The migration of mid-ocean ridges is driven by asymmetric plate motions on either ridge flank transmitted from far-field subduction forces. Within this model, the geometry and location of mid-ocean ridges are independent of lower-mantle dynamics. However, this fails to recognise the attraction between mid-ocean ridges and mantle plumes. Using numerical models of mantle convection, we show that plumes with high buoyancy flux (> 6000 kg/s) can capture mid-ocean ridges within a 1000 km radius and anchor them in place. If the plume buoyancy flux wanes below 1000 kg/s the ridge may be released, potentially resulting in rapid migration rates that trigger a major plate reorganisation. Plume-ridge interactions are commonly preserved as conjugate large igneous provinces (LIPs), which form on each flank of a mid-ocean ridge as new crust is created. The decoupling of ridges from plumes are demarcated by a switch from conjugate LIPs, formed by a plume beneath a spreading ridge, to trails of intraplate hotspot volcanoes signifying the plume and ridge have separated. We demonstrate that the waning buoyancy flux of the Kerguelen plume, inferred from the geochemistry of eruption products, resulted in its decoupling with the SE Indian Ridge spurring rapid northward migration of the Australian plate. Our modelling predicts that following plume-ridge decoupling, the waning plume can tilt 15° within the upper mantle towards the migrating ridge, providing an explanation for diffuse volcanism and low eruption volumes along the Kerguelen Archipelago. Our results have significant implications for other plume-ridge interactions globally such as the Iceland, Tristan, and Easter plumes, and the generation of intraplate hotspot volcanoes proximal to mid-ocean ridges.

How to cite: Mather, B., Seton, M., Williams, S., Whittaker, J., Carey, R., Arnould, M., Coltice, N., and Duncan, B.: The role of plume-ridge decoupling on rapid plate motion and intraplate volcanism, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10441, https://doi.org/10.5194/egusphere-egu23-10441, 2023.

It is well accepted that convection in the Earth’s mantle provides the torques to drive vertical and horizontal plate motions. Yet the precise nature of the interaction between flow and plates remains incomplete, because the strength of plates allows them to integrate over a presumably complex flow field in the mantle beneath – making it difficult to get a glimpse even on the recent Cenozoic mantle flow. Over the past years a pressure driven, so-called Poiseuille, flow model for upper mantle flux in the asthenosphere has gained increasing geodynamic attention – for a number of fluid dynamic arguments. This elegantly simple model makes a powerful testable prediction: Poiseuille flow induce plate motion changes should coincide with regional scale mantle convection induced elevation changes.

Here I will focus on Australia, which undergoes a profound directional change from westward to northward motion in the early Cenozoic. At the same time there is evidence for early Cenozoic high dynamic topography in the western part of the continent. Thus, suggesting a high-pressure source in the upper mantle to the west of Australia. Altogether these geological and geophysical observations indicate that the separation of Australia from Antarctica was largely driven by plume push torque from the Kerguelen plume.

How to cite: Stotz, I. L., Carena, S., Vílacis, B., Bunge, H.-P., and Hayek, J. N.: Plume driven plate tectonics: new insights from the Australia/Antarctica separation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12268, https://doi.org/10.5194/egusphere-egu23-12268, 2023.

Volcanic provinces within Earth's continents exhibit a wide range of characteristics that reflect the intricate nature of the dynamic interactions at their origin. To improve our understanding of the driving mechanisms at play, we address the generation of intra-plate continental volcanism by modelling the 3-D interaction between an upwelling mantle plume and a thick lithospheric block. We examine scenarios with and without plate motion and assess the spatio-temporal distribution and intensity of produced melts. Our findings demonstrate the critical role of lithospheric thickness in determining the location and volume of plume-driven magmatic provinces. Building on these results obtained using simplified lithospheric structures, we further apply our numerical methodology to simulate the inferred interaction between the Cosgrove plume and eastern Australia during the past 35 Myr. We design the Australian continent using available 3-D lithospheric architecture determined through seismic tomography and impose the inferred plate motion associated with this region. Our models incorporate updated peridotite melting parameterisations to provide quantitative estimates of generated melt volume and composition. We find that plume-driven and shallow edge-driven melting processes, modulated by the lithospheric thickness of the Australian continent, combine to explain the observed volcanic record. Our preliminary results agree well with surface observations and provide further insight into the geodynamics of eastern Australia.

How to cite: Duvernay, T. and Davies, R.: 3-D Modelling of the Dynamical Mechanisms Driving Continental Intra-Plate Volcanism, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10559, https://doi.org/10.5194/egusphere-egu23-10559, 2023.

