Displays

Several studies suggest that Mars went through an episode of Magma Ocean (MO) early in its history. When the MO crystallises, solid mantle appears. The crystallisation of this MO starts at the Core-Mantle Boundary (CMB) and continues upwards to the surface of the planet. Assuming that this process occurs by fractional crystallisation, the solid cumulates that form are progressively enriched in incompatible elements, including iron, and an unstable density stratification is developed. This stratification is thought to have resulted in a planetary-scale mantle overturn after MO crystallisation, potentially explaining the early magnetic field, crustal dichotomy and chemical heterogeneities present on martian mantle.

However, previous studies on the thermo-chemical evolution of Mars consider only fractional crystallisation of the MO, and lack the possibility of re-melting/re-freezing of material at the mantle-MO interface, before the MO is fully crystallised.

In this study we investigate the effect of re-melting/re-freezing of material at the mantle-MO interface during MO crystallisation, on the dynamics and composition of the solid mantle. We use a numerical method with the convection code StagYY. The solid mantle is represented by a 2D spherical annulus geometry, and the MO by a 0D object at top of the mantle. The boundary condition applied to the solid domain allows the parameterisation of fractional crystallisation/re-melting of material at the mantle-MO interface. We model the growth of the solid mantle from the CMB up to the surface of the planet, and we account for core cooling and the presence of an atmosphere.

We show that by taking re-melting/re-freezing of material into account, the onset of convection can start earlier in Mars history. These results bring implications for the density stratification and overturn, and to the existence of isotopically distinct reservoirs on the mantle. Moreover, our results show that the mode of convection is preferentially degree-1, which can potentially explain the crustal dichotomy. 

How to cite: Bolrão, D., Ballmer, M., Morison, A., Rozel, A., Labrosse, S., and Tackley, P.: From a magma ocean to a solid mantle: implications for the thermo-chemical evolution of Mars, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5357, https://doi.org/10.5194/egusphere-egu2020-5357, 2020.

Late accretionary bombardments in the first billion years of solar system history strongly affected the initial physical and chemical states of the Earth. Evidence of ancient impacts can be preserved in the oldest known terrestrial zircons with ages up to ca. 4.4 Ga. Here, we use the Hadean zircon record to directly assess the thermal effects of impact bombardment on the early Earth’s crust, couple the results to models of closure temperature-dependent diffusive loss and U-Pb age-resetting in zircon, derive zircon ages, and compare them to published ages.

The impact bombardment model consists of (i) a stochastic cratering model which populates the surface with craters within constraints derived from the lunar cratering record, the size/frequency distribution of the asteroid belt, and dynamical models; (ii) analytical expressions that calculate a temperature field for each crater; and (iii) a three-dimensional thermal model of the terrestrial lithosphere, where craters are allowed to cool by conduction and radiation. Equations for diffusion in zircon are coupled to these thermal models to estimate the amount of age-resetting.

We present modeling results for the Earth between 4.5 Ga and 3.5 Ga based new mass-production functions. Mean surface temperatures and geothermal gradients were assumed as 20 °C and 70 °C/km. Total delivered mass was estimated at 0.0013(M_planet), or 7.8 × 10²¹ kg. The size-frequency distributions of the impacts were derived from dynamical modeling. We begin model runs with a global magma ocean, which would have been formed by the Moon-forming impact. Mean impactor density of 3000 kg/m³ and impactor velocity distribution from [1,2] was used, and impact angle of each impactor was stochastically generated from a gaussian centered at 45 degrees. The typical impact velocity of the Earth is ~21 km s^-1.

It is important to note that the model age outputs we report omit normal processes of generation of zircon-saturated magmas that were operative in the Hadean. We find that as the impact flux decreases with time and becomes negligible for the purposes of thermal modeling by ca. 3.5 Ga. We find that the probability of randomly selecting a zircon of a given age increases with increasing age, predicting a large number of very old zircons. This contrasts with the actual age distribution of Hadean zircons, which, for >4 Ga, indicates the opposite case: the probability of selecting a zircon of a given age decreases with increasing age. We interpret this discrepancy to mean that impacts were not the dominant process in determining the ages of Hadean zircons. This is consistent with observations that the majority of Hadean zircons had formation temperature significantly lower than those expected for melt sheets and thermobarometry measurements suggesting formation of some Hadean zircons in a plate boundary environment.

[1] Mojzsis, S.J. et al. (2019). Astrophys. J., 881, 44. [2] Brasser, R. et al. (2020) Icarus 338, 113514. 

How to cite: Mojzsis, S. J. and Abramov, O.: Analytical thermochronometric models of Earth’s crust during the late accretionary bombardment epoch (4.5-3.5 Ga), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3675, https://doi.org/10.5194/egusphere-egu2020-3675, 2020.

Terrestrial planet mantles cannot transport the very high heat production in their early stages through subsolidus convection and instead produce voluminous melt that makes its way to the surface to transport the heat. This heat-pipe mode of heat transport implies a very different tectonics than either the rigid or mobile-lid tectonics driven by subsolidus convection. Although  similar to rigid-lid convection in that there is relatively little horizontal motion, heat-pipe lithospheres are by no means stagnant. Vertical transport through the continuous eruption of new material on the surface reaches rates of several mm/year (with significant spatial and temporal variations). This strongly impacts the shape of the geotherm, producing a cold and strong lid (despite the high heat flow). In addition, this vertical transport produces global compressional stresses as old surfaces are buried and forced downward to smaller radii. The horizontal variations in burial rates will lead to stress concentrations and ultimately plastic failure and thrusting (see Io’s numerous tectonic uplifts as an example). The transition from the advectively dominated heat-pipe lithosphere to a thin conductive lithosphere reverses this process, resulting in a period of global extension (again with large horizontal variations) as global volcanism wanes. An additional aspect of vertical transport in the heat-pipe lithosphere is the cycling of water and other volatiles into the lithosphere and mantle as surface materials are buried. This material is available for metamorphic reactions and will interact with rocks at the wet solidus, producing evolved rock compositions and volatile by-products (e.g. methane) that will contribute to the early atmospheres of these planets. Evidence of vertical transport in ancient Earth rocks has generally been attributed to subduction but heat-pipe advection provides a more global opportunity for such cycling.

