Geodynamic modelling is increasingly dependent on the initial conditions. Non-steady-state planetary evolution models are complex enough that several solutions can be achieved with small variations in the initial temperature, composition, etc. Models of magma ocean evolution predict an overturn of a layer enriched in iron and incompatible elements. The absence of this layer in any tomographic inversion of the Earth suggests that our models of magma ocean evolution are missing key phenomena. Since these models are the basis for the initial conditions of Earth-applied geodynamic planetary models, there is an urgency to find the origin of the discrepancies between our idea of magma ocean crystallization and the current state of the Earth.

To advance our understanding of the consequences of magma ocean crystallization, we carry out high resolution models of mantle flow coupled with (1D) magma ocean evolution (including melting and crystallization processes). We allow two different forms of topography to arise with a sticky air (sticky magma ocean) approximation: dynamic topography and thermodynamic topography. We explore how different equilibration times affect melt segregation and magma ocean composition, as well as how crystal settling influences solid state convection and differentiation. We calculate, as well, chemical exchange between the solid and liquid parts of our model by expanding on previous work, studying different equilibrium constants and allowing the system to self-regulate.

Preliminary results suggest a competition effect between the two forms of topography mentioned above. This competition implies that the equilibrium constant of chemical exchange, as well as melting segregation speed and crystal growth and settling, will have an essential role in the equilibration between the solid mantle and the liquid magma ocean. This and other results have implications for Earth, but also for other discovered magma oceans such as those on the Moon, Mars or even exoplanets.

How to cite: Manjon Cabeza Cordoba, A., Ballmer, M. D., and Shorttle, O.: Melting and Remelting in a Crystallizing Magma Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19267, https://doi.org/10.5194/egusphere-egu24-19267, 2024.

We present a numerical model of a cooling magma ocean (MO) and the atmosphere degassing from it. The solidification of the MO leads to the enrichment of the silicate melt in volatiles, thus favoring degassing. Both reservoirs interact via heat and volatile exchange, where the volatiles are H2O and CO2. The aim of this model is to explore the influence of the atmosphere on the surface conditions after the MO stage, and especially the conditions required for the condensation of a water ocean to occur. For example, for an early Earth at 1 AU initially containing 1 Earth's water ocean mass, a water ocean could form for initial CO2 content as large as 1,000 bars. Moreover, a tenth of the actual Earth's water ocean mass would be sufficient to generate a water ocean on early Venus. Liquid water could also be present on the surface of the two exoplanets Trappist-1e and 1f. Comparing our results with other recent models, we discuss the relative influence of the model hypotheses, such as mantle composition, the treatment of the heat transfer in the atmosphere, and the treatment of the last stages of the MO solidification.

How to cite: Massol, H., Davaille, A., Sarda, P., and Delpech, G.: Early formation of a water ocean as a function of initial CO2 and H2O contents in a solidifying rocky planet, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16653, https://doi.org/10.5194/egusphere-egu24-16653, 2024.

Crystallization processes, a partial or complete overturn of the early mantle and other processes can lead to a compositionally stably stratified mantle which may hinder of even prevent convective currents. In the classical view, the layer will be stable if the restoring force, generated by the compositional stratification will exceed the driving force as provided by heating the layer from the core. The situation resembles the diffusive scenario of double diffusive convection, characterized by the fast diffusing component (heat) being the driving force, while the slowly diffusing component (composition) acts as the restoring force. If perturbed, subcritical convection can eventually take plain ce.  While in  pure thermal convection , subcritical flow only develops as localized patterns, in the double diffusive case, a perturbation can lead to a global destabilization (blue sky bifurcation) of the system, due to a sufficient finite  amplitude perturbation. . In this study we have investigated the influence of a finite amplitude perturbation of varying magnitudes , namely realze by an impact on a stably stratified mantle. Numerical experiments in 2- and 3D, cartesian and spherical simulations, based on a Finite Volume scheme  have been conducted to study the evolution of such a system. Key parameters  which characterize the evolution are the (1) initial stratification and the structure f the perturbation. The viscosity structure is a further influencing factor.

The experiments show that an impact into a stably stratified mantle can lead to a global destabilization, giving rise to complex flow patterns, including local and transient layering of the mantle flow. An evolutionary path of a planet from a stably stratified state to a complex layered period and/or to a full convection mode seems sensitive.

How to cite: Hansen, U. and Dude, S.: Spontaneous destabilization of a compositionally stably stratified mantle in the Early Earth, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16401, https://doi.org/10.5194/egusphere-egu24-16401, 2024.

