Lowermost Mantle Research Articles

Grain Boundary Diffusion of Ferropericlase: Implications for the Core‐Mantle Interaction

AbstractGeophysical observations indicate that iron enrichment of various spatial scales may be present in the lowermost mantle. Various mechanisms have been proposed to explain the process of iron infiltration from the core to the mantle, each with its own inherent limitations. Grain boundary (GB) diffusion significantly outpaces bulk diffusion within crystal interiors, and may facilitate iron transport across the core‐mantle boundary (CMB). In this study, we investigate diffusion in two symmetric tilt GBs of ferropericlase and ferropericlase single crystals using large‐scale molecular dynamics simulations. The GB diffusivities in pure periclase and their temperature dependence agree well with previous studies. In addition, we study the GB diffusion of Fe in (Mg, Fe)O GBs for the first time. The results suggest that GB diffusion of Fe is likely to be sluggish near the CMB, and thus may not be an effective mechanism for transporting iron from the core to the mantle.

Subducted Slab Slipping Underneath the Northern Edge of the Pacific Large Low‐Shear‐Velocity Province in D″

AbstractWe conduct waveform inversion for the 3‐D seismic shear wave (S‐wave) velocity structure in the lowermost mantle near the northern edge of the Pacific large low‐shear‐velocity province (LLSVP). We image a slab‐like high‐velocity anomaly slipping beneath the Pacific LLSVP in the lowermost 200 km of the mantle, extending toward an ultra‐low velocity zone (ULVZ) beneath a point about 2,000 km southwest of Hawaii. Another strong low‐velocity anomaly exists along the edge of the LLSVP just above the slab‐like sheet, 50–200 km above the core‐mantle boundary (CMB). These results suggest in general that (a) slabs can gather ULVZ materials scattered on the CMB and push them into LLSVPs, creating concentrated ULVZs near LLSVP edges, (b) slabs can uplift hot material from the CMB to create strong anomalies along the edges of LLSVPs, and (c) large seismic‐wave velocity contrasts between strong low‐velocity anomalies and slabs create sharp LLSVP boundaries.

Why Are Plume Excess Temperatures Much Less Than the Temperature Drop Across the Lowermost‐Mantle Thermal Boundary Layer?

AbstractWhile temperature drop across the mantle's basal thermal boundary layer (TBL) is likely 1,000 K, the temperature anomaly of plumes believed to rise from that TBL is only up to a few hundred Kelvins. Reasons for that discrepancy are still poorly understood and a number of causes have been proposed. Here, we use the ASPECT software to model plumes from the lowermost mantle and study their excess temperatures. We use a mantle viscosity that depends on temperature and depth with a strong viscosity increase from below the lithosphere toward the lower mantle, reaching about Pas above the basal TBL, consistent with geoid modeling and slow motion of mantle plumes. With a mineral physics‐derived pyrolite material model, the difference between a plume adiabat and an ambient mantle adiabat just below the lithosphere is about two thirds of that at the base of the mantle, for example, 1,280 versus 835 K. 3D models of isolated plumes become nearly steady‐state 10–20 Myr after the plume head has reached the surface, with excess temperature drop compared to an adiabat for material directly from the core‐mantle boundary (CMB) usually less than 100 K. In the Earth, plumes are likely triggered by slabs and probably rise preferably above the margins of chemically distinct piles. This could lead to reduced excess temperatures, if plumes are more sheet‐like, similar to 2D models, or temperature at their source depth is less than at the CMB. Excess temperatures are further reduced when averaged over the plume conduit or melting region.

Micropolar Modeling of Shear Wave Dispersion in Marine Sediments and Deep Earth Materials: Deriving Scaling Laws

We draw connections between eight different theories used to describe microscopic (atomic) and macroscopic (seismological) scales. In particular, we show that all these different theories belong to a particular case of a single partial differential equation, allowing us to gain new physical insights and draw connection among them. With this general understanding, we apply the micropolar theory to the description of shear-wave dispersion in marine sediments, showing how we can reproduce observations by only using two micropolar parameters in contrast to the seventeen parameters required by modifications of Biot’s theory. We next establish direct connections between the micro (laboratory) and macro (seismological) scales, allowing us to predict (and confirm) the presence of post-perovskite in the lowermost mantle based on laboratory experiments and to predict the characteristic length Lc at which shear deformation becomes significant at seismological scales in the lowermost mantle.

