Ultra-low Velocity Zones Research Articles

Hydrous minerals and the storage of water in the deep mantle

Water is transported into the deep mantle via hydrous minerals in subducting slabs. During subduction, a series of minerals in these slabs such as serpentine or chlorite, Mg–sursassite and/or the 10Å phase, and phase A can be stable at different pressures within the slab geotherms, and may transport significant amount of water into the Earth's interior. The transition zone has a large water storage capacity because of the high solubility of water in wadsleyite and ringwoodite. The recent discovery of hydrous ringwoodite and phase Egg as inclusions in ultra deep diamonds from Juina, Brazil suggests that the transition zone may indeed contain water. Seismic tomographic studies and electrical conductivity observations suggest that the transition zone may contain large amount of water, at least locally, beneath the subduction zones. The discovery of a new hydrous phase H, MgSiO2(OH)2, and its solid solution with isostructural phase δ-AlOOH, suggests that a significant amount of water could be stored in this hydrous magnesium silicate phase which is stable down to the lower mantle. Water may be transported into the bottom of the lower mantle via phase H–δ solid solution in descending slabs. This new high pressure hydrous phase solid solution has a high bulk modulus and sound velocity owing to strong O–H bonding due to hydrogen bond symmetrization in the lower mantle. Therefore, water stored in this hydrous phase would not reduce the seismic wave velocity in the lower mantle, and is seismically invisible. Dehydration melting could then occur at the base of the lower mantle, providing a potential explanation for the ultralow-velocity zone at the core–mantle boundary. When this hydrous magnesium silicate phase or hydrous melt makes contact with the metallic outer core at the core–mantle boundary, then hydrogen is likely to dissolve into the core.

Seismic evidence for an Iceland thermo-chemical plume in the Earth's lowermost mantle

We constrain the geographic extent, geometry and velocity structure of the seismic anomaly near the Earth's core–mantle boundary (CMB) beneath Iceland, based on travel time and three-dimensional waveform modeling of the seismic data sampling the lowermost mantle beneath Iceland. Our analysis suggests a mushroom-shaped low velocity anomaly situated in the lowermost mantle beneath Iceland surrounded by a high velocity province. The best fitting mushroom-shaped model is 600 km high and has a stem with a radius of 350 km in the lowermost 250 km of the mantle and a cap with increasing radii from 550 km at 250 km above the CMB to 650 km at 600 km above the CMB. The shear velocity structure varies from 0% at the top to −3% at 250 km above the CMB and to −6% at the CMB. These inferred seismic features, in combination with the previous evidence of existence of ultra-low velocity zones at the base of the mantle beneath the region, suggest that Iceland represents a thermo-chemical plume generated by interaction of downwelling and a localized chemical anomaly at the base of the mantle.

A compositional origin to ultralow‐velocity zones

AbstractWe analyzed vertical component short‐period ScP waveforms for 26 earthquakes occurring in the Tonga‐Fiji trench recorded at the Alice Springs Array in central Australia. These waveforms show strong precursory and postcursory seismic arrivals consistent with ultralow‐velocity zone (ULVZ) layering beneath the Coral Sea. We used the Viterbi sparse spike detection method to measure differential travel times and amplitudes of the postcursor arrival ScSP and the precursor arrival SPcP relative to ScP. We compare our measurements to a database of 340,000 synthetic seismograms finding that these data are best fit by a ULVZ model with an S wave velocity reduction of 24%, a P wave velocity reduction of 23%, a thickness of 8.5 km, and a density increase of 6%. This 1:1 VS:VP velocity decrease is commensurate with a ULVZ compositional origin and is most consistent with highly iron enriched ferropericlase.

