GD3.2 | Dynamics, structure and evolution of Earth and rocky planets from formation to the present day
PICO
Dynamics, structure and evolution of Earth and rocky planets from formation to the present day
Co-organized by GMPV3/PS4
Convener: Paul Tackley | Co-conveners: Gregor Golabek, Lena Noack, Paolo Sossi
PICO
| Wed, 26 Apr, 14:00–18:00 (CEST)
 
PICO spot 3a
Wed, 14:00
Dynamical processes shape the Earth and other rocky planets throughout their history; their present state is a result of this long-term evolution. Early on, processes and lifetimes of magma oceans establish the initial conditions for their long-term development; subsequently their long-term evolution is shaped by the dynamics of the mantle-lithosphere system, compositional differentiation or mixing, possible core-mantle reactions, etc.. These processes can be interrogated through observations of the rock record, geochemistry, seismology, gravity, magnetism and planetary remote sensing all linked through geodynamical modelling constrained by physical properties of relevant phases.

This session aims to provide a holistic view of the dynamics, structure, composition and evolution of Earth and rocky planets (including exoplanets) on temporal scales ranging from the present day to billions of years, and on spatial scales ranging from microscopic to global, by bringing together constraints from geodynamics, mineral physics, geochemistry, petrology, planetary science and astronomy.

PICO: Wed, 26 Apr | PICO spot 3a

Chairpersons: Paul Tackley, Gregor Golabek
Small bodies (up to Mars) and experimental constraints
14:00–14:02
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PICO3a.1
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EGU23-326
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ECS
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On-site presentation
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Yaozhu Li, Phil J. A. McCausland, Roberta L. Flemming, and Noriko T. Kita

Ureilites are ultramafic achondrite meteorites that likely represent a large parent body. Large olivine and pyroxene grains display a high degree of textural equilibrium, forming “triple-junction” contacts at their grain boundaries. However, ureilites also have primitive characteristics, for example high siderophile and carbon content, high noble gas content, and unequilibrated olivine and pyroxene compositions. So far, the origin of ureilites and their parent body are still debated as it is difficult to explain the observation of textural equilibrium juxtaposed with such primitive properties. Conventionally, ureilites are considered to be mantle residues from within an unknown, large rocky body. Because feldspar is completely depleted from most ureilite samples, it has been thought that the parent body accreted early and experienced extensive igneous differentiation processes, with primary heating attributed to short-lived 26Al decay in the early solar system. Here we report on polymict ureilite breccia Elephant Moraine 87720. We found that the sample has several unusually magnesian-rich olivine clasts with mg# (Mg/(Mg+Fe)) up to 98.7 and calcium-poor pyroxene with Wo as low as to 1.0. Moreover, we discovered two coarse-grained aluminous spinel grains with over 56.4-58.7 wt% Al2O3 and 11.3-11.8wt% Cr2O3, in contact with olivine and pyroxene grains. These aluminous spinel clasts are unique among ureilite samples. To determine the provenance of the spinel grains and other clasts (e.g., high magnesian olivine and low calcium pyroxene) in this sample, we conducted in situ oxygen 3-isotope analyses by Secondary Ion Mass Spectrometry SIMS (IMS 1280), University of Wisconsin-Madison. SIMS mineral data plot along the slope ~1 line in the oxygen 3-isotope diagram, similar to those of bulk ureilites (Greenwood et al., 2017, Chemie der Erde 77, 1-43) including ureilitic samples found in Almahata Sitta, with the same range of ∆17O (from –2.3‰ to –0.2‰). These grains follow the Fe-loss/addition trend defined by a molar plot of Fe/Mn versus molar Fe/Mg, showing a near constant and chondritic Mn/Mg ratio, falling in among common ureilitic compositions. We conclude that the origin of these clasts, including the aluminous spinel, is primarily ureilitic, but they extend the δ18O measurement for ureilites up to 9.7 ‰. We hypothesize a magmatic origin for these clasts that they were formed under low-oxygen fugacity, in a high Al/Si ratio hot melt, favouring the crystallization of Al-spinel instead of a Cr-rich endmember. The clasts in this EET 87720 specimen may possibly represent a new type of high Al, low Ca, low Cr lithic material within the ureilite parent body. Finally, we calculated a possible crystallization temperature of 1379 K using spinel-olivine equilibrium crystallization (Roeder et al 1979, Contrib. Min. Petrol. 6, 325-334). Our estimate corresponds well with the theoretical model proposed by Goodrich et al. (2004, Chemie der Erde 64, 283-327) that the UPB was hot, with a temperature above 1100 °C (1373 K). Our results are consistent with other petrological evidence and olivine-pigeonite-melt thermometry (Singletary and Grove, 2003, Met. Planet. Sci. 38, 95-108) which constrain smelting temperatures within the ureilite parent body.