How to cite: Moore, W. and Webb, A.: Heat Pipes and Vertical Tectonics in Terrestrial Planets, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22657, https://doi.org/10.5194/egusphere-egu2020-22657, 2020.

The plate tectonics is an essential geophysical/geological process on the deep mantle water and carbon cycling, which may also control the long-term climate evolution because the volcanic degassing induced by the plate subduction seems to change the atmospheric condition. However, as suggested by the geological evidence on the onset timing of the plate tectonics in early Earth, which is modeled by the transition from the stagnant lid tectonics to the plate subduction, this timing may have great uncertainty. Here, two questions are addressed: 1. How can the deep mantle volatile cycling would be affected by the onset timing of the plate tectonics in the planetary system evolution?; 2. As a result of the successful scenario of the deep mantle volatile cycling explained for the observational constraints of the subduction flux of the water and carbon, how can the climate evolution be responded as a function of the history of the deep mantle volatile cycling such as the subduction flux? To address these questions, a simplified model of whole planetary system evolution based on the thermal history computation of the silicate mantle coupled with the energy balance climate evolution and deep mantle volatile is used with controlling both heat transfer and volatile cycling associated with the transition between stagnant lid and plate tectonics.

The main result indicates that plate tectonics may be essential for the mild and stable climate that allows having liquid water over billions of years of the time scale. This is because a sufficient amount of volcanic degassing can be found for the vigorous plate tectonics rather than the stagnant lid state to get the long-term mild climate. For the stagnant lid state, the snowball limit cycle can be found. Thus, the vigorous plate motion may contribute to stabilizing the warm climate.

To find out the constraint on the present-day surface environment, the transition timing from the stagnant lid to the vigorous plate subduction for explaining the present-day amount of volatiles and their subduction flux would range from 1 to 3 Ga. And, around 5 to 10 ocean masses of the water in the total planetary system is required so that the deep mantle melting should be continuously found to supply the volatile component to the atmosphere associated with the plate subduction, which is worked for the reducing the melting temperature of the silicate mantle. However, the subduction flux for finding the mild climate is one to two orders of magnitude larger than the expected from the geological constraint – 10¹² to 10¹³ kg/yr as well as some difficulty for explaining the global sea-level change. In the presentation, some improvements on including the big storage capacity of the volatiles in the mantle transition zone will be provided for giving a better understanding of both the deep mantle volatile cycle and climate evolution in the plate-mantle evolution system.

How to cite: Nakagawa, T.: The long-term climate evolution and planetary habitability – Onset timing of the plate tectonics in early Earth, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9209, https://doi.org/10.5194/egusphere-egu2020-9209, 2020.

The collision of large bolides with planets with substantial atmospheres, such as Earth and early Mars, results in significant climatic and surface effects. For very large impacts, forming basins >~500 km in diameter, these post-impact effects would be global and include [1]: (1) transient high atmospheric and surface temperatures, (2) deposition of material that was vaporized by the impact event and subsequently condensed (e.g. terrestrial spherule layers), (3) a transient, vigorous hydrologic cycle characterized by rainfall rates sufficient to produce flooding, and (4) surface aqueous alteration, made possible by the hot rainfall and high temperatures. On Mars, the formation of such large basins, including Hellas, Isidis, and Argyre, occurred in the early- to mid-Noachian [2]; while younger, smaller basins would have influenced the climate and surface on a local or regional scale, such intense, global effects would have occurred only during the earliest parts of Mars history. Previous work has qualitatively [1] and quantitatively [in 3D; 3,4] constrained the effects from large basin-scale impacts on Mars, but lacks detailed application to any specific impact.

The fact that these drastic, global effects would occur following each large basin-scale impact [1,3,4] implies that the effects from formation of the youngest of the large basins would be best preserved and closest to the present-day surface. Here, we build upon previous work [1,3,4] by qualitatively and quantitatively exploring the climatic and surface effects from the formation of the youngest large basin, Argyre. We find that: (1) a tens of meters thick, near-globally-distributed, olivine and glass-rich spherule layer should be preserved on or very near the surface, (2) the induced hydrologic cycle would have been characterized by rainfall rates akin to Earth rainforests and would have lasted for decades to centuries, (3) the intense rainfall would have caused flooding, significant erosion, and smoothing of landforms, and (4) hot rainfall and high temperatures would have caused surface aqueous alteration, including partial alteration of the olivine-rich layer to carbonates as well as alteration of basaltic material to Fe/Mg-smectites and Al-phyllosilicates, which would present in a leaching profile.

Implications of these findings include: (1) distinguishing the role of impact-induced aqueous alteration from that associated with normal climate conditions, (2) predictions of areas where the spherule layer and alteration products may be observed, (3) the transition from a basin-scale impact-dominated regime to a basin-free regime in martian climate evolution, and (4) guidelines for exploration and recognition of these impact-related units at rover and sample return scale.

