Mineral phase transitions can either hinder or accelerate mantle flow. In the present day, the formation of the bridgmanite + ferropericlase assemblage from ringwoodite at 660 km depth has been found to cause weak and intermittent layering of mantle convection. However, for the higher temperatures in Earth’s past, different phase transitions could have controlled mantle dynamics. We investigate the potential changes in convection style during Earth’s secular cooling using a new numerical technique that reformulates the energy conservation equation in terms of specific entropy instead of temperature. This approach enables us to accurately include the latent heat effect of phase transitions for mantle temperatures different from the average geotherm, and therefore fully incorporate the thermodynamic effects of realistic phase transitions in global-scale mantle convection modeling. We set up 2-D models with the geodynamics software ASPECT, using thermodynamic properties computed by HeFESTo, while applying a viscosity profile constrained by the geoid and mineral physics data and a visco-plastic rheology to reproduce self-consistent plate tectonics and Earth-like subduction morphologies. Our model results reveal the layering of plumes induced by the wadsleyite to garnet (majorite) + ferropericlase endothermic transition (between 420–600 km depth and over the 2000–2500 K temperature range). They show that this phase transition causes a large-scale and long-lasting temperature elevation in a depth range of 500–650 km depth if the potential temperature is higher than 1800 K, indicating that mantle convection may have been partially layered in Earth’s early history.

The interaction between aging oceanic plates and their underlying mantle is a crucial component of the plate tectonic cycle. Sub-lithospheric small-scale convection explains why plates appear not to thicken after a certain age. Yet, many open questions still surround this process. Here, we link grain-scale processes, dynamic models of asthenospheric flow, and seismic observations to gain new insights into the mechanisms of small-scale convection. We present high-resolution 3D geodynamic models of oceanic plate evolution using the community modeling software ASPECT. These simulations use an Earth-like rheology including coupled diffusion and dislocation creep as well as their interplay with evolving olivine grain size. Our models quantify how the balance between diffusion and dislocation creep affects the morphology and temporal stability of small-scale sub-lithospheric convection, including its onset age, the average depth and wavelength of the small-scale convection rolls, and the amplitude of the temperature and grain size anomalies within the rolls. We directly relate these quantities predicted by the dynamic models to geophysical observables through laboratory-derived constitutive relations, converting variations in temperature, pressure, grain size, water content and stable melt fraction to seismic velocity and attenuation. By creating synthetic seismic tomography models of different dynamic scenarios and comparing them to observations from the Pacific OBS Research into Convecting Asthenosphere (ORCA) experiment, we determine the parameter range in which geodynamic models fit these seismic observations. This provides new constraints on oceanic asthenosphere rheology beneath this part of the Pacific Plate, with potentially broad implications for Earth dynamics.

Simulating present-day solid Earth deformation and volatile cycling requires integrating diverse geophysical datasets and advanced numerical techniques to model complex multiphysics processes at high resolutions. Subduction zone modeling is particularly challenging due to the large geographic extent, localized deformation zones, and the strong feedbacks between reactive fluid transport and solid deformation. Here, we develop new workflows for simulating 3-dimensional thermal-mechanical subduction and patterns of volatile dehydration at convergent margins, adaptable to include reactive fluid transport. We apply these workflows to the Hikurangi margin, where recent geophysical investigations have offered unprecedented insight into the structure and processes coupling fluid transport and solid deformation across broad spatiotemporal scales. Geophysical data sets constraining the downgoing and overriding plate structure are collated with the Geodynamic World Builder, which provides the initial conditions for forward simulations using the open-source geodynamic modeling software ASPECT. We systematically examine how plate interface rheology and hydration of the downgoing plate and upper mantle influence Pacific–Australian convergence and seismic anisotropy. Models prescribing a dry rheology and a plate boundary viscosity of 5e20 Pa s best reproduce observed plate velocities. Conversely, models considering hydrated olivine fabrics best reproduce observations of seismic anisotropy. Predicted patterns of slab dehydration and mantle melting correlate well with observations of seismic attenuation and arc volcanism. These results suggest that hydration-related rheological heterogeneity and related fluid weakening may strongly influence slab dynamics. Future investigations integrating coupled fluid transport and global mantle flow will provide insight into the feedbacks between subduction dynamics, fluid pathways, and arc volcanism.

Mantle convection models based on geophysical constraints have provided us with a basic understanding of the forces driving and resisting plate motions on Earth. However, existing studies computing the balance of underlying forces are contradicting, and the impact of plate boundary geometry on surface deformation remains unknown. We address these issues by developing global instantaneous 3-D mantle convection models with a heterogeneous density and viscosity distribution and weak plate boundaries prescribed using different geometries. We find that the plate boundary geometry of the Global Earthquake Model (GEM, Pagani et al., 2018), featuring open plate boundaries with discrete lithospheric-depth weak zones in the oceans and distributed crustal faults within continents, achieves the best fit to the observed GPS data with a directional correlation of 95.1% and a global point-wise velocity residual of 1.87 cm/year. A good fit also requires plate boundaries being 3 to 4 orders of magnitude weaker than the surrounding lithosphere and low asthenospheric viscosities between 5e17 and 5e18 Pa s. Models without asthenospheric and lower mantle heterogeneities retain on average 30% and 70% of the plate speeds, respectively. Our results show that Earth’s plate boundaries are not uniform and better described by more discrete plate boundaries within the oceans and distributed faults within continents. Furthermore, they emphasize the impact of plate boundary geometry on the direction and speed of plate motions and reaffirm the importance of slab pull in the uppermost mantle as a major plate driving force.

Mantle plumes are thought to recycle material from the Earth’s deep interior. One constraint on the nature and quantity of this recycled material comes from the observation of seismic discontinuities. The detection of the X-discontinuity beneath Hawaii, interpreted as the coesite-stishovite transition, requires the presence of at least 40% basalt. However, previous geodynamic models have predicted that the percentage of high-density basaltic material that mantle plumes can carry to the surface is no higher than 15–20%. We propose this contradiction can be resolved by taking into account the length scale of chemical heterogeneities. While previous modeling studies assumed mechanical mixing on length scales smaller than the model resolution, we here model basaltic heterogeneities with length scales of 30–40~km, allowing for their segregation relative to the pyrolitic background plume material. Our models show that larger basalt fractions than previously thought possible—exceeding 40%—can accumulate within plumes at the depth of the X-discontinuity. Two key mechanisms facilitate this process: (1) The random distribution of basaltic heterogeneities induces large temporal variations in the basalt fraction with cyclical highs and lows. (2) The high density contrast between basalt and pyrolite below the coesite-stishovite transition causes ponding and accumulation of basalt at that depth, an effect that only occurs for intermediate viscosities of pyrolite. These results further constrain the chemical composition of the Hawaiian plume. Beyond that, they provide a geodynamic mechanism that explains the seismologic detection of the X-discontinuity and highlights how recycled material is carried towards the surface.