The Use of Azimuthal Variation in ScS–S Differential Travel Times to Investigate Possible Anisotropy in the Lowermost Mantle Beneath the Philippines

The Use of Azimuthal Variation in ScS–S Differential Travel Times to Investigate Possible Anisotropy in the Lowermost Mantle Beneath the Philippines

Reduced Thermal Conductivity of Hydrous Aluminous Silica and Calcium Ferrite‐Type Phase Promote Water Transportation to Earth's Deep Mantle

AbstractSubduction of oceanic slabs introduces chemical heterogeneities in the Earth's interior, which could further induce thermal, seismic, and geodynamical anomalies. Thermal conductivity of slab minerals crucially controls the thermal evolution and dynamics of the subducted slab and ambient mantle, while such an important transport property remains poorly constrained. Here we have precisely measured high pressure‐temperature thermal conductivity of hydrous aluminous post‐stishovite (ΛHy‐Al‐pSt) and aluminum‐rich calcium ferrite‐type phase (ΛCF), two important minerals in the subducted basaltic crust in the lower mantle. Compared to the dry aluminous stishovite and pure stishovite, hydration substantially reduces the ΛHy‐Al‐pSt, resulting in ∼9.7–13.3 W m−1 K−1 throughout the lower mantle. Surprisingly, the ΛCF remains at ∼3–3.8 W m−1 K−1 in the lower mantle, few‐folds lower than previously assumed. Our data modeling offers better constraints on the thermal conductivity of the subducted oceanic crust from mantle transition zone to the lowermost mantle region, which is less thermally conductive than previously modeled. Our findings suggest that if the post‐stishovite carries large amounts of water to the lower mantle, the poorer heat conduction through the basaltic crust reduces the slab's temperature, which not only allows the slab bringing more hydrous minerals to greater depth, but also increases slab's density and viscosity, potentially impacting the stability of heterogeneous structures at the lowermost mantle.

Fe‐FeH Eutectic Melting Curve and the Estimates of Earth's Core Temperature and Composition

AbstractFe and FeH form a binary eutectic system above ∼40 GPa. Here we performed melting experiments in a laser‐heated diamond‐anvil cell and obtained the Fe‐FeH eutectic melting curve between 52 and 175 GPa. Its extrapolation shows the eutectic temperature to be 4,350 K at the inner core boundary (ICB), which is lower than that in Fe‐FeSi but is higher than those in the Fe‐S, Fe‐O, and Fe‐C systems. In addition, its dT/dP slope is comparable to those of the melting curves of Fe and FeH endmembers, suggesting that the eutectic liquid composition changes little with increasing pressure and is about FeH0.6 at the ICB pressure. We also estimated the effect of each light element on depressing the liquidus temperature at 330 GPa based on a combination of binary eutectic temperature and composition and found that the effect is large for C and S and small for H, O, and Si when considering the amount of each element that reduces a certain percentage of a liquid iron density. Furthermore, we searched for a set of possible outer core liquid composition and ICB temperature (the liquidus temperature of the former at 330 GPa should match the latter), which explains the outer core density deficit that depends on core temperature. The results demonstrate that relatively low core temperatures, lower than the solidus temperature of a pyrolitic lowermost mantle at the core‐mantle boundary (CMB), are possible.

