Marla Metternich

Earth’s interior plays a fundamental role in the long-term evolution of the surface, climate, and biosphere. However, Earth's mantle evolution remains largely ambiguous, as imaging techniques are limited to present-day observations and geochemical or geological constraints apply to a non-global scale. Plate tectonic reconstructions coupled with convection models could provide constraints on the evolution of mantle structure. In this study, we employ both fully self-consistent and kinematically constrained mantle convection models[1]. The mantle rheology is temperature-, pressure-, phase-, and stress-dependent, with the latter represented through pseudo-plasticity. The novelty of this work lies in employing a composite rheology with “realistic” rheological parameters[2] in a fully three-dimensional geometry. By using both fully self-consistent models and plate-driven models, we aim to address the discrepancies in terms of long-term convective and tectonic behaviour that arise when forcing plate velocities onto the surface. The latter is done by imposing time-dependent surface velocity boundary conditions provided by a plate tectonic reconstruction[3].To evaluate the extent to which the models reproduce plate-like tectonics, we explore several independent constraints. In particular, we compute slab sinking rates and compare them to estimates inferred from seismic tomography[4]. Slab sinking rates in self-consistent models provide insight into the mantle’s rheology. For example, sinking rates that are lower than those based on tomographic and geological data may indicate an overly viscous mantle. Our results show that the slab sinking rate is generally higher in models with imposed plate velocities compared to fully self-consistent models. Furthermore, a tessellation algorithm[5] will be applied to the surface of the models to detect plates in the self-consistent models with plate-like behaviour. Based on these results, a plate-size frequency distribution can be calculated and compared to present-day Earth[6]. Results show that low yield stresses generate too many small plates, and too few large plates [Fig. 1]. In order to generate Earth-like plate tectonics, yield stress needs to be sufficiently high to reproduce the plate-size frequency distribution of present-day Earth, but also sufficiently low to facilitate a long-term mobile lid regime.[1] Tackley, P. J. (2008). Phys. Earth Planet. Inter. 171, 1–4.[2] Tackley, P. J., Ammann, M., Brodholt, J. P., Dobson, D. P., & Valencia, D. (2013). Icarus 225, 50–61.[3] Merdith, A. S., Williams, S. E., Collins, A. S., et al. (2021). Earth-Sci. Rev. 214, 103477.[4] Van der Meer, D. G., van Hinsbergen, D. J. J., & Spakman, W. (2018). Tectonophysics 723, 309–448.[5] Janin, A., Coltice, N., Chamot-Rooke, N., & Tierny, J. (2025). Nat. Geosci. 18, 1041–1047.[6] Bird, P. (2003). Geochem. Geophys. Geosyst. 4, 2001GC000252.

Earth’s interior plays an important role in the long-term evolution of the surface, climate and biosphere. Rheology is the cornerstone of mantle convection and tectonics, and constraining mantle viscosity has been a priority in the geodynamic community. In this study, we employ fully self-consistent and three-dimensional Earth-like mantle convection models[1]. The mantle rheology is temperature-, pressure- and stress-dependent. Plate-like behaviour in global mantle models can be obtained using a pseudo-plastic rheology[2]. Rheology in some of our models also depends on phase and creep mechanism. As in previous work[3], this is implemented by using laboratory values for activation energy and activation volume for the upper mantle and an analytical fit to experimental data for the lower mantle. The novelty of this work lies in employing a composite rheology with “realistic” rheological parameters in a fully three-dimensional geometry. Using these more realistic models, we aim to improve our understanding of mantle rheology in the context of self-consistent generation of plate-like behaviour. To achieve this, slab sinking rates will be computed that can be compared to estimates based on tomography[4], which is a relatively new source of constraint[5]. The tectonic mode depends on the plastic yield stress. In turn, the yield stress parameter space for a plate-like regime depends on whether continents, phase-dependent rheology and dislocation creep are considered. Thus, the yield stress and reference viscosity parameter spaces must first be explored for each rheological model. Generally, we observe that lower yield stresses lead to higher surface mobilities. On top of high surface mobility (deformation), plate-like behaviour asks for localisation of deformation in narrow zones. Plateness is a widely used measure for this, which we find to be high for models with sufficiently low yield stresses. Furthermore, preliminary results show that models with phase-dependent rheology are more likely to be in a plate-like regime compared to models without a viscosity jump between the upper and lower mantle. Lastly, we hypothesise that the slab sinking speed may be highly sensitive to rheology and may be affected by the presence of continents. [1] Tackley, P. J. (2008). Modelling compressible mantle convection with large viscosity contrasts in a three-dimensional spherical shell using the yin-yang grid. Physics of the Earth and Planetary Interiors, 171(1–4), Article 1–4.[2] Moresi, L., & Solomatov, V. (1998). Mantle convection with a brittle lithosphere: Thoughts on the global tectonic styles of the Earth and Venus. Geophysical Journal International, 133(3), 669–682.[3] Tackley, P. J., Ammann, M., Brodholt, J. P., Dobson, D. P., & Valencia, D. (2013). Mantle dynamics in super-Earths: Post-perovskite rheology and self-regulation of viscosity. Icarus, 225(1), 50–61.[4] Van der Meer, D. G., Van Hinsbergen, D. J., & Spakman, W. (2018). Atlas of the underworld: Slab remnants in the mantle, their sinking history, and a new outlook on lower mantle viscosity. Tectonophysics, 723, 309-448.[5] Van Der Wiel, E., Van Hinsbergen, D. J. J., Thieulot, C., & Spakman, W. (2024). Linking rates of slab sinking to long-term lower mantle flow and mixing. Earth and Planetary Science Letters, 625, 118471.

