Ultra-low Velocity Zones Research Articles

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

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

Effects of 2.5-D ultra-low and ultra-high velocity zones on flip-reverse-stacking (FRS) of the ScS wavefield

SUMMARY Within the last decade, thin ultra-low velocity zone (ULVZ) layering, sitting directly on top of the core–mantle boundary (CMB), has begun to be investigated using the flip-reverse-stack (FRS) method. In this method, pre- and post-cursor arrivals that are symmetrical in time about the ScS arrival, but with opposite polarities, are stacked. This same methodology has also been applied to high velocity layering, with indications that ultra-high velocity zones (UHVZs) may also exist. Thus far, studies using the FRS technique have relied on 1-D synthetic predictions to infer material properties of ULVZs. 1-D ULVZ models predominantly show a SdS precursor that reflects off the top of the ULVZ and an ScscS reverberation within the ULVZ that arrives as a post-cursor. 1-D UHVZ models are more complex and have a different number of arrivals depending on a variety of factors including UHVZ thickness, velocity contrast, and lateral extent. 1-D modelling approaches assume that lower mantle heterogeneity is constant and continuous everywhere across the lower mantle. However, lower mantle features display lateral heterogeneity and are either finite in extent or display local thickness variations. We examine the interaction of the ScS wavefield with ULVZs and UHVZs in 2.5-D geometries of finite extent. We show that multiple additional arrivals exist that are not present in 1-D predictions. In particular, multipath ScS arrivals as well as additional post-cursor arrivals are generated. Subsequent processing by the FRS method generates complicated FRS traces with multiple peaks. Furthermore, post-cursor arrivals can be generated even when the ScS ray path does not directly strike the heterogeneity from above. Analysing these predictions for 2.5-D models using 1-D modelling techniques demonstrates that a cautious approach must be adopted in utilization and interpretation of FRS traces to determine if the ScS wavefield is interacting with a ULVZ or UHVZ through a direct strike on the top of the feature. In particular, traveltime delays or advances of the ScS arrival should be documented and symmetrical opposite polarity arrivals should be demonstrated to exist around ScS. The latter can be quantified by calculation of a time domain multiplication trace. Because multiple post-cursor arrivals are generated by finite length heterogeneities, interpretation should be confined to single layer models rather than to interpret the additional peaks as internal layering. Furthermore, strong trade-offs exist between S-wave velocity perturbation and thickness making estimations of ULVZ or UHVZ elastic parameters highly uncertain. We test our analysis methods using data from an event occurring in the Fiji-Tonga region recorded in North America. The ScS bounce points for this event sample the CMB region to the southeast of Hawaii, in a region where ULVZs have been identified in several recent studies. We see additional evidence for a ULVZ in this region centred at 14°N and 153°W with a lateral scale of at least 250 km × 360 km. Assuming a constant S-wave velocity decrease of −10 or −20 per cent with respect to the PREM model implies a ULVZ thickness of up to 16 or 9 km, respectively.

Superionic iron hydride shapes ultralow-velocity zones at Earth’s core–mantle boundary

Seismological studies have exposed numerous ultralow velocity zones (ULVZs) exhibiting extraordinary physical attributes at Earth's core-mantle boundary, yet their compositions and origins remain controversial. Water-iron reaction can generate unique phases under lowermost-mantle conditions and likely plays a crucial role in forming ULVZs. Through first-principles molecular dynamic simulations with machine learning techniques, we determine that iron hydride, the product of water-iron reaction, is stable as a superionic phase at the core-mantle boundary. This superionic iron hydride has much slower velocities and a higher density than the ambient mantle under lowermost-mantle conditions. Accumulation of iron hydride, created through either a chemical reaction between subducted water and iron or solidification of core material entrained in the lower mantle by convection, can explain the seismic observations of ULVZs particularly those associated with subduction. This work suggests that water may have a substantial role in creating seismic heterogeneities at the core-mantle boundary.

