Deep Lower Mantle Research Articles

Structural Evolution of Basaltic Melts in the Deep Earth: Insights From High‐Pressure Sound Velocity of Glass

AbstractThe densification mechanisms of silicate melts under high pressure are of key interest in understanding the evolution of the early Earth and its present‐day internal structure. Here, we report Brillouin spectroscopy‐derived transverse acoustic wave velocities from a basaltic glass at high pressures up to 163 GPa and ambient temperature to provide insight into pressure‐induced changes in its elasticity and, by extension, its density. We find that the pressure dependence of below 110–140 GPa follows a trend nearly tantamount to those of pyrolite and Fe‐ and (Fe,Al)‐bearing MgSiO3 glasses, indicating that the large compositional differences among these glasses do not exert variable acoustic wave velocity trends. However, at higher pressures we observe a small departure from the profiles of the Al‐poor compositions toward higher acoustic wave velocities to eventually become stiffer. This pressure‐induced steepening in is comparable to that of (Mg, Fe, Al)(Si, Al)O3 glass, and suggests a possible structural change toward a denser state caused by more rapidly changing Al–O coordination in network‐forming Al. Coupled with the high Fe content in basalt, this may render basaltic melt denser than surrounding minerals in the deep lower mantle, and may provide an additional mechanism for the existence of ultralow‐velocity zones.

Stability of H2O‐Rich Fluid in the Deep Mantle Indicated by the MgO‐SiO2‐H2O Phase Relations at 23 GPa and 2,000 K

AbstractThe Earth's mantle contains significant amounts of water in the form of hydroxyl in hydrous minerals, nominally anhydrous minerals, and hydrous silicate melts. H2O fluid is thought to be present only in the shallow regions because it will always dissolve tens of weight percent of silicates by forming hydrous silicate melt in the deep mantle. Here I investigated the phase relations in the MgO‐SiO2‐H2O system by high‐pressure experiments at a pressure of 23 GPa and a temperature of 2,000 K, corresponding to the conditions at the bottom of the mantle transition zone and the topmost lower mantle. The experimental results indicate that hydrous melt can contain more than 90 wt.% of H2O, that is, it becomes H2O‐rich fluid when coexists only with stishovite. In contrast, silicate‐rich hydrous melt is formed when the system is enriched with MgO component. Therefore, H2O‐rich fluid may be stabilized in locally SiO2‐enriched rocks even at the topmost lower mantle, acting as a water source for the deep lower mantle by slab subduction. The H2O fluid also provide a possible cause for the occurrence of natural ice‐VII originated from 660 km depth.

Pressure stabilizes ferrous iron in bridgmanite under hydrous deep lower mantle conditions

Earth’s lower mantle is a potential water reservoir. The physical and chemical properties of the region are in part controlled by the Fe3+/ΣFe ratio and total iron content in bridgmanite. However, the water effect on the chemistry of bridgmanite remains unclear. We carry out laser-heated diamond anvil cell experiments under hydrous conditions and observe dominant Fe2+ in bridgmanite (Mg, Fe)SiO3 above 105 GPa under the normal geotherm conditions corresponding to depth > 2300 km, whereas Fe3+-rich bridgmanite is obtained at lower pressures. We further observe FeO in coexistence with hydrous NiAs-type SiO2 under similar conditions, indicating that the stability of ferrous iron is a combined result of H2O effect and high pressure. The stability of ferrous iron in bridgmanite under hydrous conditions would provide an explanation for the nature of the low-shear-velocity anomalies in the deep lower mantle. In addition, entrainment from a hydrous dense layer may influence mantle plume dynamics and contribute to variations in the redox conditions of the mantle.

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.

Variation in bridgmanite grain size accounts for the mid-mantle viscosity jump.