Bayesian inference for ultralow velocity zones in the Earth's lowermost mantle: Complex ULVZ beneath the east of the Philippines

AbstractUltralow velocity zones (ULVZs) are small‐scale structures with a sharp decrease in S and P wave velocity, and an increase in the density on the top of the Earth's core‐mantle boundary. The ratio of S and P wave velocity reduction and density anomaly are important to understanding whether ULVZs consist of partial melt or chemically distinct material. However, existing methods such as forward waveform modeling that utilize 1‐D and 2‐D Earth‐structure models face challenges when trying to uniquely quantify ULVZ properties because of inherent nonuniqueness and nonlinearity. This paper develops a Bayesian inversion for ULVZ parameters and uncertainties with rigorous noise treatment to address these challenges. The posterior probability density of the ULVZ parameters (the solution to the inverse problem) is sampled by the Metropolis‐Hastings algorithm. To improve sampling efficiency, parallel tempering is applied by simulating a sequence of tempered Markov chains in parallel and allowing information exchange between chains. First, the Bayesian inversion is applied to simulated noisy data for a realistic ULVZ model. Then, measured data sampling the lowermost mantle under the Philippine Sea are considered. Cluster analysis and visual waveform inspection suggest that two distinct classes of ScP (S waves converted to, and reflected as, P waves) waves exist in this region. The distinct waves likely correspond to lateral variability in the lowermost mantle properties in a NE‐SW direction. For the NE area, Bayesian model selection identifies a two‐layer model with a gradual density increase as a function of depth as optimal. This complex ULVZ structure can be due to the percolation of iron‐enriched, molten material in the lowermost mantle. The results for the SW area are more difficult to interpret, which may be due to the limited number of data available (too few waveforms to appropriately reduce noise) and/or complex 2‐D and 3‐D structures that cannot be explained properly by the 1‐D models required by our inversion approach. In particular, the complex waveforms require highly layered 1‐D models to fit the data. These models appear physically unreasonable and suggest that the SW region cannot be explained by 1‐D structure.

High-Fe (Mg, Fe)O inclusion in diamond apparently from the lowermost mantle

The only direct samples of Earth's lower mantle are rare inclusions in diamonds. The most abundant are (Mg, Fe)O, showing highly magnesian compositions. Here we report a transmission electron microscopy study of an exceptional inclusion with composition ∼(Mg0.35Fe0.65)O, with 5–7 vol% nanometer-sized magnesioferrite spinel precipitated on dislocations and the interface with diamond. The magnesioferrite apparently precipitated as a high-pressure polymorph from the (Mg, Fe)O host that contained an extraordinarily large concentration of Fe3+. Paradoxically, the crystal also contains blebs of Fe–Ni alloy. These observations plus the recently documented very low seismic velocities of high-Fe (Mg, Fe)O and other experimental results suggest that this inclusion may have originated in one of the ultra-low velocity zones at the base of the mantle and come to the surface through diapiric upwelling culminating in kimberlite eruption.

The P-wave boundary of the Large-Low Shear Velocity Province beneath the Pacific

The Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle represent volumetrically significant thermal or chemical or thermo-chemical heterogeneities. Their structure and boundaries have been widely studied, mainly using S-waves, but much less is known about their signature in the P-wavefield. We use an extensive dataset recorded at USArray to create, for the first time, a high-resolution map of the location, shape, sharpness, and extent of the boundary of the Pacific LLSVP using P(Pdiff)-waves. We find that the northern edge of the Pacific LLSVP is shallow dipping (26° relative to the horizontal) and diffuse (∼120 km wide transition zone) whereas the eastern edge is steeper dipping (70°) and apparently sharp (∼40 km wide). We trace the LLSVP boundary up to ∼500 km above the CMB in most areas, and 700 km between 120° and 90°W at the eastern extent of the boundary. Apparent P-wave velocity drops are ∼1–3% relative to PREM, indicating a strong influence of LLSVPs on P-wave velocity, at least in the high-frequency wavefield, in contrast to previous studies. A localised patch with a greater velocity drop of ∼15–25% is detected, defined by large magnitude gradients of the travel-time residuals. We identify this as a likely location of an Ultra-Low Velocity Zone (ULVZ), matching the location of a previously detected ULVZ in this area. The boundary of a separate low velocity anomaly, of a similar height to the LLSVP, is detected in the north–west Pacific, matching tomographic images. This outlier appears to be connected to the main LLSVP through a narrow channel close to the CMB and may be in the process of joining or splitting from the main LLSVP. We also see strong velocity increases in the lower mantle to the east of the LLSVP, likely detecting subducted material beneath central America. The LLSVP P-wave boundary is similar to that determined in high-resolution S-wave studies and follows the −0.4% ΔVS iso-velocity contour in the S40RTS tomography model. Additionally, the LLSVP boundary roughly matches the shape of the −0.4% ΔVP iso-velocity contour of the P-wave model GyPSuM but defines an area more similar to that defined by the 0.0% VP iso-velocity contour. High resolution P-wave velocity determination allows for estimation of the ratio of P- and S-wave velocity anomalies (RS,P) which can be used to indicate dominantly thermal or chemical control of seismic velocities. Although the RS,P is found here to be approximately 2.4, which is indicative of a thermo-chemical anomaly. However, this result contains a large amount of uncertainty and the implications for the origin of LLSVPs likely remain inconclusive. Nonetheless, other observations of the Pacific LLSVP are consistent with a thermo-chemical anomaly whose shape and boundary sharpness are controlled by proximity to active and past subduction.

