The origins of ultra-low velocity zones (ULVZs) detected at Earth’s core-mantle boundary, potentially linked to large low shear velocity provinces (LLSVPs), have been a subject of ongoing debate. Recent experiments have demonstrated the formation of iron hydride through a water-iron reaction under lowermost-mantle conditions; however, the stability of this compound has been inadequately constrained. By integrating first-principles molecular dynamic simulations with a machine learning approach, we have determined the stability and elastic properties of iron hydride under core-mantle boundary conditions.

Our results reveal that iron hydride is a stable superionic phase, in which hydrogen diffuses akin to a liquid while iron vibrates at lattice sites. Significantly, this phase exhibits markedly slower velocities and a higher density compared to the ambient mantle under lowermost-mantle conditions. The accumulation of iron hydride, through either water-iron reaction or the solidification of core material, provides a plausible explanation for seismic observations of ULVZs, particularly those associated with subduction. This work underscores the substantial role of water in generating seismic heterogeneities at the core-mantle boundary.

How to cite: Zhang, Y., Wang, W., and Wu, Z.: Superionic Iron Hydride originated Ultra-low Velocity Zones at Earth’s Core Mantle Boundary, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2430, https://doi.org/10.5194/egusphere-egu24-2430, 2024.

Two large low shear velocity provinces (LLSVPs) near the core-mantle boundary beneath the Pacific Ocean and Africa were discovered about 40 years ago and are characterized by tomographic images as reductions of shear and compression wave velocities by up to 3-4% and 1%, respectively, over an area of several thousand kilometers and a height of more than a thousand kilometers. It was recently proposed that the Fe³⁺ enrichment in bridgmanite could reduce its shear wave velocity and thus account for the formation of LLSVPs (Wang et al., 2021). However, the viscosity of the Fe³⁺-enriched bridgmanite is unknown, while it is critical for the long-term stability of LLSVPs at the base of the lower mantle against mantle convection. Considering the operation of diffusion creep in the lower mantle, viscosity will be positively correlated with grain size. Therefore, we measured the grain growth rate of bridgmanite under lower mantle conditions as a function of Fe³⁺ content. The experimental results show a significant enhancement of grain growth by Fe³⁺ incorporation. Thus, a larger grain size of bridgmanite is expected in the Fe³⁺-enriched LLSVPs than in the Fe³⁺-poor surrounding mantle, leading to highly viscous LLSVPs. As a result, they are stabilized at the base of the lower mantle over geological time.

How to cite: Fei, H., Wang, F., McCammon, C., and Katsura, T.: The effect of Fe3+ on the grain growth kinetics of bridgmanite and implications for the lower mantle seismic velocity anomalies, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5736, https://doi.org/10.5194/egusphere-egu24-5736, 2024.

Minor amount of water may alter the chemical properties of minerals. In this talk, we review the role of water as an oxidant and explore the chemistry between water and mineral at Earth’s deep lower mantle. We consider the addition of pyrite-type (Mg,Fe)O₂, which is stable at depth of 1800 through the reaction between water and ferropericlase. Among the major lower mantle components, we find that this pyrite-type phase holds the strongest affinity to ferrous Fe. This is consistent with quenched experiments in which we observed amorphous bands enriched with Fe and an excessive amount of O, possibly derived from the decompression-induced amorphization of high-pressure samples. Our results support the existence of Fe-rich and dense pyrite-type (Mg,Fe)O₂ near the core mantle boundary. It features high density due to its Fe concentration. At the meantime, the pyrite-type phase is known to exhibit low seismic velocity. The formation of this phase suggests some seismic heterogeneities, for example the LLSVP, may have originated from the chemistry between water and surrounding minerals.

How to cite: Hu, Q.: Water modulate the Fe-Mg partitioning in Earth’s deep lower mantle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5856, https://doi.org/10.5194/egusphere-egu24-5856, 2024.