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

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

Ultra‐Low Velocity Zone Beneath the Atlantic Near St. Helena

AbstractThere are various hotspots in the Atlantic Ocean, which are underlain by mantle plumes that likely cross the mantle and originate at the core‐mantle boundary. We use teleseismic core‐diffracted shear waves to look for an Ultra‐Low Velocity Zone (ULVZ) at the potential base of central Atlantic mantle plumes. Our data set shows delayed postcursory phases after the core‐diffracted shear waves. The observed patterns are consistent in frequency dependence, delay time, and scatter pattern with those caused by mega‐ULVZs previously modeled elsewhere. Synthetic modeling of a cylindrical structure on the core‐mantle boundary below St. Helena provides a good fit to the data. The preferred model is 600 km across and 20 km high, centered at approximately 15° South, 15° West, and with a 30% S‐wave velocity reduction. Significant uncertainties and trade‐offs do remain to these parameters, but a large ULVZ is needed to explain the data. The location is west of St. Helena and south of Ascension. Helium and neon isotopic systematics observed in samples from this region could point to a less‐outgassed mantle component mixed in with the dominant signature of recycled material. These observations could be explained by a contribution from the Large Low Shear Velocity Province (LLSVP). Tungsten isotopic measurements would be needed to understand whether a contribution from the mega‐ULVZ is also required at St. Helena or Ascension.

Stability and wave velocity of novel iron magnesium oxide (Mg, Fe)2O3.5 in the lower mantle

The mineral composition of the anomalous wave velocity zone in the lower mantle is indispensable to understanding seismic wave velocity anomalies and the genesis as well as driving forces of plate tectonics. We here systematically investigate the structural stability, density, elastic wave velocity and anisotropy of the wave velocity for (Mg, Fe)2O3.5, a new type of the low wave velocity mineral, in the 0–136 GPa range by means of first-principles calculations. Calculations show that (Mg, Fe)2O3.5 possesses high density, with a value higher than that in the Preliminary Reference Earth Model at the core-mantle boundary by up to 17.31 %, and its longitudinal and transverse wave velocities are reduced by about 9.23 % and 17.41 %, respectively. Moreover, the elastic wave velocity is characterized by anisotropy, and is consistent with the characteristics of the seismic wave velocity in ultra–low velocity Zone (ULVZ). Our study elucidates that (Mg, Fe)2O3.5 is a candidate mineral for the wave velocity anomalies in the ULVZ that survive in the deep mantle. Such findings may pave the way towards exploring the origin of the ULVZ from the viewpoint of chemical composition anomalies.

Evidence for Ultra-Low Velocity Zone Genesis in Downwelling Subducted Slabs at the Core–Mantle Boundary

Abstract We investigate broadband SPdKS waveforms from earthquakes occurring beneath Myanmar. These paths sample the core–mantle boundary beneath northwestern China. Waveform modeling shows that two ∼250 × 250 km wide ultra-low velocity zones (ULVZs) with a thickness of roughly 10 km exist in the region. The ULVZ models fitting these data have large S-wave velocity drops of 55% but relatively small 14% P-wave velocity reductions. This is almost a 4:1 S- to P-wave velocity ratio and is suggestive of a partial melt origin. These ULVZs exist in a region of the Circum-Pacific with a long history of subduction and far from large low-velocity province (LLVP) boundaries where ULVZs are more commonly observed. It is possible that these ULVZs are generated by partial melting of mid-ocean ridge basalt.