The Evolution of Dense ULVZs Originating Outside LLSVPs and Implications for Dynamics at LLSVP Margins

AbstractInteractions between multiple‐scale thermochemical heterogeneities in the lowermost mantle, specifically ultralow velocity zones (ULVZs) and large low shear velocity provinces (LLSVPs), are critical in lower mantle dynamics. However, the evolution of ULVZs formed outside LLSVPs has not been thoroughly explored. Here we perform two‐dimensional numerical experiments to examine the evolution of highly dense ULVZs originating beneath cold downwellings and their interactions with the LLSVP. We find that ULVZs with an intrinsic density anomaly more than 500 kg/m3 compared with the ambient lowermost mantle cannot fully enter the LLSVP and would dwell at LLSVP margins for an indefinitely long time. This suggests that dense ULVZ within LLSVPs might have different sources from those outside LLSVPs. The buoyancy number and compositional viscosity of ULVZs are controlling factors on their dynamics and imprints on the core‐mantle boundary (CMB), such as how much the ULVZ protrudes into the LLSVP and the CMB topography beneath the ULVZ. The excess density of ULVZs dictates their width but not their thickness. The oscillations of ULVZ morphology suggest that various types of plumes occur at the LLSVP margin. The mobility of ULVZ implies that the bottom margin of the LLSVP moves much faster than its center. Hot zones exist within the LLSVP near its margins, which may affect the evolution of ULVZs and subducted material nearby. The CMB topography under dense ULVZs are positive unless the buoyancy number of ULVZs exceeds 6.0. These results have intriguing implications for the distribution of ULVZs as well as thermochemical evolution in the lowermost mantle.

The Iron Spin Transitions in Hydrous Fe3+‐Bearing Bridgmanite and Its Geophysical Properties in the Lower Mantle

AbstractHydrous Fe3+‐bearing bridgmanite (Bdg) is potentially a critical water host in the lowermost mantle. The spin transition behaviors of such materials are pivotal for understanding geophysical heterogeneity in the deep Earth but are poorly understood. Here, we investigated the spin transition and related geophysical properties of Fe3+ with associated H defects [Fe3+‐H] at high P‐T conditions using first‐principles simulations. Our calculations predict that the presence of hydrogen reduces the onset pressure of the spin transition of Fe3+ in Bdg, leading to higher fractions of low spin Fe3+. Along standard geotherms, spin transition is predicted to remain incomplete even at the core‐mantle boundary (CMB), and lateral temperature variations would significantly affect the proportions of high and low spin Fe and related properties like elasticity. The thermoelastic property of hydrous Fe3+‐bearing bridgmanite exhibit stronger softening anomalies at the lower mantle conditions compared to dry system, which potentially enhancing the seismic detectability of the hydrous Bdg in the deep earth. Density profile of hydrous Fe3+‐bearing bridgmanite indicates that the [Fe3+‐H] defect modestly increases the system's density, but much less than that caused by incorporating an equivalent amount of iron alone. This is crucial for understanding regions like Large Low Shear Velocity Provinces (LLSVPs), which exhibits large velocity drops but only minor density changes. The co‐adsorption of Fe and H allows for the introduction of Fe to induce velocity drops without the concomitant sharp increase in density, as pure iron would, thus enabling Fe‐H enrichment as a potential source of LLSVPs.

Flow and Deformation in Earth's Deepest Mantle: Insights From Geodynamic Modeling and Comparisons With Seismic Observations

AbstractThe dynamics of Earth's D″ layer at the base of the mantle plays an essential role in Earth's thermal and chemical evolution. Mantle convection in D″ is thought to result in seismic anisotropy; therefore, observations of anisotropy may be used to infer lowermost mantle flow. However, the connections between mantle flow and seismic anisotropy in D″ remain ambiguous. Here, we calculate the present‐day mantle flow field in D″ using 3D global geodynamic models. We then compute strain, a measure of deformation, outside the two large‐low velocity provinces (LLVPs) and compare the distribution of strain with previous observations of anisotropy. We find that, on a global scale, D″ materials are advected toward the LLVPs. The strains of D″ materials generally increase with time along their paths toward the LLVPs and toward deeper depths, but regions far from LLVPs may develop relative high strain as well. Materials in D″ outside the LLVPs mostly undergo lateral stretching, with the stretching direction often aligning with mantle flow direction, especially in fast flow regions. In most models, the depth‐averaged strain in D″ is >0.5 outside the LLVPs, consistent with widespread observations of seismic anisotropy. Flow directions inferred from anisotropy observations often (but not always) align with predictions from geodynamic modeling calculations.

