Seismic Scatterers Research Articles

Elasticity of Hydrous SiO2 Across the Post‐Stishovite Transition in the Lower Mantle

AbstractElastic properties of Si0.95H0.21O2 with hydrogarnet substitution (4H+ = Si4+) across the post‐stishovite transition (28–42 GPa) are determined up to 70 GPa using ab initio calculations and a pseudo‐proper type Landau model. At 28 GPa, elastic coefficients C11 and C12 converge, and the average shear and compressional velocity (VS and VP) decrease by a maximum of 25.5% and 5.2%, respectively. Hydrogarnet substitution reduces ambient elastic moduli and sound velocities, and shifts shear softening to lower pressure. 2%–13% Si0.95H0.21O2 may cause a VS anomaly of −0.5% to −2.6% at 700–820 km depth, explaining low VS layers beneath North America and the European Alps. Additionally, 20 vol % SiO2 in subducted basalt, with decreasing water content from 3.2 wt% to zero, could cause a VS anomaly of up to −7(4) % from 700 to 1,900 km depth, aligning with seismic scatterers identified in some subduction regions.

Hydrogen partitioning between stishovite and hydrous phase δ: implications for water cycle and distribution in the lower mantle

Water is transported into the deep mantle by subducting slabs, playing important roles in mantle dynamics and evolution. An aluminous hydrous mineral, phase δ with a main component of AlOOH, has been considered an important water carrier in the lower mantle. Recent studies reported that SiO2 stishovite can accommodate weight percent levels of water, indicating another important water carrier in the lower mantle. However, which mineral can mainly carry water is not clear yet. Recent hydrous phase relation studies reported that stishovite is depleted in alumina when coexisting with hydrous phase δ, in which water content of stishovite was not investigated. In this study, we investigated hydrogen partitioning between stishovite and hydrous phase δ at 24–28 GPa and 1000–1200 °C by means of Kawai-type multi-anvil press in combination with Fourier-transform infrared spectroscopy at ambient conditions on recovered samples. Fourier-transform infrared spectra of recovered stishovites showed that water contents of stishovite coexisting with hydrous phase δ were limited to up to ~ 500 ppm. This indicates that coexisting hydrous phase δ causes not only depletion in alumina but also in hydrogen in stishovite and therefore mainly transports water in a cold subducting slab. Once hydrous phase δ becomes thermally unstable, alumina and water contents in silica minerals are increased by the chemical reaction between SiO2 and AlOOH, and aluminous silica minerals such as stishovite and CaCl2-type phase will be a main water carrier in the lower mantle. Presence of small-scale seismic scatterers observed around 1900 km depth, which was considered to be caused by a transition from almost pure SiO2 stishovite to CaCl2-type phase, might also be able to be explained by the phase transition of stishovite coexisting with hydrous phase δ.

Thermal Equations of State of Magnesite: Implication for the Complex Mid‐Lower Mantle Seismic Scatterers

AbstractMagnesite (MgCO3) entering the lower mantle together with the subducted oceanic crust is an important carbon carrier. The reaction between magnesite and mantle minerals has been documented, but its influence on the density and velocity profiles of lower mantle remains unexplored. To decipher the deep carbon transportation and its associated effect, here we determined the thermal equations of state of magnesite up to 120 GPa and 2600 K using X‐ray diffraction in laser‐heated diamond anvil cells. The obtained thermal elastic parameters of magnesite facilitated a comprehensive understanding on the influence of magnesite‐SiO2 reaction, variation of carbon and SiO2 content, and temperature on the origin of lower‐mantle scatterers at 1,000–1,800 km depth. Our modeling revealed that the depth of the lower‐mantle VS scatterers is mainly controlled by the Al2O3 content in SiO2, while its magnitude depends on the SiO2 content. Along normal geotherm, the magnesite‐SiO2 reaction would occur before the post‐stishovite transition, consuming substantial SiO2 in the subducted oceanic crust. Depending on the amount of residual SiO2, the post‐stishovite transition can produce a 2.5–5.2 (2)% VS reduction, compatible with the observed seismic scatterers in Izu‐Bonin and Mariana subduction zones. Along slab geotherm, this reaction occurs after the post‐stishovite transition, generating a greater VS reduction of 4.4–6.4 (4)%. We thus propose that the reaction between sinking MgCO3 and SiO2 in the slab is one of the potential factors influencing the magnitude of the lower‐Vs scatterers at 1,000–1,900 km depth. Our results provide new insights into the deep‐mantle carbonate transportation influencing regional geophysics.

