The Diversity of Exoplanets: From Interior Dynamics to Surface Expressions

The coupled interior–atmosphere system of terrestrial exoplanets remains poorly understood. Exoplanets show a wide variety of sizes, densities, surface temperatures, and interior structures, with important knock-on effects for this coupled system. Many exoplanets are predicted to have a “stagnant lid” at the surface, with a rigid stationary crust, sluggish mantle convection, and only minor volcanism. However, if exoplanets have Earth-like plate tectonics, which involves several discrete, slowly moving plates and vigorous tectono-magmatic activity, then this may be critical for planetary habitability and have implications for the development (and evolution) of life in the galaxy. Here, we summarize our current knowledge of coupled planetary dynamics in the context of exoplanet diversity

The evolution and distribution of recycled oceanic crust in the Earth's mantle: Insight from geodynamic models

A better understanding of the Earth's compositional structure is needed to place the geochemical record of surface rocks into the context of Earth accretion and evolution. Cosmochemical constraints imply that lower-mantle rocks may be enriched in silica relative to upper-mantle pyrolite, whereas geophysical observations support whole-mantle convection and mixing. To resolve this discrepancy, it has been suggested that subducted mid-ocean ridge basalt (MORB) segregates from subducted harzburgite to accumulate in the mantle transition zone (MTZ) and/or the lower mantle. However, the key parameters that control basalt segregation and accumulation remain poorly constrained. Here, we use global-scale 2D thermochemical convection models to investigate the influence of mantle-viscosity profile, planetary-tectonic style and bulk composition on the evolution and distribution of mantle heterogeneity. Our models robustly predict that, for all cases with Earth-like tectonics, a basalt-enriched reservoir is formed in the MTZ, and a harzburgite-enriched reservoir is sustained at 660∼800 km depth, despite ongoing whole-mantle circulation. The enhancement of basalt and harzburgite in and beneath the MTZ, respectively, are laterally variable, ranging from ∼30% to 50% basalt fraction, and from ∼40% to 80% harzburgite enrichment relative to pyrolite. Models also predict an accumulation of basalt near the core mantle boundary (CMB) as thermochemical piles, as well as moderate enhancement of most of the lower mantle by basalt. While the accumulation of basalt in the MTZ does not strongly depend on the mantle-viscosity profile (explained by a balance between basalt delivery by plumes and removal by slabs at the given MTZ capacity), that of the lowermost mantle does: lower-mantle viscosity directly controls the efficiency of basalt segregation (and entrainment) near the CMB; upper-mantle viscosity has an indirect effect through controlling slab thickness. Finally, the composition of the bulk-silicate Earth may be shifted relative to that of upper-mantle pyrolite, if indeed significant reservoirs of basalt exist in the MTZ and lower mantle

Variable dynamic styles of primordial heterogeneity preservation in the Earth's lower mantle

The evolution of the system Earth is critically influenced by the long-term dynamics, composition and structure of the mantle. While cosmochemical and geochemical constraints indicate that the lower mantle hosts an ancient primordial reservoir that may be enriched in SiO2 with respect to the upper mantle, geophysical observations and models point to efficient mass transfer and convective mixing across the entire mantle. Recent hypotheses of primordial-material preservation in a convecting mantle involve delayed mixing of intrinsically dense and/or intrinsically strong heterogeneity. Yet, the effects of composition-dependent rheology and density upon heterogeneity preservation and the dynamics of mantle mixing remain poorly understood. Here, we present two-dimensional numerical models in spherical geometry, investigating the preservation styles of primordial material as a function of its physical properties (i.e., viscosity and density contrasts). We establish multiple regimes of primordial-material preservation that can occur in terrestrial planets. These include (1) efficient mixing, (2) double-layered convection with or without topography, and (3) variable styles of partial heterogeneity preservation (e.g., as diffuse domains, piles or viscous blobs in the lower mantle). Some of these regimes are here characterised for the first time, and all regimes are put into context with each other as a function of model parameters. The viscous-blobs and diffuse-domains regimes can reconcile the preservation of primordial domains in a convecting mantle, potentially resolving the discrepancy between geochemical and geophysical constraints for planet Earth. Several, if not all, regimes characterised here may be relevant to understand the long-term evolution of terrestrial planets in general

Mantle melting and intraplate volcanism due to self‐buoyant hydrous upwellings from the stagnant slab that are conveyed by small‐scale convection