Isotopic systems and trace elements are ideal proxies to constrain the production and recycling of crust (both mafic and felsic) over time. Within the Rubidium-Strontium (Rb-Sr) system, Rb-87 decays to Sr-87 and due to the preferential partitioning of Rb into the crust (relative to Sr) during partial melting, ⁸⁷Sr/⁸⁶Sr ratio of the crust is higher than that of the mantle over time. Trace elements such as Niobium (Nb) and Uranium (U) do not fractionate when mantle melts to form mafic magma (oceanic crust) but they do fractionate when oceanic crust is recycled and undergoes fluid-present melting, i.e., during the production of felsic magmas (continental crust) [Hofmann et al., 1986], thereby resulting in a lower Nb/U of the felsic crust compared to the mantle. In this work, we couple the evolution of the above-mentioned geochemical proxies with the melting processes in global convection models using the code StagYY [Tackley, 2008]. Results from these geodynamic models are then compared with geochemical data obtained from olivine-hosted melt inclusions extracted from komatiites of 3.27 Ga Weltevreden formation (Barberton Greenstone Belt, South Africa).

These models self-consistently generate oceanic and continental crust while considering both plutonic and volcanic magmatism [Jain et al., 2019] and incorporate a composite rheology for the upper mantle. Pressure-, temperature-, and composition-dependent water solubility maps calculated with Perple_X [Connolly, 2009] control the ingassing and outgassing of water between the mantle and surface [Jain et al., 2022]. These models show intense production and recycling of continental crust during the Hadean and the early Archean, which is in agreement with new geochemical data [Vezinet et al., in review] and previous geochemical box models [Rosas & Korenaga, 2018; Guo & Korenaga, 2020]. The thermal evolution is also consistent with cooling history of the Earth inferred from petrological observations [Herzberg et al., 2010].

As the estimates of total amount of water (at the surface and in the deep interior) vary from 5-15 ocean masses (OMs) based on magma ocean solidification models to 1.2-3.3 OMs based on petrological models [Nakagawa et al., 2018], different initial values of water are also tested, which show a strong influence on the amount of felsic melts produced. Ongoing work includes incorporating the effect of water on the density and viscosity of mantle minerals and adapting the lithospheric strength with surface topography.

How to cite: Jain, C., Sobolev, S., Sobolev, A., and Vezinet, A.: Thermochemical models of early Earth evolution constrained by geochemical data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9734, https://doi.org/10.5194/egusphere-egu24-9734, 2024.

Plate tectonics is a fundamental framework for understanding the geodynamic processes shaping our planet, including seismicity, volcanism, mountain building, and even the long-term climate system and habitability of our planet. However, how plate tectonics evolved over 4.5 billion years remains a major unanswered question. In this study, we employ dynamically self-consistent three-dimensional thermochemical geodynamic models to simulate plate tectonics evolution with physically realistic parameters. Our results demonstrate that plate tectonics undergoes three main stages due to differing dominant mantle cooling modes. Initially, magmatism dominates surface heat transport, with extrusive volcanism leading to a mobile heat-pipe mode, characterised by a high level of volcanism, large surface heat flux, and highly mobile plates in the first 1.5 billion years. This differs from the "heat-pipe" mode occurring on Jupiter's satellite Io by additionally having plate-like behaviour as well as crustal delamination and lithospheric dripping. As the mantle cools, it transitions to a stable mode where mantle convection patterns and their surface expressions become stable for around 1-2 billion years, followed by a smooth evolution to present-day plate tectonics. Our model matches key observations of the surface heat flux, strain rate, plate velocity, and plate distribution patterns, indicating that the early mobile heat pipe mode plays a crucial role in efficiently extracting heat from the mantle. Magmatic intrusion is also expected to have important effects, which we will examine. This study may provide insights into the Earth's dynamic processes and mantle-atmosphere feedback related to plate tectonics and the dynamic evolution of other terrestrial planets, which ultimately affect the long-term climate system and habitability of our planet.

How to cite: Yan, J. and Tackley, P.: How did plate tectonics evolve? Insights from 3-D spherical thermochemical convection simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11554, https://doi.org/10.5194/egusphere-egu24-11554, 2024.

The Earth is the only planet in our Solar System with active plate tectonics. Answering questions such as why plate tectonics started on Earth and which tectonic regime came before are fundamentally important for understanding the evolution of the early Earth. Currently, the most popular answers are (i) before plate tectonics on Earth, there was stagnant lid or plutonic-squishy-lid tectonic regime with no or minor contribution of subduction; (ii) plate tectonics took over when the initially high mantle temperature on Earth dropped by 100-200K due to secular cooling. In this work, we challenge both these statements based on published and new data and models.

Plenty of observations suggest that plate tectonics started on Earth during mid-Archean. At the same time, the numerical models and petrological data on the thermal evolution of the Earth show that mid-Archean was likely the time of the highest temperature of the Earth’s mantle and that significant secular cooling took place later in the Proterozoic. Moreover, previously published and our new thermochemical models also suggest that among all proposed tectonic regimes, only mobile-lid regime (i.e. plate tectonics) can lead to significant cooling of the Earth. Therefore, we conclude that plate tectonics in mid- or early Archean was unlikely to be initiated due to the significant secular cooling of the Earth’s mantle. The existence of no-subduction regimes, such as stagnant-lid or plutonic-squishy-lid, prior to plate tectonics are challenged by the new geochemical data which suggest extensive subduction and continental crust production already in the Hadean and early Archean.

