositional heterogeneity was present in the Palaeoarchaean deep mantle, most probably introduced by recycled crustal material. Deep mantle heterogeneity and large-scale deep mantle convection has been operating since the Palaeoarchaean, according to enriched plume signatures found in a 3.45-billion-year-old ultramafic–mafic suite from the North China Craton.

Do mantle plumes preserve the heterogeneous structure of their deep-mantle source?

It has been proposed that the spatial variations recorded in the geochemistry of hotspot lavas, such as the bilateral asymmetry recorded at Hawaii, can be directly mapped as the heterogeneous structure and composition of their deep-mantle source. This would imply that source-region heterogeneities are transported into, and preserved within, a plume conduit, as the plume rises from the deep-mantle to Earth's surface. Previous laboratory and numerical studies, which neglect density and rheological variations between different chemical components, support this view. However, in this paper, we demonstrate that this interpretation cannot be extended to distinct chemical domains that differ from surrounding mantle in their density and viscosity. By numerically simulating thermo-chemical mantle plumes across a broad parameter space, in 2-D and 3-D, we identify two conduit structures: (i) bilaterally asymmetric conduits, which occur exclusively for cases where the chemical effect on buoyancy is negligible, in which the spatial distribution of deep-mantle heterogeneities is preserved during plume ascent; and (ii) concentric conduits, which occur for all other cases, with dense material preferentially sampled within the conduit's centre. In the latter regime, the spatial distribution of geochemical domains in the lowermost mantle is not preserved during plume ascent. Our results imply that the heterogeneous structure and composition of Earth's lowermost mantle can only be mapped from geochemical observations at Earth's surface if chemical heterogeneity is a passive component of lowermost mantle dynamics (i.e. its effect on density is outweighed by, or is secondary to, the effect of temperature). The implications of our results for: (i) why oceanic crust should be the prevalent component of ocean island basalts; and (ii) how we interpret the geochemical evolution of Earth's deep-mantle are also discussed.

Analysis of the Refined CRUST1.0 Crustal Model and its Gravity Field

The global crustal model CRUST1.0 (refined using additional global datasets of the solid topography, polar ice sheets and geoid) is used in this study to estimate the average densities of major crustal structures. We further use this refined model to compile the gravity field quantities generated by the Earth's crustal structures and to investigate their spatial and spectral characteristics and their correlation with the crustal geometry in context of the gravimetric Moho determination. The analysis shows that the average crustal density is 2,830 kg/m(3), while it decreases to 2,490 kg/m(3) when including the seawater. The average density of the oceanic crust (without the seawater) is 2,860 kg/m(3), and the average continental crustal density (including the continental shelves) is 2,790 kg/m(3). The correlation analysis reveals that the gravity field corrected for major known anomalous crustal density structures has a maximum (absolute) correlation with the Moho geometry. The Moho signature in these gravity data is seen mainly at the long-to-medium wavelengths. At higher frequencies, the Moho signature is weakening due to a noise in gravity data, which is mainly attributed to crustal model uncertainties. The Moho determination thus requires a combination of gravity and seismic data. In global studies, gravimetric methods can help improving seismic results, because (1) large parts of the world are not yet sufficiently covered by seismic surveys and (2) global gravity models have a relatively high accuracy and resolution. In regional and local studies, the gravimetric Moho determination requires either a detailed crustal density model or seismic data (for a combined gravity and seismic data inversion). We also demonstrate that the Earth's long-wavelength gravity spectrum comprises not only the gravitational signal of deep mantle heterogeneities (including the core-mantle boundary zone), but also shallow crustal structures. Consequently, the application of spectral filtering in the gravimetric Moho determination will remove not only the gravitational signal of (unknown) mantle heterogeneities, but also the Moho signature at the long-wavelength gravity spectrum.

