Superplume Research Articles

Origin of ULVZs near the African LLSVP: Implications from their distribution and characteristics

Ultra-low velocity zones (ULVZs) provide important information on the composition and dynamics of the core-mantle boundary (CMB). However, their global distribution and characteristics are not well constrained, especially near African large low-shear velocity provinces (LLSVPs). Here, we used ScS precursor (SdS) and postcursor (ScscS) phases recorded by various seismic networks in Africa and South America to investigate the ULVZ characteristics underlying the South Atlantic Ocean. We found no evidence of ULVZs near the SE boundary of South America, but an ULVZ was found within the SW boundary of the African LLSVP, with thicknesses ranging from 11–18 km and reductions in S-wave velocities of 18%–34%. Our results, combined with the global distribution of ULVZs, suggest that thermal activity may be essential to ULVZ formation. Moreover, subducted slab and mantle flow may also play a key role, depending on the location of the ULVZs.

Multi‐Mode Waveform Tomography of the Indian Ocean Upper and Mid‐Mantle Around the Réunion Hotspot

AbstractRéunion Island in the western Indian Ocean is well known as one of the most active volcanic hotspots on Earth. Its birth, Ma ago, created the Deccan volcanic traps in India (almost 2 million ), associated with the Cretaceous‐Tertiary boundary and with the extinction of about 90% of life on the Earth, including dinosaurs. However, the deep structure of the underlying mantle, the potential presence of a rising plume and its exact geometry in the lower and in the upper mantle are still subjects of debates. The use of seismic data acquired by the French‐German RHUM‐RUM experiment in the Indian Ocean around the Réunion volcanic hotspot (2012–2013) and the collection of broadband seismic data from temporary experiments and from the FDSN (Federation of Digital Seismograph Networks) data center make it possible to investigate the deep structure of the Réunion mantle plume along its complete track, from its birth to its present stage, with a lateral resolution of km. So far, global seismic tomography models cannot provide such high resolution images of the transition zone or lower mantle in this region. In this study, we used the spectral element method (SEM) to perform waveform forward modeling for several thousand paths beneath the Indian Ocean, and normal mode perturbation theory to compute the gradient and the Hessian for the inverse part of the tomography. Using this hybrid method, we derived a regional tomographic model (including teleseismic and regional events) beneath the Indian Ocean, down to km depth, from simultaneous inversion of fundamental and higher mode three components waveforms down to 40 s period. Our model retrieves a low‐velocity channel extending from West to East in the western side of the Central Indian Ridge, in the depth range of 150–250 km. It also reveals a plume conduit with a broad head in the upper mantle and narrow tail anchored in the lower mantle at 1,200 km depth or deeper. The connection between the Réunion hotspot and the South‐Africa Large Low‐Shear Velocity Province (LLSVP) is also brought to light. Our findings suggest a long‐lived Réunion hotspot, since the lower part of the conduit appears to be anchored in the lower mantle, likely fed by the African LLSVP. Our results will guide further geochemical and geodynamic studies on the interaction between the lower transition zone (660–1,000 km) and the deep lower mantle beneath the Réunion hotspot.

Comparing global seismic tomography models using varimax principal component analysis

Abstract. Global seismic tomography has greatly progressed in the past decades, with many global Earth models being produced by different research groups. Objective, statistical methods are crucial for the quantitative interpretation of the large amount of information encapsulated by the models and for unbiased model comparisons. Here we propose using a rotated version of principal component analysis (PCA) to compress the information in order to ease the geological interpretation and model comparison. The method generates between 7 and 15 principal components (PCs) for each of the seven tested global tomography models, capturing more than 97 % of the total variance of the model. Each PC consists of a vertical profile, with which a horizontal pattern is associated by projection. The depth profiles and the horizontal patterns enable examining the key characteristics of the main components of the models. Most of the information in the models is associated with a few features: large low-shear-velocity provinces (LLSVPs) in the lowermost mantle, subduction signals and low-velocity anomalies likely associated with mantle plumes in the upper and lower mantle, and ridges and cratons in the uppermost mantle. Importantly, all models highlight several independent components in the lower mantle that make between 36 % and 69 % of the total variance, depending on the model, which suggests that the lower mantle is more complex than traditionally assumed. Overall, we find that varimax PCA is a useful additional tool for the quantitative comparison and interpretation of tomography models.