A viscosity jump of one to two orders of magnitude in the lower mantle of Earth at 800-1,200-km depth is inferred from geoid inversions and slab-subducting speeds. This jump is known as the mid-mantle viscosity jump1,2. The mid-mantle viscosity jump is a key component of lower-mantle dynamics and evolution because it decelerates slab subduction3, accelerates plume ascent4 and inhibits chemical mixing5. However, because phase transitions of the main lower-mantle minerals do not occur at this depth, the origin of the viscosity jump remains unknown. Here we show that bridgmanite-enriched rocks in the deep lower mantle have a grain size that is more than one order of magnitude larger and a viscosity that is at least one order of magnitude higher than those of the overlying pyrolitic rocks. This contrast is sufficient to explain the mid-mantle viscosity jump1,2. The rapid growth in bridgmanite-enriched rocks at the early stage of the history of Earth and the resulting high viscosity account for their preservation against mantle convection5-7. The high Mg:Si ratio of the upper mantle relative to chondrites8, the anomalous 142Nd:144Nd, 182W:184W and 3He:4He isotopic ratios in hot-spot magmas9,10, the plume deflection4 and slab stagnation in the mid-mantle3 as well as the sparse observations of seismic anisotropy11,12 can be explained by the long-term preservation of bridgmanite-enriched rocks in the deep lower mantle as promoted by their fast grain growth.

First-principles investigation on diffusion in stishovite and CaCl2-type silica: Implication for MORB viscosity in the lower mantle

Subducted oceanic crust is thought to play a key role in generating the compositional heterogeneities in the lower mantle. Its rheological properties are essential for understanding the segregation of oceanic crust from the slab and the further interaction with the surrounding mantle. Here, we report first-principles results for Si and O diffusions in stishovite and CaCl2-type silica and evaluate the viscosity of the oceanic crust under lower-mantle conditions. Our results show that the Si diffusion in CaCl2-type silica is enhanced by pressure, with a negative activation volume of ∼−0.2 cm3/mol along the 〈111〉 direction. We find that the Si diffusivity of CaCl2-type silica is two orders of magnitude faster than that of bridgmanite in the deep lower mantle, and therefore, it becomes a rheological weaker phase compared to bridgmanite. In addition, CaCl2-type silica could substantially reduce the crustal viscosity and facilitate the segregation of upper oceanic crust from the lower lithosphere with increasing depths. The delaminated crustal fragments may be sufficiently stirred and stretched by the vigorous mantle convection and account for the ubiquitous small-scale heterogeneities in the lower mantle. Our results provide new constraints on the crustal rheology data for future geodynamic simulations to better understand the crustal recycling in the lower mantle.

A first-principles study of the structural, electronic and elastic properties of the FeO2-FeO2He system under high pressure.

The origin of the wave velocity anomalies at the core-mantle boundary (CMB) has been controversial. The primordial helium reservoir in the deep lower mantle remains elusive even with geochemical evidence for its existence. Here, we calculated the density and wave velocity of the FeO2-FeO2He system under the CMB conditions using first principles. The FeO2 and FeO2He of pyrite-type can exist stably under the CMB conditions without melting, and the incorporated helium increases the stability of the system. The electrical properties of FeO2 and FeO2He are not related to pressure. Doped helium reduces the density of the system but increases the elastic modulus. Our results suggest that FeO2 can be used as a viable material composition of ultra-low velocity zones (ULVZs), and FeO2He can explain the D'' seismic discontinuity instead of ULVZs. The primordial helium reservoir possibly formed by the accumulation of FeO2He, the only stable solid helium-bearing compound under the CMB conditions, may coincide with the location of the D'' layer.

Hydrous Melting Driven Upwelling From the Mantle Transition Zone in the Mongolia Plateau Revealed by Receiver Function Analysis