Northeast China is a very typical area for studying intra-plate volcanism in the back-arc setting. It is commonly proposed that the subduction of the Pacific plate has been responsible for widespread Holocene volcanoes in NE China. Yet, how this process drives volcanism remains a topic of vigorous debate. Investigation of seismic anisotropy can provide important evidence for the cause-and-effect relationship between mantle flow, lithospheric deformation and shallow structures. In this study, using seismic data from four networks across NE China and north Korea, we analyze shear wave splitting in converted P- to S-waves at the Moho (Pms), S-waves from the subducted slab interface (local S), and SKS phases. The Pms phases show a relatively weak crustal anisotropy (~0.25 s), with fast polarization directions aligned sub-parallel to major tectonic features. For the local S and SKS phases, fast polarization directions show significant lateral variations. We further perform a quantitative inversion to show that the depth of the anisotropy is ~150 km, thus driven by flow within the asthenosphere associated with Pacific subduction. However, the presence of many null SKS splitting phases, together with scattered local S-wave anisotropy suggests a localized region of vertical flow directly beneath Changbaishan volcano. Such patterns correspond well to regional upper-mantle seismic velocity structure, and suggest that a localized upwelling with a relatively deep origin drives volcanism in the Changbaishan region. Furthermore, we infer that mantle upwelling is deflected to the SW beneath Changbaishan and spreads asymmetrically at the base of the lithosphere, possibly because of the long history of volcanism in the region.

How to cite: Han, C., Hammond, J., and Ballmer, M.: Multi-scale anisotropy in NE China: Implications for intra-plate volcanism, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4570, https://doi.org/10.5194/egusphere-egu23-4570, 2023.

Many vertical seismic velocity anomalies originate in the transition zone between the upper and lower mantle (410–660 km) and form so-called secondary plumes. These anomalies are interpreted as the result of thermal effects of large-scale thermal upwelling (primary plume) in the lower mantle and/or deep dehydration of fluid-rich subducting oceanic plates. We present the results of thermo-mechanical modelling to investigate the dynamics of such small-scale thermal and chemical (hydrous) anomalies rising from the lower part of the Earth’s upper mantle. Our goal is to determine the conditions that allow thermo-chemical secondary plumes of moderate size (initial radius of 50 km) to penetrate the overlying lithosphere, as detected in seismo-tomographic studies in such intra-continental areas as the Tengchong volcano in south-western China and the Eifel volcanic fields in north-western Germany. To this end, we investigate the effect of the compositional deficit of the plume density due to the presence of water and hydrous silicate melts. In our models, secondary plumes of purely thermal origin do not penetrate the overlying plate, but flatten at its base, forming “mushroom”-shaped structures at the level of the lithosphere-asthenosphere boundary. On the contrary, plumes with enhanced density contrast due to a chemical (hydrous) component are shown to be able to penetrate upward through the lithospheric mantle to shallow depths near the Moho. Our findings can explain the enigmatic observations of columnar (“finger”-shaped) anomalies in the intraplate lithospheric mantle discovered in Europe and China. We argue that a chemical component is a characteristic feature not only of conventional hydrous plumes developed in the big mantle wedge over presently descending oceanic slabs, but also of upper mantle plumes in other tectonic settings.

How to cite: Cloetingh, S., Koptev, A., Lavecchia, A., Kovács, I., and Beekman, F.: Hydrous secondary plumes: towards understanding the enigmatic «finger» structures in the intraplate lithospheric mantle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2386, https://doi.org/10.5194/egusphere-egu23-2386, 2023.

