The effects of pressure and temperature on the phase transformation of olivine to wadsleyite and then ringwoodite within the mantle is well understood. However, the extent to which stress affects this phase transformation is not clear. Understanding how stress influences the kinetics of the olivine to spinel phase transformation and the mechanism in which it does so at grain scale, will have broader implications for mantle dynamics. Deformation experiments using Mg₂GeO₄ have been used as an approximate analogue for fayalite as it transforms from olivine to ringwoodite at lower pressures and temperatures rather than the conditions found at d410 (Vaughan, 1981). This enables the use of larger samples than possible for the silicate system, and allows for extensive microstructural investigations. This session aims to discuss high pressure deformation experiments on Mg₂GeO₄ (olivine) during the transformation to ringwoodite using a Griggs-type, solid medium, deformation apparatus. These experiments expand on (Vaughan 1984) which linked kinetics of the reaction in a model that matches other stressed reactions in the mantle (Wheeler, 2020). Experiments were conducted at a range of confining pressures 0.8 - 1.2 GPa at a fixed temperature of 900 °C and a strain rate of 10^-6 /s. The four samples were deformed to finite strains ranging from 10 to 45 %. The aim of the conditions chosen was to apply varying amounts of differential stress and therefore differing the σ₁ stress on the sample as a whole. Samples were characterised down to the level of individual interfaces using Electron Backscatter Diffraction (EBSD) to understand the physical mechanism of the reaction and the kinetics that govern it.

How to cite: Akhtar-Lewis, S.: Effects of Stress on the Olivine–Spinel Phase Transformation in the Mantle: Griggs-Type Deformation Experiments Using Mg2GeO4 as an Analogue., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6130, https://doi.org/10.5194/egusphere-egu24-6130, 2024.

A main objective in geodynamics is to create models that provide quantitative information to other Earth science disciplines. In order to assess the validity of the underlying assumptions and chosen input parameters related to different geodynamic hypotheses, it is crucial to test these models against observations. In this, thermodynamic models of mantle mineralogy represent an essential tool. On the one hand, they enable the linking of temperature fields from mantle circulation models (MCMs) to seismic observations. On the other hand, they provide critical information on material behaviour in response to changing temperature and pressure conditions that occur over time within such mantle convection simulations. Some of the most interesting aspects in this context relate to mineral phase transitions and associated dynamic effects on mantle flow.

The mantle transition zone (TZ) in particular is expected to influence vertical mass flow between upper and lower mantle as it hosts a complex set of mineral phase transitions as well as an increase in viscosity with depth. Still, neither its seismic structure nor the associated dynamic effects have conclusively been constrained. The seismic discontinuities at around 410 and 660 km depth (‘410’ and ‘660’) have classically been related to phase transitions between olivine polymorphs, the pressure of which is modulated by lateral temperature variations. The resulting topography of these discontinuities 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.

Here, we present hypothetical realizations of TZ seismic structure and major discontinuities based on the 3-D temperature field of a published MCM 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., Thomas, C., and Bunge, H.-P.: Geodynamic-mineralogical predictions of mantle transition zone seismic structure, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2139, https://doi.org/10.5194/egusphere-egu24-2139, 2024.