Here we present global geodynamic models of Earth’s evolution computed using StagYY and ASPECT codes in 2D spherical annulus and 3D geometries respectively. StagYY models suggest that during the Hadean and the early Archean, the tectonic regime was oscillating between plume-induced subduction and plutonic-squishy-lid. The mantle temperature remains high during this time, but significant amount of continental crust is produced in these models, which is in agreement with the new geochemical data (melt inclusions from Weltevreden komatiites). After the emergence of continents in the mid- to late Archean, we decrease the effective friction of the oceanic lithosphere in the models to mimic the lubricating effect of continental sediments in subduction channels. This leads to a transition of the tectonic regime from oscillatory to continuous mobile-lid and to an efficient secular cooling of Earth, which is consistent with petrological observations. Being 2D, all plume-induced subduction zones in StagYY models are global. With the preliminary 3D ASPECT models, we show how a number of plume-induced regional subduction zones in early Earth evolve into a global network of plate boundaries and result in plate tectonics.

How to cite: Sobolev, S., Jain, C., and Pons, M.: Why does Earth have plate tectonics and what was before?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12766, https://doi.org/10.5194/egusphere-egu24-12766, 2024.

Plate tectonics is the most prominent surface manifestation of mantle convection in Earth. Current observational results suggest that Earth is the only planet displaying plate tectonics. However, whether plate tectonics has accompanied the entire evolutionary history of Earth is a key question. If not, questions such as when plate tectonics began and what kind of tectonic mode prevailed before plate tectonics, have always been at the forefront and hot topics in the field of Earth science.

In this study, we employed three-dimensional spherical mantle convection numerical simulations to explore various mantle convection modes. Results indicate that, under different parameter frameworks, mantle convection modes can be categorized into five types: I) non-plate active lid convection mode, where the surface exhibits multiple concentrated weak zones, resulting in relatively fragmented plates; II) plate-like mobile lid convection mode, characterized by a higher number of subduction zones and mid-ocean ridges on the surface, which spontaneously form, develop and disappear over time, dividing the surface into about 10 plates; III) episodic plate-like mobile lid convection mode, where the surface experiences plate-like mobile lid mode for most of the time, interspersed with transient surface stagnation; IV) episodic stagnant lid convection mode, characterized by long periods of surface stagnation interspersed with short periods of surface movement with the surface mostly featuring only one subduction zone. V) stagnant lid convection mode, where the surface appears as a single rigid layer.

We mainly analyze the influence of lithospheric strength, i.e., yielding stress, Rayleigh number and internal heat rate on these five mantle convection modes. We can better explain the plate tectonics of the present Earth using mode Ⅱ. Because of the higher internal heat rate, higher mantle temperature and lower mantle viscosity, resulting in a larger Rayleigh number, our research suggests that the early Earth was in mode III or IV. Our results suggest that even if there was some type of plate tectonics in the early Earth, it is different from present plate tectonics. Before the onset of plate tectonics, the Earth might have experienced episodic lid convection. The results hold important scientific significance for understanding the evolution of the Earth's plate tectonics.

How to cite: Xiang, S. and Huang, J.: Mantle convection modes in 3D mantle convection simulations and its implications for the evolution of Earth’s plate tectonics, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13588, https://doi.org/10.5194/egusphere-egu24-13588, 2024.

The rock and rock-ice mixtures of the core-enveloping spherical shells comprising terrestrial body interiors have thermally determined viscosities well described by an Arrhenius dependence. Accordingly, the implied viscosity contrasts determined from the activation energies (E) characterizing such bodies can reach values exceeding 10⁴⁰, for a temperature range that spans the conditions found from the lower mantle to the surface. In this study, we first explore the impact of implementing a cut-off to limit viscosity magnitude in cold regions. Using a spherical annulus geometry, we investigate the influence of core radius, surface temperature, and convective vigour on stagnant lid formation resulting from the extreme thermally induced viscosity contrasts. We demonstrate that the cut-off viscosity must be increased with decreasing curvature factor, ƒ (=r_in/r_out, where r_in and r_out are the inner and outer radii of the annulus, respectively), in order to obtain physically valid solutions. We find that for statistically-steady systems, the mean temperature decreases with core size, and that a viscosity contrast of at least 10⁷ is required for stagnant lid formation as ƒ decreases below 0.5. Inverting the results from over 80 calculations featuring stagnant lids (from a total of approximately 180 calculations), we apply an energy balance model for heat flow across the thermal boundary layers and find that the non-dimensionalized temperature in the Approximately Isothermal Layer (AIL) in the convecting layer under a stagnant lid is well predicted by T'_AIL=½{ -(2T'_out+γ) + √[γ² + 4γ(1+T'_out)] } where γ is a function of E and ƒ, and T'_out is the non-dimensionalized surface temperature. Moreover, the normalized (i.e., non-dimensional) thickness of the stagnant lid, L', can be obtained from a measurement of the non-dimensional surface heat flux once T'_AIL is determined. Stagnant-lid thicknesses increase from 10 to 30 percent of the shell thickness as ƒ is decreased, and thick lids can overlie vigorously convecting underlying layers in small core bodies, potentially delaying secular cooling and suggesting that small objects with small cores may have developed thick elastic outer shells early in the solar system's history while maintaining vigorously convecting interiors. However, we also find that for the small number of 3-D calculations that we examined, parametrizations based on 2-D calculations overestimate the temperature of the convecting layer and the thickness of the conductive lid when ƒ is small (less than 0.4).