Observations of Venus imply ongoing tectonic and volcanic activity, suggesting the planet is dynamically active[1,2]. Tectonically altered regions, such as ridges or tesserae, indicate surface mobility. However, unlike Earth, no evidence of active plate tectonics has been identified. The tectonics and volcanism of terrestrial planets are closely tied to active mantle convection modes. Rheology, a crucial element in tectonics, is influenced by the presence of water[3]. Despite this, the impact of water has largely been overlooked in Venus studies, as its interior is typically assumed to be dry. This assumption is being challenged by indications of significant hydrodynamic escape into space, requiring volcanic replenishment. Consequently, water is likely still present in Venus' interior, even if the concentrations are unknown. Importantly, the potential effects of water on Venus' dynamics and evolution remain poorly understood. The interplay between water, mantle dynamics, and volcanic activity would likely contribute to a more comprehensive understanding of Venus' evolution. This underlines the need to consider complex dynamic thermo-magmatic models that account for water, including composition-dependent finite water solubilities. In this study, we use the numerical code StagYY to perform state-of-the-art 2D models in a spherical annulus geometry to assess the effects of water on the tectono-magmatic evolution of Venus[4,5]. Particular attention will be given to the way water influences mantle convection and tectonics. Indeed, results show that the presence of water can dramatically change the geodynamic regime through the rheology, melting and outgassing. With the introduction of composition-dependent water solubility maps, dehydration processes will redistribute water throughout the mantle[6]. Since water content is directly related to the viscosity structure, the convective regime is expected to change as well. The main question we want to address is how dehydration processes and water distribution influence the convective and tectonic regimes of Venus. Studying the impact of water on Venus's interior may not only unveil insights into its tectonic evolution but also sets the stage for crucial future research, advancing our broader understanding of planetary processes and habitability. [1] Smrekar, S. E., Stofan, E. R., Mueller, N., Treiman, A., Elkins-Tanton, L., Helbert, J., ... & Drossart, P. (2010). Recent hotspot volcanism on Venus from VIRTIS emissivity data. Science, 328(5978), 605-608.[2]Gülcher, A. J., Gerya, T. V., Montési, L. G., & Munch, J. (2020). Corona structures driven by plume–lithosphere interactions and evidence for ongoing plume activity on Venus. Nature Geoscience, 13(8), 547-554.[3]Karato, S. I. (2015). Water in the evolution of the Earth and other terrestrial planets. Treatise on Geophysics, 9, 105-144.[4] Tackley, P. J. (2008). Modelling compressible mantle convection with large viscosity contrasts in a three-dimensional spherical shell using the yin-yang grid. Physics of the Earth and Planetary Interiors, 171(1-4), 7-18.[5]Tian, J., Tackley, P. J., & Lourenço, D. L. (2023). The tectonics and volcanism of Venus: New modes facilitated by realistic crustal rheology and intrusive magmatism. Icarus, 399, 115539.[6]Nakagawa, T. (2017). On the numerical modeling of the deep mantle water cycle in global-scale mantle dynamics: The effects of the water solubility limit of lower mantle minerals. Journal of Earth Science, 28(4), Article 4. https://doi.org/10.1007/s12583-017-0755-3

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.

Observations of Venus reveal tectonic expressions and recent volcanism, showing that the planet is still active. Tectonically deformed areas such as ridges or tesserae indicate surface mobility, however, no signs of active plate tectonics like on Earth have been found. The tectonics and volcanism of Venus and other terrestrial planets are defined by the active mantle convection mode. A key component of tectonics is rheology, which is affected by water as shown by numerous studies[1].  However, the effects of water have been mostly ignored when studying Venus because its interior has been assumed to be dry. This notion is being challenged by indications of strong hydrodynamic escape to space that requires volcanic replenishment[2]. Therefore, water should be present in Venus’ interior, even if its content is not known. Importantly, the potential effects of water in the dynamics and evolution of Venus are poorly understood. This calls for the consideration of complex dynamic thermo-magmatic models that track water and take into account intrusive and extrusive magmatism. In this study, we use the code StagYY to perform state-of-the-art 2D numerical models in a spherical annulus geometry to assess the effects of water on the tectono-magmatic evolution of Venus[3]. Particular attention will be given to changes in mantle viscosity, melt generation and crustal properties such as thickness and surface age. We explore model settings related to melting, intrusive magmatism, and water presence. Results show that intrusion depth influences the thermal evolution and related magmatism. Moreover, preliminary results show that the rate of water outgassing is directly related to changes in the thermo-magmatic evolution of Venus. Water outgassing rates have further implications on surface conditions and atmospheric compositions over time. In the future, coupling these improved mantle convection models to atmospheric evolution models may unveil new insights into the thermal and tectonic history that has shaped Venus into the planet we observe today.