Gregor J. Golabek

Continental crust formation on early Earth controlled by intrusive magmatism

The global geodynamic regime of early Earth, which operated before the onset of plate tectonics, remains contentious. As geological and geochemical data suggest hotter Archean mantle temperature and more intense juvenile magmatism than in the present-day Earth, two crust–mantle interaction modes differing in melt eruption efficiency have been proposed: the Io-like heat-pipe tectonics regime dominated by volcanism and the “Plutonic squishy lid” tectonics regime governed by intrusive magmatism, which is thought to apply to the dynamics of Venus. Both tectonics regimes are capable of producing primordial tonalite–trondhjemite–granodiorite (TTG) continental crust but lithospheric geotherms and crust production rates as well as proportions of various TTG compositions differ greatly, which implies that the heat-pipe and Plutonic squishy lid hypotheses can be tested using natural data. Here we investigate the creation of primordial TTG-like continental crust using self-consistent numerical models of global thermochemical convection associated with magmatic processes. We show that the volcanism-dominated heat-pipe tectonics model results in cold crustal geotherms and is not able to produce Earth-like primordial continental crust. In contrast, the Plutonic squishy lid tectonics regime dominated by intrusive magmatism results in hotter crustal geotherms and is capable of reproducing the observed proportions of various TTG rocks. Using a systematic parameter study, we show that the typical modern eruption efficiency of less than 40 per cent leads to the production of the expected amounts of the three main primordial crustal compositions previously reported from field data (low-, medium- and high-pressure TTG). Our study thus suggests that the pre-plate-tectonics Archean Earth operated globally in the Plutonic squishy lid regime rather than in an Io-like heat-pipe regime.

N-body simulations of oligarchic growth of Mars: Implications for Hf–W chronology

Dauphas and Pourmand [2011. Hf–W–Th evidence for rapid growth of Mars and its status as a planetary embryo. Nature 473, 489–492] estimated the accretion timescale of Mars to be 1.8−1.0+0.9Myr from the W isotopes of Martian meteorites. This timescale was derived assuming perfect metal–silicate equilibration between the impactor and the targets mantle. However, in the case of a small impactor most likely only a fraction of the targets mantle is involved in the equilibration, while only a small part of the impactors core equilibrates in the case of a giant impact. We examined the effects of imperfect equilibration using results of high-resolution N-body simulations for the oligarchic growth stage. These effects were found to be small as long as a planetary embryo has a deep liquid magma ocean during its accretion. The effect due to partial involvement of the targets mantle in equilibration is small due to the low metal–silicate partition coefficient for W suggested from the low Hf/W ratio of the Martian mantle. The effect due to partial involvement of the impactors core is also small because a large fraction of the embryo mass is delivered from small planetesimals, which are likely to fully equilibrate in the deep magma ocean on the embryo. The accretion timescale of Mars estimated by the Hf–W chronology is shorter than that expected for the minimum mass solar nebula model as long as more than 10% of each impactors core re-equilibrates with the Martian mantle and the final stages of accretion are prolonged. This probably indicates that accretion of Mars rapidly proceeded due to solid and gas surface densities significantly larger than those for the minimum mass solar nebula or due to accretion of small fragments or pebbles.

Rheological controls on the terrestrial core formation mechanism

Iron core differentiation of terrestrial planetary bodies is thought to have occurred simultaneously with planetary accretion. The exact mechanisms of core formation, however, remain incompletely understood. One model proposes that cores are formed from numerous smaller iron cores from predifferentiated planetesimals. To further understand this mechanism for forming Mars- and Earth-sized bodies, we present here systematic numerical simulations. Our models include a non-Newtonian temperature-, pressure- and strain rate–dependent viscoplastic rheology. Four different core formation regimes are being observed in the study, as a function of activation volume, friction angle, Peierls stress, and the initial temperature state of the body. We derive scaling laws, which show the importance of shear heating localization and plastic yielding as mechanisms to drive planetary differentiation in planetary interiors, that are in good agreement with numerical simulations. Results indicate that the effective rheology of the planetary body has a major effect on the core formation mechanism: while bodies with a weak rheology generally show a diapiric mode of core formation, the interior of planetary bodies with a stiff rheology can be fractured or displaced toward the surface. On Earth-sized protoplanets, the water content seems also to have a significant influence on the mode of core formation. Results indicate a time scale of differentiation of a few million years, significantly shorter than expected from the Stokes sinking time in a Newtonian medium.