How to cite: Javaheri, P., Lowman, J., and Tackley, P.: Spherical geometry convection in a fluid with an Arrhenius thermal viscosity dependence: the impact of core size and surface temperature on the scaling of stagnant-lid thickness and internal temperature, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3226, https://doi.org/10.5194/egusphere-egu24-3226, 2024.

How to cite: Tian, J. and Tackley, P.: The influence of deep mantle thermal conductivity on the long-term thermal evolution of Earth's mantle and core, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12993, https://doi.org/10.5194/egusphere-egu24-12993, 2024.

The Large Low-Velocity Provinces (LLVPs) beneath West Africa and the Southern Pacific are characterized by low seismic wave velocities and are associated with plate-unrelated magmatism such as hotspots, large igneous provinces, and kimberlite. Despite their significance, the structure, origin, and feeding processes of LLVPs remain elusive.

Previous studies have suggested that the LLVPs have remained stationary for over 300 million years, but their morphology appears to have changed. While geodynamic simulations favor denser LLVPs, a recent free-oscillation analysis has suggested lighter ones. The High-Velocity Region (HVR), which surrounds the LLVPs, is located beneath present and past subduction zones. Plumes of varying morphology are imaged between hotspots and LLVP margins, with intensive plumes revealing ultra-low velocity zones (ULVZs) at their roots. Ocean island basalts (OIBs) from hotspots are geochemically enriched and originate from multiple reservoirs. Interestingly, OIB chemistry does not correlate with seismic plume imaging and differs between the two LLVPs. OIB near the African LLVP is influenced by fluid-related subducted materials.

In the light of these results, I propose the following new model for the LLVPs. The LLVPs are at higher temperatures than the HVR and the surrounding mantle. They are composed of Fe-enriched mono-mineral bridgmanite rock, called bridgmanitite. The large grain size of bridgmanitite results in high viscosity despite the high temperatures, thereby stabilizing the LLVPs. The LLVPs are block-structured and the relative movement of the blocks changes the LLVP morphology. Bridgmanitite was formed by the solidification of a primordial magma ocean. Its deposition at the core-mantle boundary forms LLVP precursors in the early mantle. These LLVP precursor blocks can be moved by the thrust and sweep of subducted slabs to form the present-day LLVPs. Erosion and plume formation have reduced the volume of the LLVPs and resulted in different LLVP heights. The HVR consists of harzburgite brought from the surface by subduction. It contains only a limited amount of basaltic rock because the basaltic rock was detached from slabs in the mid-mantle due to suppressed grain growth. As a result, the HVR material is high-density due to the low temperature but is intrinsically low-density due to its chemistry. The HVR material has been heated using the LLVP heat to form plumes. Plumes are geochemically depleted when they have formed in the deep mantle. However, they are enriched in incompatible elements and volatiles in the shallow mantle. This enrichment results from melt migration due to the temperature gradient around the plume. Thus, although LLVP heat drives plume formation, the plate-unrelated magmas such as OIB are not directly derived from the LLVPs.

How to cite: Katsura, T.: Large Low-Velocity Provinces (LLVPs): a new model for their structure, origin, and evolution, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8054, https://doi.org/10.5194/egusphere-egu24-8054, 2024.