Equations of State and Phase Boundaries of SiO2 Polymorphs Under Lower Mantle Conditions

AbstractSiO2 is an important modally abundant component of terrestrial planets, while there are few studies that quantitatively determine the properties of silica under simultaneously high‐temperature and high‐pressure conditions. In this study, thermal properties of stishovite, CaCl2‐type SiO2 and seifertite have been obtained by using extensive first‐principles simulations. The quasi‐harmonic approximation and intrinsic anharmonic effects were considered, and a generalized rescaling method was adopted to eliminate systematic errors in the simulation results. Based on the simulation data, we have established new equations of state for the SiO2 polymorphs and determined their phase boundaries. Anharmonicity has been demonstrated to show noticeable effects on their thermal properties and phase boundaries. With the new results, we found that SiO2 polymorphs show increasing buoyancy in the subducted basaltic slabs at depths greater than ∼800–900 km. In the normal lower mantle, they are readily stable over a broad range of depths approximately from 1,500 km to 2,600 km. This implies that SiO2 polymorphs may contribute to the lateral heterogeneity in the deep lower mantle as observed by small‐scale seismic scatterers.

Mid-mantle seismic scatterers beneath the Samoan hotspot

We analyze teleseismic array data of deep and intermediate-depth earthquakes in the Fiji-Tonga-Kermadec region recorded at seismic networks at the west coast of US and Alaska with the aim of mapping seismic scatterers in the lower mantle and in the upper mantle transition zone beneath the Samoan hotspot. Short period and broadband waveform data are array-processed in order to detect coherent but anomalous phases in the P coda up to nearly 120 s after direct P waves and locate where they arrive from. Updating the data set reported in our previous study (Kaneshima and Helffrich, 2010; Kaneshima, 2018; Kaneshima, 2019), we examine nine scatterers near the hotspot, seven in the lower mantle and two in the transition zone, with the main focus on three lower mantle scatterers nearest to the hotspot at 1000 to 1200 km depths. We observe S-to-P waves generated from the three lower mantle scatterers for earthquakes with a wide range of hypocenters from 17°S to 24°S, while hardly any signs of corresponding P-to-P scattering waves are observed. The pulse shapes of the S-to-P waves from the three scatterers are simple and their pulse widths do not significantly differ from those of direct P waves when the signal quality is high enough. The geometry of the scatterers suggested by these observations is a steeply dipping sheet-like structure with the thickness less than several kilometers. The material consisting of the sheet needs to have a Vs value at least several percent lower than the surrounding mantle, as required by the amplitudes of S-to-P scattering waves relative to P, which is typically several percent. The low Vs anomaly of basaltic rocks relative to pyrolite under the shallow lower mantle conditions may be consistent with the observations (Wang et al., 2020). Seismic tomography models suggest that the nine scatterers tend to be located around the edge of a plume that ascends from the Pacific LLSVP to the Samoan hotspot. Numerical simulations of ascending plumes from a thermo-chemical pile at the bottom of the mantle show that the plumes tend to rise preferentially in the vicinity of the edge of the pile. Other simulations also show that blobs of recycled former oceanic crust subducting to the bottom of mantle are entrained into a plume and stretched vertically to a significant degree, consistently with the images suggested by geochemical studies of OIB. The scattering observations therefore may elucidate the structure of entrained basaltic blobs on their way of returning back to the surface hotspot.

Superhydrous aluminous silica phases as major water hosts in high-temperature lower mantle