Seismicity of the East African Rift System for the period 2013 to 2016

Space-time distribution of earthquakes in the East African Rift System (EARS) has been investigated for the years 2013–2016. The spatial earthquakes distribution in the time window considered in this study has revealed that the Main Ethiopian Rift (MER) is structurally linked to the eastern branch of the EARS. A relatively high level of seismicity has been obtained in this study compared to the ISC (International Seismological Centre) catalog for the same time period which implies the region is seismically more active than we thought. In addition to the rift floor, the rift margins and the surrounding plateaus are found to be seismically active. The western and eastern branches of the EARS are found to be more active than the other rift segments for the time period considered. The frequency-magnitude distribution (FMD) of events during this study period provided an average b- and a -values of ~1.01 and ~6.5, respectively. The b-value result is implying relatively high stress with tectonic origin earthquakes occurrence dominating the region. On the other hand, threshold magnitude completeness of Mc 3.0 is obtained which is the minimum value that has never been observed previously for such large area. The seismic energy map of the region has been systematically investigated and peaks energy-release is observed at Afar depression, eastern and western branches. The energy mapping shows that the MER is structurally connected to the eastern branch of the EARS which is consistent with seismicity distribution. The overall depth of occurrence of events in the eastern and the western branches of the EARS are relatively deeper than the one found for Afar and MER. This implies that the seismogenic thickness increases in EARS when we go from north to south which is in agreement with rift age, magmatism and influence of the African Super plume on the crust and upper mantle deformation in EARS.

Elastic Properties of the Pyrite‐Type FeOOH‐AlOOH System From First‐Principles Calculations

AbstractThe stability, structure, and elastic properties of pyrite‐type (FeS2 structured) FeO2H were determined using density functional theory‐based computations with an internally consistent Coulombic self‐interaction term (Ueff). The properties of pyrite‐type FeO2H are compared to that of pyrite‐type AlO2H, with which it likely forms a solid solution at high temperature, as well as the respective lower pressure CaCl2‐type polymorphs of both endmembers: ϵ‐FeOOH and δ‐AlOOH. Due to substantial differences in the CaCl2‐type → pyrite‐type structural transition pressures of these endmembers, the stabilities of the (Al,Fe)O2H solid solution polymorphs are anticipated to be compositionally driven at lower mantle pressures. As the geophysical properties of (Al,Fe)OOH are structurally dependant, interpretations regarding the contribution of pyrite‐type FeO2H to seismically observed features must take into account the importance of this broad phase loop. With this in mind, Fe‐rich pyrite‐type (Al,Fe)OOH may coexist with Al‐dominant CaCl2‐type δ‐(Al,Fe)OOH in the deep Earth. Furthermore, pyrite‐type (Al0.5–0.6,Fe0.4–0.5)O2H can reproduce the reduced compressional and shear velocities characteristic of seismically observed ultra low velocity zones in the Earth's lowermost mantle while Al‐dominant but Fe‐bearing CaCl2‐type δ‐(Al,Fe)OOH may contribute to large low shear velocity provinces.

Formation of large low shear velocity provinces through the decomposition of oxidized mantle

Large Low Shear Velocity Provinces (LLSVPs) in the lowermost mantle are key to understanding the chemical composition and thermal structure of the deep Earth, but their origins have long been debated. Bridgmanite, the most abundant lower-mantle mineral, can incorporate extensive amounts of iron (Fe) with effects on various geophysical properties. Here our high-pressure experiments and ab initio calculations reveal that a ferric-iron-rich bridgmanite coexists with an Fe-poor bridgmanite in the 90 mol% MgSiO3–10 mol% Fe2O3 system, rather than forming a homogeneous single phase. The Fe3+-rich bridgmanite has substantially lower velocities and a higher VP/VS ratio than MgSiO3 bridgmanite under lowermost-mantle conditions. Our modeling shows that the enrichment of Fe3+-rich bridgmanite in a pyrolitic composition can explain the observed features of the LLSVPs. The presence of Fe3+-rich materials within LLSVPs may have profound effects on the deep reservoirs of redox-sensitive elements and their isotopes.