Although the geoid is usually displayed with respect to the reference ellipsoid, the difference between geoid and the Earth's hydrostatic equilibrium figure is geodynamically more meaningful, and has its deepest low in the Ross Sea area. Nearby in West Antarctica, there is also a residual topography high. This region is characterized by thin lithosphere, and a mantle plume has been suggested beneath. Hence upper mantle viscosity could be regionally reduced, allowing for faster rebound than elsewhere upon melting of the West Antarctic Ice Sheet (WAIS) which is one of the tipping elements of the global climate system. To study the possible causes of the geoid low / topography high combination, we compute the effects of density anomalies with the shape of a cylindrical disk of a given radius and depth range. With a density anomaly of -1% we find that a geoid low of the right size and magnitude can be explained with a disk radius of about 10° of arc and the base of the disk in the lower transition zone or even lower mantle; with a shallower base the amplitude is under-predicted. On the other hand, if in this case the top of the disk is shallower than ~150 km, dynamic topography amplitude is over-predicted. The fact that the residual topography high (more sensitive to density anomalies at shallower depth) is laterally displaced relative to the geoid low (more sensitive to greater depths) could indicate a plume or upwelling that is tilted due to large-scale flow. Alternatively, there may be two separate disks somewhat laterally displaced, one just below the lithosphere and mainly causing a dynamic topography high and one below the transition zone causing the geoid low.
In order to test the feasibility of such density models, we perform computations of a plume that enters at the base of a box corresponding to a 3300 km x 3300 km region in the upper mantle, as well as some whole-mantle plume models, with the Aspect mantle convection code. However, these plume models have typically a narrow conduit (much narrower than ~10° of arc) and the plume tends to only become wider as it spreads beneath the lithosphere, i.e.\ at depths typically shallower than about 300 km, hence it would tend to rather under-predict the amplitude of the geoid compared to dynamic topography. We discuss how to possibly overcome the discrepancy between what is required to explain geoid and dynamic topography, and the outcome of numerical forward models.

How to cite: Steinberger, B.: The deepest geoid low on Earth and its possible relation to the instability of the West Antarctic Ice Sheet, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2410, https://doi.org/10.5194/egusphere-egu23-2410, 2023.

Geothermal Heat Flow (GHF) is a crucial boundary condition governing ice sheet stability, due to the positive relationship between thermal input into the ice sheet and basal sliding rates. Tectonic history biases the crustal distribution of heat-producing elements, and the pattern of mantle convection influences regional thermal structure, leading to significant intracontinental variations in Antarctic GHF of order 100 mW/m².  However, extensive ice cover across Antarctica severely limits the ability to directly measure GHF or crustal composition. Geophysical proxies are therefore required to access information pertaining to the lateral structure of GHF and its potential impact on ice sheet dynamics.

Previous studies have used geomagnetic data to infer the depth above which ferromagnetic structure is locked in, corresponding to the ~850 K isotherm. Others have relied on the sensitivity of seismic velocity to thermal structure to model local variations in surface temperature gradient. Both approaches require assumptions on crustal properties, which are typically chosen ad-hoc, and may affect GHF estimates in a significant and non-systematic manner. Other studies have used the observed covariation between lithospheric seismic velocity and GHF in regions with high measurement densities (e.g., continental USA) to map Antarctic seismic structure into GHF. This introduces a dependency of inferred Antarctic GHF on the range of tectonic environments sampled by the continental region used to derive the empirical relationship.

Here, we adopt a distinct approach, in which Monte Carlo sampling is used to include crustal conductivity and heat production as free parameters in a numerical modelling procedure that fits theoretical geotherms to new probabilistic seismic inferences of upper mantle temperature structure beneath Antarctica. By integrating empirical constraints on crustal conductivity derived from P-wave velocity data, we are able to build distributions of covarying crustal conductivity, heat production, and GHF. This allows us to generate a model of Antarctic GHF which is complementary to that of other studies, and includes an estimate of lateral uncertainty structure based on the sensitivity of thermal gradients to crustal composition and anelastic deformation at seismic frequencies.

How to cite: Hazzard, J., Richards, F., and Roberts, G.: Refining Estimates of Antarctic Geothermal Heat Flow Using Seismological Constraints on Crustal Composition and Lithospheric Thermal Structure, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3400, https://doi.org/10.5194/egusphere-egu23-3400, 2023.

The investigation of mantle-derived products coming from Sub Continental Lithospheric Mantle (SCLM) is crucial for understanding its geochemical features and evolution, the mantle-crust interaction, and the volatiles composition. In this respect, mineral-hosted fluid inclusions (FI) in mantle xenoliths play a fundamental role, as their composition provides useful insights about the extent and timing of mobilization of volatiles during melt extraction and melt/fluid-rock reactions in the mantle, especially when their composition is combined with the information extracted from mineral chemistry and texture.