Water transported by subducted oceanic plates changes mineral and rock properties at high pressures and temperatures, affecting the dynamics and evolution of the Earth's interior. Although geochemical observations imply that water should be stored in the lower mantle, the limited amounts of water incorporation in pyrolitic lower-mantle minerals suggest that water in the lower mantle may be stored in the basaltic fragments of subducted slabs. Here, we performed multianvil experiments to investigate the stability and water solubility of aluminous stishovite and CaCl2-structured silica, referred to as poststishovite, in the SiO2-Al2O3-H2O systems at 24 to 28 GPa and 1,000 to 2,000 °C, representing the pressure-temperature conditions of cold subducting slabs to hot upwelling plumes in the top lower mantle. The results indicate that both alumina and water contents in these silica minerals increase with increasing temperature under hydrous conditions due to the strong Al3+-H+ charge coupling substitution, resulting in the storage of water up to 1.1 wt %. The increase of water solubility in these hydrous aluminous silica phases at high temperatures is opposite of that of other nominally anhydrous minerals and of the stability of the hydrous minerals. This feature prevents the releasing of water from the subducting slabs and enhances the transport water into the deep lower mantle, allowing significant amounts of water storage in the high-temperature lower mantle and circulating water between the upper mantle and the lower mantle through subduction and plume upwelling. The shallower depths of midmantle seismic scatterers than expected from the pure SiO2 stishovite-poststishovite transition pressure support this scenario.

Elasticity of Hydrated Al‐Bearing Stishovite and Post‐Stishovite: Implications for Understanding Regional Seismic VS Anomalies Along Subducting Slabs in the Lower Mantle

AbstractSeismic studies have found seismic scatterers with −2 to −12% shear velocity anomalies along some subducting slabs at 700–1900 km depth. The ferroelastic post‐stishovite transition in subducted mid‐ocean ridge basalt (MORB) has been linked to these seismic features, but compressional and shear wave velocities (VP and VS) and full elastic moduli (Cij) of Al,H‐bearing stishovite and post‐stishovite at high pressure remain uncertain. Here we have determined Raman shifts of optic modes and equation of state parameters of two hydrated Al‐bearing stishovite crystals, Al1.3‐SiO2 (1.34 mol% Al and 0.55 mol% H) and Al2.1‐SiO2 (2.10 mol% Al and 0.59 mol% H), up to ∼70 GPa in diamond anvil cells coupled with Raman spectroscopy and X‐ray diffraction. The experimental data are modeled using a pseudoproper Landau theory to derive full Cij and sound velocities across the post‐stishovite transition at high pressure. The Al and H dissolution in stishovite significantly reduces the transition pressure to 21.1 GPa in Al1.3‐SiO2 and to 16.1 GPa in Al2.1‐SiO2, where the transition is manifested by approximately 29% VS reduction. Considering that stishovite with approximately 1.3 mol% Al and 0.6 mol% H could account for 20 vol% in subducted MORB at the top‐lower mantle, the Al,H‐bearing post‐stishovite transition with a Clapeyron slope of 65 K/GPa would occur at about 1060 km depth with −7(4)% VS anomaly. The VS anomalies across the Al,H‐bearing post‐stishovite transition can help explain the seismically‐observed depth‐dependent VS anomalies along some subducting slabs in the top‐ to mid‐lower‐mantle depths including the Tonga subducting slab.

Evidence for basalt enrichment in the mantle transition zone from inversion of triplicated P- and S-waveforms

Recycling of chemically stratified oceanic lithosphere at subduction zones and mantle mixing is the main source for the production and long-term maintenance of heterogeneities as evidenced through mantle samples and the presence of seismic scatterers. The mantle transition zone (MTZ), which is bound by seismic discontinuities around 410 and 660 km depth that reflect mantle mineral phase transformations in the olivine system, is considered to play a significant role in reprocessing and remixing of subducted oceanic plates. However, the compositional nature and mixing state of the MTZ is generally unconstrained with models ranging from a fully-equilibrated pyrolitic to a mechanically-mixed MTZ enriched in basalt. Here, we consider regional P- and S-waveforms from different regions that have interacted extensively with the MTZ and are triplicated at the seismic discontinuities. Because of the strong interaction with the MTZ and the sensitivity to mantle temperature and composition, triplicated waveforms can be used as probes of the thermochemical conditions in and around the MTZ. Here, we model the MTZ by considering two-endmember models: a fully-equilibrated and a mechanical mixture of depleted (harzburgite) and fertile (basalt) lithologies, whose properties are computed self-consistently using petrologic phase equilibria in combination with equation-of-state modeling as a function of pressure, temperature, and composition. Based on this method, we invert carefully selected triplicated P- and S-waveforms for local one-dimensional profiles of MTZ structure. In some of the studied regions, we find both radial and lateral heterogeneities in composition that show a trend from more fertile lithologies in the mid-MTZ to more pyrolitic compositions below the MTZ. This is in line with evidence provided by global geodynamic models that support the segregation and accumulation of basalt toward the mid-MTZ. Because a number of the regions studied here encompass subduction zone settings, we also observe lower-than-average mantle temperatures (∼1200∘C) and, as a consequence, thicker-than-average MTZ (∼240–280 km). While some areas appear to be better fit with a particular mixing model, rigorous statistical analysis of the datafit shows that on average, and at the level of the resolving power of the data, an equilibrated mantle across a large part of the studied subduction zone settings is favored.