The effect of plate motion history on the longevity of deep mantle heterogeneities

Understanding the first-order dynamical structure and evolution of Earth's mantle is a fundamental goal in solid-earth geophysics. Tomographic observations reveal a lower mantle characterised by higher-than-average shear-wave speeds beneath Asia and encircling the Pacific, consistent with cold slabs beneath regions of ancient subduction, and lower-than-average shear-wave speeds in broad regional areas beneath Africa and the Central Pacific (termed LLSVPs). The LLSVPs are not well understood from a dynamical perspective and their origin and evolution remain enigmatic. Some numerical studies propose that the LLSVP beneath Africa is post-Pangean in origin, formed as a result of return flow in the mantle due to circum-Pangean subduction, countered by an older Pacific LLSVP, suggested to have formed during the break up of Rodinia. This propounds that, prior to the formation of Pangea, the lower mantle was dominated by a degree-1 convection pattern with a major upwelling centred close to the present-day Pacific LLSVP and subduction concentrated mainly in the antipodal hemisphere. In contrast, palaeomagnetic observations which proffer a link between the reconstructed eruption sites of Phanerozoic kimberlites and Large Igneous Provinces with regions on the margins of the present-day LLSVPs suggest that the anomalies may have remained stationary for at least the last 540 Myr and further that the anomalies were largely insensitive to the formation and subsequent break-up of Pangea. Here we investigate the evolution and long-term stability of LLSVP-like structures in Earth's mantle by integrating plate tectonics and numerical models of global thermochemical mantle dynamics. We explore the possibility that either one or both LLSVPs existed prior to the formation of Pangea and improve upon previous studies by using a new, true polar wander-corrected global plate model to impose surface velocity boundary conditions for a time interval that spans the amalgamation and subsequent break-up of the supercontinent. We find that, were only the Pacific LLSVP to exist prior to the formation of Pangea, the African LLSVP would not have been created within the lifetime of the supercontinent. We also find that, were the mantle to be dominated by two antipodal LLSVP-like structures prior to the formation of Pangea, the structures would remain relatively unchanged to the present day and would be insensitive to the formation and break-up of the supercontinent. Our results suggest that both the African and Pacific LLSVPs have remained close to their present-day positions for at least the past 410 Myr.

Constraints on core‐mantle boundary topography from normal mode splitting

The frequencies of Earth's normal modes are split by rotation, ellipticity, and internal structure of the Earth. Thus, models of mantle heterogeneity and discontinuity topography generate splitting that may be tested against observations. We insert maps of core‐mantle boundary (CMB) topography, which are derived via either a purely seismic or a joint tomographic/geodynamic inversion of body waves data, on top of tomographic model S20RTS. We then calculate synthetic splitting functions for normal modes that have been shown to be sensitive to CMB topography and compare these to observed normal mode splitting data. The CMB topography maps obtained via geodynamically constrained tomography fit normal mode data better than purely seismic maps, in particular when the geodynamic constraint also accounts for the presence of post‐perovskite in the D″ region. We test the significance of the reduction in misfit using the concept of observability which suggests that normal modes are able to observe the difference between the different CMB topography maps. In addition, we find that the statistical significance, assessed by checking what fraction of 1000 randomly generated CMB models achieve a comparatively good fit as the desired model, is higher than 92% for degree 2 and 98% for all degrees. In summary, we have identified a model of CMB topography that fits body wave data and improves, at least to some extent, the fit to normal mode data, and is coherent with the large‐scale pattern of deep mantle heterogeneity expected on the basis of convection modeling.

Elasticity of subducted basaltic crust at the lower mantle pressures: Insights on the nature of deep mantle heterogeneity

The origin of the large low velocity structures observed in the lowermost mantle is still unclear. To make new insights into deep mantle heterogeneity, we performed ab initio modeling of the elasticity of representative petrology with the pyrolitic and basaltic compositions based on the elasticities of the following mineral phases: magnesium silicate perovskite (post-perovskite), ferropericlase, calcium silicate perovskite, stishovite (CaCl 2), and calcium ferrite (titanate)-type MgAl 2O 4. Calculations elucidated that the basalt composite is ∼2% denser than the pyrolitic one through all of the pressures in the lower mantle. In addition, the shear ( V S) and bulk sound ( V Φ) velocities were found to be ∼2% slower and faster in the basalt composition in particular at ∼30 GPa and >120 GPa respectively, while both the pyrolitic and basaltic composites have almost the same compressional wave speed ( V P). Thermochemically distinct piles which are thought to be related to accumulated hot and dense subducted crusts would therefore be expected to produce as much as a −5% to −6% low velocity anomaly in the S wave but only ∼−1% in the P wave. These suggest that the basaltic pile seems less compatible to the seismological signatures of the large low velocity structures, if accompanied by thermal anomalies reaching +1000 to +1500 K.