Neoproterozoic magmatic and metamorphic imprints in the East Kunlun Orogenic Belt, North Tibetan Plateau, NW China: Implications for the assembly and initial breakup of the Rodinia supercontinent

The East Kunlun Orogenic Belt (EKOB), located on the northern margin of the Tibetan Plateau, experienced intense, long-lasting,and complicatedmulti-stage orogeny from the Neoproterozoic to Early Mesozoic. Neoproterozoic magmatic and metamorphic events recorded critical information on Precambrian tectonic evolutionary history. Zircon U-Pb ages of 914 ± 6.5, 902 ± 5.3, and 856 ± 6.8 Ma were determined for granitic gneiss, monzogranite, and charnockite, respectively. Geochemical and petrological studies reveal high SiO2, Al2O3, and K2O content with abundant Al-rich minerals, such as garnet and muscovite, which show typical S-type granite properties. The εHf(t) values ranging from − 6.5 to + 5.6 (TDM2 = 1.4–2.2 Ga) indicate a primarily Paleoproterozoic metagreywackes and metapelitic derivation with mantle contribution. Zircon U-Pb results show that amphibolite yielded ages of 846 ± 5.2 Ma. Geochemistry andεHf(t) values ranging from 10.5 to 15.9 (TDM1 = 0.8–0.9 Ga) suggest that the mafic magma originated from depleted mantle metasomatized by subduction fluid. We propose that the Neoproterozoic rocks in this study were generated in syn-orogenesis and incipient continental rift-related extension environment, whichcorresponds to the assembly and initial breakup of the Rodinia supercontinent. In the EKOB, The initial breakup-related extension of the supercontinent affected by super plume occurred at ca. ~ 0.85 Ga. East Kunlun and its adjacent blocks were within a single tectonic domain and recorded important evolutionary history of the Rodinia supercontinent.

Lateral variations of electrical conductivity in the lower mantle constrained by Swarm and CryoSat-2 missions

The electrical conductivity is an important geophysical parameter connected to the thermal, chemical, and mineralogical state of the Earth’s mantle. In this paper, we apply the previously developed methodology of forward and inverse EM induction modeling to the latest version of satellite-derived spherical harmonic coefficients of external and internal magnetic field, and obtain the first 3-D mantle conductivity models with contributions from Swarm and CryoSat-2 satellite data. We recover degree 3 conductivity structures which partially overlap with the shape of the large low-shear velocity provinces in the lower mantle.

Constraints on the composition and temperature of LLSVPs from seismic properties of lower mantle minerals

Seismic observations have suggested the presence of two Large Low Shear Velocity Provinces (LLSVPs) in the lowermost mantle whose nature and origin are still debated. Several studies have tried to infer their potential composition using seismic observations with the hope to identify their formation mechanism. In particular, compositions enriched in iron (∼12–14 wt%) and bridgmanite (∼90 vol%) have been identified as promising candidates. Interestingly, these characteristics are somewhat consistent with the cumulates produced by the solidification of a primitive magma ocean, except that the iron enrichment should be much larger (>20 wt%). Here, we provide a reappraisal of potential LLSVPs compositions based on an improved mineralogical model including, in particular, the effects of alumina. We systematically investigate the effects of six parameters: FeO and Al2O3 content, proportion of CaSiO3 and bridgmanite (so that the proportion of ferropericlase is implicitly investigated), Fe3+/∑Fe and temperature contrast between far-field mantle and LLSVPs. From the 81 millions cases studied, only 79000 cases explain the seismic observations. Nevertheless, these successful cases involve a large range of parameters with, for instance, FeO content between 12–25 wt% and Al2O3 content between 3–17 wt%. We then apply a principal component analysis (PCA) to these cases and find two robust results: (i) the proportion of ferropericlase should be low (<6 vol%); (ii) the formation of Fe3+-bearing bridgmanite is much more favored compared to other iron-bearing components. Following these results, we identify two end-member compositions: a Bm-rich and a CaPv-rich one. For each end-member composition, a large range of parameters is possible. We note, however, that a low temperature contrast (<500 K) is favored, and that a certain proportion between FeO content, Al2O3 and oxidation state should be maintained. Finally, we discuss different scenarios for the formation of LLSVPs and propose that investigating the mineral proportion produced by each scenario is the best way to evaluate their relevance. For instance, the solidification of a primitive magma ocean may produce FeO and Al2O3 contents similar to those suggested by our analysis. However, the mineral proportion of such reservoirs is not well-constrained and may contain a larger proportion of ferropericlase than what is allowed by our results.