AbstractThe detailed images of the mantle transition zone (MTZ) give insights into the nature of mantle upwelling from the deep Earth. We use receiver function to investigate the MTZ structure beneath the Mongolia Plateau using earthquakes recorded by 223 stations deployed in the Central Asian Orogenic Belt. The topographies of the 410 km (D410) and 660 km (D660) discontinuities are both characterized by slight depressions in most parts of the Mongolia Plateau. An obvious low‐velocity layer (LVL) atop the MTZ with an average thickness of ∼40 km is observed beneath this study area, indicating the hydrous partial molten layer atop the D410 and a hydrous MTZ. The D410 depth of ∼410 km and the LVL up to ∼60 km thick beneath the Dariganga volcano imply that there is a chemically distinct hydrous upwelling related to the subducted Pacific slab rather than a classic thermal‐dominated upwelling. The ∼10 km thinned MTZs are found beneath the western Hangay Dome and the Middle Gobi volcano. A narrow NS‐trending MTZ thickening zone extending from the Hovsgol rift to the central Hangay Dome is mainly caused by a ∼10 km depression of D660. We speculate that the mantle upwelling beneath the Hangay Dome may be related to the stagnant slab descending through the MTZ, rather than being rooted from the deep lower mantle. Hydrous melting‐driven upwellings from the MTZ beneath the Mongolia Plateau play a key role in intraplate volcanism.

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.

Formation of an Al‐Rich Niccolite‐Type Silica in Subducted Oceanic Crust: Implications for Water Transport to the Deep Lower Mantle

AbstractSubducted oceanic crust is enriched in free silica. Although being one of the silica polymorphs at lower‐mantle pressures, niccolite‐type phase (Nt‐phase) has not been documented in multicomponent metabasaltic or metasediment compositions relevant to subducting oceanic crust. Here, we report the formation of an Al‐rich Nt‐phase (∼24.4 to 32.4 wt% Al2O3), coexisting with Al‐depleted bridgmanite (∼6.4 to 7.6 wt% Al2O3), δ‐phase, and iron‐rich phase in model hydrated basalts over the pressure‐temperature range of 84–113 GPa and 1,800–2,200 K. Infrared spectroscopy of a pure synthetic Al‐rich Nt‐phase shows OH bending and stretching vibrations at high pressures, indicative of its hydrous nature. This study suggests that Al‐rich Nt‐phase can serve as a potential water carrier in subducted oceanic crust to the deep lower mantle.

Stability of a Mixed‐Valence Hydrous Iron‐Rich Oxide: Implications for Water Storage and Dynamics in the Deep Lower Mantle

AbstractIncorporation of water into mantle compositions can have significant effects on the phase relations in the systems. In this study, we synthesized an iron‐rich hexagonal hydrous phase (referred to as “HH1‐phase”) under the high pressure‐temperature (P‐T) conditions of the deep lower mantle and determined the crystal structure of the HH1‐phase at 79 GPa using the multigrain crystallography method. The chemical formula obtained was Fe12.76O18Hx (x ∼ 4.5) in the Fe‐O‐H system. To demonstrate the role of HH1‐phase for water storage in multicomponent systems relevant to mantle compositions, we investigated the stability of HH1‐phase in both MgO‐rich pyrolitic and SiO2‐rich basaltic compositions. Our results indicate that the HH1‐phase serves as major water storage in a pyrolitic composition, whereas the Al‐rich CaCl2‐type δ‐phase and SiO2 phase are major water storage phases in a SiO2‐rich basaltic composition. Incorporation of considerable amounts of SiO2, MgO, and Al2O3 into the HH1‐phase expands its stability field from 98 GPa in the Fe‐Al‐O‐H system to at least 108 GPa (corresponding to ∼2,400 km depth) in the Mg‐Si‐Al‐Fe‐O‐H system. Plumes of hot upwelling rock rooted at the base of the lower mantle have been proposed as a possible origin of hotspot volcanoes. The hydrous Fe‐rich HH1‐phase, if included into the material of upwelling plumes, will decompose on its rising to the upper part of the lower mantle and release water. Our results should provide constraints on water storage in the deep lower mantle and have implications for deep mantle dynamics.

Chemical reaction between ferropericlase (Mg,Fe)O and water under high pressure-temperature conditions of the deep lower mantle

AbstractThe presence of water may contribute to compositional heterogeneities observed in the deep lower mantle. Mg-rich ferropericlase (Fp) (Mg,Fe)O in the rock-salt structure is the second most abundant phase in a pyrolitic lower mantle model. To constrain water storage in the deep lower mantle, experiments on the chemical reaction between (Mg,Fe)O and H2O were performed in a laser-heated diamond-anvil cell at 95–121 GPa and 2000–2250 K, and the run products were characterized combining in situ synchrotron X-ray diffraction measurements with ex-situ chemical analysis on the recovered samples. The pyrite-structured phase FeO2Hx (x ≤ 1, Py-phase) containing a negligible amount of Mg (<1 at%) was formed at the expense of iron content in the Fp-phase through the reaction between (Mg,Fe)O and H2O, thus serving as water storage in the deepest lower mantle. The formation and segregation of nearly Mg-free Py-phase to the base of the lower mantle might provide a new insight into the deep oxygen and hydrogen cycles.