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.

Melting of subducted basalt at the core-mantle boundary

The geological materials in Earth's lowermost mantle control the characteristics and interpretation of seismic ultra-low velocity zones at the base of the core-mantle boundary. Partial melting of the bulk lower mantle is often advocated as the cause, but this does not explain the nonubiquitous character of these regional seismic features. We explored the melting properties of mid-oceanic ridge basalt (MORB), which can reach the lowermost mantle after subduction of oceanic crust. At a pressure representative of the core-mantle boundary (135 gigapascals), the onset of melting occurs at ~3800 kelvin, which is ~350 kelvin below the mantle solidus. The SiO2-rich liquid generated either remains trapped in the MORB material or solidifies after reacting with the surrounding MgO-rich mantle, remixing subducted MORB with the lowermost mantle.

Melt redistribution by pulsed compaction within UltraLow Velocity Zones

This article investigates the melt distribution and resultant seismic signature within UltraLow Velocity Zones (ULVZs) forced by pulsed compaction at the mantle–ULVZ interface. Transient flow in the ambient mantle causes periodic compaction in the ULVZ matrix. For a neutrally buoyant melt, an initially uniform melt distribution is modified by the formation of a thin, decompacting, melt-rich layer near the top and a wide, melt-poor, compacting layer near the bottom. Such a structure is reflected in large reductions in S and P wave velocities near the top and smaller reductions near the bottom of the ULVZ. A dense melt pools near the bottom of the ULVZ, leading to larger reductions in seismic wave speed near the bottom. The magnitude of melt segregation in the decompaction layer is controlled by the viscosity of the ULVZ matrix in a nonlinear fashion. At high ULVZ viscosities, the compaction length becomes substantially larger than the dimension of thin ULVZs, leading to a reduction in the magnitude of melt segregation in the decompaction layer. In a ULVZ of matrix viscosity 1020Pas containing an average melt volume fraction of 0.05, formation of decompacting, melt-rich layers reduce the S and P wave velocities by 25% and 8%, respectively. Vertical variation in seismic velocity reduction within the ULVZ column is a consequence of melt redistribution by compaction, rather than variation of melt microstructure within the ULVZ.

Direct shock compression experiments on premolten forsterite and progress toward a consistent high‐pressure equation of state for CaO‐MgO‐Al2O3‐SiO2‐FeO liquids