Thermoelastic Properties of B2‐Type FeSi Under Deep Earth Conditions: Implications for the Compositions of the Ultralow‐Velocity Zones and the Inner Core

AbstractThe CsCl‐type (B2) phase of FeSi (B2‐FeSi) has been proposed as a candidate phase in the ultralow‐velocity zones (ULVZs) at the base of the lower mantle and in the Earth's inner core. However, the elastic properties of B2‐FeSi under relevant conditions remain unclear. Here we determine the density, elastic constants, and velocities of B2‐FeSi at high pressures (90–390 GPa) and temperatures (3,000–6,000 K) relevant to the Earth's lower most mantle and the inner core, using first‐principles molecular dynamics simulations. At the base of the lower mantle, B2‐FeSi shows significantly lower velocities and a higher density than those of the ambient mantle. Mechanical mixing models suggest the presence of ∼27–39 vol% B2‐FeSi in the silicate mantle is consistent with the reduced velocities and the elevated density of ULVZs observed seismically. On the other hand, the hcp‐Fe and B2‐FeSi mixture exhibits higher bulk sound velocity compared to the PREM under inner core conditions. Adding superionic H in the interstitial sites of B2‐FeSi lowers its density but has little effect on the bulk sound velocity of B2‐FeSi, precluding H‐bearing B2‐FeSi as a major component in the Earth's inner core.

Seismic Observation of a New ULVZ Beneath the Southern Pacific

AbstractWe present new observations of core‐diffracted shear waves which contain anomalous waveforms sampling the lowermost mantle beneath the southern Pacific region. Data in two distinct geometries, one from New Zealand to North America and the other from the Fiji and Solomon Islands to South America, show evidence of postcursor phases. The postcursor delays and move‐outs imply that they are caused by an ultra‐low velocity zone (ULVZ). Beamforming analyses of the observed diffracted postcursors show a strong backazimuthal deviation, suggesting this new ULVZ is likely to have a cylindrical shape similar to broad ULVZs sampled by shear diffracted waves elsewhere. Full‐waveform modeling suggests that the postcursors seen in North America might be due to the previously modeled ULVZ located to the west of the Galápagos, while those seen in South America are due to a previously unknown ULVZ beneath the Southern Pacific. We cannot fit observations in both geometries by a single ULVZ. For the new location, we propose one cylindrical ULVZ model with a radius of 400 km and a shear wave velocity decrease of 20% centered at geographical coordinates (−33.6, 130) close to the Pitcairn hotspot. Despite some uncertainty in the west‐east direction, this new ULVZ observation likely provides another example to support the hypothesis that ULVZs exist at the base of mantle plumes where primordial signatures are observed in the ocean island basalts.

Detections of ultralow velocity zones in high-velocity lowermost mantle linked to subducted slabs

Detections of ultralow velocity zones in high-velocity lowermost mantle linked to subducted slabs

Ultralow velocity zone and deep mantle flow beneath the Himalayas are linked to a subducted slab

Ultralow velocity zone and deep mantle flow beneath the Himalayas are linked to a subducted slab

Exceptionally Low Thermal Conduction of Basaltic Glasses and Implications for the Thermo‐Chemical Evolution of the Earth's Primitive Magma Ocean

AbstractThe thermal properties of the Earth's primordial magma are the key factors that constrained crystallization and other thermo‐chemical processes in Earth's primitive magma ocean and therefore controlled the Earth's long‐term evolution. Thermal conductivity of the primordial magma is conventionally assumed to be a constant of about 4 W m−1 K−1 under the high pressure‐temperature conditions of the primitive magma ocean. Here we measured the lattice thermal conductivity of a variety of basaltic and silicate glasses at high pressures and a wide range of temperatures. Our results suggest that the primordial magma, if it is indeed represented by basaltic melts, had a thermal conductivity of ∼1.0–1.9 W m−1 K−1, much lower than previously thought. Such low thermal conduction reduced heat loss and thus prolonged the cooling time of the early magma ocean, promoting convection in the solidifying mantle and preventing a global overturn. Moreover, if the seismic ultralow velocity zones presently observed in the lowermost mantle are made of basaltic melts, originating either from remnants of the primitive magma ocean or pieces of subducted crust, the material in these zones must have an ultralow thermal conductivity, which would reduce cooling and thus influence the thermo‐chemical evolution of the present day core‐mantle boundary.