High pressure-temperature phase relations of basaltic crust up to mid-mantle conditions

A substantial amount of subducted basaltic crusts may exist in the lower mantle. It features distinct chemical composition from the peridotitic mantle and plays important roles in the chemical and dynamic evolution of Earth's interior. However, the chemical composition of mineral phases present in basaltic crust in the lower mantle is still poorly constrained. Here, we determined phase relations of normal mid-ocean ridge basalt (MORB) up to 52 GPa at 2000 K by multi-anvil press. Throughout our experiments, the mineral assemblages consist of five phases including bridgmanite, stishovite, calcium perovskite (davemaoite), calcium ferrite and new hexagonal aluminous phases. The density of MORB calculated from our phase assemblages and previously published thermoelastic data is 2-3% higher than that of the average mantle, which is consistent with literature. The new hexagonal aluminous phase is a host of potassium but only occupies 1-2 vol.% due to limited abundance of potassium in normal MORB. This indicates that the new hexagonal aluminous phase doesn't affect the elastic properties of basalt. Electron energy-loss spectra of recovered basaltic Al-rich bridgmanite show significant enrichment of ferrous iron (75-85%) compared with peridotitic Al-poor bridgmanite (∼30%), which is against previous studies showing that ferric iron ratio in bridgmanite increases with Al content. Ferric iron exhibits strong partitioning into the new hexagonal aluminous phase (41-64%), whereas bridgmanite (14-28%) and calcium ferrite phase (5-27%) remain Fe2+-enriched. The oxygen vacancy component of MgAlO2.5 in bridgmanite is ∼11% up to 40 GPa, which is much higher than that in peridotitic bridgmanite (2-3%), possibly producing a viscosity contrast in the mid-mantle that would explain slab stagnation and plume thinning between 660 km and 1000 km depth. The presence of ferrous iron-rich bridgmanite in the deep lower mantle may contribute to seismic features of large low-shear-velocity provinces.

Stabilization of S3O4 at high pressure: implications for the sulfur-excess paradox

The amount of sulfur in SO2 discharged in volcanic eruptions exceeds that available for degassing from the erupted magma. This geological conundrum, known as the “sulfur excess”, has been the subject of considerable interests but remains an open question. Here, in a systematic computational investigation of sulfur-oxygen compounds under pressure, a hitherto unknown S3O4 compound containing a mixture of sulfur oxidation states +II and +IV is predicted to be stable at pressures above 79 GPa. We speculate that S3O4 may be produced via redox reactions involving subducted S-bearing minerals (e.g., sulfates and sulfides) with iron and goethite under high-pressure conditions of the deep lower mantle, decomposing to SO2 and S at shallow depths. S3O4 may thus be a key intermediate in promoting decomposition of sulfates to release SO2, offering an alternative source of excess sulfur released during explosive eruptions. These findings provide a possible resolution of the “excess sulfur degassing” paradox and a viable mechanism for the exchange of S between Earth’s surface and the lower mantle in the deep sulfur cycle.