AbstractWe performed shock compression experiments on preheated forsterite liquid (Mg2SiO4) at an initial temperature of 2273 K and have revised the equation of state (EOS) that was previously determined by shock melting of initially solid Mg2SiO4 (300 K). The linear Hugoniot, US = 2.674 ± 0.188 + 1.64 ± 0.06 up km/s, constrains the bulk sound speed within a temperature and composition space as yet unexplored by 1 bar ultrasonic experiments. We have also revised the EOS for enstatite liquid (MgSiO3) to exclude experiments that may have been only partially melted upon shock compression and also the EOS for anorthite (CaAl2SiO6) liquid, which now excludes potentially unrelaxed experiments at low pressure. The revised fits and the previously determined EOS of fayalite and diopside (CaMg2SiO6) were used to produce isentropes in the multicomponent CaO‐MgO‐Al2O3‐SiO2‐FeO system at elevated temperatures and pressures. Our results are similar to those previously presented for peridotite and simplified “chondrite” liquids such that regardless of where crystallization first occurs, the liquidus solid sinks upon formation. This process is not conducive to the formation of a basal magma ocean. We also examined the chemical and physical plausibility of the partial melt hypothesis to explain the occurrence and characteristics of ultra‐low velocity zones (ULVZ). We determined that the ambient mantle cannot produce an equilibrium partial melt and residue that is sufficiently dense to be an ultra‐low velocity zone mush. The partial melt would need to be segregated from its equilibrium residue and combined with a denser solid component to achieve a sufficiently large aggregate density.

SPdKS analysis of ultralow‐velocity zones beneath the western Pacific

We collected a new data set of 1354 broadband SPdKS waveforms sampling the western Pacific Ocean region. These data indicate that multiple ultralow‐velocity zones (ULVZs) exist in this region. We compared these data to 2.5‐D synthetic seismograms computed with the PSVaxi method for a suite of 517 ULVZ models. The region beneath the North and South Philippine Sea shows no evidence for ULVZ presence. The region beneath the Coral Sea shows a large ULVZ which is approximately 700 × 700 km in lateral dimensions and up to 20 km thick. Multiple small‐scale ULVZs approximately 180 × 180 km and up to 10 km thick are inferred beneath the South China Sea. Our findings are consistent with previous efforts utilizing the ScP seismic phase and provide additional constraints on ULVZ position and size.

Does partial melting explain geophysical anomalies?

The existence of partial melt is frequently invoked to explain geophysical anomalies such as low seismic wave velocity and high electrical conductivity. I review various experimental and theoretical studies to evaluate the plausibility of this explanation. In order for a partial melt model to work, not only the presence of melt, but also the presence of appropriate amount of melt needs to be explained. Using the mineral physics observations on the influence of melt on physical properties and the physics and chemistry of melt generation and transport, I conclude that partial melt model for the asthenosphere with homogeneous melt distribution does not work. One needs to invoke inhomogeneous distribution of melt if one wishes to explain observed geophysical anomalies by partial melting. However, most of models with inhomogeneous melt distribution are either inconsistent with some geophysical observations or the assumed structures are geodynamically unstable and/or implausible. Therefore partial melt models for the geophysical anomalies of the asthenosphere are unlikely to be valid, and some solid-state mechanisms must be invoked. The situation is different in the deep upper mantle where melt could completely wet grain-boundaries and continuous production of melt is likely by “dehydration melting” at around 410-km. In the ultralow velocity zone in the D″ layer, where continuous production of melt is unlikely, easy separation of melt from solid precludes the partial melt model for low velocities and high electrical conductivity unless the melt density is extremely close to the density of co-existing solid minerals or if there is a strong convective current to support the topography of the ULVZ region. Compositional variation such as Fe-enrichment is an alternative cause for the anomalies in the D″ layer.

Evaluation of 1‐D and 3‐D seismic models of the Pacific lower mantle with S, SKS, and SKKS traveltimes and amplitudes