The mechanisms sustaining basaltic continental intraplate volcanism remain controversial. Continental intraplate volcanism is often geographically associated with slab stagnation in the mantle transition zone (MTZ), for example, in eastern Asia, central Europe, and western North America. Using 2‐D geodynamic models, we here explore the role of the stagnation of a slab and an associated hydrous layer in the MTZ on the formation and evolution of intraplate volcanism. Due to the intrinsic buoyancy of the hydrous layer atop the stagnant slab, upwellings develop within a few million years and rise to ~410‐km depth. At these depths, they partly lose their intrinsic buoyancy due to dehydration and stall intermittently. However, they are readily entrained by sublithospheric small‐scale convection to reach the base of lithosphere, sustaining mantle melting and intraplate volcanism. Water contents of >0.3 wt.‐% in a ≥ 60‐km‐thick layer atop the slab are sufficient for an early (<~20 Myr) onset of melting to account for volcanism, for example, in NE China. Thus, significant amounts of hydrous materials are not expected to remain stable in the MTZ for geological timescales, consistent with geophysical estimates. To explain the geochemical signatures of the Cenozoic basaltic volcanism in northern China, a mixed composition of the hydrous layer, including an enriched mantle‐type and a hybrid depleted mid‐ocean ridge basalts mantle/high μ‐type component, is required

Plutonic‐Squishy Lid: A New Global Tectonic Regime Generated by Intrusive Magmatism on Earth‐Like Planets

The thermal and chemical evolution of rocky planets is controlled by their surface tectonics and magmatic processes. On Earth, magmatism is dominated by plutonism/intrusion versus volcanism/extrusion. However, the role of plutonism on planetary tectonics and long‐term evolution of rocky planets has not been systematically studied. We use numerical simulations to systematically investigate the effect of plutonism combined with eruptive volcanism. At low‐to‐intermediate intrusion efficiencies, results reproduce the three common tectonic/convective regimes as are usually obtained in simulations using a viscoplastic rheology: stagnant‐lid (a one‐plate planet), episodic (where the lithosphere is usually stagnant and sometimes overturns into the mantle), and mobile‐lid (similar to plate tectonics). At high intrusion efficiencies, we observe a new additional regime called “plutonic‐squishy lid.” This regime is characterized by a set of small, strong plates separated by warm and weak regions generated by plutonism. Eclogitic drippings and lithospheric delaminations often occur close to these weak regions, which leads to significant surface velocities toward the focus of delamination, even if subduction is not active. The location of the plate boundaries is strongly time dependent and mainly occurs in regions of magma intrusion, leading to small, ephemeral plates. The plutonic‐squishy‐lid regime is also distinctive from other regimes because it generates a thin lithosphere, which results in high conductive heat fluxes and lower internal mantle temperatures when compared to a stagnant lid. This regime has the potential to be applicable to the Early Archean Earth and present‐day Venus, as it combines elements of both protoplate tectonic and vertical tectonic models

The influence of bulk composition on the long-term interior-atmosphere evolution of terrestrial exoplanets

Aims: The secondary atmospheres of terrestrial planets form and evolve as a consequence of interaction with the interior over geological time. We aim to quantify the influence of planetary bulk composition on the interior–atmosphere evolution for Earth-sized terrestrial planets to aid in the interpretation of future observations of terrestrial exoplanet atmospheres. Methods: We used a geochemical model to determine the major-element composition of planetary interiors (MgO, FeO, and SiO2) following the crystallization of a magma ocean after planet formation, predicting a compositional profile of the interior as an initial condition for our long-term thermal evolution model. Our 1D evolution model predicts the pressure–temperature structure of the interior, which we used to evaluate near-surface melt production and subsequent volatile outgassing. Volatiles are exchanged between the interior and atmosphere according to mass conservation. Results: Based on stellar compositions reported in the Hypatia catalog, we predict that about half of rocky exoplanets have a mantle that convects as a single layer (whole-mantle convection), and the other half exhibit double-layered convection due to the presence of a mid-mantle compositional boundary. Double-layered convection is more likely for planets with high bulk planetary Fe-content and low Mg/Si-ratio. We find that planets with low Mg/Si-ratio tend to cool slowly because their mantle viscosity is high. Accordingly, low-Mg/Si planets also tend to lose volatiles swiftly through extensive melting. Moreover, the dynamic regime of the lithosphere (plate tectonics vs. stagnant lid) has a first-order influence on the thermal evolution and volatile cycling. These results suggest that the composition of terrestrial exoplanetary atmospheres can provide information on the dynamic regime of the lithosphere and the thermo-chemical evolution of the interior

A poorly mixed mantle transition zone and its thermal state inferred from seismic waves

The abrupt changes in mineralogical properties across the Earth’s mantle transition zone substantially impact convection and thermochemical fluxes between the upper and lower mantle. While the 410-km discontinuity at the top of the mantle transition zone is detected with all types of seismic waves, the 660-km boundary is mostly invisible to underside P-wave reflections (P660P). The cause for this observation is debated. The dissociation of ringwoodite and garnet into lower-mantle minerals both contribute to the ‘660’ visibility; only the garnet reaction favours material exchanges across the discontinuity. Here, we combine large datasets of SS and PP precursors, mineralogical modelling and data-mining techniques to obtain a global thermal map of the mantle transition zone, and explain the lack of P660P visibility. We find that its prevalent absence requires a chemically unequilibrated mantle, and its visibility in few locations is associated with potential temperatures greater than 1,800 K. Such temperatures occur in approximately 0.6% of Earth, indicating that the 660 is dominated by ringwoodite decomposition, which tends to impede mantle flow. We find broad regions with elevated temperatures beneath the Pacific surrounded by major volcanic hotspots, indicating plume retention and ponding of hot materials in the mantle transition zone