Sluggish thermochemical basal mantle structures support their long-lived stability.

Large low shear-wave velocity provinces (LLSVPs) in the lowermost mantle are the largest geological structures on Earth, but their origin and age remain highly enigmatic. Geological constraints suggest the stability of the LLSVPs since at least 200 million years ago. Here, we conduct numerical modeling of mantle convection with plate-like behavior that yields a Pacific-like girdle of mantle downwelling which successfully forms two antipodal basal mantle structures similar to the LLSVPs. Our parameterized results optimized to reflect LLSVP features exhibit velocities for the basal mantle structures that are ~ 4 times slower than the ambient mantle if they are thermochemical, while the velocity is similar to the ambient mantle if purely thermal. The sluggish motion of the thermochemical basal mantle structures in our models permits the notion that geological data from hundreds of millions of years ago are related to modern LLSVPs as they are essentially stationary over such time scales.

Iron Content‐Dependence of Ferropericlase Elastic Properties Across the Spin Crossover From Novel Experiments and Machine Learning

AbstractThe iron spin crossover in (Mg1‐xFex)O ferropericlase causes changes to its physical properties that are expected to affect seismic velocities in Earth's lower mantle. We present new time‐resolved pressure‐volume measurements of iron‐rich ferropericlase (xFe = 0.40, 0.59) and combine the results with literature data with xFe = 0.04–0.6 to investigate the dependence of ferropericlase elastic properties on iron content. We infer the relationship between unit‐cell volume, pressure and iron content directly from the data by training Mixture Density Networks and derive bulk modulus, density and bulk sound velocity from the outputs. This allows us to constrain the effect of the spin crossover on these properties and estimate their uncertainties for different iron contents. Our findings indicate that the spin crossover may significantly alter the physical properties of ferropericlase in iron‐enriched regions in the lowermost mantle, with implications for the interpretation of seismic heterogeneities observed near the core‐mantle boundary.

Dʺ Structures Beneath the East China Sea Resolved by P‐Wave Slowness Anomalies

AbstractThe Dʺ layer, defined as 200–400 km in the lowermost mantle, is a thermal and chemical boundary layer between the solid silicate mantle and the liquid outer core. Deciphering the detailed structures of the Dʺ region is essential for unlocking the thermal and chemical states in the deep Earth. Here, we precisely measure the slowness and back‐azimuth of the direct P‐waves by beamforming based on the F‐trace stack at the KZ Array in Kazakhstan, to investigate the detailed Dʺ structures beneath the East China Sea. The P‐wave slowness for rays turning beneath the East China Sea exhibits a significant anomaly as a function of the P‐wave turning depth. Strong correlations between slowness and back‐azimuth anomalies for rays from different directions suggest a tilted Moho, with a tilting direction of ∼103° and a dip angle of ∼15°, beneath the KZ Array, further supported by radial receiver functions. After correcting for the slowness anomalies caused by the tilted Moho and heterogeneities outside the Dʺ layer, we construct a series of Vp Dʺ models to fit the remaining slowness anomalies for rays sampling the East China Sea. We obtain the best Dʺ model with a height of 360 km, a maximum δVp of +1.4%, a Dʺ discontinuity thickness of 120 km, and an 80‐km low‐velocity layer at the base of the mantle by minimizing residuals between the predicted and observed slowness anomalies. Combining the sharpness of the Dʺ discontinuity imaged here with mineralogical analysis suggests a Fe‐enriched region in a cold subduction environment beneath the East China Sea.