AbstractIn this study, we analyzed the seismic phases S, SKS, and SKKS from 31 deep‐focus earthquakes in the Tonga‐Fiji region recorded in North America between epicentral distances of 85° and 120°. The differential traveltimes and amplitude ratios for these phases reveal clear epicentral distance trends not predicted by standard one‐dimensional (1‐D) reference Earth models. The increase of the S/SKS amplitude ratio up to a factor of 10 is accompanied by an increase of the S‐SKS differential traveltime of up to 10 s. SKKS‐SKS differential traveltimes of 2–3 s and SKKS/SKS amplitude ratios of a factor of 2–4 across the epicentral range have maxima near 107°. We examined these observations using full (1‐D and 3‐D) waveforms for three 1‐D seismic velocity profiles for the central Pacific region and for the tomographic model S40RTS including modifications: different regularization parameters, great‐circle path azimuthal variation, strength of S wave velocity perturbations, S wave velocity gradients in the lower mantle, and ultra–low velocity zones. To explain these data, we constructed a hybrid model that combines both features of S40RTS and short‐wavelength features from the 1‐D profiles. The large‐scale seismic structure is represented by S40RTS. Embedded within S40RTS are a 20 km thick ultra–low velocity zone at the core‐mantle boundary near the source side and a 200 km thick negative velocity gradient zone near the receiver side of the paths. Our analysis demonstrates that the S wave velocity structure of the Pacific large low shear‐velocity province cannot be interpreted solely by global tomographic or regional modeling approaches in exclusion of each other.

Mega ultra low velocity zone and mantle flow

Mantle flow in Earth's interior has been inferred from a variety of geo-disciplines. Two continental-scale, nearly antipodal, large low shear velocity provinces (LLSVPs) at the base of the mantle, thought to be dense and chemically distinct likely play a significant role in mantle dynamics and plume generation, and hence are targeted in a high-resolution seismic study. We analyze broadband SPdKS waveforms using a 2.5D axi-symmetric finite difference wave propagation algorithm PSVaxi. Here we find patches of greatly reduced seismic wave speeds at the core–mantle boundary, i.e., ultra-low velocity zones (ULVZs), within the Pacific LLSVP, including the largest ULVZ detected to date, roughly 250×800km in lateral dimension and 10–15km thick, in an apparent hole in the LLSVP. The presence of this ULVZ in the LLSVP hole is well explained by dynamically merging, chemically-distinct piles containing ULVZs at their margins. The consequence of these merging piles may be to initiate anomalously large, infrequent plumes, as well as to provide a means to transfer isotopes to the surface.

Extreme‐pressure research explores how Earth's mantle solidified

During the earliest stages of the Earth's formation, the planet's mantle may have taken the form of a giant magma ocean, being fully or partially molten all the way down to the core‐mantle boundary. Though mantle material is predominantly solid today, some scientists suggest that regions of anomalously low seismic wave velocity deep within the mantle, known as ultralow velocity zones (ULVZs), may be indicative of a remnant magma ocean or of partial melting of minerals near the core‐mantle boundary. To understand how the early mantle solidified or whether modern melt could be the source of ULVZs, scientists need to know how various minerals and melts behave under the extreme conditions found near the center of the Earth.

A melting model for the lowermost mantle using Clapeyron slopes derived from experimental data: Consequences for the thickness of ultralow velocity zones (ULVZs)

Ultralow velocity zones (ULVZs) are relatively thin regions directly above the core‐mantle boundary (CMB) that exhibit marked seismic P‐ and S‐velocity reductions. A viable explanation for the reduction is the presence of melt fractions within ULVZs. Since melt was found to be denser than solid in melting experiments at lowermost mantle pressures, partially molten ULVZs should exhibit negative buoyancy. Using published experimental data, we present a melting model based on Clapeyron slopes for the formation of ULVZs as partially molten regions above the CMB and apply the resulting melting curves and latent heat effects in fully dynamic, regionally constant viscosity convection simulations of the lowermost mantle. We find that the height of the ULVZs depends only moderately on Rayleigh number but strongly decreases with increasing excess density of melt over solid. The models predict excess density of at least 1% to explain observed heights. The combined effect of topography and latent heat of melting reduces the vigor of mantle convection only very slightly, while if combined with a decrease of the ULVZ viscosity, mantle flow velocities are significantly enhanced near the CMB, and overall mantle temperatures are notably increased. ULVZ heights are found to be insensitive to ULVZ viscosity (for the range isoviscous to 1/100 the viscosity of the ambient mantle).