Xenolith data reveal lateral and vertical compositional variations of the upper mantle of the Precambrian cratons, indicating a different degree of refertilization with respect to the most depleted mantle in iron components, characterizing the oldest Archean cratons. The South African cratonic region is composed of the Kaapvaal and Zimbabwe craton, both of Archean age, having deep and fast lithospheric roots, which are likely depleted in heavy constituents. In contrast, there exist regions, such as the Limpopo belt, a terrane that was trapped between the Kaapvaal and Zimbabwe cratons during their collision (2.6–2.7 Ga), and Bushveld Complex, an area characterized by intraplate magmatism occurred 2.05 Ga, whose negative velocity anomalies in the upper mantle, indicate a more fertile composition due to metasomatism. To unravel the origin of these anomalies and link them to the tectonic history of the area, we apply an integrative technique based on a joint interpretation of the seismic tomography and gravity data, which can discern temperature and compositional variations. To this aim, we combine the global surface seismic tomography model [1] with the embedded regional model [2], derived from teleseismic tomographic inversion of the S-body wave dataset recorded by the Southern African Seismic Experiment. The combined seismic model is inverted for temperature, assuming an initial composition, representative of a refertilized upper mantle [3], using a mineral physics approach [4]. The composition and temperature of the upper mantle are iteratively changed, increasing progressively the amount of iron depletion, to fit the residual density, obtained from the joint inversion of the residual gravity and residual topography. The great advantage of using both the gravity field and residual topography lies in their different dependence on the distribution of density heterogeneities (depth and size). In a second type of inversion we included the GOCE gravity gradient [5]. The obtained results show that the most depleted lithosphere is confined at depth lower than 100 km, generating a temperature higher than ~200, with respect to that of a refertilized lithosphere. The Southeastern Terrane of the Kaapval craton are characterized by thicker and more depleted cratonic roots than the Zimbawe craton. The presence of a depleted mantle below the cratonic crust may indicate that the crust and mantle have been connected since the craton formation. These results, related to the different structures and properties of the upper mantle, improve our understanding of the evolution of the South African cratonic lithosphere.

References

[1] Schaeffer and Lebedev, 2013. https://doi.org/10.1093/gji/ggt095

[2] Youssof et al., 2015. http://dx.doi.org/10.1016/j.epsl.2015.01.034

[3] Griffin et al., 2004. doi:10.1016/j.chemgeo.2004.04.007

[4] Conolly, 2005. doi:10.1016/j.epsl.2005.04.033

[5] Kaban et al., 2022. doi.org/10.1007/s00024-021-02925-6

How to cite: Tesauro, M., Kaban, M., and Youssof, M.: Thermo-compositional model of the South African cratonic mantle obtained from seismic and gravity data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11790, https://doi.org/10.5194/egusphere-egu24-11790, 2024.

The presence of lithologies derived from recycled oceanic lithosphere in the convective mantle is an expected consequence of subduction. Geochemical studies have provided compelling evidence of the contribution of recycled eclogite and pyroxenite in the mantle source of oceanic basalts, particularly ocean island basalts (OIB). However, identifying their signatures in mid-ocean ridge basalts (MORB) is challenging due to more intricate melting and mixing processes. Furthermore, the use of elemental and isotopic proxies of different geochemical affinities provides contrasting pictures on their source heterogeneity. Understanding the role of pyroxenite and eclogite during partial melting bears critical information regarding the fate of recycled lithospheric material, the dynamics and timescales of mantle convection and the thermal regime of mid-ocean ridges.

We present a numerical approach based on the thermodynamically constrained Mixed-Source Melting model (MSM3), enabling a coherent assessment of the role of recycled lithologies. Within a comprehensive plate tectonic cycle, the MSM3 model simulates the two-stage recycling of eclogites derived from subducted oceanic crust in a marble-cake mantle.

• Stage 1 corresponds to the formation of secondary pyroxenite from the hybridization of high-degree eclogite-derived melts interacting at high pressure with peridotite in the convective mantle.
• Stage 2 corresponds to the formation of MORB in a triangular melting regime from the adiabatic decompression melting of a 3-lithology source of peridotite, pyroxenite and residual eclogite obtained from stage 1.

To tackle the diversity of geochemical proxies applied to oceanic basalts, MSM3 recovers melt and residual compositions in terms of major elements and sulfur, as well as any lithophile and chalcophile trace elements and isotope systems. This is achieved thanks to the integration of melting models with pMELTS calculations constrained by a thermodynamic parametrization specific to pyroxene-rich lithologies (Melt-PX), calculations of sulfur concentration at sulfide saturation (SCSS), and composition-dependent partition coefficients. To take into account the inherent variability of most parameters (e.g., potential temperature, source proportions, sulfur contents) and avoid arbitrary choices, we use a stochastic approach by running the MSM3 model as an inversion based on adaptative Monte Carlo simulations.