Bayesian Inference of Mantle Viscosity From Whole‐Mantle Density Models

AbstractThe long‐wavelength geoid is sensitive to Earth's mantle density structure as well as radial variations in mantle viscosity. We present a suite of inversions for the radial viscosity profile using whole‐mantle models that jointly constrain the variations in density, shear‐ and compressional‐wavespeeds using full‐spectrum tomography. We use a Bayesian approach to identify a collection of viscosity profiles compatible with the geoid, while enabling uncertainties to be quantified. Depending on tomographic model parameterization and data weighting, it is possible to obtain models with either positive‐ or negative‐buoyancy in the large low shear velocity provinces. We demonstrate that whole‐mantle density models in which density and VS variations are correlated imply an increase in viscosity below the transition zone, often near 1,000 km. Many solutions also contain a low‐viscosity channel below 650 km. Alternatively, models in which density is less‐correlated with VS—which better fit normal mode data—require a reduced viscosity region in the lower mantle. This feature appears in solutions because it reduces the sensitivity of the geoid to buoyancy variations in the lowermost mantle. The variability among the viscosity profiles obtained using different density models is indicative of the strong nonlinearities in modeling the geoid and the limited resolving power of the geoid kernels. We demonstrate that linearized analyses of model resolution do not adequately capture the posterior uncertainty on viscosity. Joint and iterative inversions of viscosity, wavespeeds, and density using seismic and geodynamic observations are required to reduce bias from prior assumptions on viscosity variation and scalings between material properties.

Fine‐Scale Structure of Earth's Deep Mantle Resolved Through Statistical Analysis of Hawaiian Basalt Geochemistry

AbstractHawaiian volcanoes belong to two geographically and geochemically distinct trends, the Loa and Kea trends. The cause of this dichotomy is still strongly debated. One of the prevailing hypotheses is that the two trends originate in the deep mantle where the Hawaiian mantle plume straddles two geophysically and geochemically distinct domains at the core‐mantle boundary (CMB). New high‐precision multi‐isotopic (Pb, Hf, Nd, and Sr) compositions of lavas from three key volcanoes, Lō‘ihi, Kohala, and Haleakalā, show transitional signatures between Loa and Kea compositions that call into question the degree of physical independence between the two trends. Statistical analysis of multi‐collector inductively coupled plasma mass spectrometer or triple‐spike Pb (n &gt; 800) and Sr, Nd, and Hf data (n &gt; 400) for shield tholeiites from the entire Hawaiian Islands (&lt;5.5 Ma) identifies six unique geochemical groups, and for the first time, documents large‐scale heterogeneities in the Kea trend. The spatial orientation of the six geochemical groups shows that the bilateral zonation of the plume source at the CMB is gradational, and that the Hawaiian mantle plume periodically entrains large‐scale ephemeral geochemical heterogeneities on million‐year, regional timescales. These geochemical heterogeneities are stretched vertically during transit of the plume to the surface and are observed in the lavas from the Hawaiian Islands. These results provide evidence that the large low shear velocity province in the deep Pacific is thermochemical and highly heterogeneous.