References

[1] Palumbo, Head (2017), Impact cratering as a cause of climate change, surface alteration, and resurfacing during the early history of Mars, MAPS, 53, p687.

[2] Fassett, Head (2011), Sequence and timing of conditions on early Mars, Icarus, 211, p1204.

[3] Turbet, Gillman, Forget, Baudin, Palumbo, Head, Karatekin (2019), The environmental effects of very large bolide impacts on early Mars explored with a hierarchy of numerical models, Icarus, 335, p113419.

[4] Steakley, Murphy, Kahre, Haberle, Kling (2019), Testing the impact heating hypothesis for early Mars with a 3D GCM, Icarus, 330, p169.

How to cite: Palumbo, A. and Head, J.: Large impact basin-related climatic and surface effects on Mars: Argyre basin as a case study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11078, https://doi.org/10.5194/egusphere-egu2020-11078, 2020.

One of the main goals of Exoplanetary Sciences is to reconcile our theoretical knowledge of terrestrial exoplanet systems with observations. To reach this goal, the interaction between it and the planetary interior needs to be studied, since the atmosphere is the only observable part of a terrestrial exoplanet. This atmosphere-interior interaction depends on properties of the interior, many of which are directly (density, viscosity) or indirectly (thermal evolution, layering) affected by the bulk composition of the planet. In order to better understand the variability in atmosphere-interior interaction between exoplanets, as well as the properties of the resulting atmosphere, we here constrain the range of terrestrial exoplanet bulk compositions.

To constrain this range, we approximate exoplanet composition by applying devolatilization to the bulk composition of the host star. We approximate exoplanet compositions by adjusting host-star compositions from a stellar catalogue according to the condensation temperature. We consider planetary differentiation by distributing elements between the core and mantle according to their tendency to stabilize oxide, thus obtaining a proxy for bulk silicate compositions. We include partitioning of light elements into the core. Lastly, we explore the effects of these compositions on the tendency to promote stable mantle stratification in the aftermath of magma-ocean freezing, using a thermodynamic model of crystallization, and on thermal evolution using a 1D parametrized convection model.

We find that mantle Mg/Si is an important control on mantle properties, since increased Mg/Si-ratios tend to decrease mantle viscosity by stabilizing soft minerals, such as olivine and ferropericlase, at the expense of pyroxene and stishovite (and corresponding high-pressure polymorphs). The Mg/Si of planets is shifted towards higher values by the slightly higher volatility of Si, and by the partitioning of Si into the core. We find that the Earth's mantle is below average in terms of bulk-silicate Mg/Si for planets in the galactic neighborhood. This result indicates that most terrestrial planets have a mantle viscosity lower than that of Earth. Earth is average in terms of bulk Fe/Si, and above average in terms of bulk Fe/Mg. We find that planets with relatively low Mg/Si and high Fe/Mg in their silicate envelopes cool slower because of high mantle viscosities, and because of their tendency to sustain double-layered convection in a stratified mantle.

Finally, we identify a number of end-member bulk planet compositions, which we recommend for use in modelling of terrestrial exoplanet interiors. These end-member compositions cover most of the variability in bulk terrestrial exoplanet compositions based on available stellar composition data. We also present mineralogical mantle profiles for these end-member compositions. In the future, we intend to explore the effects of these bulk-silicate planet compositions on surface tectonic style, and the related feedback on planetary cooling and volatile cycling.

How to cite: Spaargaren, R., Wang, H., Mojzsis, S., Ballmer, M., and Tackley, P.: Exoplanet bulk silicate composition as a function of host stellar elemental abundances, and its effects on long-term planetary evolution, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20378, https://doi.org/10.5194/egusphere-egu2020-20378, 2020.

The vigour and style of mantle convection in tidally-locked super-Earths may be substantially different from Earth's regime. Earth's surface temperature is spatially uniform at 300 K, which is sufficiently cold to drive strong downwellings into the interior (i.e. subduction). In contrast, a tidally-locked super-Earth can have a large temperature contrast between the dayside and nightside, which we infer could lead to a dichotomy of the interior dynamics. We therefore use constraints from astrophysical observations to infer the possible pattern of flow in the interior of a tidally-locked super-Earth, using super-Earth LHS 3844b as a case study. We run mantle convection models using the code StagYY with two-dimensional spherical annulus geometry and parameters from the literature that are appropriate for LHS 3844b. The majority of the mantle is either perovskite or post-perovskite with the phase transition occurring around 1700 km depth (the total mantle depth is 3757 km). An upper and lower bound for the viscosity of post-perovskite is provided by previous theoretical calculations. We include plastic yielding to model the brittle nature of the lithosphere; plastic yielding occurs when the local stress state exceeds a prescribed yielding criteria and is commonly applied in studies of Earth to produce surface behaviour similar to plate tectonics.

For a low yield stress criteria (promoting a weak lithosphere), we find that plumes are generally evenly distributed between the dayside and nightside, albeit strong downwellings form on the nightside. Plumes on the nightside have less lateral mobility than on the dayside because they are confined by downwellings either side. In contrast, for a high yield stress criteria, the interior dynamics are mostly driven by a prominent downwelling on the dayside which flushes hot material from the lower thermal boundary layer around the CMB towards the nightside where plumes preferentially arise. This, in turn, leads to a return flow of colder material from the near surface of the nightside towards the dayside. This seemingly counterintuitive pattern of flow is a consequence of weak lithosphere (due to temperature) on the dayside that is able to deform and thereby subduct, whereas lithosphere on the nightside is too stiff to subduct.