The effects of short-lived radionuclides and porosity on the early thermo-mechanical evolution of planetesimals

Abstract The thermal history and internal structure of chondritic planetesimals, assembled before the giant impact phase of chaotic growth, potentially yield important implications for the final composition and evolution of terrestrial planets. These parameters critically depend on the internal balance of heating versus cooling, which is mostly determined by the presence of short-lived radionuclides (SLRs), such as 26 Al and 60 Fe, as well as the heat conductivity of the material. The heating by SLRs depends on their initial abundances, the formation time of the planetesimal and its size. It has been argued that the cooling history is determined by the porosity of the granular material, which undergoes dramatic changes via compaction processes and tends to decrease with time. In this study we assess the influence of these parameters on the thermo-mechanical evolution of young planetesimals with both 2D and 3D simulations. Using the code family i2elvis/i3elvis we have run numerous 2D and 3D numerical finite-difference fluid dynamic models with varying planetesimal radius, formation time and initial porosity. Our results indicate that powdery materials lowered the threshold for melting and convection in planetesimals, depending on the amount of SLRs present. A subset of planetesimals retained a powdery surface layer which lowered the thermal conductivity and hindered cooling. The effect of initial porosity was small, however, compared to those of planetesimal size and formation time, which dominated the thermo-mechanical evolution and were the primary factors for the onset of melting and differentiation. We comment on the implications of this work concerning the structure and evolution of these planetesimals, as well as their behavior as possible building blocks of terrestrial planets.

Self-consistent generation of single-plume state for Enceladus using non-Newtonian rheology

The thermal dichotomy of Enceladus suggests an asymmetrical structure in its global heat transfer. So far, most of the models proposed that obtained such a distribution have prescribed an a priori asymmetry, i.e., some anomaly in or below the south polar ice shell. We present here the first set of numerical models of convection that yield a stable single-plume state for Enceladus without prescribed mechanical asymmetry. Using the convection code StagYY in a 2-D spherical annulus geometry, we show that a non-Newtonian ice rheology is sufficient to create a localized, single hot plume surrounded by a conductive ice mantle. We obtain a self-sustained state in which a region of small angular extent has a sufficiently low viscosity to allow subcritical to weak convection to occur due to the stress-dependent part of the rheological law. We find that the single-plume state is very unlikely to remain stable if the rheology is Newtonian, confirming what has been found by previous studies. In a second set of numerical simulations, we also investigate the first-order effect of tidal heating on the stability of the single-plume state. Tidal heating reinforces the stability of the single-plume state if it is generated in the plume itself. Lastly, we show that the likelihood of a stable single-plume state does not depend on the thickness of the ice shell.

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

Abstract 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.

Coupling SPH and thermochemical models of planets: Methodology and example of a Mars-sized body

Abstract Giant impacts have been suggested to explain various characteristics of terrestrial planets and their moons. However, so far in most models only the immediate effects of the collisions have been considered, while the long-term interior evolution of the impacted planets was not studied. Here we present a new approach, combining 3-D shock physics collision calculations with 3-D thermochemical interior evolution models. We apply the combined methods to a demonstration example of a giant impact on a Mars-sized body, using typical collisional parameters from previous studies. While the material parameters (equation of state, rheology model) used in the impact simulations can have some effect on the long-term evolution, we find that the impact angle is the most crucial parameter for the resulting spatial distribution of the newly formed crust. The results indicate that a dichotomous crustal pattern can form after a head-on collision, while this is not the case when considering a more likely grazing collision. Our results underline that end-to-end 3-D calculations of the entire process are required to study in the future the effects of large-scale impacts on the evolution of planetary interiors.

Towards self-consistent modelling of the Martian dichotomy

1 pagina.-- Resumen del trabajo presentado en la 19th Annual V.M. Goldschmidt Conference, V.M. Goldschmidt Conference.