Deflating the LLSVPs: Bundles of Mantle Thermochemical Plumes Rather Than Thick Stagnant “Piles”

AbstractBased on SEMUCB‐WM1 tomographic model, validated by other recent models, and fluid mechanics constraints, we show that the large low shear velocity provinces (LLSVPs) present at the base of the Earth's mantle beneath the Pacific and Africa do not extend as compact, uniform structures very high above the core‐mantle boundary. In contrast, they contain a number of well‐separated, low‐velocity conduits that extend vertically throughout most of the lower mantle. The conceptual model of compact piles, continuously covering the areal extent of the LLSVPs, is therefore not correct. Instead, each LLSVP is composed of a bundle of thermochemical upwellings probably enriched in denser than average material. It is only when the tomographic model is filtered to long wavelengths that the two bundles of plumes appear as uniform provinces. Furthermore, the overall shape of the LLSVPs is probably controlled by the distribution of subducted slabs, and due to their thermochemical nature, the position of both LLSVPs and individual upwelling dynamics should be time dependent. There is also evidence for smaller plumes originating near the CMB in the faster than average regions of the voting map of Lekic et al. (2012, https://doi.org/10.1016/j.epsl.2012.09.014) as well as other, barely resolved, weaker plumes within the LLSVPs. These finer‐scale features are starting to be resolved tomographically owing to improvements in full waveform modeling of body waves, including diffracted S waves (Sdiff) and waves multiply reflected on the core‐mantle boundary (ScS) and their codas.

Insights Into the Origins and Compositions of Mantle Plumes: A Comparison of Galápagos and Hawai'i

AbstractThe Galápagos and Hawai'i archipelagos are formed by mantle plumes originating at the large low shear velocity province (LLSVP) boundary. We report new high‐precision Pb, Sr, Nd, and Hf isotopic analyses on 83 Galápagos samples and compare them with those of Hawai'i. The data confirm that like Hawai'i, Galápagos is a bilaterally asymmetric plume whose compositional boundary trends NW‐SE. On their northeast sides, the plumes share a common source, Pacific lower mantle, whose intermediate isotopic signature may be common to many plumes. The Hawaiian and Galápagos plumes' southwestern sides are anchored in the Pacific LLSVP and are compositionally distinct; in Hawai'i, Loa trend lavas reflect contributions from the EM1 mantle end‐member, whereas in Galápagos, HIMU is dominant, suggesting that the Pacific LLSVP is compositionally heterogeneous and includes different types of recycled material. Furthermore, the surficial expression of a bilaterally asymmetric plume is strongly influenced by its tectonic setting: (a) Thick Hawaiian lithosphere supports a volcano evolution process, including rejuvenated volcanism, whereas the thin Galápagos lithosphere inhibits Hawai'i‐style rejuvenated‐stage eruptive activity, instead causing extended, widespread volcanism; (b) the proximity of the Galápagos to a mid‐ocean ridge causes entrainment of the depleted upper mantle, overwhelming depleted material intrinsic to the plume and affecting volcanoes' magmatic architecture; and (c) the geometric relationship between the LLSVP boundary and plate motion influences geochemical patterns at the surface. Thus, despite striking differences in surficial expression of the Galápagos and Hawai'i plumes, they share a common generation mechanism, supplied by the Pacific LLSVP and the lower mantle.

Shallow Lower Mantle Viscosity Modulates the Pattern of Mantle Structure

AbstractTomographic images of Earth's mantle reveal that structure, even at the largest spatial scales, does not remain correlated across all mantle depths. This is a surprising result given the relationship between a dominantly spherical harmonic degree‐2 pattern in the lower mantle and the pattern of plate motions at the surface, that is, the fact that the two large, low‐shear velocity provinces (LLSVPs) beneath Africa and the Pacific Ocean are located near regions of surface divergence and away from regions of recent and ongoing subduction. Here, we combine analysis of seismic tomography models and numerical simulations of global‐scale mantle circulation to show that the change in correlation in mantle structure across the extended transition zone (400–1,000‐km depth), as revealed by the mantle radial correlation function (RCF), can be attributed primarily to the influence of seismically fast regions in the transition zone beneath the western Pacific subduction zones. Similar changes in the correlation of structure in global models of mantle circulation can be explained by a relatively large increase in viscosity (of at least a hundred times) between the upper mantle and lower mantle. We also demonstrate that a stronger endothermic phase transition at a depth of 660 km helps explain the observed pattern of the mantle RCF and the correlation between geodynamic models and seismic tomography. We account for the differences in resolution and parameterization of geodynamic versus tomographic models through the application of a tomographic resolution operator.