Our models therefore show that the vigour of convection and the distribution of upwellings and downwellings of tidally locked super-Earths are sensitive to the strength of the lithosphere: plumes can either be equally distributed around the planet or preferentially occur on the nightside. In the first case, the cold downwellings are also equally distributed but more prominent on the nightside, whereas in the second case they are preferentially on the dayside. Somewhat unexpected, we do not observe a preference for hot plumes to congregate on the dayside. Our results have implications for space missions such as TESS, CHEOPS, JWST, PLATO and ARIEL that will discover and characterise super-Earths, thereby potentially probing for signals of volatile outgassing and volcanism.

How to cite: Meier, T. G., Bower, D. J., Lichtenberg, T., and Tackley, P. J.: Interior dynamics of tidally-locked super-Earths: the case of LHS 3844b, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10961, https://doi.org/10.5194/egusphere-egu2020-10961, 2020.

Venus shares some striking similarities with Earth; at the same time, it exhibits characteristics that are widely different from that of our own planet. Indeed, it is an example of an active planet that may have followed a radically different evolutionary pathway despite the similar mechanisms at work and probably comparable initial conditions. The evolution of Venus is still poorly constrained, partly due to a lack of relevant measurements. As a result, there is currently no consensus on the history of Earth’s sister’s surface conditions. It has, however, been suggested that water could have been stable for long periods of time at the surface of Venus, depending on the specific composition of the atmosphere.

Venus observation has shown the D/H ratio in its atmosphere is consistent with water loss, possibly amounting to 100 times its atmosphere present content. Fractionation of hydrogen however depends on the mechanisms at work and the conditions of loss, meaning this estimate is still very crude and qualitative. Material on Venus has also been shown to be consistent with surface oxidation, but an oxidized small layer of 10 μm depth can explain the observed spectra. We investigate how Venus’ atmosphere, mantle and surface could have evolved in the past in light of the multiple mechanisms affecting volatile exchanges. We have developed a self-consistent coupled numerical simulation of the evolution of Venus, striving to identify and model mechanisms that are important to the behavior of the planet and its surface conditions.

Loss mechanisms are of special interest, since they provide a way to quantify how much water could have been lost over time and thus could potentially put an upper limit to the amount of water in Venus’ past. The current simulations include modeling of mantle dynamics, volcanism, atmospheric escape (both hydrodynamic and non-thermal), evolution of atmosphere composition, surface oxidation and evolution of surface conditions (greenhouse effect) and the coupling between interior and atmosphere of the planet.

Volatile fluxes between the different layers of the planet seem critical to estimate how Venus changed over time. This is especially important as we have highlighted the strong role played by mantle/atmosphere coupling in regulating both mantle dynamics and surface conditions through surface temperature evolution. It is also seemingly a major factor governing, in turn, volatile outgassing and outgassed species composition.

In recent evolution, volatile exchanges seem very limited, with low release of water in the atmosphere, especially. Loss mechanisms also appear to be able to remove very low amounts of water and oxygen, from the surface/atmosphere (4 mbar to a few bar), making it quite difficult to accommodate large bodies of water, especially during Venus’ recent past. Trapping oxygen on the surface through oxidation of newly emplaced volcanic material is more uncertain. It can certainly explain the loss of a few more bars. The process is likely to remains inefficient, but it cannot be ruled out that larger volumes of oxidized material exist on Venus and could contain oxygen from past liquid water layers of a few meters to tens of meters deep.

How to cite: Gillmann, C., Golabek, G., and Tackley, P.: Oxidizing Venus’ Surface: Consequences for Volatile Inventory, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5164, https://doi.org/10.5194/egusphere-egu2020-5164, 2020.

In its bulk properties, Venus appears similar to Earth, but both planets have developed substantially different geodynamic regimes. Earth has plate tectonics with a continuously renewed surface and its crustal distribution is very dichotomous in composition, thickness, and age. Venus, on the other hand, presently displays a period of a stagnant-lid regime, which may or may not was interrupted by catastrophic events of tectonic recycling during its history. Venus’ crustal thickness is not well constrained, but likely thicker than Earth’s oceanic crust; pronounced crustal dichotomy may be possible but evidence needs yet to be found. The age of the crust appears rather uniform, which traditionally has been taken as evidence that an episodic overturn must have taken place. However, recent arguments have challenged the episodic overturn hypothesis and favor a more continuous stagnant lid on Venus.

To resolve the problem of Venus’ geodynamic regime understanding the generation of Venus’ crust in a dynamic context that also considers the underlying mantle is necessary. This can be achieved using numerical models of mantle convection tailored to Venus, which include the basic complexities of planetary mantle convection in terms of effective rheology, mineralogy and melting processes. Still, previous models have essentially failed to predict the thickness and age characteristics of Venus’ crust. One possible reason is that these models only considered extrusive volcanism, which renews the surface directly, while intrusive magmatism does not. Yet, intrusion seems the dominant mode of magmatism at least on Earth, so we investigate its influence in our model and evaluate whether this ingredient is key to predict Venus’ crustal characteristics.