Studies of ocean island basalts (OIBs) have identified a “Prevalent Mantle” component (or PREMA) that appears to represent one of the fundamental constituents of Earth’s mantle. Recent documentation of this signature in deep-sourced kimberlitic magmatism has increasingly linked PREMA with thermochemical structures above the core mantle boundary (LLSVPs). Yet, kimberlites provide geographically limited sampling of Earth’s mantle, which makes it difficult to identify the spatial association between LLSVPs and the PREMA component sampled by these magmas. To further investigate the global distribution of the PREMA mantle component, we utilize a much more widespread class of mantle-derived magmatism – global Cenozoic alkali basalts, nephelinites and basanites (‘sodic basalts’ hereafter) – that are derived from both continental and oceanic settings. Statistical treatment of the available ~3500 geochemical and isotopic analyses of Cenozoic sodic basalts worldwide shows that at low degrees of melting these magmas exhibit similar Sr-Nd-Hf isotopic characteristics to PREMA. There is no apparent spatial relationship between the distribution of PREMA-like sodic basalts and LLSVPs, implying that the PREMA component is not exclusively associated with LLSVPs. The PREMA-like signature of low-degree sodic basalts occurs in both OIBs and continental magmas, negating an exclusive link to continental lithosphere. Geochemical modelling and mantle convection simulations indicate that PREMA could have been generated soon after Earth accretion, experiencing only minimal melting or enrichment, and then scattered throughout the mantle, including the upper mantle, rather than being the result of mixing between depleted and enriched mantle components. This work indicates that PREMA probably represent a widespread fusible component that can be mingled with other components at various scales to generate all types of mantle magmatism.

How to cite: Liu, J., Cai, R., Pearson, G., Giuliani, A., van Keken, P., and Oesch, S.: Is PREMA exclusively associated with LLSVPs?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7271, https://doi.org/10.5194/egusphere-egu24-7271, 2024.

The nature of Iceland hotspot, located on the Mid-Atlantic Ridge, has been subject of contentious debate. Earlier seismic tomographic studies have suggested the presence of a deep mantle plume or confined to the upper mantle without a clear plume affinity. In this study, we utilize SS precursors to image the Mantle Transition Zone (MTZ) beneath Iceland. We collected a large SS precursor dataset that contains teleseismic recordings from all available global broadband stations between year 1976 and 2023. The resulting dataset enables a dense sampling of the Iceland and the Mid-Atlantic Ridge (i.e., 40°-80° N and -70°W to 10° E) with more than 3000 high-quality SS precursor waveforms. We adopted a recently proposed Robust Damped Rank-Reduction method to process the SS precursor data, which enables exploiting the signal coherency in multi-dimensional (4D) data and significantly improving the quality of the weak precursory arrivals. Seismic imaging based on the processed SS precursors effectively captures regional-scale topographic variations of mantle discontinuities, eliminates contaminating noises that produce small-scale artifacts, and enhances the lateral coherency of the MTZ structure.

The resulting MTZ images reveal that the 410 km discontinuities are depressed by 5 km compared to regional average, whereas the 660 km discontinuities are uplifted by 6 km, leading to a MTZ of 230 km thick beneath Iceland. The region of depressed 410 extends southwards from the eastern edge of Greenland along the Mid-Atlantic Ridge for a distance about 20 degrees. In comparison, the elevation of 660 is more wide-spread, reaching as far as the northwestern end of Greenland. These observations suggest the interaction of a deep mantle plume with the 660 km discontinuity in a broad area covering the north Mid-Atlantic Ridge. The thick lithosphere beneath Greenland may have channeled the mantle flow towards the ridge, thereby causing the thinning of the MTZ centering on Iceland. In the southeast of the study area, our model also reveals a thinning area, isolated from the Iceland anomaly. The observed complex MTZ topography, in conjunction with multiple low-velocity centers in global tomographic models, may suggest the presence of a bifurcated, deep mantle plume originated from the lower mantle that may feed the shallow hotspots near the Iceland-Mid-Atlantic Ridge.

How to cite: Jiang, X., Chen, Y., Obou ́e, Y. A. ́. S. I., and Wang, J.: Imaging of the Mantle Transition Zone with SS Precursors in the Iceland-Mid-Atlantic Ridge Region, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9201, https://doi.org/10.5194/egusphere-egu24-9201, 2024.