Peridotite xenoliths sampled by the intra-continental rift magmatism at West Eifel (Germany) and northern Victoria Land (Antarctica) are extremely rich in FI, and bear witness to multiple metasomatic modifications taking place in the local SCLM (Rizzo et al. 2021; Casetta et al. 2022). In this study, the concentration and isotopic signature of CO₂ in mineral-hosted FI in peridotite rocks was coupled to mineral chemistry and thermo-oxy-barometric modelling, with the aim of exploring if, and how, the provenance and mobilization of C-bearing species are related to the main melt extraction and metasomatic processes that took place in the local SCLM domains or to the recycling into the mantle of old crustal material. Our findings show that the concentration of CO₂ in FI varies from 0 up to 162 µg/g, being higher in West Eifel than in Antarctica samples, and also higher in pyroxenes- than in olivine-hosted inclusions. A correlation between the CO₂ content in FI and the Mg#, Al₂O₃   and TiO₂ concentrations in mineral phases is observed. The δ¹³C ratio of CO₂ in pyroxene-hosted FI spans a wide range, from typical mantle values of -6‰ to -4‰ in peridotites from Antarctica up to higher values (-2‰ to +2‰) in peridotites from West Eifel that overlap the range of carbonates. Interestingly, a clear correlation between the δ¹³C ratio of the FI and the Al₂O₃ concentration of their host pyroxenes is displayed by all xenoliths, indicating that the signature of fluids is related to the chemical evolution of the host mineral phases. Consistently, the δ¹³C ratio is positively correlated to the temperature recorded by both olivine-spinel and orthopyroxene-clinopyroxene pairs (T = 850-1200°C) in xenoliths from both localities.

Besides potentially widening the range of δ¹³C ratios of mantle-derived products, our results confirm that coupling the chemistry of FI to that of the host mineral phases in mantle peridotites is one of the best ways to explore the cause and effects of the melt/fluid-rock reactions taking place in the SCLM.

REFERENCES

Casetta, F., Rizzo, A. L., Faccini, B., Ntaflos, T., Abart, R., Lanzafame, G., ... & Coltorti, M. (2022). CO2 storage in the Antarctica Sub-Continental Lithospheric Mantle as revealed by intra-and inter-granular fluids. Lithos, 416, 106643.

 Rizzo, A. L., Faccini, B., Casetta, F., Faccincani, L., Ntaflos, T., Italiano, F., & Coltorti, M. (2021). Melting and metasomatism in West Eifel and Siebengebirge Sub-Continental Lithospheric Mantle: Evidence from concentrations of volatiles in fluid inclusions and petrology of ultramafic xenoliths. Chemical Geology, 581, 120400.

How to cite: Rizzo, A. L., Casetta, F., Faccini, B., Faccincani, L., Sandoval-Velasquez, A., Aiuppa, A., and Coltorti, M.: The carbon cycle in the mantle below intra-continental rift settings, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15035, https://doi.org/10.5194/egusphere-egu23-15035, 2023.

The dynamic behaviour of crystals in convecting fluids determines how magma bodies solidify. In particular, it is often important to estimate how long crystals stay in suspension in the host liquid before being deposited at its bottom (or top, for light crystals and bubbles of volatiles). We perform a systematic 3D numerical study of particle-laden Rayleigh-Benard convection, and derive a robust model for the particle residence time. For Rayleigh numbers higher than 10⁷, inertial particles' trajectories exhibit a monotonic transition from fluid tracer-like to free-fall dynamics, the control parameter being the ratio between particle Stokes velocity and the mean amplitude of the fluid velocity. The average settling rate is proportional to the particle Stokes velocity in both the end-member regimes, but the distribution of residence times differs markedly from one to the other. For lower Rayleigh numbers (<10⁷), an interaction between large-scale circulation and particle motion emerges, increasing the settling rates on average. Nevertheless, the mean residence time does not exceed the terminal time, i.e. the settling time from a quiescent fluid, by a factor larger than four. An exception are simulations with only a slightly super-critical Rayleigh number (~10⁴), for which stationary convection develops and some particles become trapped indefinitely. 2D simulations of the same problem overestimate the flow-particle interaction - and hence the residence time - for both high and low Rayleigh numbers, which stresses the importance of using 3D geometries for simulating particle-laden flows. We outline how our model can be used to explain the depth changes of crystal size distribution in sedimentary layers of magmatic intrusions that are thought to have formed via settling of a crystal cargo, and discuss how the micro-structural observations of solidified intrusions can be used to infer the past convective velocity of magma.

How to cite: Patočka, V., Tosi, N., and Calzavarini, E.: Residence time of crystals in a thermally convecting magma reservoir, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1366, https://doi.org/10.5194/egusphere-egu23-1366, 2023.