Distribution of seismic scatterers in the San Jacinto Fault Zone, southeast of Anza, California, based on passive matrix imaging

Fault zones are associated with multi-scale heterogeneities of rock properties. Large scale variations may be imaged with conventional seismic reflection methods that detect offsets in geological units, and tomographic techniques that provide average seismic velocities in resolved volumes. However, characterizing elementary localized inhomogeneities of fault zones, such as cracks and fractures, constitutes a challenge for conventional techniques. Resolving these small-scale heterogeneities can provide detailed information for structural and mechanical models of fault zones. Recently, the reflection matrix approach utilizing body wave reflections in ambient noise cross-correlations was extended with the introduction of aberration corrections to handle the actual lateral velocity variations in the fault zone (Touma et al., 2021). Here this method is applied further to analyze the distribution of scatterers in the first few kilometers of the crust in the San Jacinto Fault Zone at the Sage Brush Flat (SGB) site, southeast of Anza, California. The matrix approach allows us to image not only specular reflectors but also to resolve the presence, location and reflectivity of scatterers for seismic waves starting with a simple homogeneous background velocity model of the medium. The derived three-dimensional image of the fault zone resolves lateral variations of scattering properties in the region within and around the surface fault traces, as well as differences between the Northwest (NW) and the Southeast (SE) parts of the study area. A localized intense damage zone at depth is observed in the SE section, suggesting that a geometrical complexity of the fault zone at depth induces ongoing generation of rock damage.

Seismic scatterers in the lower mantle near subduction zones

SUMMARY We investigate the global distribution of S-to-P scatterers in the shallow to mid-lower mantle beneath subduction zones, where deep seismicity extends down to the bottom of the upper mantle. By array processing broadband and short period waveform data obtained at seismic networks, we seek anomalous later phases in the P coda within about 15–150 s after direct P waves. The later phases usually arrive along off-great circle paths and significantly later than S-to-P conversion from the ‘660 km’ discontinuity, often show positive slowness anomalies relative to direct P, and do not show a conversion depth that is consistent among nearby events. They are thus adequately regarded as scattered waves, rather than conversion at a global horizontal discontinuity. The S-to-P scattered waves often show amplitudes comparable to ‘S660P’ waves, which indicates that a spatial change in elastic properties by several percent occurs at the scatterers as abruptly as the post-spinel transformation and should arise from compositional heterogeneity. We locate prominent S-to-P scatterers beneath Pacific subduction zones and beneath southern Spain. Nearly half of 137 S-to-P scatterers located in this study and previous studies by the authors are shallower than 1000 km, and the number of scatterers decreases with depth. Scatterers deeper than 1800 km are rare and mostly weak. We examine relations between the locations of the scatterers and recently subducted slabs inferred from seismic tomography. The scatterers of mid-mantle depths, deeper than about 1000 km, are located distant from tomographic slabs. On the other hand, the majority of shallower scatterers are located beneath the slabs rather than near their fastest portions, which would indicate that chemically heterogeneous materials are not extensively entrained within thickened and folded slabs when the slabs impinge on the lower mantle. We also find scatterers near the locations where basaltic rocks of recently subducted oceanic crust are expected to exist, which suggests that oceanic crust is not delaminating when slabs impinge on the lower mantle.