The impact of Pangean subducted oceans on mantle dynamics: Passive piles and the positioning of deep mantle plumes

The impact of Pangean subducted oceans on mantle dynamics: Passive piles and the positioning of deep mantle plumes

Extensive iron–water exchange at Earth’s core–mantle boundary can explain seismic anomalies

Seismological observations indicate the presence of chemical heterogeneities at the lowermost mantle, just above the core–mantle boundary (CMB), sparking debate over their origins. A plausible explanation for the enigmatic seismic wave velocities observed in ultra-low-velocity zones (ULVZs) is the process of iron enrichment from the core to the silicate mantle. However, traditional models based on diffusion of atoms and penetration of molten iron fail to account for the significant iron enrichment observed in ULVZs. Here, we show that the chemical reaction between silicate bridgmanite and iron under hydrous conditions leads to profound iron enrichment within silicate, a process not seen in anhydrous conditions. Our findings suggest that the interaction between the core and mantle facilitates deep iron enrichment over a few kilometres at the bottom of the mantle when water is present. We propose that the seismic signatures observed in ULVZs indicate whole mantle convection, accompanied by deep water cycles from the crust to the core through Earth’s history.

Widespread D″ ${\mathbf{D}}^{\mathbf{{\prime\prime}}}$ Anisotropy Beneath North America and the Northeastern Pacific and Implications for Upper Mantle Anisotropy Measurements

AbstractObservations of seismic waves that have passed through the Earth's lowermost mantle provide insight into deep mantle structure and dynamics, often on relatively small spatial scales. Here we use SKS, S2KS, S3KS, and PKS signals recorded across a large region including the United States, Mexico, and Central America to study the deepest mantle beneath large swaths of North America and the northeastern Pacific Ocean. These phases are enhanced via beamforming and then used to investigate polarization‐ and propagation direction‐dependent shear wave speeds (seismic anisotropy). A differential splitting approach enables us to robustly identify contributions from anisotropy. Our results show strong seismic anisotropy in approximately half of our study region, indicating that anisotropy may be more prevalent than commonly thought. In some regions, the anisotropy may be induced by flow driven by sinking cold slabs, and in other, more compact regions, by upwelling flow. Measured splitting due to lowermost mantle anisotropy is sufficiently strong to be non‐negligible in interpretations of SKS splitting due to upper mantle anisotropy in certain regions, which may prompt future re‐evaluations of upper mantle anisotropy beneath North and Central America.

The sensitivity of lowermost mantle anisotropy to past mantle convection

It is widely believed that seismic anisotropy in the lowermost mantle is caused by the flow-induced alignment of anisotropic crystals such as post-perovskite. What is unclear, however, is whether the anisotropy observations in the lowermost mantle hold information about past mantle flow, or if they only inform us about the present-day flow field. To investigate this, we compare the general and seismic anisotropy calculated using Earth-like mantle convection models where one has a time-varying flow, and another where the present-day flow is constant throughout time. To do this, we track a post-perovskite polycrystal through the flow fields and calculate texture development using the sampled strain rate and the visco-plastic self-consistent approach. We assume dominant slip on (001) and test the effect of the relative importance of this glide plane over others by using three different plasticity models with different efficiencies at developing texture. We compare the radial anisotropy parameters and the anisotropic components of the elastic tensors produced by the flow field test cases at the same location. We find, under all ease-of-texturing cases, the radial anisotropy is very similar (difference <2%) in the majority of locations and in some regions, the difference can be very large (>10%). The same is true when comparing the elastic tensors directly. Varying the ease-of-texture development in the crystal aggregate suggests that easier-to-texture material may hold a stronger signal from past flow than harder-to-texture material. Our results imply that broad-scale observations of seismic anisotropy such as those from seismic tomography, 1-D estimates and normal mode observations, will be mainly sensitive to present-day flow. Shear-wave splitting measurements, however, could hold information about past mantle flow. In general, mantle memory expressed in anisotropy may be dependent on path length in the post-perovskite stability field. Our work implies that, as knowledge of the exact causative mechanism of lowermost mantle anisotropy develops, we may be able to constrain both present-day and past mantle convection.