Terrestrial planets evolve through multiple magma-ocean stages during accretion and differentiation. Magma oceans become progressively enriched upon fractional crystallization (FC), which should be dominant at least in the upper mantle. The resulting upwards enrichment of the cumulate package drives gravitational overturn(s), and ultimately stabilizes a FeO- and SiO₂-enriched basal magma ocean (BMO) [1]. Alternatively, a ~pyrolitic BMO may be formed due to a liquid-solid density crossover at high pressures [2,3]. In any case, the slowly cooling BMO is very likely to freeze by FC. However, we find that the consequences of FC of the BMO are inconsistent with geophysical constraints for Earth (Ismail+, this meeting). For FC, the final-stage cumulates are expected to be strongly FeO-enriched (~eutectic), stabilizing a layer at the base of the mantle with density anomalies >2,000 kg/m³. Such a layer should be extremely long-lived, but is not detected by seismic imaging.

Using a thermodynamic model [4], we here investigate the chemical consequences of an alternative scenario, in which the BMO interacts with (partially) molten basaltic material in the lower mantle. We refer to such a scenario as reactive crystallization (RC). Even in the present-day, the core-mantle boundary may be hotter than the solidus of subducted basalt [5]. Accordingly, any recycled Hadean/Archean is likely to have undergone (partial) melting in the lowermost mantle, and mixed with the BMO. This scenario is attractive, because large volumes of crust may be readily delivered to the lowermost mantle, and will produce dense magmas there, which sink into the BMO to promote efficient reaction.

We find that the first BMO cumulates due to RC are Mg-rich bridgmanite (~MgSiO₃). With progressive addition of basaltic material, Al₂O₃ becomes enhanced in the BMO to promote FeO-disproportionation, leading to loss of elemental Fe to the core and crystallization of FeAlO₃. With ongoing cooling, the BMO starts effectively shrinking, and final BMO cumulates are similar in composition than, and slightly enriched compared to, basalt. The associated intrinsic density anomalies are 300~350 kg/m³, i.e., much more moderate than for FC of the BMO. These predicted densities and cumulate compositions (bridgmanitic with high FeAlO₃) are in very good agreement with the geophysical signatures of large low-velocity provinces [6]. In turn, the predicted final composition of the BMO itself may correspond to that of seismically-detected ultra-low velocity zones.

Our results imply that large rocky planets such as Earth, Venus or even Super-Earths may host only a rather short-lived BMO due to efficient crustal recycling. In turn, small stagnant-lid planets with limited crustal recycling, such as e.g. Mars, may host longer-lived BMOs (Cheng+, this meeting). These predictions have important implications for the long-term thermal and chemical evolution of terrestrial planets.

[1] Ballmer+, G-cubed, 2017; [2] Labrosse+, nature, 2007; [3] Caracas+, EPSL, 2019; [4] Boukare+, JGR Solid Earth, 2015; [5] Adrault+, science 2014; [6] Vilella+, EPSL, 2021

How to cite: Ballmer, M., Spaargaren, R., and Ismail, M.: Reactive Crystallization of the Basal Magma Ocean: Consequences for present-day mantle structure, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9854, https://doi.org/10.5194/egusphere-egu23-9854, 2023.

The H₂O incorporation into minerals changes the properties of minerals and rocks and affects the dynamics and evolution of the Earth’s interior. The higher H₂O contents in plume-related magmas than in mid-oceanic ridge magmas suggest that deeper regions store more significant amounts of H₂O than shallower regions in the mantle. Paradoxically, however, the H₂O solubility in the lower-mantle minerals in ultramafic systems is limited. Therefore, we expect basaltic fragments of subducted slabs to store H₂O in the lower mantle. It has been suggested that silica minerals can be H₂O hosts in the basaltic systems under lower-mantle conditions, and alumina incorporation enhances their H₂O solubility. To determine the stability and water solubility of silica minerals under top-most lower-mantle conditions, the current study synthesised silica minerals in the SiO₂-Al₂O₃-H₂O systems at pressures of 24 and 28 GPa and temperatures of 1000 to 2000°C using a multi-anvil press. We identified phases present in the run products using a micro-focused X-ray diffractometer and measured their water solubility using an FT-IR spectrometer.

We found that the Al₂O₃ contents in the silica minerals increased with increasing temperature from 0.7~0.8 wt.% at 1000~1200°C to 10 wt.% at 2000°C. Their H₂O contents also increased with increasing temperature from 0.3 at 1700°C to 1.0~1.1 wt.% at 1900°C. The silica mineral was stishovite at temperatures lower than 1600~1700°C, whereas it was CaCl₂-structured silica, referred to as post-stishovite, at higher temperatures. Thus, post-stishovite contained much more significant amounts of H₂O than stishovite whose water content is consistent with previous reports.