The Earth/Moon system likely results from a giant impact between a Mars-size object and the proto-Earth 70 to 110 Myrs after the formation of the first solids of the Solar System. This high-energy context leads to extreme conditions under which volatile elements would not normally be preserved in the protolunar disk. However, recent measurements of lunar samples highlight the presence of a significant amount of water in the Moon's interior (1.2 to 74 ppm). The aim of the present work is to quantify the water contribution of the late accretion on the early Moon. Here, we use a 2D axisymmetric model with the hydrocode iSALE-Dellen to study the fate of a large impactor on a target body similar to the early Moon with a crust, a magma ocean, and a mantle. For this purpose, we compute different models to monitor the depth to which the impacted material is buried at the end of the impact event and the degree of devolatilisation of the impactor. Three parameters are explored: the crustal thickness (ranging from 10 to 80 km), the impactor radius (ranging from 25 to 200 km) and the impactor velocity (ranging from 1 to 4 times the target escape velocity). Our models show that impactors with a radius greater than 50 km impacting a partially molten lunar body with a crust thinner than 40 km could significantly contribute to the water content of the lunar mantle even for impact velocities of less than 5 km s^-1. For larger impact velocities (≥ 10 km s^-1) the impactor material is significantly molten and its water content is devolatilised within the lunar atmosphere. Depending on the water content of the impactor material and the ability of the lunar magma ocean to maintain chemical heterogeneities, the late lunar accretion following the Moon-forming giant impact could explain the differences in water content between the lunar samples.

How to cite: Engels, T., Monteux, J., Boyet, M., and Bouhifd, A.: Large impacts and their contribution to the water budget of the Early Moon., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6343, https://doi.org/10.5194/egusphere-egu24-6343, 2024.

Dome-shaped uplifted and fractured terrain observed at the surface of the Moon and Mars includes floor-fractured impact craters. Such deformation features are inferred to form by the emplacement and inflation of sill- and laccolith-shaped magma bodies in the shallowest 1-2 km of a planetary body’s rocky crust. Only the final surface deformation features can be observed from space, modelling helps to understand the emplacement dynamics and the deformation of the overlying rock. A mismatch exists, however, between the complex mechanical response of host rocks to magma-induced stresses observed on Earth in exposed volcanic plumbing systems and the linearly elastic deformation assumed by most of the often-used numerical models.

We have implemented simulations of the inflation of a laccolith intrusion in a particle-based host medium in the two-dimensional (2D) Discrete Element Method (DEM). Our approach allows us to investigate magma-induced, highly discontinuous, deformation and dynamic fracturing and visualizes the localization of subsurface strain. We systematically varied a range of numerical model parameters that govern host rock strength (bond cohesion, bond tensile strength, bond elastic modulus), and specific gravity known for the Moon, Mars and Earth. For equal rock stiffness and amounts of intruded magma, our model results show that we can expect more vertical surface displacement on the Moon due to the lower gravity there compared to Mars, and Earth. Rock toughness and rock stiffness control the amount of fracturing more than gravity does.

We also tested how host rock strengths in our 2D DEM model could be upscaled from intact strengths of rock samples collected at Earth analogue sites, or by implementing a digital crack network that simulates the highly fractured conditions of the intensively impacted Lunar and Martian crusts. Our results show that laccolith inflation in pre-cracked host rocks results in higher surface displacements and a higher amount of magma-induced cracking in broader fractured zones. We expect that our model results will induce a better understanding of the emplacement and architecture of shallow magmatic intrusions below magma-induced uplifted terrain and floor-fractured craters on the Moon and Mars.

How to cite: Poppe, S., Morand, A., Harnett, C. E., Cornillon, A., Awdankiewicz, M., Heap, M., and Mège, D.: Modelling magma-induced surface uplift and dynamic fracturing around laccoliths on the Moon, Mars, and Earth, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2138, https://doi.org/10.5194/egusphere-egu24-2138, 2024.

High-resolution images taken by the Lunar Reconnaissance Orbiter show the existence of many boulder tracks within the Finsen crater. Current research suggests that shallow moonquakes and meteorite impacts are likely to be the cause of boulder falls. Based on a simplified model, we simulate the rolling process of the boulder along the slope when the lunar surface shakes and provide the critical PGA (peak ground acceleration) required for the boulder to start rolling under different conditions. The results show that boulders may roll down slopes within one or more cycles under strong ground shaking. The critical PGA of seismic waves required for a boulder to conduct slope rolling is related to the size of the boulder, the slope at the initial location, the dominant period of the seismic wave, and the ratio of horizontal and vertical peak ground accelerations. Except for rolling against the slope, in some cases, boulders may jump and roll downhill. Using the high-resolution images taken by the LRO to determine how the boulders rolled downhill, we can then estimate the lower limit of the magnitude of moonquakes in the region under different conditions. Finally, we provide a preliminary estimation of the lower limit of the paleo-moonquakes magnitude in the Finsen Crater.

How to cite: Tao, S., Shi, Y., and Zhu, B.: Magnitude estimation of Paleo-moonquakes from boulder tracks in Finsen crater, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6814, https://doi.org/10.5194/egusphere-egu24-6814, 2024.