Localized ultra-low velocity zones at the eastern boundary of Pacific LLSVP

It is suggested that ultra-low velocity zones (ULVZs) are often located near the boundary of large low shear velocity zones (LLSVPs). Previous studies have provided evidence of ULVZs residing along the western and northern boundary of the Pacific LLSVP. However, so far there is no detailed report on ULVZs along the eastern boundary of the Pacific LLSVP. In this study, we collected PKP precursor waveforms from earthquakes in South America that are recorded by seismic arrays in Australia. These precursors show clear and coherent phases. Slowness and migration analysis of these precursors indicate that they originate from scatterers in the lowermost mantle east of the Pacific LLSVP. Waveform modeling suggests that these seismic scatterers are localized ULVZs, with P-wave velocity reductions of ∼3–10% and thickness of several tens of kilometers. The dimension of the ULVZs and the geological correlation of the ULVZs with the Galapagos hotspot, suggest that these ULVZs are mainly due to compositionally distinct heterogeneities, although partial melting cannot be completely ruled out. The distribution of both coherent and incoherent PKP precursors also suggests vigorous small-scale mantle convections near the edges of the LLSVP.

Seismic scatterers in the mid-lower mantle beneath Tonga-Fiji

We analyze deep and intermediate-depth earthquakes at the Tonga-Fiji region in order to reveal the distribution of scattering objects in the mid-lower mantle. By array processing waveform data recorded at regional seismograph stations in the US, Alaska, and Japan, we investigate S-to-P scattering waves in the P coda, which arise from kilometer-scale chemically distinct objects in the mid-lower mantle beneath Tonga-Fiji.With ten scatterers previously reported by the author included, twenty-three mid-lower mantle scatterers have been detected below 900 km depth, while scatterers deeper than 1900 km have not been identified. Strong mid-lower mantle S-to-P scattering most frequently occurs at the scatterers located within a depth range between 1400 km and 1600 km. The number of scatterers decreases below 1600 km depth, and the deeper objects tend to be weaker. The scatterer distribution may reflect diminishing elastic anomalies of basaltic rocks with depth relative to the surrounding mantle rocks, which mineral physics has predicted to occur. The predominant occurrence of strong S-to-P scattering waves within a narrow depth range may reflect significant reduction of rigidity due to the ferro-elastic transformation of stishovite in basaltic rocks.Very large signals associated with mid-mantle scatterers are observed only for a small portion of the entire earthquake-array pairs. Such infrequent observations of large scattering signals, combined with quite large event-to-event differences in the scattering intensity for each scatterer, suggest both that the strong arrivals approximately represent ray theoretical S-to-P converted waves at objects with a plane geometry. The plane portions of the strong scatterers may often dip steeply, with the size exceeding 100 km. For a few strong scatterers, the range of receivers showing clear scattered waves varies substantially from earthquake-array pair to pair. Some of the scatterers are also observed at different arrays that have significantly different directions of incident waves to the scatterers. Furthermore, weak but coherent P-to-P scattered waves as well as S-to-P waves are observed for a few of the scatterers. These observations indicate that the locally plane scatterers also possess substantial topography.

Post-stishovite transition in hydrous aluminous SiO2

Lakshtanov et al. (2007) showed that incorporation of aluminum and some water into SiO2 significantly reduces the post-stishovite transition pressure in SiO2. This discovery suggested that the ferroelastic post-stishovite transition in subducted MORB crust could be the source of reflectors/scatterers with low shear velocities observed in the mid to upper lower mantle. A few years later, a similar effect was observed in anhydrous Al-bearing silica. In this paper, we show by first principles static calculations and by molecular dynamics using inter-atomic potentials that hydrogen bonds and hydrogen mobility play a crucial role in lowering the post-stishovite transition pressure. A cooperative redistribution of hydrogen atoms is the main mechanism responsible for the transition pressure reduction in hydrous aluminous stishovite. The effect is enhanced by increasing hydrogen concentration. This perspective suggests a potential relationship between the depth of seismic scatterers and the water content in stishovite.

Multi-mode conversion imaging of the subducted Gorda and Juan de Fuca plates below the North American continent

Receiver function analysis and seismic tomography show tectonic structures dipping eastward in the mantle below the Cascadia volcanic arc (western US) that have been related to the subduction of the Gorda and Juan de Fuca oceanic micro-plates. Inconsistencies in the dip angle and depth extent of the slab between the two methods undermine the interpretation of the structure and processes at work. Receiver function imaging is biased by multiple reflection phases that interfere with converted phases, and produce spurious discontinuities in images. Here, we correct the interference using a multiple mode conversion imaging technique that efficiently removes artifacts under dipping structures. The method has the advantage of being applicable to large aperture arrays, and can image large-scale structures down to the transition zone. With this approach, the interfaces between the subducting and overriding plates and the oceanic Moho are imaged at shallow depths (<120 km) with a dip angle of ∼20°, consistently with former studies. In addition, several important features are imaged with the present method. Faint converters located between 100 and 400 km depth in the mantle wedge, and strong sub-horizontal seismic scatterers near 160 km depth, may highlight dehydration and metasomatism processes in the Cascadia subduction zone. A discontinuity located at ∼15 km depth in the lithospheric mantle of the subducted plates and associated with a negative impedance contrast is interpreted as the fossil fabric of the plates acquired at the spreading ridges.