The effect of Ti on Ca-pv and Mg-pv phase stability

Magnesium silicate perovskite in the form of bridgmanite (bdg) and Calcium silicate perovskite (Ca-pv) have similar chemical structures and may mix into a single perovskite phase in the lower mantle which would have profound effects on many seismic properties. While we have previously found that this is unlikely to occur in pure bdg and ca-pv in this paper we examine whether phase mixing can be induced by Titanium.We predict that even small amounts of Ti can cause significant increases in mixing of the two phases. Miscibility of the phases has a strong dependence upon how Ti is partitioned between the two phases before mixing and thus the source and history of introduced Ti is important in determining miscibility.We predict basalts, even those with heavy Ti enrichment (10%), will not form a single phase in subducted slabs as their mixing temperatures remain above 2500 K for most compositions throughout lower mantle pressures. In pyrolytic mantle it is predicted that at shallow depths large amounts of Ti are needed to induce phase mixing (~40% Ti at 25 GPa and geotherm temperatures) but less Ti is needed to induce mixing with depth (~1% Ti at 125 GPa and geotherm temperatures). Thus we predict that enriched Ti regions will see perovskite mixing near the bottom of the lower mantle. These mixed perovskite regions partition Ti out of unmixed regions and thus provide a mechanism for Ti enriched regions to form in the deep lower mantle. Both ferrous iron and the Ca:Mg ratio are predicted to have a larger control on the mixing temperature of pyrolytic systems than Ti, however. For Ti and Ca rich pyroxene megacrysts we find that they should become a single phase along a lower mantle geotherm at around 40–115 GPa depending heavily upon Ti and Ca concentration.

HP-PdF2-type FeCl2 as a potential Cl-carrier in the deep Earth

Abstract We report for the first time the formation of a HP-PdF2-type FeCl2 phase (space group Pa3), through high pressure-temperature (P-T) reactions in the hydrous systems (Mg0.6Fe0.4)SiO3–H2O–NaCl and FeO2H–NaCl in a laser-heated diamond-anvil cell up to 108 GPa and 2000 K. Applying single-crystal X-ray diffraction (XRD) analysis to individual submicrometer-sized grains, we have successfully determined the crystal structure of the as-synthesized FeCl2 phase, in agreement with our theoretical structure search results. In situ high P-T XRD data revealed the substitution of Cl for OH(O) in such a cubic Pa3 structure, demonstrating that this topology is a potential host for both H and Cl in the deep Earth. The chemical analysis of the recovered sample showed that the post-perovskite phase contains considerable amounts of Na2O and Fe2O3. The coexistence of the cubic FeCl2 phase and post-perovskite suggests that the lowermost mantle could be a potential reservoir of Cl. The possible presence of volatiles such as H and Cl in the deep lower mantle would impact the composition and iron valence state of the post-perovskite phase.

Role of hydrogen and proton transportation in Earth’s deep mantle

Hydrogen (H) is the most abundant element in the known universe, and on the Earth’s surface it bonds with oxygen to form water, which is a distinguishing feature of this planet. In the Earth’s deep mantle, H is stored hydroxyl (OH−) in hydrous or nominally anhydrous minerals. Despite its ubiquity on the surface, the abundance of H in the Earth’s deep interior is uncertain. Estimates of the total H budget in the Earth’s interior have ranged from less than one hydrosphere, which assumes an H-depleted interior, to hundreds of hydrospheres, which assumes that H is siderophile (iron-loving) in the core. This discrepancy raises the questions of how H is stored and transported in the Earth’s deep interior, the answers to which will constrain its behavior in the deep lower mantle, which is defined as the layer between 1700 km depth and the core–mantle boundary.

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.

Spectral proxies for bonding transitions in SiO2 and MgSiO3 polymorphs at high pressure up to 270 GPa by O K -edge x-ray Raman scattering