 New high-pressure, high-temperature layered partial melting experiments have been performed to simulate the interaction between the last stage, dense Fe-Ti-rich cumulate layer that remained after about 99.5% crystallization of the lunar magma ocean (LMO), and the underlying solidified Mg-rich mantle. For this, a synthetic assemblage of an upper Fe-Ti-rich cumulate layer (6.1 wt.% TiO₂ and 39.7 wt.% FeO) and a lower forsteritic olivine layer (Mg# = 92.8) has been taken in 1:4 weight ratio, respectively, and subjected to experiments at pressures ranging between 1 to 3 GPa and temperatures varying between 1100 ⁰C and 1525 ⁰C using a piston cylinder apparatus. In the top cumulate layer, phases such as Fe-rich clinopyroxene, Fe-poor clinopyroxene, pigeonite, orthopyroxene, rutile, ilmenite, quartz and melt were formed, depending upon different P-T conditions. The Fe-Ti-rich basaltic melts (5-18.5 wt.% TiO₂, 13-28.6 wt.% FeO, and 35-59 wt.% SiO₂) produced in this cumulate layer at different degrees of partial melting approach the lunar mare basalts in their compositions and can be used to explain the huge variation in TiO₂ enrichment that is observed in lunar basalts (between 0 to ~17 wt. %). Following LMO crystallization, the last stage dense mineral-melt cumulate layer is expected to undergo a gravitational overturn due to density instability. This work aims to simulate the partial melting of the cumulates in this layer as a result of the overturn. The consequent compositional heterogeneity of the lunar mantle is used to justify the observed variation in Ti-rich basalt compositions on the lunar surface. The basaltic melts produced in these experiments are mostly Al, Mg-poor compared to available lunar basalt samples. However, this deficiency may be addressed by assimilation of these melts into low-Ti, Mg-rich basalt magmas that would have subsequently erupted from the underlying mantle. Simulations of such assimilation using thermodynamic modelling have also been done and the results support this theory. The possible fate of the last stage melt of the LMO has thus been studied and used to understand the variable compositions of lunar basalts.

How to cite: Moitra, H., Ghosh, S., Mukherjee, T., and Gupta, S.: Understanding the variability in the titanium contents of lunar basalts using high-pressure, high-temperature layered partial melting experiments and thermodynamic modelling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17525, https://doi.org/10.5194/egusphere-egu24-17525, 2024.

The InSight mission's SEIS instrument has provided a unique opportunity to probe the deep interior of Mars. This seismic exploration of the Martian interior has emerged as a promising avenue for revealing the enigmatic geophysical properties and dynamic processes within the planet's mantle.

In this study, we present an analysis of the seismic signature of marsquakes which transit deep into the mantle, providing crucial information on the seismic velocity profile and potential heterogeneities. The quakes show a characteristic late emergence of the first energy at higher frequencies which can be analysed as due to the scattering of seismic energy as it transits the mantle. From this we are able to quantify the size distribution of the mantle's small-scale heterogeneity as well as to constrain the rheological properties and convective vigor of the Martian mantle.

As unlike Earth, Mars has sealed its mantle contents under a stagnant lid, we use our observations to provide evidence about the early stages of planetary formation and differentiation on Mars. Our findings contribute to the better understanding of the Martian mantle's geodynamics and allow a comparative assessment of the evolution of planetary interiors that likely apply to other planets that lack plate tectonics.

How to cite: Charalambous, C., Pike, T., Fernando, B., Samuel, H., Bill, C., Lognonné, P., and Banerdt, B.: Small-scale Structure of the Martian Mantle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17640, https://doi.org/10.5194/egusphere-egu24-17640, 2024.

The origin of the martian crustal dichotomy is a long-standing mystery since its discovery in the Mariner 9 era. Among various proposed hypotheses, a single giant impact origin (i.e. the Borealis impact) is the most well known, and the most studied. However, studies that include realistic impact models often adapt a simplified geological and geophysical model for predicting the final crustal distribution, while long-term mantle convection studies have mostly employed an over-simplified parametrization of the impact. Here we use a coupled SPH-thermochemical approach to first simulate an impact event, and then use the result of this realistic model as the initial condition for the long-term mantle convection model. We demonstrate that a giant impact collision results in a mantle-deep magma pond, which upon crystallisation leads to a thicker crust production on the impacted hemisphere. In other words, an impact-origin of Mars's southern highlands requires the giant impact to occur in the southern hemisphere. We find that both the impact scenario and the mantle properties affect the geometry of the impact-induced crust ("highlands'') and the subsequent state of the interior, and that the formation of "highlands'' extends beyond the initial magma pond.  We show that a near head-on (15^o from the normal) impact event with impactor radius of 750 km, together with a mantle viscosity of 10²⁰ Pa s, can best reproduce the southern highlands of Mars with a geometry similar to that of present-day observations.

How to cite: Cheng, K. W., Ballantyne, H., Golabek, G., Jutzi, M., Rozel, A., and Tackley, P.: Combined impact and interior evolution models in three dimensions indicate a southern impact origin of the Martian Dichotomy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22090, https://doi.org/10.5194/egusphere-egu24-22090, 2024.