The concomitant increases in H₂O and Al₂O₃ contents suggest that H₂O is incorporated via charge-coupled substitution of Si⁴⁺ — Al³⁺+H⁺ in these silica minerals. The current stability of post-stishovite in H₂O- and Al₂O₃-bearing systems is located at much lower pressures than in pure SiO₂ and H₂O-poor, Al₂O₃-bearing systems. In addition, the OH bands are more intense in the E//[010] direction than in the E//[100] direction. These observations imply that tilting of (Si, Al)O₆ octahedra around the c axis by the hydrogen bonding in the [010] direction may have stabilised poststishovite at lower pressures.

The increases in H₂O solubility in aluminous stishovite and poststishovite with temperature have a tremendous impact on the H₂O storage and transport in the mantle. The H₂O solubility in the other nominally anhydrous minerals decreases with increasing temperature. Dense hydrous magnesium silicates decompose with increasing temperature. Therefore, these minerals cannot be H₂O hosts or carriers in the deep mantle except for cold subduction zones. On the other hand, hydrous stishovite and poststishovite can store and transport H₂O in ambient mantle and even in plumes.

It has been considered that the stishovite-poststishovite transition causes seismic scattering in the mid-mantle. However, many seismic scatterers are located at 700 to 1900 km depths, which are too shallow for the stishovite-poststishovite transition in the pure SiO₂ system. However, we found that the Al₂O₃ and H₂O incorporations lower the transition pressure to 24 GPa, i.e., 700 km depth. Hence, observing the seismic scatterers in the mid-mantle supports significant H₂O storages in aluminous poststishovite.

How to cite: Katsura, T., Ishii, T., Criniti, G., Ohtani, E., Purevjav, N., Fei, H., and Mao, H.: Hydrous aluminous silicas as major water hosts in the lower mantle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2770, https://doi.org/10.5194/egusphere-egu23-2770, 2023.

Mantle shear-wave velocity models derived from seismic full waveform inversion methods reveal a very heterogeneous lithosphere-asthenosphere system beneath intracontinental Western and Central Europe north of the Alps. To better understand the physical state of the upper mantle in this region, we convert shear-wave velocity models to thermodynamically consistent temperature and density configurations using a Gibbs's free energy minimization approach. The inferred physical state of the lithosphere-asthenosphere system is then investigated for its consequences on past and present-day crustal deformation. For instance, a thermal lithosphere-asthenosphere boundary that varies in depth between > 200 km in the southern North Sea and < 80 km close to the Alpine deformation front raises important questions regarding the causes for this thermal disequilibrium and its effects on the thermomechanical stability of the crust. In particular, we will discuss the imaged mantle thermal anomalies in light of the inherited crustal structure and its effects on ongoing deformation (including seismicity) in this intracontinental setting.

How to cite: Bott, J., Scheck-Wenderoth, M., Kumar, A., Cacace, M., Noe, S., and Faleide, J. I.: Relationships between upper mantle thermal structure and crustal deformation in Western and Central Europe – new interpretations of seismic tomography models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6574, https://doi.org/10.5194/egusphere-egu23-6574, 2023.

Geophysical methods such as seismology, magnetotellurics and gravity are key to reconstructing the structure of the upper mantle and inferring its composition. However, the relationship between composition and geophysical parameters, e.g. seismic velocity, resistivity or density, is complex and depends on other factors such as temperature, for example. This makes it difficult to untangle the various effects from inversions based on single parameters. Joint inversion establishes quantitative relationships between different geophysical parameters and thus provides additional information that can be interpreted in terms of composition and temperature. I use 3D joint inversion of surface wave, gravity and magnetotelluric data to construct integrated models in a data driven way. The relationship between the different quantities is recovered as part of the inversion through a Variation of Information based constraint. This constraint aims at establishing a one-to-one relationship between each parameter pair. Results from the western United States, Germany, Southern Africa and Australia show that this approach can retrieve highly detailed and strongly coupled results that can be interpreted, for example, in terms of hydration of the lithosphere. Comparison of results from different geologic domains indicates significantly different relationships depending on formation age. I will discuss how we can use data driven parameter relationships to infer the state of the lithosphere. In addition, I will outline the road towards robust quantitative inference based on these relationships.

How to cite: Moorkamp, M.: The structure and composition of the upper mantle from joint inversion derived parameter relationships, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7037, https://doi.org/10.5194/egusphere-egu23-7037, 2023.