While the ultralow velocity zones (ULVZs) may hold key information about deep earth dynamics, their elusive state and origin remain as an enigma. Recently, high pressure experiments indicate that ULVZs may form from the B2 FeSi phase crystallized from the outer core containing H and Si. The possibility and impact of the B2 FeSi at the core mantle boundary (CMB) awaits further exploration. The conductivity of the B2 phase can be used to understand its role in deep mantle dynamics and also served as a test for its existence at the CMB. By using first-principles molecular dynamics and Kubo-Greenwood formula, we calculated the thermal and electrical conductivities of B2 Fe_xSi_1-x at 135 GPa up to 4,500 K. Our results show that at the CMB conditions, the thermal and electrical conductivities of B2 FeSi are significantly higher than other lower mantle minerals and the core. This would lead to a thermal anomaly region at the CMB, promote the core dynamo and enhance the cooling of the core if the ULVZs are dominated by B2 phase.

How to cite: Huang, Y., Li, Y., and Ni, H.: Ab initio study of the conductivities of B2 FeSi under core-mantle boundary conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13850, https://doi.org/10.5194/egusphere-egu24-13850, 2024.

Large igneous provinces (LIPs), characterized by massive basaltic lava flows and rapid formation over short time frames, exert profound effects on Earth's environment, climate, and mineral resources. Although prior studies have revealed that high water content in the mantle sources would be a critical factor for the genesis of LIPs, the origin of water in their mantle sources remains debated, posing a critical challenge for understanding the geodynamic context of LIP formation and associated deep mantle processes. The water can derive from the primordial components that build the earth, or from the earth surface carried by the subducted oceanic slabs. Here, we conducted coupling analysis of water contents and hydrogen isotopes of the melt inclusions and quenched glasses from the high ³He/⁴He picrites in Western Greenland and low-Ti type picrites from Emeishan large igneous province. The results show that the high H₂O/Ce (>1300) and normal mantle like D/H ratio of the primary magma are distinct to that of the oceanic island basalts and Archean komatiites. Combining the high mantle potential temperature and non-arc like trace elemental patterns, we suggest that the ELIP taps highly hydrous reservoirs in the deep mantle, potentially with water inherited from earth-building materials or the less dehydrated subducted oceanic lithosphere. Our work also raises the caution that the traditionally combined water content and δD analysis for OHMs does not necessarily allow the accurate determination of water concentration of the primary magma. For Western Greenland picrites, the results show water contents from 850 to 1050 ppm by weight, but large variable δD from -75± to -140±8 ‰, which forms the trends well modeled by the kinetic fractionation driven by inward water diffusion into the melt inclusions. The primary δD of the these picrites were inferred to be around -80 ‰, consistent with the recent results for the high 3He/4He submarine basaltic glasses from Loihi, Hawaii. Thus, our work may support a globally consistent primordial water in the origin of chondritic material.

How to cite: Liu, J., Xia, Q., and Zhou, J.: The origin of water in the large igneous provinces: constraints from hydrogen isotope, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14584, https://doi.org/10.5194/egusphere-egu24-14584, 2024.

Seismic images of Earth’s interior have revealed two continent-sized anomalies with low seismic velocities, known as the large low-velocity provinces (LLVPs), in the lowermost mantle. The LLVPs are often interpreted as intrinsically dense heterogeneities that are compositionally distinct from the surrounding mantle. Here we show that LLVPs may represent buried relics of Theia mantle material (TMM) that was preserved in proto-Earth’s mantle after the Moon-forming giant impact. Our canonical giant-impact simulations show that a fraction of Theia’s mantle could have been delivered to proto-Earth’s solid lower mantle. We find that TMM is intrinsically 2.0–3.5% denser than proto-Earth’s mantle based on models of Theia’s mantle and the observed higher FeO content of the Moon. Our mantle convection models show that dense TMM blobs with a size of tens of kilometres after the impact can later sink and accumulate into LLVP-like thermochemical piles atop Earth’s core and survive to the present day. The LLVPs may, thus, be a natural consequence of the Moon-forming giant impact. Because giant impacts are common at the end stages of planet accretion, similar mantle heterogeneities caused by impacts may also exist in the interiors of other planetary bodies.

How to cite: Yuan, Q., Li, M., Desch, S. J., Ko, B., Deng, H., Garnero, E. J., Gabriel, T. S. J., Kegerreis, J. A., Miyazaki, Y., Eke, V., and Asimow, P. D.: Moon-forming impactor as a source ofEarth’s basal mantle anomalies, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4206, https://doi.org/10.5194/egusphere-egu24-4206, 2024.