An unsually large ULVZ at the base of the mantle near Hawaii

Previous studies have documented the presence of ultra-low-velocity-zones (ULVZs) at the base of the mantle, through observations of body wave complexities. Geometrically their heights are in the range of ∼5–30km, while little is known about their lateral extent beyond about 102km, due to limitations in sampling. Here we show remarkable features in the waveforms of S/Sdiff phases of western Pacific events observed at stations in North America that indicate the presence of a very large ultra-low-velocity-zone (ULVZ) at the base of the mantle, centered ∼11° to the southwest of Hawaii, within and near the northern border of the Pacific LLSVP. Waveform complexities include strongly delayed (>30s) postcursors. Measurements of travel times, beamforming analysis of out-of-plane energy, and full waveform comparisons with 3D numerical simulations, constrain the location, lateral extent, height and velocity reduction of the ULVZ with some level of trade-off. The simplified 3D model consists of a cylindrical ULVZ with a large aspect ratio of ∼20km in height and ∼910km in diameter at the CMB. The shear wave velocity reduction is ∼20%. This is to our knowledge the largest ULVZ mapped to date and the first time the lateral extent of a ULVZ has been constrained with some precision. Its location suggests that it may be the root of a long-lived, stable plume responsible for the Hawaiian volcanic chain, the hotspot track with the largest buoyancy flux.

Normal mode sensitivity to Earth’s D″ layer and topography on the core-mantle boundary: what we can and cannot see

The core–mantle boundary (CMB) is Earth’s most profound internal boundary separating the liquid iron outer core and the solid silicate mantle. The detailed structure near the CMB has a major influence on mantle convection and the evolution of the core. Seismic observations, such as topography on the CMB, thin ultra-low velocity zones (ULVZs), seismic anisotropy and the anticorrelation between shear wave and bulk sound velocity heterogeneities have mainly been made using body waves and are still poorly constrained. We investigate the sensitivity of Earth’s free oscillations to these features and specifically show how large individual anomalies must be for them to be observable. In addition, we discuss the possible trade-offs between these different lowermost mantle structures. Although modes have strong sensitivity to all the structures inserted, the results illustrate the limits of what normal modes can resolve. Our tests show that: (i) Even small scale features, such as ULVZs, with a thickness larger than 19 km can be observed as long as their distribution contains a long wavelength component. (ii) The peak-to-peak amplitude of CMB topography has a larger influence than its pattern and has to be smaller than 5 km to fit the data. (iii) The effect of scaling between shear wave velocity and density anomalies is less constrained, but a laterally varying pattern is implied by a simple test, suggesting the presence of chemical variations. (iv) A strong trade-off exists between anisotropy in compressional wave velocity and incidence angle whereas shear wave anisotropy is less observable. These findings provide valuable information for future normal mode studies on structures in Earth’s lowermost mantle and their trade-offs.

A three‐dimensional microgeodynamic model of melt geometry in the Earth's deep interior

This article presents a three‐dimensional microgeodynamic model of grain‐melt geometry in partially molten rocks. The isotropic unit cell of the partially molten rock is characterized by a face‐centered‐cubic symmetry, consisting of rhombic dodecahedral grains. The variation of surface tension between grain‐grain and grain‐melt contacts excites a coupled viscous flow within grains and the interstitial melt, leading to a steady state grain‐melt geometry. We obtain the fractional area of intergranular contact, contiguity, from these models as a function of melt volume fraction, between melt fractions of 0.05 and 0.25. Comparison with previous results indicates that the contiguity in three‐dimensional models is lower than in two‐dimensional models. The contrast between two‐ and three‐dimensional values of contiguity increases at high melt volume fractions. We apply our model to the ultralow‐velocity zones (ULVZs) and the very low velocity province (VLVP) in the Earth's core‐mantle boundary. The observed seismic signature of the ULVZ and VLVP can be explained by peridotite melt volume fractions between 0.08 and 0.12 and between 0.01 and 0.05, respectively, in a matrix of elastic properties similar to the Preliminary Earth Reference Model.