How to cite: Li, Y., McCausland, P. J. A., Flemming, R. L., and Kita, N. T.: Thermal constraints on the ureilite parent body (UPB): Evidence from the refractory spinel in polymict ureilite EET 87720 using in situ SIMS, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-326, https://doi.org/10.5194/egusphere-egu23-326, 2023.

14:02–14:04
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PICO3a.2
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EGU23-3530
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On-site presentation
David E Smith, Sander Goossens, and Maria T Zuber

Analysis of the lunar Bouguer gravity field under major basins reveals how gravity varies with spherical harmonic degree L and, potentially, with depth (to relate the two we use a relationship based on point masses).  We have studied 19 lunar basins based upon a GRAIL 1200 degree and order gravity model (GRGM1200B).  The vertical component of Bouguer gravity shows how the gravity is distributed in spherical harmonic degree between the lowest degree, 2, and the highest degree, 1200. Under each basin, this gravity spectrum of accelerations per individual spherical harmonic degree shows a benign region for L from 800 to 100, a range of approximately 20 km immediately below the surface, consistent with the observation that the upper crust is largely homogenous (Zuber et al., 2013). A region of more varied gravity signal occurs down to L~20, approximately 60 km deeper. The basin gravity signal merges with the deep interior at L~10, approximately 150 km below the surface. A set of profiles over latitude or longitude through an individual basin anomaly shows how the magnitude of the gravity signal changes with depth as it passes from the annular moat to the central high of the anomaly; all of which takes place between L~100-20, a depth range estimated to be ~20-80 km.  However, all basins are different to some extent. Outside of the basin anomaly the gravity spectra are relatively benign from just below the surface to L~40, a depth of approximately 45 km and consistent with the approximate average thickness of the lunar crust.  An exception to the general characteristics of the spectra of basins is South Pole-Aitken (SPA) which indicates a structure with few variations that is very similar to the regions that have near zero Bouguer gravity at the surface with no large anomalies in the top 100 km. We interpret this result for SP-A as a result of its largely compensated state.

How to cite: Smith, D. E., Goossens, S., and Zuber, M. T.: Implications of Bouguer Gravity Structure Under Major Lunar Basins, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3530, https://doi.org/10.5194/egusphere-egu23-3530, 2023.

14:04–14:06
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PICO3a.3
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EGU23-14366
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On-site presentation
Arthur Briaud, Clément Ganino, Agnès Fienga, Nicolas Rambaux, Anthony Mémin, Hauke Hussmann, Alexander Stark, and Xyanyu Hu

The Moon is the most well-known extraterrestrial planetary body thanks to observations from ground-based, space-borne instruments and lunar surface missions. Data from Lunar Laser Ranging (LLR), magnetic, gravity, surface observations and seismic Apollo ground stations help us to quantify the deformation undergone by the Moon due to body tides. These observations provide one of the most significant constraints that can be employed to unravel the deep interior. The Moon deforms in response to tidal forcing exerted by, to first order, the Earth, the Sun and, to a lesser extent, by other planetary bodies. We use the degree-2 tidal Love number as a tool for studying the inner structure of our satellite. Based on measurements of the tidal Love numbers k2 and h2 and quality factors from the GRAIL mission, LLR and Laser Altimetry on board the LRO spacecraft, we perform a random walk Monte Carlo samplings for combinations of thicknesses and viscosities for models of Moon with and without inner core. By comparing predicted and observed parameters of the lunar tidal deformations, we infer constraints on the outer core viscosity, for a Moon with a thin outer core and a thick inner core, and a Moon with a thicker outer core but a denser and thinner inner core. In addition, by deducing the temperature and assuming the chemical composition of the low-viscosity zone, we obtain stringent constraints on its radius, viscosity and density. 

How to cite: Briaud, A., Ganino, C., Fienga, A., Rambaux, N., Mémin, A., Hussmann, H., Stark, A., and Hu, X.: Exploration of the lunar deep interior through global deformation modeling., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14366, https://doi.org/10.5194/egusphere-egu23-14366, 2023.