Physical 1D-average reference models of the Earth offer valuable summaries of the radial variations in rock properties and a reference for geophysical studies. PREM, in particular, has been used widely for >40 years and comprises Vp, Vs, density, radial anisotropy and attenuation profiles, while also fitting the Earth’s mass and moment of inertia. Many of PREM’s features have proven remarkably accurate, despite the limited amount of data used to construct it, but some features are inconsistent with now available data. Also, the upper mantle structure differs so much between Earth’s different tectonic environments that a global average is not quite representative of any of them.  The recent growth in seismic station coverage yields very dense data sampling, globally and over different tectonic environments. Here, we use a large global dataset to construct ten 1D, multi-parameter, reference models of the upper mantle, for the globe and for 9 basic tectonic types: cratons; stable platforms; Phanerozoic continents with normal (<46.5 km) and thick (>46.5 km) crust; rifts and continental hotspots; old oceans; intermediate oceans; young oceans; backarcs.

The dataset comprises Love and Rayleigh-wave phase velocities, measured using waveform inversion and all available data since 1990s; surface heat flow measurements; topography/bathymetry. With tomography-based tectonic regionalization, we identify areas within each tectonic environment and compute average dispersion curves in the 20-30 to 310 s period range, which constrain shear velocity and anisotropy in the entire upper mantle.

We then use computational-petrology-based inversion to calculate 1D physical models for the globe and the 9 basic tectonic types. Our non-linear gradient search converges to true best-fitting models. The main unknowns in the inversion are the depth of the lithosphere-asthenosphere boundary (LAB); the geotherm from the LAB down to 400 km depth; radial anisotropy (0-800 km). The steady-state geotherm in the lithosphere is computed from the LAB depth and the radiogenic heat production and thermal conductivity profiles by solving the conductive heat transfer equation. Rock composition and the geotherm determine the density, seismic velocities and attenuation down to 400 km. Seismic velocities in the crust, transition zone (410-660 km) and shallow lower mantle can vary to fit the data. Density below 410 km and all parameters in the core and most of the lower mantle are from PREM. Like PREM, our reference models honour the Earth's mass and moment of inertia.

Small phase-velocity errors and relative data-synthetic misfits (<~0.1%) are necessary to resolve radial trade-offs in the upper-mantle structure. We achieved this by obtaining very accurate dispersion curves and by meticulously tuning the inversion, its parameterisation and regularisation.

The best-fitting models have slightly depleted lithospheric mantle and fertile asthenosphere for most tectonic types. In Archean and Proterozoic continents, the mantle lithosphere is more depleted. No other compositional heterogeneities are required to fit the data. Isotropic-average seismic velocities decrease monotonically from the Moho to the LAB. The geotherms follow the mantle adiabatic temperature gradient in the asthenosphere. Our results provide useful, accurate new reference models for global and regional seismic imaging and other geophysical studies. 

How to cite: Xu, Y., Lebedev, S., Civiero, C., and Fullea, J.: Global and tectonic-type physical reference models of the upper mantle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5459, https://doi.org/10.5194/egusphere-egu23-5459, 2023.

The mantle transition zone (TZ) is expected to influence vertical mass flow between upper and lower mantle as it hosts a complex set of mineral phase transitions and an increase in viscosity with depth. Still, neither its seismic structure nor its dynamic effects have conclusively been constrained. The seismic discontinuities at around 410 and 660 km depth ('410' and '660') are classically associated with phase transitions between olivine polymorphs, the pressure of which is modulated by lateral temperature variations. Resulting discontinuity topography is seismically visible and can thus potentially provide insight on temperature and phase composition at depth. Besides the olivine phase changes, the disassociation of garnet may additionally impact the 660 at higher temperatures. However, the volume of material affected by this garnet transition and its dynamic implications have not yet been quantified.

This study presents hypothetical realizations of TZ seismic structure and major discontinuities based on the temperature field of a published 3-D mantle circulation model for a range of relevant mineralogies, including pyrolite and mechanical mixtures (MM). Systematic analysis of these models provides a framework for dynamically informed interpretations of seismic observations and gives insights into the potential dynamic behaviour of the TZ. Using our geodynamic-mineralogical approach we can identify which phase transitions induce specific topographic features of 410 and 660 and quantify their relative impact. Areal proportions of the garnet transition at the 660 are ∼3 and ∼1 per cent for pyrolite and MM, respectively. This proportion could be significantly higher (up to ∼39 per cent) in a hotter mantle for pyrolite, but remains low (< 2 per cent) for MM. In pyrolite, both slabs and plumes are found to depress the 660 —with average deflections of 14 and 6 km, respectively— due to the influence of garnet at high temperatures indicating its complex dynamic effects on mantle upwellings. Pronounced differences in model characteristics for pyrolite and MM, particularly their relative garnet proportions and associated topography features, could serve to discriminate between the two scenarios in Earth.