Melting in the FeO[sbnd]SiO2 system to deep lower-mantle pressures: Implications for subducted Banded Iron Formations

Banded iron formations (BIFs), consisting of layers of iron oxide and silica, are far denser than normal mantle material and should have been subducted and sunk into the deep lower mantle. We performed melting experiments on Fe2SiO4 from 26 to 131 GPa in a laser-heated diamond-anvil cell (DAC). The textural and chemical characterization of a sample recovered from the DAC revealed that SiO2 is the liquidus phase for the whole pressure range examined in this study. The chemical compositions of partial melts are very rich in FeO, indicating that the eutectic melt compositions in the FeOSiO2 binary system are very close to the FeO end-member. The eutectic temperature is estimated to be 3540±150 K at the core–mantle boundary (CMB), which is likely to be lower than the temperature at the top of the core at least in the Archean and Paleoproterozoic eons, suggesting that subducted BIFs underwent partial melting in a thermal boundary layer above the CMB. The FeO-rich melts formed by partial melting of the BIFs were exceedingly dense and therefore migrated downward. We infer that such partial melts have caused iron enrichment in the bottom part of the mantle, which may have contributed to the formation of ultralow velocity zones (ULVZs) observed today. On the other hand, solid residues left after the segregation of the FeO-rich partial melts have been almost pure SiO2, and therefore buoyant in the deep lower mantle to be entrained in mantle upwellings. They have likely been stretched and folded repeatedly by mantle flow, forming SiO2 streaks within the mantle “marble cake”. Mantle packages enhanced by SiO2 streaks may be the origin of seismic scatterers in the mid-lower mantle.

Strong seismic scatterers near the core–mantle boundary north of the Pacific Anomaly

Tomographic images have shown that there are clear high-velocity heterogeneities to the north of the Pacific Anomaly near the core–mantle boundary (CMB), but the detailed structure and origin of these heterogeneities are poorly known. In this study, we analyze PKP precursors from earthquakes in the Aleutian Islands and Kamchatka Peninsula recorded by seismic arrays in Antarctica, and find that these heterogeneities extend ∼400km above the CMB and are distributed between 30° and 45°N in latitude. The scatterers show the largest P-wave velocity perturbation of 1.0–1.2% in the center (160–180°E) and ∼0.5% to the west and east (140–160°E, 180–200°E). ScS–S differential travel-time residuals reveal similar features. We suggest that these seismic scatterers are the remnants of ancient subducted slab material. The lateral variations may be caused either by different slabs, or by variations in slab composition resulting from their segregation process.

Strong, Multi-Scale Heterogeneity in Earth's Lowermost Mantle.

The core mantle boundary (CMB) separates Earth’s liquid iron outer core from the solid but slowly convecting mantle. The detailed structure and dynamics of the mantle within ~300 km of this interface remain enigmatic: it is a complex region, which exhibits thermal, compositional and phase-related heterogeneity, isolated pockets of partial melt and strong variations in seismic velocity and anisotropy. Nonetheless, characterising the structure of this region is crucial to a better understanding of the mantle’s thermo-chemical evolution and the nature of core-mantle interactions. In this study, we examine the heterogeneity spectrum from a recent P-wave tomographic model, which is based upon trans-dimensional and hierarchical Bayesian imaging. Our tomographic technique avoids explicit model parameterization, smoothing and damping. Spectral analyses reveal a multi-scale wavelength content and a power of heterogeneity that is three times larger than previous estimates. Inter alia, the resulting heterogeneity spectrum gives a more complete picture of the lowermost mantle and provides a bridge between the long-wavelength features obtained in global S-wave models and the short-scale dimensions of seismic scatterers. The evidence that we present for strong, multi-scale lowermost mantle heterogeneity has important implications for the nature of lower mantle dynamics and prescribes complex boundary conditions for Earth’s geodynamo.