We here demonstrate the flexibility of this approach, even for systems controlled by sulfides. We show that, over potential temperatures ranging between 1280 and 1420 ºC, the generation of 0-10% of pyroxenitic heterogeneities from subducted eclogite, and the contribution of both eclogite and pyroxenite in the melting regime of MORB produce 20-95 % of the melts aggregated at the ridge. Such proportions correspond to up to 30 times the proportion of these lithologies in the mantle. This over-contribution is controlled by the melting regime properties and is enhanced or attenuated by the mass balance specific to the elements and isotope systems considered (concentrations, partitioning behavior, modal evolution of the main host minerals in the different lithologies). In other words, the more-fusible pyroxenite and eclogite act as chocolate in the marble-cake mantle, giving the dominant flavor to its melting products, although different geochemical proxies may "taste" it differently.

How to cite: Tilhac, R., Garrido, C., König, S., and Varas-Reus, M. I.: Chocolate in the marble cake: the fate of eclogite and pyroxenite during mantle convection and melting, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7341, https://doi.org/10.5194/egusphere-egu24-7341, 2024.

The Icelandic mantle plume is regarded as one of the most significant mantle upwellings on Earth, however the dynamics of its interaction with the surrounding asthenosphere and mid-oceanic ridge systems in the North Atlantic are poorly understood. The clearest manifestation of this plume-ridge interaction are the Reykjanes V-shaped ridges and V-shaped troughs (VSRs and VSTs) that straddle the Reykjanes Ridge axis south of Iceland. These time-transgressive linear features are particularly well exposed by short-wavelength gravity data and are thought to represent the progressive sampling of thermal asthenospheric pulses that horizontally advect away from the Icelandic mantle plume conduit. The Reykjanes Ridge therefore acts as a ‘window-sampler’ into the temporal and spatial dynamics of plume outflow. International Ocean Discovery Program (IODP) Expedition 395C drilled into two VSR and VST pairs along a plate-spreading flow line approximately 600 km south of Iceland in summer 2021. Over 400 m of basalt was recovered, which represents a magmatic record over 15 Ma of plate spreading at a fixed distance from the mantle plume conduit. We present Nd isotopic analysis of recovered whole-rock that reveals a linear isotopic evolution from ƐNd of 7.5 to 10.5 over 14 Ma (n = 50), which implies that the ‘plume-like’ enriched component of the mantle source has been progressively diluted by mixing with depleted upper mantle material. This evolution occurred synchronously with the entire timeframe of VSR formation as defined by free-air gravity anomalies, and a long-wavelength increase in crustal thickness implied by wide-angle seismic experiments. It is therefore apparent that the dynamics of plume-ridge interaction are directly interlinked with changes in magmatism, structural tectonics and crustal production. Furthermore, major and trace elements of both whole-rock and glass samples have been measured, by multiple analytical techniques, revealing distinct compositions between and within boreholes. These observations can be understood in terms of temporal changes in the depth and degree of melting. In summary, petrological, petrophysical and geochemical analysis of this rock core in conjunction with consideration and modelling of wide-angle seismic surveys, gravity and bathymetric data can be used to develop a quantitative understanding of the dynamics of plume-ridge interaction, test hypotheses for the formation of VSRs, and constrain the temporal evolution of the North Atlantic mantle domain.

How to cite: Pearman, C., White, N., Maclennan, J., and Tien, C.-Y. and the IODP Expedition 395 science party: Temporal Record of Plume-Ridge Interaction in the North Atlantic: Interdisciplinary Insights from IODP Expedition 395C, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-557, https://doi.org/10.5194/egusphere-egu24-557, 2024.

The origin of Large Low-Velocity Provinces (LLVPs) at the base of mantle remains a mystery, but particle physics may be able to provide another piece of the puzzle. Using a phenomenon known as neutrino oscillation, atmospheric neutrino experiments are sensitive to the electron number density inside the Earth, which is complementary to the information seismology can provide. In order to reveal lateral heterogeneities in density and chemical composition such as those expected for LLSVPs across the Earth’s lower mantle, we have developed a numerical method that allows us to compute the sensitivity of neutrino oscillation data to 3D Earth structure. Based on this approach, we will present some preliminary assessment of the potential resolving power of ongoing and future neutrino experiments.