Decoupling of short-lived radiogenic and helium isotopes in the Marquesas hotspot

The short-lived radiogenic 182Hf/182W (t½ = 9 Ma) and 146Sm/142Nd (t½ = 103 Ma) systems are useful tools to investigate differentiation processes in Earth's early history. Mantle reservoirs that have been preserved over long geological timescales could carry signatures formed early on in Earth's history and later could act as sources to modern mantle plumes. Negative μ182W anomalies in combination with high 3He/4He ratios have been detected globally as differently sloping negative trends in plume-derived ocean island basalts (OIB) from individual hotspots. Large low shear velocity provinces (LLSVP) and ultra-low velocity zones (ULVZ) are seismically anomalous regions in the lower mantle that have been suggested as sources of primordial mantle components near the core-mantle boundary (CMB) and accessed by deep-rooted mantle plumes.In this work, seven samples from the islands Hiva Oa and Fatu Hiva of the Marquesas Archipelago, showing a range in 3He/4He ratios, have been investigated motivated by its location above the Pacific LLSVP and a recently discovered “mega-ULVZ”. Despite elevated 3He/4He ratios of up to 14.4 R/RA, new high precision measurements using thermal ionization mass spectrometry revealed non-anomalous μ182W (−0.9 ± 4.9 ppm) and μ142Nd (1.3 ± 2.4 ppm, 95% confidence interval (c.i.)). These mark the Marquesas Archipelago as the first hotspot to show a decoupling of 3He/4He ratios and μ182W in OIB. A mixing model between an elevated 3He/4He-negative-μ182W and an EM2 or a HIMU endmember suggest that the addition of recycled material is not responsible for this decoupling. Instead, like many previously studied OIB, the Marquesas plume is most likely dominated by an LLSVP component proposed to contain high 3He/4He ratios and μ182W of 0. The unique observed absence of negative μ182W anomalies, on the other hand, implies a lack of involvement of the ULVZ source in the plume's melting region, despite the hotspot being located above a recently discovered mega-ULVZ. This may imply that the expression of ancient mantle domains in hotspot geochemistry is temporally variable, thus, delaying the arrival or involvement of ULVZ material in the origin of the Marquesas OIB.

Studying inner core and lower mantle structure with a combination of PKP and converted SKP and PKS waves

SUMMARYStructure of the inner core is often measured through traveltime differences between waves that enter the inner core (PKPdf) and waves that travel through the outer core only (PKPab and PKPbc). Here we extend the method to converted waves PKSdf and SKPdf and compare results to PKP wave measurements. PKSdf and SKPdf have a very similar path to PKPdf and if velocity variations are present in the inner core, all three wave types should experience them equally. Since traveltime deviations can be due to velocity changes (either isotropic or anisotropy) as well as wave path deviations born from heterogeneity, we simultaneously investigate wave path directions and traveltimes of PKP, SKP and PKS waves for several source-array combinations. One of the path geometries is the anomalous polar corridor from South Sandwich to Alaska, which has strong traveltimes anomalies for PKPdf relative to more normal equatorial path geometries. Here we use array methods and determine slowness, traveltime and backazimuth deviations and compare them to synthetic data. We find that path deviations from theoretical values are present in all wave types and paths, but also that large scatter exists. Although some of the path deviations can be explained by mislocation vectors and crustal variations, our measurements argue that mantle structure has to be considered when investigating inner core anisotropy. Our polar path data show similar traveltime residuals as previously published, but we also find slowness residuals for this path. Interestingly, SKPdf and PKSdf for the South Sandwich to Alaska path show traveltime residuals that are partly opposite to those for PKPdf, possibly due to an interaction with a localized ultra-low velocity zone where waves enter the core.

Mapping structures on the core–mantle boundary using Sdiff postcursors: Part II. Application to the Hawaiian ULVZ

SUMMARY We present a new data set of nearly 100 earthquakes which show clear evidence of the Hawaiian ultra-low velocity zone (ULVZ) in the S core-diffracted phase (Sdiff), representing the most comprehensive Sdiff data set of a ULVZ to date. Using a Bayesian inversion approach, as outlined in Martin et al., and a subset of the data set, we characterise the 2-D morphology and velocity of the Hawaiian ULVZ. The results suggest that the ULVZ is smaller than previously estimated, with an elliptical shape, and oriented along the direction of the large low-shear velocity province boundary. Using forward modelling, we infer that the ULVZ has a height of 25 km and shear velocity reduction of 25 %.