Using probabilistic seismic tomography to test mantle velocity–density relationships

We use a neighborhood algorithm to explore the fit to long period seismic data of a wide variety of long wavelength mantle models. This approach to the global tomographic inverse problem yields probability distributions for seismic velocities, density, and related properties as functions of depth. Such distributions can be robust even when individual models are not, and allow us to test several assumptions about the Earth that have long been enforced a priori in inversions. In particular, we are able to test the paradigm of deep mantle heterogeneity that is dominantly thermal in origin, producing velocity and density perturbations that are well correlated and have relative amplitudes given by δlnρ/δlnvs<0.5. Our distributions show that such relationships are unlikely, and even though the results are consistent with recent best fitting models from damped seismic inversions, they demonstrate that many specific properties of such models are not robust. The data clearly favor density perturbations that are poorly or negatively correlated with velocity heterogeneity and have amplitudes several times larger (yielding δlnρ/δlnvs>1.0) than damped inversions allow. These characteristics are most pronounced in the upper mantle transition zone and the base of the lower mantle, suggesting layered convection. The negative density–velocity correlations favored at these depths imply dominantly chemical heterogeneity, while the likelihood of relatively high amplitude density variations suggests that variable iron content is an important component of this heterogeneity. These results, which we show to be consistent with independent gravity constraints, represent a profound change in the interpretation of seismic constraints. In addition, the distributions show that even though best fitting density models from recent inversions or our sampling are consistent with the data, most specific properties of such models are not robust. This implies that it is more appropriate to use seismic model distributions, rather than individual models, to make geodynamic and geochemical inferences.

Topography of the ocean floor: Thermal evolution of the lithosphere and interaction of deep mantle heterogeneities with the lithosphere

Topography associated with large wave‐length geoid is derived by two methods. First, the bathymetry corrected for sediment loading is decomposed using a least‐squares method as a function of age plus terms proportional to the geoid. The topography at old ages is flatter predicted by the law. At degrees 2 and 3, very little topography is associated with the geoid. The transfer function is not even certain in sign but is never smaller than 0.6. More topography seems associated with higher degree harmonics (6 and 7). Second, the function of age alone best fitting the topography is determined. The correlation coefficients of the residual topography with the geoid are then computed. A slight correlation appears for degree 6, but not for degrees 2 and 3. These results suggest a, mechanical decoupling between the lower mantle mass heterogeneities and the lithosphere.

Whole-mantle convection and plate tectonics

Summary. It seems unlikely that the lower mantle is not involved in motions related to plate tectonics. The evidence relating to several alleged obstacles to lower-mantle convection is reviewed. The evidence for a chemical composition difference between the upper and lower mantle is not compelling. There now seems to be good evidence that the viscosity of the mantle is fairly uniform, contrary to a widely-held view, except possibly in the oceanic upper mantle. Phase changes may locally enhance or retard thermal convection, but are unlikely to prevent it. It is not necessary to assume that descending lithospheric slabs cannot penetrate below 700 km depth in order to explain the distribution and source mechanisms of deep earthquakes. Other recent seismic evidence suggests deep-mantle heterogeneities which may be related to large-scale flow. The behaviour of some simple layered-fluid models is analysed to show the effect of viscosity contrasts. The results indicate that the lower mantle would have to be 10000 times more viscous than the upper mantle in order to confine thermal convection to the upper mantle. The lower mantle would have to be at least 1000 times more viscous than the upper mantle in order to exclude viscous flow entrained by the moving plates. Alternatively, a shallow low-viscosity hyer would have to be about 10 000 times less viscous than the underlying mantle for the 'return' flow to be confined to it. These and other results indicate that only extreme viscosity contrasts (arising from non-linear or temperature-dependent rheology, for instance, as well as from different physical states) significantly affect large-scale flow patterns, and that flow patterns are strongly affected by moving boundaries. A version of whole-mantle thermal convection is proposed which seems to be capable of explaining the main features of plate tectonics: the buoyancy forces are concentrated in, but not confined to, the descending lithospheric slabs. Descending slabs would then cause their attached plates to move rapidly, as in the Elsasser model. The flow entrained by the fast plates would dominate the flow pattern in the mantle: convection cells under fast plates would be amplified; those under other plates would have smaller amplitudes and would fit between the fast cells. This model can qualitatively account for