Using the code StagYY, we compute a suite of mantle convection models in 2D spherical annulus geometry that run through the entire solid-state history of Venus. We vary the partitioning of intrusive and extrusive volcanism from purely extrusive to dominantly intrusive and predict the present-day distributions of crustal thickness and surface age in the stagnant lid regime. With more intrusive magmatism, average crustal thickness is reduced by 20-25%, but mean crustal thickness still exceeds other independent estimates. The surface is on average much older, which is more consistent with mean age estimates from crater counting. However, lateral age variations also become stronger with dominantly intrusive volcanism, which indicates that volcanism keeps going on, but is more restricted spatially. Governing parameters like mantle reference viscosity and relative enrichment of heat-producing elements into the crust change the absolute values of mean crustal thickness and surface age, but do not improve surface age uniformity. This is somewhat at odds with Venus’ seemingly uniform surface age, so suitable conditions for this possibility are further evaluated in models featuring episodic overturn events.

How to cite: Uppalapati, S., Rolf, T., and Werner, S.: The role of intrusive magmatism in shaping Venus’ present-day crust and its age distribution , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16621, https://doi.org/10.5194/egusphere-egu2020-16621, 2020.

It has been shown that melting and crust production strongly influences the convection regime of terrestrial planets, potentially even more than the vigor of convection. A planet producing and erupting a lot of crust can hardly remain in the stagnant lid regime and produces resurfacings or even reaches some mobile-lid regime. On the other hand, a planet that intrudes its melt in the lithosphere tends to have a larger conductive heat flux and cools efficiently without much lid mobility. Thus, the question of the amount of melts being erupted or intruded might dominate the cooling of terrestrial planets. So far, an "eruption efficiency", which gives the ratio of melt that erupts over the remaining melt fraction, has been imposed in numerical simulations. The eruption efficiency in the convection code StagYY has thus far been treated as a constant in time and space. Here, we explore the effects of a time- and space-dependent eruption efficiency on planetary evolution in the planetary convection code StagYY. An equation was devised that describes how eruptive a system is, based on the main characteristics of lithospheric melt transport: the amount of melt and the local stress state. In a range of systematic simulations, we explore the consequences of this parameter. 

In a first set of simulations this parameter is explored while keeping the eruption efficiency constant. Results show that the most important parameters are the amount of melt, where the stress has smaller local effects. Additionally, changing the yield stress, viscosity or constant eruption efficiency has a large effect on what the eruptivity should be based on this equation. Parameters that govern the global mantle temperature are less important for the eruptivity. 

A second set of simulations was performed with the eruption efficiency behaving in a fully self-consistent manner. These models tend to behave like intrusive systems, except during resurfacing episodes when the models become very extrusive. Models that show mobile behaviour at almost all times in the planetary evolution will have an almost constant spatially averaged eruption efficiency. In these models the eruption efficiency does vary locally however.

How to cite: Arts, M.: Exploring the effects of a time- and space-dependent eruption efficiency on planetary evolution., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20076, https://doi.org/10.5194/egusphere-egu2020-20076, 2020.

A major shift in Earth’s crustal generation processes at ~3.2 to 2.5 Ga has been inferred from mineralogical, geological, and geochemical records, particularly those recorded by fine-grained sediments and zircon crystals. The most common hypothesis to explain this shift is the onset of plate tectonic recycling following some form of hot stagnant lid geodynamics. However, all prior detailed geologic studies of our best-preserved Eoarchean terrane, the ~3.85 - 3.60 Ga Isua supracrustal belt of SW Greenland, interpret this site to record terrane collision within the context of plate tectonics. This represents a significant counterweight to the assumption underpinning the ~3 Ga tectonic-mode-change models, i.e., the idea that early Earth’s record is broadly representative. The Isua belt is divided into ~3.8 and ~3.7 Ga halves, and these have been interpreted as plate fragments which collided by ~3.6 Ga. Here, we examine the evidence used to support plate tectonic interpretations, focusing on 1) reanalysis of prior geochronological results and associated cross-cutting relationships which have previously been interpreted to record as many as eight tectonic events, and 2) new field observations leading to reinterpretation of basic structural relationships. Simpler interpretations of the geochronological and deformation data are viable: the belt may have experienced nearly homogeneous metamorphic conditions and strain during a single deformation event prior to intrusion of ~3.5 Ga mafic dikes. Curtain and sheath folds occur at multiple scales throughout the belt, with the entire belt potentially representing Earth’s largest a-type fold. We propose a new model: two cycles of volcanic burial and resultant melting and TTG intrusion produced first the ~3.8 Ga rocks and then the ~3.7 Ga rocks above, after which the whole belt was deformed and thinned in a shear zone, producing the multi-scale a-type folding patterns. The Eoarchean assembly of the Isua supracrustal belt is therefore most simply explained by vertical-stacking volcanic and instrusive processes followed by a single shearing event. In combination with well-preserved Paleoarchean terranes, these rocks record the waning downward advection of lithosphere inherent in volcanism-dominated heat-pipe tectonic models for early Earth. These interpretations are consistent with recent findings that early crust-mantle dynamics are remarkably similar across the solar system’s terrestrial bodies.

How to cite: Webb, A. A. G., Müller, T., Zuo, J., Haproff, P., and Ramírez-Salazar, A.: Eoarchean formation of the Isua supracrustal belt , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13268, https://doi.org/10.5194/egusphere-egu2020-13268, 2020.