One of the most informative ways of studying the interior structure and geodynamics of terrestrial planets is the joint investigation of gravity and topography data. In the case of Venus, this is in fact one of the only sources of information about the planet's interior, along with estimations of the tidal Love number [1], moment of inertia factor [2], and the absence of an internally generated magnetic field [3].

Early gravity and topography studies that made use of Pioneer Venus and later Magellan data revealed unique properties of Venus's interior. They showed that Venus has a notably higher gravity-topography correlation for long wavelengths compared to Earth and a globally large apparent depth of compensation [4]. Considering these characteristics and analyzing the wavelength-dependent ratio between gravity and topography, the so-called spectral admittance, [5] concluded that the long-wavelength topography of the planet was supported dynamically, i.e., through convection in the mantle.

Since then, several studies have investigated the gravity and topography signature of Venus with the goal of understanding the planet’s interior by constraining geophysical properties of the mantle, such as the mantle viscosity. Some estimated the dynamic geoid contribution from three-dimensional geodynamic simulations [6,7]. Others adopted the analytical viscous flow model by [11] to study the viscosity structure of Venus's mantle [8,9,10]. The main advantage of the latter method is that it is computationally inexpensive, allowing for the performance of inversions. However, it is a simplified model which neglects lateral viscosity variations.

In this study, we estimate the dynamic topography and geoid signatures from the geodynamical models by [12], which include a strongly temperature-dependent viscosity, hence lateral viscosity variations, to evaluate the influence of different parameters such as the increase of viscosity with depth, the presence of viscosity jumps, and the ratio between intrusive and extrusive magmatism. In a second step, we plan to systematically evaluate the influence of lateral viscosity variations on the geoid and topography and thus to quantify the importance of the simplifications adopted in the analytical model.

Our first results show that the increase of viscosity with depth should be no more than 2 orders of magnitude, since larger values strongly decrease the spectral correlation and admittance at long wavelengths which is inconsistent with the observations. In addition, scenarios where extrusive magmatism dominates tend to overestimate the admittance due to the generation of thick thermal lithospheres in excess of 300 km thickness. These results underscore the importance of gravity and topography analyses for deciphering the geodynamical evolution and tectonic style of Venus.

[1] Konopliv and Yoder (1996) GRL, 23; [2] Margot et al. (2021) Nat Astron; [3] Phillips and Russel (1987) JGR, 92; [4] Sjogren et al. (1980) JGR, 85;  [5] Kiefer et al. (1986) GRL, 13; [6] Huang et al. (2013) EPSL, 362; [7] Rolf et al. (2018) Icarus, 313; [8] Pauer et al. (2006) JGR, 111; [9] Steinberger et al. (2010) Icarus, 207; [10] Maia et al. (2023) GRL, 50; [11] Hager & Clayton (1989) Mantle Convection; [12] Plesa et al. (2023) EGU.

How to cite: Maia, J., Plesa, A.-C., Tosi, N., and Wieczorek, M.: Constraining Venus's interior with gravity and topography predictions from geodynamic models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15952, https://doi.org/10.5194/egusphere-egu24-15952, 2024.

So far, most numerical models focused on understanding different aspects of the Earth’s system, such as mantle convection, tectonics, or atmospheric dynamics, have typically adopted a limited perspective constrained to the specific region of interest. Recently, efforts have been made to couple these simulations, enabling them to interact seamlessly with one another. Abstaining from doing so may lead to results that don't accurately represent how the various systems interact.

Achieving the successful integration of the deep carbon cycle and water transport into our mantle dynamics code (StagYY) represents a crucial step towards this goal - a comprehensive, large-scale model of our planet, encompassing phenomena from the Earth's core to the uppermost layers of the atmosphere resulting from a collaborative effort between different research groups. We adopt a thermodynamic approach to quantify the H₂O solubility of important water-carrying minerals within the mantle, with the goal of faithfully coupling geodynamic models and realistic transport/incorporation of water across the silicate part of our planet. The details of the numerical implementation are yet the subject of ongoing discussion; however, several crucial considerations must be thoroughly evaluated. These include the assessment of density variations resulting from water integration into nominally anhydrous minerals, the complex multi-stage degassing process of subducting slabs, and the inclusion of exotic phases that are currently absent from the thermodynamic databases being utilized.

The anticipated effects of a water-bearing mantle on the overarching dynamics are still under investigation. Nevertheless in-gassing, degassing, and the internal transport of water within the mantle exert a direct influence on various aspects. These include the governing viscosity field, the extent of H₂O-induced melting, and atmospheric CO₂ concentrations, among others. If water can, in fact, permeate deep into the mantle, it has the potential to introduce significant deviations in the dynamics of lower mantle convection compared to what current models predict. Furthermore, recent considerations of the stability regime of hydrous phases also point towards interesting implications in the study of water-rich exoplanets as stronger gravity profiles should result in colder geotherms, significantly expanding the thermodynamical stability of water-transporting phases across the P-T parameter space.