How to cite: Pestes, R., Coelho, J., Deniz Hernandez, Y., Durand, S., Fuji, N., Mittelstaedt, E., and Van Elewyck, V.: Neutrino oscillations investigation of the base-of-the-mantle structure: simulation tools and preliminary results, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17413, https://doi.org/10.5194/egusphere-egu24-17413, 2024.

The study of the Earth’s interior has been traditionally based on seismological and geodynamic modelling, the first one providing important information about its present-day structure, composition and state, while the second about its dynamics and compositional evolution. Seismological and geodynamical modelling are very often conducted independently, which creates mechanical and geometrical inconsistencies across the models, hampers the interpretation of seismic observations in terms of geodynamic processes and enhances the non-uniqueness of geodynamic model predictions.

An alternative approach is combining computational seismology and geodynamics with mineral physics, which provides a comprehensive understanding of the Earth's interior processes, seismic behavior, and material properties. In this multidisciplinary methodology, the geodynamic flow calculations are used to compute the rock elastic properties as a function of strain-induced mantle fabrics through micro-mechanical models of crystal aggregate deformation, and of the local P-T conditions with thermodynamically self-consistent models of mantle mineralogy. The obtained seismic mantle structure is then used for seismological synthetics, such that specific hypotheses on mantle dynamics can be tested directly against seismic data. Examples from the South American, North American, and the Central Mediterranean convergent margins will be discussed.

Finally, I will introduce ECOMAN, a recently developed, open-source software package that is intended to overcome the computationally intensive nature of this approach and the lack of a dedicated and comprehensive computational framework for modelling strain-/stress-induced rock fabrics and testing the effects of the resulting mechanical (elastic and viscous) anisotropy on seismic imaging and mantle convection.

How to cite: Faccenda, M.: Constraining Mantle Convection Patterns by Joint Geodynamic and Seismological Modelling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5439, https://doi.org/10.5194/egusphere-egu24-5439, 2024.

In order to improve our understanding of mantle flow, we require a joint collaboration between all fields of Earth Sciences. Seismic tomography provides key information on the current state of the mantle and therefore can constrain mantle circulation models. We present high-resolution (degree-60) global models of frequency-dependent phase and group velocity measurements from huge a huge dataset of ~47 million Rayleigh and Love waves. These include fundamental mode measurements, which are sensitive to the uppermost mantle and up to 6^th overtone, adding sensitivity to the transition zone, covering a period range of 16-375 s. We also present global models of mantle attenuation (degree-20), made from ~10 million Rayleigh wave amplitude measurements, including fundamental and up to 4^th overtone measurements (35-275 s). All seismic maps presented also have associated uncertainty maps, which are essential for robust interpretation but also for multidisciplinary interpretations of mantle circulation models.

We constrain 3D mantle circulation models, known as TERRA models, at the present day. In order to do this, we construct 1D profiles of velocities and density from a suite of TERRA models on a 2x2 degree grid. Forward modelling of each 1D profile using MINEOS provides global predictions of seismic observables at all seismic wave periods, including phase velocity and group velocity. A misfit can then be calculated between the seismic models and predictions from the suite of TERRA models. This provides constraints on which TERRA models are most Earth-like, which will improve our understanding of mantle flow.

How to cite: Sturgeon, W., Ferreira, A. M. G., Panton, J., and Davies, J. H.: Constraining global mantle circulation models with global seismic observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10944, https://doi.org/10.5194/egusphere-egu24-10944, 2024.