A multi-siderophile element connection between volcanic hotspots and Earth's core

The existence of resolvable 182W/184W deficits in modern ocean island basalts (OIB) relative to the bulk silicate Earth has raised questions about the relationship of these rocks to Earth's core. However, because the core is expected to host high abundances of highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re), it would be expected that such heterogeneity is accompanied by correlating variability in HSE abundances among OIB, but this has not been observed. We report instead a relationship between the W isotopic compositions and Ru/Ir ratios of Hawai‘i and Iceland OIB, which represent two of Earth's primary mantle plumes. Previous studies have highlighted the unique behavior of Ru relative to Os and Ir during metal-silicate fractionation, particularly when sulfide phases are segregated with metal. Using the information from these studies, we construct models predicting the consequences for HSE fractionation of various scenarios in which 182W/184W deficits can be created. These models show that the observed trends are likely inconsistent with modern, active core-mantle interaction at the CMB, and instead the observed low-Ru/Ir, low-182W/184W OIB are best explained by metal-silicate interaction that happened at significantly lower pressures. Such conditions may reflect what is expected for metal-silicate equilibration during the process of core formation itself, meaning that the deep mantle sources of OIB, such as ultra-low velocity zones, may instead reflect preserved relics of core formation. An ancient origin for distinct domains now residing at the core-mantle boundary is consistent with geophysical and petrological observations, for example that the Mg/Fe ratio of ferropericlase in the D″ layer is in significant disequilibrium with the modern core. Additional work is required to constrain the behavior of HSE during silicate differentiation processes that may also generate low 182W/184W ratios. However, if modern OIB represent a direct link to the ancient processes of core formation, future geochemical studies may be able to unlock new information about the formation and evolution of the Earth using OIB.

Carbon isotope composition of basalts from Loihi Seamount: Primordial or recycled carbon in the Hawaiian mantle plume?

We analyzed the carbon isotope composition of vesicle CO2, plus He isotopes and He and CO2 concentrations in the vesicle (vapor) and glass (melt) phases of 37 submarine basalts from the summit, north and south rifts, and east flank of Loihi Seamount. Tholeiites and transitional basalts lie in a narrow range of vesicle δ13C = −4.6 to −0.9‰, while alkali basalts range from −7.2 to −2.1‰. Calculated total (vesicle+glass) δ13C for the majority of the basalts ranges from −6 to −2‰ assuming the vapor-melt fractionation factor Δ (= δvapor - δmelt) is +2 to +4‰ as measured in basaltic systems. This relatively narrow range of δ13C resembles mantle source values deduced from gas-rich mid-ocean ridge basalts and basalts from Iceland, and for Kilauea volcano deduced from its fumarole gas. However, this similarity presents a conundrum because Loihi basalts have degassed >97% of their initial CO2 as deduced from CO2 - Ba systematics and crystal fractionation modeling.Loihi parental magma (MgO=18 wt.%) had initial CO2 concentrations of 0.6 to 1.9 wt.%. Most tholeiitic and transitional basalts appear to have followed a quasi closed-system degassing history. Correcting for this degassing indicates the median δ13C for Loihi undegassed parental magmas is −1.5‰ and the δ13C range is −4 to +1‰. Estimates of this δ13C range are only weakly dependent on the choice of Δ and initial [CO2] in closed-system degassing scenarios. The Loihi mantle plume source is therefore characterized by δ13C values that are higher than the range of −6 to −4‰ prevalent in mantle-derived basalts and many diamonds. This could be due to primordial carbon isotope heterogeneity in Earth's mantle, exchange of carbon at the core-mantle boundary between ultra-low velocity zone silicates and the metallic liquid outer core, or to the presence of a small fraction (<1% by mass) of surficial carbonate that was tectonically recycled to the Hawaiian plume source region. Currently, an origin from recycled carbonate has the most supporting evidence.