14:06–14:08
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PICO3a.4
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EGU23-14392
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ECS
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On-site presentation
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Laurine Rey, Tobias Keller, Ying-Qi Wong, Paul Tackley, Christian Liebske, and Max Schmidt

Understanding the dynamics of magma ocean crystallisation during planetary cooling can elucidate the initial mantle structure and subsequent evolution of early planetary bodies. However, most studies on magma ocean crystallisation focus on either the thermo-chemistry (e.g., Johnson et al. 2021) or the fluid dynamics of a cooling magma ocean (e.g., Maurice et al. 2017). This precludes investigations into coupled thermo-mechanical processes, such as the effect of convection and phase segregation on chemical differentiation. However, coupled models are challenging to implement due to their numerical complexity and limited experimental constraints on magma ocean crystallisation for model calibration.

We develop a two-phase, 6-component model in a 2D rectangular domain based on a multi-phase, multi-component reactive transport model framework (Keller & Suckale, 2019). Magma ocean convection is modelled using Stokes equations while crystal settling is calculated using a form of hindered Stokes law. The fluid mechanics model is coupled with a thermo-chemical model of evolving temperature, phase proportions, and phase compositions to form a reactive transport model, following Keller & Katz (2016). We apply this model to the lunar magma ocean (LMO) by describing the melt and crystal compositions with 6 pseudo-components (approximating forsterite-fayalite, orthopyroxene-clinopyroxene and anorthite-albite mineral systems). To calibrate the melting temperature and composition of each component, we fit data from fractional crystallisation experiments for a Taylor Whole Moon composition (Schmidt & Krättli 2022) using a transitional Markov Chain Monte Carlo method.

The 6-component melting model calibrated to experimental data is successfully implemented in the reactive transport model. First results indicate the importance of crystal settling speed and magma convection speed on convective mixing, magma ocean stratification, and crystal cumulate formation. The small size of the Moon and its relatively well-constrained magma ocean history, make the LMO an excellent case study to apply the model. However, with the aid of new experimental data for larger and chemically different planets, such as Mars, this model can provide more general insight into the early evolution of terrestrial bodies.

REFERENCES: Maurice et al. (2017) doi:10.1002/2016JE005250, Johnson et al. (2021) doi: 10.1016/j.epsl.2020.116721,  Keller & Suckale (2019) doi:10.1093/gji/ggz287, Keller & Katz (2016)  doi: 10.1093/petrology/egw030,  Schmidt & Krättli (2022) doi:10.1029/2022JE007187

How to cite: Rey, L., Keller, T., Wong, Y.-Q., Tackley, P., Liebske, C., and Schmidt, M.: Magma ocean crystallization model coupling fluid mechanics and thermo-chemistry: application to the lunar magma ocean., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14392, https://doi.org/10.5194/egusphere-egu23-14392, 2023.

14:08–14:10
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PICO3a.5
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EGU23-6480
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ECS
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On-site presentation
Peiyan Wu, Yongjiang Xu, Yanhao Lin, and Bernard Charlier

The compositional diversity of volcanic rocks revealed by NASA’s MESSENGER at the surface of Mercury has been interpreted to result from partial melting of a heterogenous sulfur-rich Mercurian mantle. However, melting relations and the composition of partial melts for iron-free and sodium-rich mantle, together with the effect of sulfur as a key volatile, have not yet been studied in detail. In this study we present results from high-pressure and high-temperature experiments on the mineralogical and geochemical evolution of the mantle residue and melting products of primitive deep Mercury’s mantle with two starting compositions differing by their Mg/Si ratios. Both compositions have sulfur added as FeS. Experiments were conducted using a multi-anvil press under reduced conditions (by controlling the Si/SiO2 ratio of the starting composition) at pressures of 3 and 5 GPa.

The residual mantle of Mercury with the lower Mg/Si ratio of 1.02 contains olivine + orthopyroxene above ~15 wt% melting at 3 and 5 GPa, and olivine disappears at melting over ~30 wt.% at 5 GPa. The Mercurian mantle with the Mg/Si of 1.35 contains olivine + orthopyroxene in the residue above ~15 wt% melting at 3 and 5 GPa, and olivine only when the melting degree is over ~50 wt.%. Our experiments also show that the majority of chemical composition of the High-Magnesium region (HMR) can result from ~25±15 wt.% melting of a deep primitive mantle. Further work will enable us to evaluate the compositional diversity of the mantle that is needed to explain the broad range of surface lavas. We also aim at understanding the role of the highly refractory residual mantle as a controlling factor for the end of major volcanic activity on Mercury at 3.5 Ga.