Quantifying this process will provide valuable insights for the geodynamic modeling community and help advance our understanding of the deep-Earth system. Furthermore, the distinct chemical exchange dynamics arising from our applications can be investigated further and prove advantageous for the study of exoplanetary atmospheres, especially those around bodies characterized by abundant water, such as water worlds and hycean planets.

How to cite: Moccetti Bardi, N., Tackley, P. J., and Metternich, M. A.: Mantle hydration and implications for Earth and exoplanetary research, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16140, https://doi.org/10.5194/egusphere-egu24-16140, 2024.

The radial density of planets increases with depth due to compressibility, leading to impacts on their convective dynamics. To account for these effects, including the presence of a quasi-adiabatic temperature profile and entropy sources due to dissipation, the compressibility is expressed through a dissipation number proportional to the planet's radius and gravity. In Earth's mantle, compressibility effects are moderate, but in large rocky or liquid exoplanets (super-earths), the dissipation number can become very large. We explore the properties of compressible convection when the dissipation number is significant. We start by selecting a simple Murnaghan equation of state that embodies the fundamental properties of condensed matter at planetary conditions. Next, we analyze the characteristics of adiabatic profiles and demonstrate that the ratio between the bottom and top adiabatic temperatures is relatively small and probably less than 2. We examine the marginal stability of compressible mantles and reveal that they can undergo convection with either positive or negative superadiabatic Rayleigh numbers. Lastly, we delve into simulations of convection in 2D Cartesian geometry performed using the exact equations of mechanics, neglecting inertia (infinite Prandtl number case), and examine their consequences for super-earth dynamics.

How to cite: Ricard, Y., Alboussière, T., and Labrosse, S.: Compressible convection in large rocky planets, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9939, https://doi.org/10.5194/egusphere-egu24-9939, 2024.

Outgassing from the interior is a key process influencing the evolution of the atmospheres of rocky planets. For planets with a stagnant lid tectonic mode, previous models have indicated that increasing planet size very strongly reduces the amount of outgassing, even to zero above a certain planet mass (Dorn et al., A&A 2018). This is because melt is buoyant only above a certain depth, which becomes shallower with increasing planet size (hence "g"); for large enough planets this depth may even lie within the lithosphere, preventing eruption and outgassing.

            However, uncertainties in rheology strongly influence the temperature structure of planets, hence (i) the depth at which melt is generated and (ii) the thickness of the lithosphere. One major uncertainty is the rheology of post-perovskite, which constitutes a large fraction of the mantle in large super-Earths. Ammann et al. (Nature 2010) find that diffusion is anisotropic; it is not clear whether the "upper bound" or "lower bound" is relevant to large-scale deformation, but both result in high viscosity at very high pressures, strongly influencing the radial temperature profile (Tackley et al., Icarus 2013). In contrast, Karato (2011, Icarus) argues that a different mechanism - interstitial diffusion - acts to make viscosity almost independent of pressure and relatively low in the post-perovskite regime.

            Another uncertainty is the reference viscosity (the viscosity at a reference temperature, pressure and stress), as this depends on bulk composition, water content, grain size and other properties. Lower reference viscosity results in thinner lithosphere and crust (e.g., Armann & Tackley, JGR 2012).

            Thus, numerical simulations are performed of the long-term (10 Gyr) thermo-chemical evolution of stagnant-lid planets (coupled mantle and core) with masses between 1 to 10 Earth masses, varying the reference viscosity and the rheology of post-perovskite. The simulations are based on the setup of Tackley et al. (2013 Icarus) with the addition of partial melting and basaltic crustal production, and outgassing of a passive tracer that partitions into the melt and outgasses 100% upon eruption.

            Results indicate that:

• the previously-found trend of lower percentage outgassing with larger planet size is reproduced, but
• outgassing does not fall to zero even in a 10 Earth mass planet. Outgassing of between 15% and 70% is found for 10 Earth mass planets (up to ~100% for Earth mass planets).
• Post-perovskite rheology (interstitial, lower-bound or upper-bound) makes only a minor difference to long-term outgassing, but does influence the timing of outgassing.
• Reference viscosity makes a large difference to outgassing, with lower viscosities leading to substantially larger outgassing percentages.
• Internal heating plays a major role: stagnant-lid planets initially heat up due to low heat transfer efficiency, thinning the lithosphere and producing widespread melting.

How to cite: Tackley, P.: Outgassing on Stagnant-Lid Planets: Influence of Rheology, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13059, https://doi.org/10.5194/egusphere-egu24-13059, 2024.