비정질 알칼리 규산염 원자구조의 철 함량 효과에 관한 고체 NMR 분광학 연구

철을 포함한 비정질 규산염 용융체의 원자 구조 규명은 지표 환경의 화성활동 및 맨틀 심부의 초저속도층의 속도구조에 이르는 광범위한 지질과정의 미시적인 원인에 대한 단서를 제공한다. 본 연구에서는 철을 포함한 비정질 규산염의 원자 구조 규명에 가장 적합한 고상 핵자기공명분광분석(NMR)을 이용하여 최대 16.07 wt%의 <TEX>$Fe_2O_3$</TEX>가 포함된 비정질 알칼리 규산염(iron-bearing alkali silicate glasses)의 철의 함량 변화가 원자구조에 미치는 영향을 규명하였다. <TEX>$^{29}Si$</TEX> 스핀-격자 완화시간(<TEX>$T_1$</TEX>)을 측정한 결과, 철의 함량에 따라 스핀-격자 완화시간이 짧아지는데 이는 철이 가지고 있는 홀전자(unpaired electron)와 핵 스핀(nuclear spin)간의 상호작용으로부터 기인한다. <TEX>$^{29}Si$</TEX> MAS NMR 실험 결과, 철이 포함되지 않은 시료의 경우 <TEX>$Q^2$</TEX>, <TEX>$Q^3$</TEX> 그리고 <TEX>$Q^4$</TEX>의 환경을 지시하는 피크가 분리됨에 반하여, 철이 포함된 시료의 경우 NMR 신호의 급격한 감소와 피크 폭이 넓어짐으로써 각각의 규소 환경이 거의 분리되지 않았다. 그러나 철의 함량에 따라 스펙트럼이 넓어지고 화학적 차폐값(chemical shift)이 높아지는 현상을 확인하였는데, 이는 <TEX>$Q^4$</TEX>의 규소 환경을 나타내는 방향으로서 철 주변의 <TEX>$Q^n$</TEX>이 불균질하게 분포하고 있음을 지시한다. <TEX>$^{17}O$</TEX> MAS NMR 실험에서도 철이 포함되지 않은 시료에서는 연결산소(Si-O-Si)와 비연결산소(Na-O-Si)가 부분적으로 분리되지만, 철의 함량이 증가하면서 각각의 산소 환경이 거의 분리되지 않는다. 이러한 연구결과는 고상 핵자기공명분광분석이 철을 포함한 비정질 규산염의 상세한 구조 연구에 효과적인 도구임을 지시한다. The study on the atomic structure of iron-bearing silicate glasses has significant geological implications for both diverse igneous processes on Earth surface and ultra-low velocity zones at the core-mantle boundary. Here, we report experimental results on the effect of iron content on the atomic structure in iron-bearing alkali silicate glasses (<TEX>$Na_2O-Fe_2O_3-SiO_2$</TEX> glasses, up to 16.07 wt% <TEX>$Fe_2O_3$</TEX>) using <TEX>$^{29}Si$</TEX> and <TEX>$^{17}O$</TEX> solid-state NMR spectroscopy. <TEX>$^{29}Si$</TEX> spin-lattice (<TEX>$T_1$</TEX>) relaxation time for the glasses decreases with increasing iron content due to an enhanced interaction between nuclear spin and unpaired electron in iron. <TEX>$^{29}Si$</TEX> MAS NMR spectra for the glasses show a decrease in signal intensity and an increase in peak width with increasing iron content. However, the heterogeneous peak broa-dening in <TEX>$^{29}Si$</TEX> MAS NMR spectra suggests the heterogeneous distribution of <TEX>$Q^n$</TEX> species around iron in iron-bearing silicate glasses. While nonbridging oxygen (<TEX>$Na-O-Si$</TEX>) and bridging oxygen (Si-O-Si) peaks are partially resolved in <TEX>$^{17}O$</TEX> MAS NMR spectrum for iron-free silicate glass, it is difficult to distinguish the oxygen clusters in iron-bearing silicate glass. The Lorentzian peak shape for <TEX>$^{29}Si$</TEX> and <TEX>$^{17}O$</TEX> MAS NMR spectra may reflect life-time broadening due to spin-electron interaction. These results demonstrate that solid-state NMR can be an effective probe of the detailed structure in iron-bearing silicate glasses.