Advances in synchrotron x-ray Raman scattering (XRS) techniques enable probing of pressure-induced element-specific bonding transitions in diverse oxides beyond megabar pressures. Evolution of the electronic structures under extreme pressure involves an emergence of additional oxygen $1s$ electron excitation pattern and shifts in the electronic states toward high-energy ranges in an XRS spectrum. Structural origins of such changes and proper spectral proxy for the corresponding evolution in electronic density of states remain to be established. Here, we calculated the O $K$-edge features in the XRS spectrum for diverse crystalline ${\mathrm{SiO}}_{2}$ polymorphs, from \ensuremath{\alpha}-quartz (1 atm) to a pyrite-type structure (at \ensuremath{\sim}271 GPa), beyond the current experimental pressure limit (\ensuremath{\sim}160 GPa). The ab initio results unravel the electronic origins of the evolution in the XRS patterns, such as the emergence of the bimodal feature with decreasing oxygen-oxygen distance $({d}_{\mathrm{O}\text{\ensuremath{-}}\mathrm{O}}) <\ensuremath{\sim}2.5 \AA{}$ and a subsequent merging of the doublet patterns with a further decrease in ${d}_{\mathrm{O}\text{\ensuremath{-}}\mathrm{O}}$ down to $\ensuremath{\sim}2.3--2.4 \AA{}$. Ab initio simulations of the XRS spectrum of pyrite-type ${\mathrm{SiO}}_{2}$ phase at $\ensuremath{\sim}271 \mathrm{GPa}$ revealed the significant electron dispersion in $2{p}^{*}$ states and the pronounced electron density between edge-sharing oxygen atoms with ${d}_{\mathrm{O}\text{\ensuremath{-}}\mathrm{O}}$ of \ensuremath{\sim}2.1 \AA{}, in contrast to an earlier prediction with a negligible electron density between those oxygen atoms. The pressure-induced increase in electron interactions leads to a systematic change in the edge energy at half of the spectrum area $({E}_{M})$ and the edge energy at the center of mass of the spectrum $({E}_{C})$ of XRS patterns beyond multimegabar pressures. Particularly, the ${E}_{M}$ and ${E}_{C}$ increase linearly with decreasing oxygen proximity $({d}_{\mathrm{O}\text{\ensuremath{-}}\mathrm{O}})$ and bulk density up to 270 GPa. The ${E}_{M}$ and ${E}_{C}$ for ${\mathrm{MgSiO}}_{3}$ phases show larger changes with varying ${d}_{\mathrm{O}\text{\ensuremath{-}}\mathrm{O}}$ than those in ${\mathrm{SiO}}_{2}$ phases, demonstrating the effect of the electronic hybridization between Mg and O on the evolution of the XRS patterns. The strong linear correlation among ${E}_{M}$, ${E}_{C}$, and ${d}_{\mathrm{O}\text{\ensuremath{-}}\mathrm{O}}$ (and density) up to \ensuremath{\sim}270 GPa confirms that the ${E}_{M}$ and ${E}_{C}$ can be effective spectral proxies to quantify the densification of crystalline and noncrystalline oxides. The results with the statistical evaluations of the electronic density of states provide improved prospects for the prediction of the evolution in core-level excitation features of diverse complex, multicomponent oxides under multimegabar pressure conditions toward the deep lower mantle of Earth and other super-Earth bodies.

Composition and Pressure Effects on Partitioning of Ferrous Iron in Iron-Rich Lower Mantle Heterogeneities

Both seismic observations of dense low shear velocity regions and models of magma ocean crystallization and mantle dynamics support enrichment of iron in Earth’s lowermost mantle. Physical properties of iron-rich lower mantle heterogeneities in the modern Earth depend on distribution of iron between coexisting lower mantle phases (Mg,Fe)O magnesiowüstite, (Mg,Fe)SiO3 bridgmanite, and (Mg,Fe)SiO3 post-perovskite. The partitioning of iron between these phases was investigated in synthetic ferrous-iron-rich olivine compositions (Mg0.55Fe0.45)2SiO4 and (Mg0.28Fe0.72)2SiO4 at lower mantle conditions ranging from 33–128 GPa and 1900–3000 K in the laser-heated diamond anvil cell. The resulting phase assemblages were characterized by a combination of in situ X-ray diffraction and ex situ transmission electron microscopy. The exchange coefficient between bridgmanite and magnesiowüstite decreases with pressure and bulk Fe# and increases with temperature. Thermodynamic modeling determines that incorporation and partitioning of iron in bridgmanite are explained well by excess volume associated with Mg-Fe exchange. Partitioning results are used to model compositions and densities of mantle phase assemblages as a function of pressure, FeO-content and SiO2-content. Unlike average mantle compositions, iron-rich compositions in the mantle exhibit negative dependence of density on SiO2-content at all mantle depths, an important finding for interpretation of deep lower mantle structures.