Globally distributed subducted materials along the Earth's core-mantle boundary: Implications for ultralow velocity zones.

Ultralow velocity zones (ULVZs) are the most anomalous structures within the Earth's interior; however, given the wide range of associated characteristics (thickness and composition) reported by previous studies, the origins of ULVZs have been debated for decades. Using a recently developed seismic analysis approach, we find widespread, variable ULVZs along the core-mantle boundary (CMB) beneath a largely unsampled portion of the Southern Hemisphere. Our study region is not beneath current or recent subduction zones, but our mantle convection simulations demonstrate how heterogeneous accumulations of previously subducted materials could form on the CMB and explain our seismic observations. We further show that subducted materials can be globally distributed throughout the lowermost mantle with variable concentrations. These subducted materials, advected along the CMB, can provide an explanation for the distribution and range of reported ULVZ properties.

Effect of Iron Content on Thermal Conductivity of Ferropericlase: Implications for Planetary Mantle Dynamics

AbstractWe have simultaneously measured thermal conductivity (λ) and thermal diffusivity (κ) for polycrystalline ferropericlase with different Fe contents (Fp3, Fp5, Fp10, Fp20, Fp30 and Fp50) up to 23 GPa and 1100 K by a pulse heating method. Experiment results reveals that even small amounts of Fe in ferropericlase can strongly reduce the thermal conductivity by several times at low temperature compared with MgO periclase. With increasing Fe in ferropericlase, both their pressure and temperature dependence decrease. λ of Fe‐bearing ferropericlase is not sensitive to temperature compared with silicates and shows a tendency of first increasing and then decreasing with increasing temperature. A universal equation was derived to estimate λ of ferropericlase with arbitrary composition under mantle conditions. The low λ values of Fe‐rich ferropericlase can explain one possible origin of ultralow velocity zones. This study suggests that Fe content in ferropericlase is an important factor controlling the cooling history of terrestrial planets.

Geochemical characteristics of spherules from the Pivdenna kimberlite pipe, East Azov region (Ukraine): implications for their sources and origin

The composition of spherules and particles of native metals from the Pivdenna kimberlite pipe, Ukraine, was studied using the SEM/EDS method. Three varieties of spherules have been distinguished: titanium-manganese-iron-silicate (TMIS) spherules, Ca-rich silicate spherules, and magnetite-wustite-iron (MW-I) spherules. TMIS spherules are composed of homogeneous glass, some having a native iron core. Large TMIS spherules may contain a crystalline phase with needle-like armalcolite. Ca-rich silicate spherules can be subdivided into two subtypes: calcium-silicate (CS) spherules where SiO2 and CaO are the dominant constituents, and calcium-iron-silicate (CIS) spherules with significant FeO content. CS spherules may contain a core consisting of native phases (Fe, Fe-Si, and Mn-Si-Fe). Native metal particles are represented by native Cu and native Zn. The spherule varieties from the Pivdenna pipe are similar to those from other kimberlite pipes in the world. We infer that the formation of spherules occurred in gas-melt streams, separately from the kimberlites, and propose a model for the formation of the most common variety of spherules (TMIS and MW-I varieties) in the region of the core-mantle boundary (CMB). First, a melt of the Fe-Ti-Mn-Si-O system was formed in ultra-low-velocity zones (ULVZ) as a result of thermochemical reactions (reduction) between the molten core and solid oxide-silicate rocks. The melt then migrates to shallower levels, where a decrease in temperature initiates oxidation with the formation of SiO2-TiO2-FeO-MnO-Fe0 melt, i.e. parent melt of TMIS and MW-I spherules. We interpret the formation of native metals in kimberlites as a result of the decomposition of nitrides, which came from the Earth’s core via intratelluric flows