Seismic tomography, a critical tool for studying Earth's interior structure and dynamics, has revealed positive seismic wave speed anomalies in the mantle that are commonly interpreted as slabs, the remnants of subducted lithosphere. However, classical travel-time tomography relies on the inversion of travel times of a few easily identifiable body wave phases along ray paths or volumetric sensitivity kernels, which is strongly dependent on the geometry of seismic sources and receivers. Since both of these are primarily clustered on modern convergent plate boundaries, the resulting tomographic resolution is highly variable across the mantle. Full-waveform inversion (FWI) attempts to reduce this dependence by fitting whole seismograms, thereby including many reflected and refracted body wave phases to enhance the volumetric sensitivity of the inversion.

Here, we analyse a new global tomographic model constructed using FWI. The mantle structure imaged in this model reveals significantly more positive seismic wave speed anomalies in the mantle when compared to travel-time tomography, particularly in regions with low seismic activity and limited station coverage. Notably, FWI detects positive wave speed anomalies with slab-like morphologies at ~1000 km depth beneath the Pacific Ocean that fall outside the coverage of classical travel-time tomography. We demonstrate the sensitivity of FWI to wave speed anomalies below the western Pacific using forward wavefield modelling. Importantly, we find that these newly imaged positive wave speed anomalies do not correspond to reconstructed subduction zones in existing global plate reconstructions.

Our work challenges the widespread assumption that positive wave speed anomalies (exclusively) represent subducted slabs, highlighting potential gaps in either global plate reconstructions or the current understanding of the nature of seismic anomalies in the mantle.

How to cite: Schouten, T., Gebraad, L., Noe, S., Gülcher, A., Thrastarson, S., van Herwaarden, D.-P., and Fichtner, A.: Global full-waveform inversion reveals previously undetected positive wave speed anomalies beneath the Pacific Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14786, https://doi.org/10.5194/egusphere-egu24-14786, 2024.

New insights from models of thermo-chemical mantle convection featuring heterogeneous thermal conductivity indicate that heat-producing element (HPE) enrichment in large low shear velocity provinces (LLSVPs) significantly impacts the long-term stability of these regions. Because the internal heating rate was more significant in the past, thermal conductivity's influence on thermal buoyancy (and bulk erosion) must have also been more substantial. As a consequence, the initial volume of the LLSVPs may have been significantly larger than their present-day volume. In numerical models, the evolution and stability of LLSVPs are often initiated by considering a dense and uniformly distributed layer on top of the core-mantle boundary. From energy balance calculations, a thin layer of LLSVP material (small mantle volume fraction) supports more HPE enrichment than a thicker layer (larger mantle volume fraction) to maintain the mantle's heat budget. For example, an initial layer thickness of 160km (~3% mantle volume) implies present-day HPE enrichment factors up to ~70 times the ambient mantle heating rate. This should be compared with more conservative factors of 10 to 20 for similar dense layer thicknesses employed in previous studies of thermochemical pile stability. Thus, HPE enrichment may have been significantly underestimated in earlier models of LLSVP evolution. Conversely, and assuming that LLSVPs formed from a much larger reservoir, HPE enrichment may be overestimated based on the present-day LLSVP volume. Our study considers LLSVPs with a primordial geochemical reservoir composition (consistent with an undegassed ⁴He/³He signature and HPE enrichment). We examine models of thermo-chemical mantle convection models with time-dependent internal heating rates and HPE enrichment (implied by the initial dense layer thicknesses). In this new context, we re-examine, in particular, the impact of a fully heterogeneous lattice thermal conductivity (derived from conductivity measurements of upper and lower mantle minerals). Furthermore, in light of recent developments with radiative conductivity, we also examine the added effect of a strongly temperature-dependent radiative conductivity component on the stability of LLSVPs. Using LLSVPs' present-day volume and core-mantle boundary coverage as a constraint, we recover potential initial conditions, heating scenarios, and thermal conductivity for an Earth-like model.

How to cite: Guerrero, J., Deschamps, F., Hsieh, W.-P., and Tackley, P.: Constraining LLSVPs initial conditions and heating scenarios from simulations of mantle convection with heterogeneous thermal conductivity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14060, https://doi.org/10.5194/egusphere-egu24-14060, 2024.