Compressional and shear wave velocities of Fe-bearing silicate post-perovskite in Earth’s lowermost mantle

Compressional and shear wave velocities of Fe-bearing silicate post-perovskite in Earth’s lowermost mantle

Trace element partitioning in a deep magma ocean and the origin of the Hf-Nd mantle array.

Crystallization in Earth's deep magma ocean could have caused trace element fractionation in the lower mantle that might be inherited to the isotopic compositions of the present-day mantle. However, the trace element partitioning has been experimentally investigated only up to the uppermost lower-mantle pressures. Here, we determined the bridgmanite/melt partition coefficients D of La, Nd, Sm, Lu, and Hf from 24 to 115 gigapascals, covering the wide pressure range of the lower mantle. Results demonstrate substantial reductions in DLu and DHf from >1 to ≪1 with increasing pressure to 91 gigapascals. We also found DLu/DHf > 1 and DSm/DNd < 1 under deep lower-mantle conditions, evolving melts toward low Lu/Hf and high Sm/Nd ratios by crystallizing bridgmanite. If residual melts form a dense hidden reservoir in the lowermost mantle, the complementary accessible mantle has the Hf and Nd isotopic compositions matching the observed terrestrial mantle array that deviates from the bulk silicate Earth reference.

Examining the influence of 2.5-D ultra-low velocity zone morphology on ScP waveforms and estimated elastic parameters

Summary Ultra-low velocity zones (ULVZs) have been identified as regions of extremely low velocity anomalies in the Earth's lowermost mantle using seismic observations from reflected, refracted, and diffracted arrivals along the mantle side of the core-mantle boundary (CMB). Estimation of ULVZ geometrical (i.e., shape and size) and elastic (i.e., velocity and density) parameters with uncertainties is crucial in understanding the role of ULVZs in the ongoing dynamic processes within the Earth's mantle; however, these parameters are still poorly known due to uncertainties and tradeoffs of the seismically resolved ULVZ geometries and elastic parameters. Computation of synthetic waveforms for 2-D and 3-D ULVZs shapes is currently computationally feasible, but past studies utilize higher dimensional waveform modeling of mostly only low-frequency diffracted waves. Most studies focusing on high-frequency core-reflected waveforms (e.g., ScP) still use 1-D modeling approaches to determine ULVZ properties. This approach might lead to wrong results if the imaged structures have inherently 3-D geometries. This study investigates high-frequency synthetic ScP waveforms for various 2.5-D ULVZ geometries showing that additional seismic arrivals are generated even when the ScP geometrical ray path does not directly strike the location of the ULVZ. The largest amplitude additional phases in the 2.5-D models are post-cursor arrivals that are generated at the edges of the finite-length ULVZs. These newly identified ScP post-cursors can arrive within the ScsP post-cursor time window traditionally analyzed in 1-D ULVZ studies. These post-cursors might then be misidentified or constructively/destructively interfere with the ScsP postcursor, leading to incorrect estimation of ULVZ parameters. In this study we investigate the bias introduced by the 2.5-D morphologies on the 1D estimated ULVZ elastic properties in a Bayesian waveform inversion scheme. We further expand the Bayesian method by including the data noise covariance matrix in the inversion and compare it to an autoregressive noise model that was utilized in previous studies. From the application to the observed ScP data, we find that the new approach converges faster, particularly for the inversion of data from multiple events, and the new algorithm retrieves ULVZ parameters with more realistic uncertainties. The inversion of 2.5-D synthetic ScP waveforms suggests that the retrieved ULVZ parameters can be misleading with unrealistically high confidence if we do not consider the data noise covariance matrix in the inversion. Our new approach can also retrieve the shape of a multi-dimensional Gaussian ULVZ if its length is 12o or longer in the great circle arc direction. However, 2.5-D synthetic waveforms show additional waveform complexity which can constructively interfere with the ScP wavefield. Hence, in many cases the estimation of ULVZ properties using 1-D forward modeling can provide incorrect ULVZ parameters. Hence previous ULVZ modeling efforts using 1-D parameter estimation methods may be incorrect.