Fifty years after the main discovery period for plate tectonics, we still lack a consensus understanding of a critical question: how did the plate tectonic system initiate? For the period before initiation of plate tectonics, models increasingly call upon a stagnant lid (i.e., a single-plate lithosphere) atop a mantle which was hotter by a few hundred degrees than the present mantle. How was this lid first broken into plates? Various hypotheses suggest that the strength of the lid was overcome by (a) mantle convective forcing, potentially along locally pre-weakened zones, (b) lithospheric gravitational instabilities between oceanic lithosphere and either adjacent oceanic plateau lithosphere or adjacent overthickened (i.e., gravitationally collapsing) continental lithosphere, or (c) one or more large bolides. These models have not converged on a mechanism or a typical early plate scale. Here, we use a new solid-mechanics based approach to the problem of the origin of plate tectonics and the processes by which plate boundaries are initiated. Specifically, we employ 3D spherical shell models of a brittle lithosphere via the three-dimensional finite element code RFPA (Rock Failure Process Analysis code). The models are subjected to quasi-static, slowly increasing interior pressure in a displacement-controlled manner (e.g., induced by gradual thermal expansion). Brittle failure is implemented through a strength criterion representing a stress limit at which the strength drops and fracture occurs. To account for local randomness, each element is assigned a failure threshold obtained from a Weibull probability distribution which contains a parameter describing the degree of material homogeneity. Globe-spanning rifting occurs as a consequence of horizontal extension. Resultant fracture spacing is a function of lithospheric thickness and rheology, such that geometrically-regular, polygonal-shaped tessellation is energetically favored because it minimizes total crack length. Therefore, anticipated warming of the early lithosphere itself (as lithospheric chilling from downwards advection due to rapid volcanism wanes) should lead to failure, propagating fractures, and the conditions necessary for the onset of multi-plate tectonics.

How to cite: Tang, C., Webb, A. G., Moore, W. B., Wang, Y., Ma, T., and Chen, T.: Breaking a single-plate Earth into a global plate network, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4617, https://doi.org/10.5194/egusphere-egu2020-4617, 2020.

The initiation of plate tectonics remains enigmatic, with the proposed onset timing ranging from Hadean to Proterozoic. Recently, many mineralogical, petrological and geochemical studies suggest onset of plate tectonics at ~3 Ga. For example, the geology of East Pilbara Terrane (~3.55 to 2.70 Ga; Australia) is widely interpreted as representing Paleoarchean non-plate tectonics, followed by plate tectonics after a ~3.2 Ga transition. In contrast, Isua supracrustal belt (3.85 to 3.55 Ga; Greenland) has been dominantly interpreted via plate tectonics. There, two ultramafic lenses have been interpreted as depleted mantle slices, emplaced via thrusting in an Eoarchean subduction zone, implying early plate tectonics. We present new petrological and geochemical data of ultramafic samples from the Isua lenses and from the East Pilbara Terrane to explore their origins. Pilbara samples appear to preserve cumulate textures; protolith textures of Isua samples are altered beyond recognition. Samples with low chemical alteration show similar whole-rock chemistry, including up to 5.0 wt.% Al₂O₃ and up to 0.25 wt.% TiO₂ that both covary negatively with MgO (37.1 to 47.5 wt.%); these variations suggest cogenetic relationships with local lavas. Flat trace-element fractionation trends parallel those of local lavas in the primitive-mantle normalized spider diagram. Spinel crystals from Pilbara samples yield ~20-60 Mg#, relatively constant Cr# at ~70, and 0.61-4.81 wt.% TiO₂. Our data are consistent with crustal cumulate emplacement. In contrast with depleted mantle rocks, our samples have higher whole-rock Al₂O₃ and TiO₂, flat (vs. upward) trace-element fractionation trends from less to more compatible elements, and spinel crystals with higher TiO₂ and relatively constant (vs. varied) Cr#. Therefore, Isua and Pilbara ultramafic rocks may have similar, non-plate tectonic origins, and the Isua record allows a ~3 Ga onset of plate tectonics.

How to cite: Zuo, J., Webb, A., Harvey, J., Haproff, P., Mueller, T., Byerly, G., Hickman, A., and Wang, Q.: Ultramafic rocks at the Isua supracrustal belt and East Pilbara Terrane are crustal cumulates, not slices of early mantle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6395, https://doi.org/10.5194/egusphere-egu2020-6395, 2020.

The Eoarchean Isua Supracrustal Belt (ISB) is one of the few locations where it is possible to study the tectono-metamorphic evolution of a young planet. The ISB is thought to represent meta-volcano-sedimentary units from two different embryotic continental segments/terranes associated with two large TTG bodies of contrasting crystallization age. Until recently, geochemical and metamorphic signatures have been interpreted to be consistent with a subduction-collision event, thereby matching Earth’s active ‘horizontal’ tectonic regime. This interpretation is often cited as evidence that plate tectonics has operated since the Early Archean. New structural, field, isotopic and geochemical data, however, suggest that the ISB is rather a continuous volcano-sedimentary sequence with a rock record that could be explained by ‘vertical’ tectonic models involving extensive volcanic resurfacing and single-plate tectonics. In this work, we present metamorphic data retrieved from a new set of samples from the eastern ISB to evaluate the two contrasting hypotheses. Throughout the ISB, two major Archean medium grade metamorphic events (M₁, M₂) can be identified, overprinted partially by near-pervasive low-temperature retrogression. The pre-Ameralik dykes (≈ 3500 Ma) event M₁, is characterized by a strong foliation and typically lineation that plunges towards the SE with development of amphibolite facies assemblages, with common appearance of syn-tectonic garnet and amphibole porphyroblasts. Phase equilibria modelling, classic and isopleth geothermobarometry show that M₁ evolved as a nearly isothermal prograde metamorphism that culminated in an amphibolite facies peak (0.65 GPa and 550-580 °C) common to the entire belt. M₂, probably Neoarchean in age, is identified by the frequent appearance of post-tectonic garnet rims with estimated lower grade conditions. Low temperature retrogression is widespread along the ISB, however, it seems more penetrative in the northern area occurring as garnet pseudomorphism and retrograde chlorite commonly mimicking the foliation by replacing biotite, with some samples showing complete chloritization. We argue that the retrogression textures could be responsible for the apparent zones of lower metamorphism previously reported as prograde, a conclusion also supported by our geothermobarometric data, and that the tectonic models supported by previuos interpretations need to be revised. The isothermal prograde path as well as the high geothermal gradient associated with peak conditions (≈ 900 °C/GPa) is consistent with vertical tectonics models during the Eoarchean. This interpretation is in agreement with global data analysis that suggest non-uniformitarian geodynamics in the Early Archean, as well as the viability of early vertical tectonics on the other terrestrial bodies of our solar system. It follows that studies like this can shed light on not just the cooling of early Earth, but also on the cooling of terrestrial planets universally.