Paired EMI-HIMU hotspots in the South Atlantic-Starting plume heads trigger compositionally distinct secondary plumes?

Age-progressive volcanism is generally accepted as the surface expression of deep-rooted mantle plumes, which are enigmatically linked with the African and Pacific large low-shear velocity provinces (LLSVPs). We present geochemical and geochronological data collected from the oldest portions of the age-progressive enriched mantle one (EMI)-type Tristan-Gough track. They are part of a 30- to 40-million year younger age-progressive hotspot track with St. Helena HIMU (high time-integrated 238U/204Pb) composition, which is also observed at the EMI-type Shona hotspot track in the southernmost Atlantic. Whereas the primary EMI-type hotspots overlie the margin of the African LLSVP, the HIMU-type hotspots are located above a central portion of the African LLSVP, reflecting a large-scale geochemical zonation. We propose that extraction of large volumes of EMI-type mantle from the margin of the LLSVP by primary plume heads triggered upwelling of HIMU material from a more internal domain of the LLSVP, forming secondary plumes.

Seismic Evidence for a Hot Mantle Transition Zone Beneath the Indian Ocean Geoid Low

AbstractThe Indian Ocean Geoid Low (IOGL) is the most prominent geoid anomaly (−106 m) on the globe, whose origin remains elusive. In the present study, we investigate the mantle transition zone (MTZ) structure beneath the region using P receiver functions (PRFs), to examine its role in the genesis of this feature. Results from 3‐D time to depth migration of PRFs reveal a thin MTZ primarily due to an elevation of the 660 km discontinuity. This is suggestive of anomalously hot temperatures in the mid mantle beneath the IOGL region, possibly sourced from the African Large Low Shear Velocity Province (LLSVP). The combined effect of the hot (low‐density) material in the MTZ proposed in this study and the (high‐density) cold slab graves atop the core‐mantle boundary inferred from previous studies can possibly explain this geoid low.

On the self-regulating effect of grain size evolution in mantle convection models: application to thermochemical piles

Abstract. Seismic studies show two antipodal regions of lower shear velocity at the core–mantle boundary (CMB) called large low-shear-velocity provinces (LLSVPs). They are thought to be thermally and chemically distinct and therefore might have a different density and viscosity than the ambient mantle. Employing a composite rheology, using both diffusion and dislocation creep, we investigate the influence of grain size evolution on the dynamics of thermochemical piles in evolutionary geodynamic models. We consider a primordial layer and a time-dependent basalt production at the surface to dynamically form the present-day chemical heterogeneities, similar to earlier studies, e.g. by Nakagawa and Tackley (2014). Our results show that, relative to the ambient mantle, grain size is higher inside the piles, but, due to the high temperature at the CMB, the viscosity is not remarkably different from ambient mantle viscosity. We further find that although the average viscosity of the detected piles is buffered by both grain size and temperature, the viscosity is influenced predominantly by grain size. In the ambient mantle, however, depending on the convection regime, viscosity can also be predominantly controlled by temperature. All pile properties, except for temperature, show a self-regulating behaviour: although grain size and viscosity decrease when downwellings or overturns occur, these properties quickly recover and return to values prior to the downwelling. We compute the necessary recovery time and find that it takes approximately 400 Myr for the properties to recover after a resurfacing event. Extrapolating to Earth values, we estimate a much smaller recovery time. We observe that dynamic recrystallisation counteracts grain growth inside the piles when downwellings form. Venus-type resurfacing episodes reduce the grain size in piles and ambient mantle to a few millimetres. More continuous mobile-lid-type downwellings limit the grain size to a centimetre. Consequently, we find that grain-size-dependent viscosity does not increase the resistance of thermochemical piles to downgoing slabs. Mostly, piles deform in grain-size-sensitive diffusion creep, but they are not stiff enough to counteract the force of downwellings. Hence, we conclude that the location of subduction zones could be responsible for the location and stability of the thermochemical piles of the Earth because of dynamic recrystallisation.

