Outgassing on stagnant-lid super-Earths

We explore volcanic outgassing on purely rocky, stagnant-lid exoplanets of different interior structures, compositions, thermal states, and age. We focus on planets in the mass range of 1-8 ME (Earth masses). We derive scaling laws to quantify first- and second-order influences of these parameters on volcanic outgassing after 4.5 Gyrs of evolution. Given commonly observed astrophysical data of super-Earths, we identify a range of possible interior structures and compositions by employing Bayesian inference modelling. [..] The identified interiors are subsequently used as input for two-dimensional (2-D) convection models to study partial melting, depletion, and outgassing rates of CO2. In total, we model depletion and outgassing for an extensive set of more than 2300 different super-Earth cases. We find that there is a mass range for which outgassing is most efficient (~2--3 ME, depending on thermal state) and an upper mass where outgassing becomes very inefficient (~5--7 \ME, depending on thermal state). [..] In summary, depletion and outgassing are mainly influenced by planet mass and thermal state. Interior structure and composition only moderately affect outgassing. The majority of outgassing occurs before 4.5 Gyrs, especially for planets below 3 ME. We conclude that for stagnant-lid planets, (1) compositional and structural properties have secondary influence on outgassing compared to planet mass and thermal state, and (2) confirm that there is a mass range for which outgassing is most efficient and an upper mass limit, above which no significant outgassing can occur. Our predicted trend of CO2-atmospheric masses can be observationally tested for exoplanets. These findings and our provided scaling laws are an important step in order to provide interpretative means for upcoming missions such as, e.g., JWST and E-ELT, that aim at characterizing exoplanet atmospheres.Comment: Accepted for publication in A&A, 19 Figures, 20 page

Melting-induced crustal production helps plate tectonics on Earth-like planets

AbstractWithin our Solar System, Earth is the only planet to be in a mobile-lid regime. It is generally accepted that the other terrestrial planets are currently in a stagnant-lid regime, with the possible exception of Venus that may be in an episodic-lid regime. In this study, we use numerical simulations to address the question of whether melting-induced crustal production changes the critical yield stress needed to obtain mobile-lid behaviour (plate tectonics). Our results show that melting-induced crustal production strongly influences plate tectonics on Earth-like planets by strongly enhancing the mobility of the lid, replacing a stagnant lid with an episodic lid, or greatly extending the time in which a smoothly evolving mobile lid is present in a planet. Finally, we show that our results are consistent with analytically predicted critical yield stress obtained with boundary layer theory, whether melting-induced crustal production is considered or not