How to cite: Ramírez-Salazar, A., Mueller, T., Piazolo, S., Webb, A., Hauzenberger, C., Zuo, J., Haproff, P., Harvey, J., and Charlton, C.: Eoarchean tectono-metamorphic signatures recorded on the Isua Supracrustal Belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1040, https://doi.org/10.5194/egusphere-egu2020-1040, 2020.

Terrestrial planets evolve through various stages of large-scale melting, or magma oceans, due to the energy release during accretion and differentiation. Any magma ocean is thought to become progressively enriched in FeO and incompatible elements upon freezing due to fractional crystallization. The resulting upwards enrichment of the related cumulate (=crystal) packages drives gravitational overturn(s) of the incipient mantle, and ultimately stabilizes a FeO-enriched molten layer at the core-mantle-boundary (CMB)¹. Such a molten layer, previously termed basal magma ocean (BMO)², is thought to also fractionally crystallize, but downwards instead of upwards, and over much longer timescales than the surficial magma ocean. This BMO fractional crystallization due to slow planetary cooling analogously implies the stabilization of a thick FeO-enriched layer at the CMB. Such a layer would essentially remain stable forever, as being too dense to be entrained by convection of the overlying mantle. However, at least for Earth, geophysical observations rule out the preservation of such a deep dense global layer. Here, we investigate the consequences of an alternative mechanism for BMO freezing, reactive crystallization, on the initial condition of solid-state mantle convection and long-term planetary evolution.

Based on scaling relationships, we show that any cumulates, which crystallize from the BMO (e.g., due to initial cooling or reaction) are readily entrained by mantle convection. Once the BMO-mantle boundary is exposed, the BMO reacts with the mantle to form reactive cumulates. Reaction is driven by disequilibrium between mantle rocks and the BMO, a situation that is inevitable independent of BMO initial composition. As reactive cumulates are continuously entrained by mantle convection, the BMO continues to freeze by reactive crystallization. Based on lower-mantle mineral-melt phase equilibria³, we calculate the compositional evolution of the BMO, and the chemistry of the BMO cumulate package. We demonstrate that for a wide range of BMO initial compositions, the cumulate package consists of two discrete layers: the first is pure bridgmanite close to the MgSiO₃ end-member; the second is mostly bridgmanite+ferropericlase that is moderately enriched in FeO and incompatibles, i.e. similar in composition to FeO-enriched pyrolite. The mass or thickness of the cumulate package depends on reaction kinetics, but is significantly larger than that of the BMO. The bridgmanitic layer is expected to be entrained by mantle convection due to its intrinsic buoyancy, but resist efficient mixing due to its intrinsic strength, thereby potentially providing an explanation for seismic scatterers/reflectors and ancient geochemical reservoirs⁴. The moderately FeO-enriched layer is expected to stabilize thermochemical piles, providing a candidate origin for the seismically-observed large low shear velocity provinces (LLSVPs)⁵.

These results have implications for the long-term (thermal) evolution of planets in general. Earth-sized terrestrial (exo-)planets and super-Earths should also initially host a MgSiO₃-rich layer as well as a moderately FeO-enriched layer. In contrast, small terrestrial planets such as Mars may host a more strongly Fe-rich deep dense global layer as long as no BMO is stabilized in their histories.

[1] Ballmer+, G-cubed 2017; [2] Labrosse+, Nature 2007; [3] Boukaré+, JGR Solid-Earth 2015; [4] Ballmer+, Nat.Geosci. 2017; [5] Ballmer+, G-cubed 2016.

How to cite: Ballmer, M., Spaargaren, R., Mallik, A., Bolrão, D., Morison, A., and Nakajima, M.: The initial condition for the long-term evolution of terrestrial planets as determined by Reactive Freezing of the Basal Magma Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11800, https://doi.org/10.5194/egusphere-egu2020-11800, 2020.

How to cite: Bower, D. J., Kitzmann, D., Wolf, A., Sanan, P., Dorn, C., Oza, A., and Lichtenberg, T.: Implications of magma oceans for astrophysical observations: mass-radius and atmospheric composition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6004, https://doi.org/10.5194/egusphere-egu2020-6004, 2020.

How to cite: Way, M., Davies, H., Duarte, J., and Green, M.: The climates of Earth's next supercontinent: effects of tectonics, rotation rate, and insolation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5821, https://doi.org/10.5194/egusphere-egu2020-5821, 2020.

Large moons such as the Galilean satellites are thought to be in an equilibrium rotation state, called a Cassini state (Peale, 1969). This state is characterized by a synchronous rotation and a precession rate of the rotation axis that is equal to the precession rate of the normal to its orbit. It also implies that the spin axis, the normal to the orbit and the normal to the Laplace plane are coplanar with a (nearly) constant obliquity.