The Formation of Hot Thermal Anomalies in Cold Subduction‐Influenced Regions of Earth's Lowermost Mantle

AbstractThe Earth's lowermost mantle is characterized by two large low shear velocity provinces (LLSVPs). The regions outside the LLSVPs have been suggested to be strongly influenced by subducted slabs and, therefore, much colder than the LLSVPs. However, localized low‐velocity seismic anomalies have been detected in the subduction‐influenced regions, whose origin remains unclear. Here, three‐dimensional geodynamic calculations are performed, and they show that linear, ridge‐like hot thermal anomalies, or thermal ridges, form in the relatively cold, downwelling regions of the lowermost mantle. Like the formation of Richter rolls due to sublithosphere small‐scale convection (SSC), the thermal ridges form as a result of SSC from the basal thermal boundary layer and they extend in directions parallel to the surrounding mantle flow. The formation of thermal ridges in subduction regions of the lowermost mantle is very sensitive to the thermal structures of the subducted materials, and thermal heterogeneities brought to the bottom of the mantle by subducting slabs greatly promote the formation of thermal ridges. The formation of thermal ridges is also facilitated by the increase of core‐mantle boundary heat flux and vigor of lowermost mantle convection. The thermal ridges may explain the low‐velocity seismic anomalies outside of the LLSVPs in the lowermost mantle. The results suggest that the relatively cold, subduction‐influenced regions of the Earth's lowermost mantle may contain localized hot anomalies.

How Thermochemical Piles Can (Periodically) Generate Plumes at Their Edges

AbstractDeep‐rooted mantle plumes are thought to originate from the margins of the Large Low Shear Velocity Provinces (LLSVPs) at the base of the mantle. Visible in seismic tomography, the LLSVPs are usually interpreted to be intrinsically dense thermochemical piles in numerical models. Although piles deflect lateral mantle flow upward at their edges, the mechanism for localized plume formation is still not well understood. In this study, we develop numerical models that show plumes rising from the margin of a dense thermochemical pile, temporarily increasing its local thickness until material at the pile top cools and the pile starts to collapse back toward the core‐mantle boundary (CMB). This causes dense pile material to spread laterally along the CMB, locally thickening the lower thermal boundary layer on the CMB next to the pile, and initiating a new plume. The resulting plume cycle is reflected in both the thickness and lateral motion of the local pile margin within a few hundred km of the pile edge, while the overall thickness of the pile is not affected. The period of plume generation is mainly controlled by the rate at which slab material is transported to the CMB, and thus depends on the plate velocity and the sinking rate of slabs in the lower mantle. A pile collapse, with plumes forming along the edges of the pile's radially extending corner, may, for example, explain the observed clustering of Large Igneous Provinces (LIPs) in the southeastern corner of the African LLSVP around 95–155 Ma.

Core-mantle boundary topography and its relation to the viscosity structure of the lowermost mantle

Two large areas of anomalously low seismic velocities are visible in all tomographic models of the lowermost mantle. Depending on the density structure of these Large Low Shear Velocity Provinces (LLSVPs), the core-mantle boundary (CMB) will deform upwards or downwards due to isostatic and dynamic topography, the latter being sensitive to the viscosity structure of the lowermost mantle. Heterogeneities in the viscosity structure, although difficult to constrain, might be especially important if the LLSVPs are thermochemical piles with elevated intrinsic viscosity as suggested by mineral physics. Based on numerical models, we identify a short-wavelength (about 80-120 km wide, up to a few km deep) topographic depression that forms around the pile edges if the pile is more viscous than the surrounding mantle. The depression forms when a wedge of thermal boundary layer material becomes compressed against the viscous pile, and is enhanced by relative uplift of the CMB beneath the pile by plumes rising above it. The depth and asymmetry of the depression constrain the magnitude of the viscosity contrast between pile and the surrounding mantle. Furthermore, (periodic) plume initiation and pile collapse at the pile margin systematically modify the characteristic depression, with a maximum in asymmetry and depth at the time of plume initiation. Core-reflected waves or scattered energy may be used to detect this topographic signature of stiff thermochemical piles at the base of the mantle.