Grain Size and localization of deformation in the lithosphere

Cette thèse a pour but d’étudier l’impact de la taille des grains sur la rhéologie du manteau terrestre. Nous avons proposé un nouveau modèle d’évolution de la taille des grains et montré que les états d’équilibre qu’il prévoit sont en adéquation avec les données expérimentales obtenues sur des échantillons d’olivine. Ce modèle est illustré par des simulations numériques d’une zone de cisaillement et de convection du manteau. Nous avons montré que notre formalisme favorise la localisation la déformation et modifie le régime de convection des planètes. Notre modèle propose que l’évolution d’une distribution de tailles de grains soit entièrement contrainte par l’énergie des joints de grains. Cette énergie diminue lors de la croissance normale et augmente lors de la recristallisation dynamique. Notre modèle stipule que l’énergie nécessaire à la nucléation des nouveaux grains est soustraite à l’énergie dissipée dans le matériau par les dislocations. Même lorsque les grains de taille moyenne sont en régime diffusif, les grains les plus gros se déforment en régime dislocatif, peuvent continuer à subir de la recristallisation et ainsi peuvent concourir à la réduction de la taille moyenne. Lorsque les effets de la croissance et de la nucléation se compensent, nous pouvons comparer l’état d’équilibre obtenu avec les données expérimentales. Nous montrons que le partitionnement de l’énergie fournie dépend principalement de la température. Ce modèle est ensuite testé dans une simulation numérique de zone de cisaillement. Dans ce système, selon la température, la contrainte déviatorique et la taille des grains, la déformation peut s’effectuer en régime de diffusion ou en régime de dislocation. Contrairement à nos attentes, notre modèle montre que la déformation se localise moins efficacement lorsque le cœur de la zone de cisaillement passe en régime de diffusion. La taille des grains semble favoriser l’apparition des zones de cisaillement en y baissant la viscosité mais ne semble pas localiser intrinsèquement la déformation. Finalement, nous avons testé notre équation de l’évolution de la taille des grains à l’aide de simulations numériques de convection du manteau. La température étant élevée dans le manteau, nous avons considéré que les grains étaient tous dans l’état d’équilibre prévu par notre modèle. Nous avons observé que la taille des grains peut suffire à modifier le régime de convection des planètes en créant une couche très non-Newtonienne dans leur lithosphère. Cette thèse a montré que la taille des grains influence fortement la dynamique des planètes en localisant la déformation à leur surface. Le nouveau modèle qu’elle propose apporte une vue nouvelle sur la création et l’entretien du régime de tectonique des plaques.In this thesis, I studied the impact of grain size on the rheology of the mantle of the Earth. We propose a new model of grain size evolution and show that the equilibrium states it predicts are in good agreement with the experimental data obtained on olivine samples. This model is illustrated by numerical simulations of mantle convection and shear zones. I show that our formalism has a non-negligible effect on the localization of deformation in shear zones and strongly modifies the convection regimes of planets. In our formalism, we propose that the energy of a grain size distribution is fully constrained by the energy of the grain boundaries. This energy diminishes during grain growth and increases during dynamic recrystallization. Our model stipulates that the amount of energy required to nucleate new grains, which is a dislocation-assisted process, is substracted to the energy dissipated in the material. We also show that even when the mean grain size is located in the diffusion regime, the biggest grains of the distribution can remain in the dislocation regime and continue to nucleate smaller grains. Thus, the mean grain size can decrease, even in the diffusion regime. When the growth and recrystallization processes are equivalent, the grain size distribution reaches an equilibrium state which can be compared with experimental data (various published olivine piezometers). The experimental calibration of our model shows that the partitioning of energy is principally temperature-dependent. I test this new model in a fully viscous numerical simulation of shear zone with a non-Newtonian- grain size-dependent viscosity. In this system, depending on the temperature, grain size or thermal state, the deformation can locally operate in diffusion or dislocation regime. Paradoxally, my model shows that the deformation is more localized when the shear zone is in the dislocation regime, where the viscosity does not depend on grain size. We observe that the grain size parameter tends to favor the apparition of shear zones because it minors the stress field but is insufficient to localize deformation in the case of a constant plate velocity. A grain size-activated localization of deformation seems to require a time-dependent stimulation which is typically obtained in visco-elasto-plastic configurations. Finally, I have tested our grain size evolution model, in its static form, in a set of numerical simulations of mantle convection. Our model shows that the grain size reaches its equilibrium very quickly in sub-lithospheric conditions. Thus, I have considered that the grain size distribution is always in the equilibrium state predicted but our model. I show that the consideration of the grain size parameter can fully modify the convection regimes of telluric planets. When surface stresses are high enough to reach the diffusion regime, the stagnant lid usually obtained with very temperature-dependent viscosities is broken by a very non-Newtonian and grain size-dependent top layer. This thesis shows that grain size strongly influences the dynamics of planets in localizing the deformation in their surfaces. The model we propose brings a new view of the creation of plate tectonics at the surface of the Earth