How to cite: Papanagnou, I., Schuberth, B. S. A., and Thomas, C.: Geodynamic-mineralogical predictions of mantle transition zone seismic structure, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6820, https://doi.org/10.5194/egusphere-egu23-6820, 2023.

Seismic tomography provides valuable insights into the structure, composition and evolution of the mantle. However, the origin of structures like the Large-Low-Velocity-Provinces (LLVPs) in the lowermost mantle remains debated. Their velocity anomalies have been interpreted to be due to purely thermal or also compositional variations, with implications for mantle circulation, the evolution of the core and the Earth’s heat budget.

To uniquely interpret seismic structures such as the LLVPs, it is crucial to constrain the relationships between different seismic observables, e.g. the ratio between shear-wave velocity (Vs) and compressional-wave velocity (Vp) variations. Joint inversions of seismic velocities have been performed, but their velocity amplitudes may be biased, uncertainties are typically not provided, and the resolution of Vs and Vp structures generally differs in existing models.

To overcome these issues, we make use of the recently developed SOLA method (Zaroli, 2016), which is based on a Backus-Gilbert philosophy. Instead of finding a model with a particular data fit, we aim to construct model averages of the true Earth with uncertainties, whilst having a control on the model resolution. This direct control on resolution enables us to build Vs and Vp models that sample the same parts of the mantle, and therefore to robustly constrain the Vs/Vp ratio.

Here, we test this philosophy by applying the SOLA method to normal modes. These free oscillations of the Earth are particularly useful to study the relationships between seismic velocities as they are directly sensitive to multiple physical parameters, including Vs, Vp as well as density. We illustrate our approach and discuss the trade-off between uncertainties and resolution using synthetic tests for both Vs and Vp, before showing real data inversions. Finally, we discuss the implications of our results for the Vs/Vp ratio in terms of mantle temperature and composition.

How to cite: Restelli, F., Koelemeijer, P., and Zaroli, C.: Obtaining robust estimates of the Vs/Vp ratio in the Earth’s lowermost mantle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8416, https://doi.org/10.5194/egusphere-egu23-8416, 2023.

Seismic velocity anomalies observed in the mantle can have several origins, the main contributions being anomalies of temperature and composition. The difference between P- and S-wave models has been used to separate thermal and compositional contributions in imaged seismic structures and identify large-scale compositional heterogeneity in the Earth's mantle. According to our two-step Machine Learning (ML) analysis of 28 P- and S-wave global tomographic models, P- and S-models differences are not intrinsic and can be reduced by changing the models in their respective null spaces. Because we find, P- and S-wave images of mantle structure are not necessarily distinct from each other, a purely thermal explanation for seismic structure is sufficient at present; significant mantle compositional heterogeneities do not need to be invoked. In this study, 28 commonly used tomographic models are examined, ranging from ray theory (e.g., UU-P07, MIT-P08) to Born scattering (e.g., DETOX) and full-waveform techniques (e.g., CSEM, GLAD). Combined Varimax Principal Component Analysis is used to reduce the dataset's dimensionality (by 82%) while preserving the relevant information of each tomographic model (94% of the original variance). Reduced-sized models are followed by a hierarchical clustering analysis (HC) using Ward’s method to categorize all the models into a hierarchy of groups based on their similarities. HC divided the set of tomographies into two main clusters: the first cluster, which we named "Pure P-wave", is composed of six P-wave models that only use longitudinal body wave phases (e.g., P, PP, Pdiff); the second cluster "Mixed" includes both P- and S-wave models; P-wave models in this cluster use inversion methods that include inputs from other geophysical and geological data sources, that cause them to be more similar to S-wave models than to pure P-wave models without a significant loss of fitness to P-wave data. Results suggest that the differences between some individual P-wave and S-wave models are smaller than the differences between grouping of models that are only P-wave or S-wave. These variable differences clearly convey that no consistent separation exists between the P- and S-wave models. We have also calculated the Distance Matrices along the Principal Components. Comparing clustering results with Distance Matrices shows that the differences between the "Pure P-wave" and "Mixed" clusters are mainly in the upper mantle. Accordingly, our results indicate that P-wave structures do not need to be very distinct from a thermal interpretation of S-wave structures and support a relatively “Homogenous” mantle.

How to cite: Rahimzadeh Bajgiran, M., Colli, L., and Wu, J.: Comparing 28 global P- and S- wave tomography models by Machine Learning analysis for the interpretation of the Earth’s mantle structures, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10123, https://doi.org/10.5194/egusphere-egu23-10123, 2023.