Elasticity and phase stability of pyrope garnet from ab initio computation

We study the high-pressure stability and elastic properties of Mg3Al2Si3O12 pyrope garnet using the density functional first principles computation method. Pyrope garnet is found to dissociate into an assemblage of MgSiO3 Mg-perovskite (Pv) and Al2O3 corundum (Cor) solid solutions at ∼19.7GPa at static conditions. Then, this assemblage undergoes a phase transition to pyropic (Al-bearing) Pv at ∼65GPa. The enthalpy of an assemblage of MgAl2O4 calcium ferrite (CF), MgPv, and stishovite (St) is less stable than that of MgPv plus Cor. A continuous reaction in the MgSiO3–Al2O3 system suggested by this study is consistent with previous experimental and computational studies but not with a recently modeled phase diagram. This implies that the formation of pyropic Pv could not cause any seismic scatterers in the mid-lower mantle. The investigated anisotropy of elastic velocities further indicates that pyrope garnet is a very isotropic mineral. Our results suggest that seismological anisotropy inferred in the upper mantle could not be due to garnet.

Seismic structure and ultra-low velocity zones at the base of the Earth’s mantle beneath Southeast Asia

We constrain seismic structure and ultra-low velocity zones near the Earth’s core-mantle boundary (CMB) beneath Southeast Asia. We first determine the average shear-velocity structure near the CMB in the region based on travel-time analysis of S, ScS, P and ScP phases. We then map seismic scattering in the lowermost mantle using the PKP precursors observed at the USArray. The inferred average shear-velocity perturbations in the lowermost 200km of the mantle range from about −6% to 6%, and exhibit a complex geographic distribution of alternate low- and high-velocity patches adjacent to each other, surrounded by a high-velocity anomaly in the south. The inferred strong seismic scatterers exhibit a crescent shape distributed from the South China Sea to the Maluku Islands and coincide with the westernmost low-velocity patch, suggesting that the strong scatterers represent ultra-low velocity zones (ULVZs). We suggest that the seismic structure in the region likely results from a complex interaction between a downwelling and a low-velocity region near the CMB. The downwelling (the high-velocity patches) displaces the low-velocity region into many low-velocity patches and pushes the ULVZs to the edge of the low-velocity region.

Spin transition of Fe3+ in Al-bearing phase D: An alternative explanation for small-scale seismic scatterers in the mid-lower mantle

Among dense-hydrous magnesium silicates potentially transporting H2O into Earthʼs deep interior, phase D (MgSi2H2O6) exhibits the highest P–T stability range, extending into the lower mantle along cold slab geotherms. We have studied the compressibility and spin state of Fe in Al-bearing phase D up to 90 GPa using synchrotron X-ray diffraction and X-ray emission spectroscopy. Fe–Al-bearing phase D was synthesized at 25 GPa and 1400 °C with approximate composition MgSi1.5Fe0.15Al0.32H2.6O6, where nearly all of the Fe is ferric (Fe3+). Analysis of Fe-Kβ emission spectra reveals a gradual, pressure-induced high-spin (HS) to low-spin (LS) transition of Fe3+ extending from 40 to 65 GPa. The fitted equation of state for high-spin Fe–Al-bearing phase D results in a bulk modulus KT0=147(2) GPa with pressure derivative K′=6.3(3). An equation of state over the entire pressure range was calculated using the observed variation in low-spin fraction with pressure and a low-spin bulk modulus of KT0=253(30) GPa, derived from the data above 65 GPa. Pronounced softening in the bulk modulus occurs during the spin transition, reaching a minimum at 50 GPa (∼1500 km) where the bulk modulus of Fe–Al phase D is about 35% lower than Fe–Al-bearing silicate perovskite. Recovery of the bulk modulus at 50–65 GPa results in a structure that has a similar incompressibility as silicate perovskite above 65 GPa. Similarly, the bulk sound velocity of Fe–Al phase D reaches a minimum at ∼50 GPa, being about 10% slower than silicate perovskite. The potential association of Fe–Al phase D with subducted slabs entering the lower mantle, along with its elastic properties through the Fe3+ spin transition predicted at 1200–1800 km, suggests that phase D may provide an alternative explanation for small-scale mid-lower mantle seismic scatterers and supports the presence of deeply recycled sediments in the lower mantle.