Taille des grains et localisation de la déformation dans la lithosphère

In this thesis, I studied the impact of grain size on the rheology of the mantle of the Earth. We propose a new model of grain size evolution and show that the equilibrium states it predicts are in good agreement with the experimental data obtained on olivine samples. This model is illustrated by numerical simulations of mantle convection and shear zones. I show that our formalism has a non-negligible effect on the localization of deformation in shear zones and strongly modifies the convection regimes of planets. In our formalism, we propose that the energy of a grain size distribution is fully constrained by the energy of the grain boundaries. This energy diminishes during grain growth and increases during dynamic recrystallization. Our model stipulates that the amount of energy required to nucleate new grains, which is a dislocation-assisted process, is substracted to the energy dissipated in the material. We also show that even when the mean grain size is located in the diffusion regime, the biggest grains of the distribution can remain in the dislocation regime and continue to nucleate smaller grains. Thus, the mean grain size can decrease, even in the diffusion regime. When the growth and recrystallization processes are equivalent, the grain size distribution reaches an equilibrium state which can be compared with experimental data (various published olivine piezometers). The experimental calibration of our model shows that the partitioning of energy is principally temperature-dependent. I test this new model in a fully viscous numerical simulation of shear zone with a non-Newtonian- grain size-dependent viscosity. In this system, depending on the temperature, grain size or thermal state, the deformation can locally operate in diffusion or dislocation regime. Paradoxally, my model shows that the deformation is more localized when the shear zone is in the dislocation regime, where the viscosity does not depend on grain size. We observe that the grain size parameter tends to favor the apparition of shear zones because it minors the stress field but is insufficient to localize deformation in the case of a constant plate velocity. A grain size-activated localization of deformation seems to require a time-dependent stimulation which is typically obtained in visco-elasto-plastic configurations. Finally, I have tested our grain size evolution model, in its static form, in a set of numerical simulations of mantle convection. Our model shows that the grain size reaches its equilibrium very quickly in sub-lithospheric conditions. Thus, I have considered that the grain size distribution is always in the equilibrium state predicted but our model. I show that the consideration of the grain size parameter can fully modify the convection regimes of telluric planets. When surface stresses are high enough to reach the diffusion regime, the stagnant lid usually obtained with very temperature-dependent viscosities is broken by a very non-Newtonian and grain size-dependent top layer. This thesis shows that grain size strongly influences the dynamics of planets in localizing the deformation in their surfaces. The model we propose brings a new view of the creation of plate tectonics at the surface of the Earth.Cette thèse a pour but d’étudier l’impact de la taille des grains sur la rhéologie du manteau terrestre. Nous avons proposé un nouveau modèle d’évolution de la taille des grains et montré que les états d’équilibre qu’il prévoit sont en adéquation avec les données expérimentales obtenues sur des échantillons d’olivine. Ce modèle est illustré par des simulations numériques d’une zone de cisaillement et de convection du manteau. Nous avons montré que notre formalisme favorise la localisation la déformation et modifie le régime de convection des planètes. Notre modèle propose que l’évolution d’une distribution de tailles de grains soit entièrement contrainte par l’énergie des joints de grains. Cette énergie diminue lors de la croissance normale et augmente lors de la recristallisation dynamique. Notre modèle stipule que l’énergie nécessaire à la nucléation des nouveaux grains est soustraite à l’énergie dissipée dans le matériau par les dislocations. Même lorsque les grains de taille moyenne sont en régime diffusif, les grains les plus gros se déforment en régime dislocatif, peuvent continuer à subir de la recristallisation et ainsi peuvent concourir à la réduction de la taille moyenne. Lorsque les effets de la croissance et de la nucléation se compensent, nous pouvons comparer l’état d’équilibre obtenu avec les données expérimentales. Nous montrons que le partitionnement de l’énergie fournie dépend principalement de la température. Ce modèle est ensuite testé dans une simulation numérique de zone de cisaillement. Dans ce système, selon la température, la contrainte déviatorique et la taille des grains, la déformation peut s’effectuer en régime de diffusion ou en régime de dislocation. Contrairement à nos attentes, notre modèle montre que la déformation se localise moins efficacement lorsque le cœur de la zone de cisaillement passe en régime de diffusion. La taille des grains semble favoriser l’apparition des zones de cisaillement en y baissant la viscosité mais ne semble pas localiser intrinsèquement la déformation. Finalement, nous avons testé notre équation de l’évolution de la taille des grains à l’aide de simulations numériques de convection du manteau. La température étant élevée dans le manteau, nous avons considéré que les grains étaient tous dans l’état d’équilibre prévu par notre modèle. Nous avons observé que la taille des grains peut suffire à modifier le régime de convection des planètes en créant une couche très non-Newtonienne dans leur lithosphère. Cette thèse a montré que la taille des grains influence fortement la dynamique des planètes en localisant la déformation à leur surface. Le nouveau modèle qu’elle propose apporte une vue nouvelle sur la création et l’entretien du régime de tectonique des plaques

A thermodynamically self-consistent damage equation for grain size evolution during dynamic recrystallization