Narrow, Fast, and "Cool" Mantle Plumes Caused by Strain-Weakening Rheology in Earth's Lower Mantle

The rheological properties of Earth's lower mantle are key for mantle dynamics and planetary evolution. The main rock-forming minerals in the lower mantle are bridgmanite (Br) and smaller amounts of ferropericlase (Fp). Previous work has suggested that the large differences in viscosity between these minerals greatly affect the bulk rock rheology. The resulting effective rheology becomes highly strain-dependent as weaker Fp minerals become elongated and eventually interconnected. This implies that strain localization may occur in Earth's lower mantle. So far, there have been no studies on global-scale mantle convection in the presence of such strain-weakening (SW) rheology. Here, we present 2D numerical models of thermo-chemical convection in spherical annulus geometry including a new strain-dependent rheology formulation for lower mantle materials, combining rheological weakening and healing terms. We find that SW rheology has several direct and indirect effects on mantle convection. The most notable direct effect is the changing dynamics of weakened plume channels as well as the formation of larger thermochemical piles at the base of the mantle. The weakened plume conduits act as lubrication channels in the mantle and exhibit a lower thermal anomaly. SW rheology also reduces the overall viscosity, notable in terms of increasing convective vigor and core-mantle boundary heat flux. Finally, we put our results into context with existing hypotheses on the style of mantle convection and mixing. Most importantly, we suggest that the new kind of plume dynamics may explain the discrepancy between expected and observed thermal anomalies of deep-seated mantle plumes on Earth

Evidence for melt leakage from the Hawaiian plume above the mantle transition zone

Dehydration reactions at the top of the mantle transition zone (MTZ) can stabilize partial melt in a seismic low-velocity layer (LVL), but the seismic effects of temperature, melt and volatile content are difficult to distinguish. We invert P-to-S receiver function phases converted at the top and bottom of a LVL above the MTZ beneath Hawaii. To separate the thermal and melting related seismic anomalies, we carry out over 10 million rock physics inversions. These inversions account for variations arising from the Clapeyron slope of phase transition, bulk solid composition, dihedral angle, and mantle potential temperature. We use two independent seismic constraints to evaluate the temperature and shear wave speed within the LVL. The thermal anomalies reveal the presence of a hot and seismically slow plume stem surrounded by a “halo” of cold and fast mantle material. In contrast to this temperature distribution, the plume stem contains less than 0.5 vol% melt, while the surrounding LVL—within the coverage area—contains up to 1.7 vol% melt, indicating possible lateral transport of the melt. When compared to the melting temperatures of mantle rocks, the temperature within the LVL, calculated from seismic observations of MTZ thickness, suggests that the observed small degrees of melting are sustained by the presence of volatiles such as CO2 and H2O. We estimate the Hawaiian plume loses up to 1.9 Mt/yr H2O and 10.7 Mt/yr CO2 to the LVL, providing a crucial missing flux for global volatile cycles

Core-Exsolved SiO2 Dispersal in the Earth's Mantle

SiO2 may have been expelled from the core directly following core formation in the early stages of Earth's accretion and onward through the present day. On account of SiO2's low density with respect to both the core and the lowermost mantle, we examine the process of SiO2 accumulation at the core‐mantle boundary (CMB) and its incorporation into the mantle by buoyant rise. Today, if SiO2 is 100–10,000 times more viscous than lower mantle material, the dimensions of SiO2 diapirs formed by the viscous Rayleigh‐Taylor instability at the CMB would cause them to be swept into the mantle as inclusions of 100 m–10 km diameter. Under early Earth conditions of rapid heat loss after core formation, SiO2 diapirs of ∼1 km diameter could have risen independently of mantle flow to their level of neutral buoyancy in the mantle, trapping them there due to a combination of intrinsically high viscosity and neutral buoyancy. We examine the SiO2 yield by assuming Si + O saturation at the conditions found at the base of a magma ocean and find that for a range of conditions, dispersed bodies could reach as high as 8.5 vol % in parts of the lower mantle. At such low concentration, their effect on aggregate seismic wave speeds is within observational seismology uncertainty. However, their presence can account for small‐scale scattering in the lower mantle due to the bodies' large‐velocity contrast. We conclude that the shallow lower mantle (700–1,500 km depth) could harbor SiO2 released in early Earth times