For rigid bodies, up to 4 possible Cassini states exist, but not all of them are stable. It is generally assumed that the Galilean satellites are in Cassini State I for which the obliquity is close to zero (see e.g. Baland et al. 2012). However, it is also theoretically possible that these satellites occupy or occupied another Cassini state.

We here investigate how the interior structure, and in particular the presence of a subsurface ocean, influences the existence and stability of the different possible Cassini states.

References :

Baland, R.M., Yseboodt, M. and Van Hoolst, T. (2012). Obliquity of the Galilean satellites: The influence of a global internal liquid layer. Icarus 220, 435-448.

Peale, S. (1969). Generalized Cassini’s laws. Astron. J. 74 (3), 483-489.

How to cite: Coyette, A., Baland, R.-M., Lemaitre, A., and Van Hoolst, T.: Cassini state of Galilean Moons: Influence of a subsurface ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8210, https://doi.org/10.5194/egusphere-egu2020-8210, 2020.

The atmospheres of rocky exoplanets are secondary and regulated by geochemical volatile cycles. Earth scientists have studied in detail the long-term inorganic carbon cycle (also known as the carbonate-silicate cycle) acting on timescales of hundreds of thousands of years. This cycle provides essential negative feedback to maintain temperate climates on Earth. With the discovery of about a thousand rocky exoplanets and ongoing hunts for an Earth-twin, it is imperative to understand the factors affecting the stability of the carbon cycle. These factors could be dependent on the orbital and stellar parameters such as stellar radiation as well as planet-specific properties such as rock composition, land and ocean fractions. On Earth, continental silicate weathering and seafloor basalt weathering act as sinks for the atmospheric carbon dioxide. In this study, we develop a novel framework to unify both weathering processes. This is done by incorporating a set of silicate weathering reactions leading to the formation of carbonates. We focus on modeling the chemistry of rock-water interaction for different rock types (depending on the planet’s surface composition), as well as pH, temperature and partial pressure of carbon dioxide. We quantify the effects of fresh rock availability for the continental weathering and landmass fractions and shallow and deep ocean fractions for the seafloor weathering. Other components of the carbon cycle such as subduction, ridge and arc volcanism are parameterized based on previous studies. The effects of planet size, redox states, and tidal locking are also investigated. Our study gives a strong control over the connection between atmospheric observables and the carbon cycle. The ultimate goal is to provide an abiotic library of geological false positives of biosignatures.

How to cite: Hakim, K.: Geochemistry of Carbon Cycles on Rocky Exoplanets, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1986, https://doi.org/10.5194/egusphere-egu2020-1986, 2020.

Atmospheric general circulation models developed for the Earth system include comprehensive parameterizations of clouds. Applying them to exoplanet atmospheres provides an opportunity to advance understanding of clouds, atmosphere dynamics, and their coupling in the context of planetary climate dynamics and habitability.

Here, we study a deep-time extreme climate of Earth as an example of the cold limit of the habitable zone. Geological evidence indicates near-global ice cover during the Neoproterozoic (1000 – 541 Million years ago) associated with considerable hysteresis of atmospheric CO₂. The Snowball Earth hypothesis provides a straightforward interpretation of Neoproterozoic proxies based on a runaway of the sea-ice albedo feedback. However, the Snowball Earth hypothesis relies on the existence of local habitats to explain the survival of photosynthetic marine species on an entirely ice-covered planet. The Jormungand hypothesis may resolve this issue by considering a weakening of the sea-ice albedo feedback by exposure of dark bare sea ice when sea ice enters the subtropics. This potentially allows the Earth system to stabilize in a climate state - the Jormungand state - with near-global ice cover. Around the equator, a narrow strip of ocean remains ice-free, where life would have easily survived during the pan-glaciations.

The weakening of the sea-ice albedo feedback is based on the change of the meridional structure of planetary albedo with a moving sea-ice edge. While previous work focused on the contribution of surface albedo to planetary albedo, we here focus on the impact of subtropical and tropical cloudiness on planetary albedo. Enhanced cloudiness generally weakens the sea-ice albedo feedback and thus decreases the climate sensitivity of the Jormungand state, i.e. it stabilizes the Jormungand state. We analyze the impact of cloudiness on the stability of the Jormungand state in the general circulation models CAM3 and ICON-AES with idealized aquaplanet setups. While CAM3 shows significant CO₂-hysteresis of the Jormungand state, ICON-AES exhibits no stable Jormungand state. Consistently, CAM3 exhibits stronger cloudiness than ICON-AES, especially in the subtropics. An analysis with a one-dimensional energy balance model shows that the Jormungand hysteresis strongly depends on the sensitivity of the planetary albedo to an advance of sea ice into the subtropics. Accordingly, we demonstrate that the absence of cloud-radiative effects within vertical columns in the subtropics drastically decreases the Jormungand hysteresis in CAM3.

Overall, the magnitude of the Jormungand hysteresis is tightly linked to the representation of cloud-radiative effects in general circulation models. Our results highlight the important role of uncertainties associated with cloud-radiative effects for climate feedbacks on planet Earth in the context of extreme climates, such as they have occurred in Earth’s deep past or might be found on Earth-like planets. In consequence, this also stresses the need and challenges of accounting for adequate cloud modeling for planetary climates.

How to cite: Braun, C., Voigt, A., Hörner, J., and Pinto, J. G.: Subtropical clouds stabilize near-Snowball Earth states, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10617, https://doi.org/10.5194/egusphere-egu2020-10617, 2020.