International audienceP>We employ basic non-equilibrium thermodynamics to propose a general equation for the mean grain size evolution in a deforming medium, under the assumption that the whole grain size distribution remains self-similar. We show that the grain size reduction is controlled by the rate of mechanical dissipation in agreement with recent findings. Our formalism is self consistent with mass and energy conservation laws and allows a mixed rheology. As an example, we consider the case where the grain size distribution is lognormal, as is often experimentally observed. This distribution can be used to compute both the kinetics of diffusion between grains and of dynamic recrystallization. The experimentally deduced kinetics of grain size coarsening indicates that large grains grow faster than what is assumed in classical normal grain growth theory. We discuss the implications of this model for a mineral that can be deformed under both dislocation creep and grain size sensitive diffusion creep using experimental data of olivine. Our predictions of the piezometric equilibrium in the dislocation-creep regime are in very good agreement with the observations for this major mantle-forming mineral. We show that grain size reduction occurs even when the average grain size is in diffusion creep, because the largest grains of the grain size distribution can still undergo recrystallization. The resulting rheology that we predict for olivine is time-dependent and more non-linear than in dislocation creep. As the deformation rate remains an increasing function of the deviatoric stress, this rheology is not localizing

On the self-regulating effect of grain size evolution in mantle convection models: application to thermochemical piles

ISSN:1869-9510ISSN:1869-952

Self-organization of magma supply controls crustal thickness variation and tectonic pattern along melt-poor mid-ocean ridges

In nature, variations in mantle sources and magmatic processes lead to significant changes of crustal thickness distribution and spreading pattern at mid-ocean ridges. Distributions in crust thicknesses are relatively uniform at fast ridges. On the contrary, great lateral fluctuations with even mantle rocks exhumation are observed at slow and ultraslow ridges. Similarity, the spreading pattern changes from symmetric configuration (at fast ridges) to highly asymmetric plates accretion with detachment faults and oceanic core complexes (at slow ridges). Recent modeling studies suggested that magmatism may play a key role in the variation. Yet, the physical mechanisms controlling spatial-temporal distribution and intensity of the magmatic activity at mid-ocean ridges remain elusive. In this study, using 3D self-consistent magmatic-thermomechanical numerical models, we systematically investigate the effect of mantle potential temperature and spreading rate on the crustal thickness lateral variations and tectonic pattern at spreading ridges of various opening rates. Our numerical results show that along melt-poor slow-ultraslow mid-ocean ridges, there are two fundamentally different types of ridge sections that spontaneously form and alternate. This is due to a tectono-magmatic instability which self-consistently partitions melt supply beneath the ridge. Two geometries form: (1) normal ridge sections (NR) with elevated topography, normal thickness of oceanic crust and a hot thermal structure, and (2) fracture zone sections (FZ) with lowered topography, thin/absent crust, exhumed mantle rocks, and a cold thermal structure. The tectono-magmatic instability along spreading ridges is triggered by the combined effects of a reduced melt supply from the mantle into crustal magma chambers and an increase in brittle layer thickness. Such bimodal structure is then maintained by the combined effects of buoyant melt partitioning along the ridge, localized latent heat release from crustal magma crystallization, and different spreading modes. This results, on one hand, in hotter, thinner and elevated magmatic sections (tectono-magmatic spreading), and, on the other hand, in colder, thicker and subsided amagmatic sections (purely tectonic spreading). This predicted bimodal distribution finds a good agreement with natural observations. This suggests that spontaneous self-organization of magma supply controlled by spreading rate and mantle potential temperature plays a critical role in shaping tectonic and crustal patterns of mid-ocean ridges.ISSN:0012-821XISSN:1385-013

Growing primordial continental crust self-consistently in global mantle convection models

ISSN:1342-937XISSN:1878-057

Formation of ridges in a stable lithosphere in mantle convection models with a viscoplastic rheology

Numerical simulations of mantle convection with a viscoplastic rheology usually display mobile, episodic or stagnant lid regimes. In this study, we report a new convective regime in which a ridge can form without destabilizing the surrounding lithosphere or forming subduction zones. Using simulations in 2-D spherical annulus geometry, we show that a depth-dependent yield stress is sufficient to reach this ridge only regime. This regime occurs when the friction coefficient is close to the critical value between mobile lid and stagnant lid regimes. Maps of convective regime as a function of the parameters friction coefficients and depth dependence of viscosity are provided for both basal heating and mixed heating situations. The ridge only regime appears for both pure basal heating and mixed heating mode. For basal heating, this regime can occur for all vertical viscosity contrasts, while for mixed heating, a highly viscous deep mantle is required.ISSN:0094-8276ISSN:1944-800