How to cite: Wu, P., Xu, Y., Lin, Y., and Charlier, B.: Melting relations for putative mantles of Mercury and the compositional diversity of the crust, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6480, https://doi.org/10.5194/egusphere-egu23-6480, 2023.

14:10–14:12
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PICO3a.6
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EGU23-11284
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ECS
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On-site presentation
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Claudia Camila Szczech, Jürgen Oberst, Hauke Hussmann, Alexander Stark, and Frank Preusker

Introduction:

Available gravity and topography data derived from MESSENGER mission provide a great opportunity to investigate the surface and the interior structures of Mercury’s impact basins. In contrast to previous studies, which focused on image data, topography, or gravity alone, we use the complementary data sets to obtain a more comprehensive picture of basins and possibly their related subsurface structures.

Methods:

In this study we use image, gravity and topography data obtained by the MESSENGER spacecraft, from the Mercury Dual Imaging System (MDIS), the Mercury Laser Altimeter (MLA) as well as a radio science experiment for gravity field modelling. Digital Terrain Models from stereo images (150m/px) [1] were used in combination with mosaiced image data (166m/px) [2] to support identification of the basins. Using the most recent gravity model [3], combined with a topography model [4], we calculated Bouguer anomalies [5] and determined a crustal thickness model[6].

Results:

We created an inventory of 319 impact basins (>150 km) classifying their morphological and gravitational characteristics, including measurements of gravity disturbance, Bouguer anomaly, crustal thickness and morphometrical measurements (Fig 1). Basins tend to undergo relaxation processes over time, which would explain the high number of modified basins.

Fig 1: A classification scheme was chosen according to rim preservation state, appearance of terraces, filling of the basin floor, depth and diameter.

 

With increasing diameter, basins were found to show more complex gravity signatures (Fig 2).  In both gravity anomalies, gravity disturbance as well as Bouguer anomaly, strong centred anomalies reflect high mass and/or density concentrations inside the impact basins, that were caused by an uplift of mantle material after the crater excavation phase [8]. The negative collar of the Bouguer anomaly profile suspected to be a consequence of depression of crust-mantle boundary, i.e. thickening of the crust. Consequently, profiles of Bouguer anomaly reflect profiles of the crust-mantle boundary.  With increasing diameter, the crustal thickness is showing a decrease in rim and centre proving a link between crustal thinning and impact basin formation (Fig 3). 

Fig. 2: [a]Gravity disturbance are mostly negative for small basins, but become positive for larger basins. [b] Bouguer anomaly showing positive centre and negative rim area (bullseye pattern).

 

Fig. 3: Bouguer anomaly contrast and crustal thickness ratio from centre and rim area. 

References:

[1]   Preusker F. et al., (2017). Planetary and Space Science, 142, 26–37.doi: 10.1016/j.pss.2017.04.012. [2] Hawkins, S.E., III, et al., (2007). Space Sci Rev 131: 247–338, DOI 10.1007/s11214-007-9266-3. [3] Konopliv, A., Park, R., & Ermakov, A. (2020). Icarus, 335, 113386 doi: 10.1016/j.icarus.2019.07.020. [4]  Neumann et al., (2016). 47th Annual Lunar and Planetary Science Conference (p. 2087). [6] Wieczorek et al., (2015). Treatise on Geophysics (pp. 153–193). Elsevier. doi: 10.1016/B978-0-444-53802-4. [7] Beuthe et al., (2020). Geophysical Research Letters, 47. doi: 10.1029/2020GL087261. [8] Melosh et al., (2013). Science, 340, 1552–1555.515 doi: 10.1126/science.1235768. 

How to cite: Szczech, C. C., Oberst, J., Hussmann, H., Stark, A., and Preusker, F.: Basin evolution and crustal structure on Mercury from gravity and topography data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11284, https://doi.org/10.5194/egusphere-egu23-11284, 2023.

14:12–14:14
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PICO3a.7
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EGU23-15144
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ECS
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On-site presentation
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Stefan Pitsch, Paolo A. Sossi, Max W. Schmidt, and Christian Liebske

Sulfide liquids in terrestrial environments are near mono-sulfidic and are FeS-rich with varying amounts of other chalcophile elements. At highly reducing conditions, such as on Mercury, elements like Ca, Mn and Mg can also form major components of sulfides and coexist with FeS [1].
Studies on the binary and ternary phase diagrams of the MgS-FeS-CaS systems have been conducted (separated from the influence of silicic melts) , owing to the limited amount of data on these systems [2,3]. With this study we also re-examine the behaviour of sulfur-enriched, highly reduced silicate melts (komatiitic and basaltic compositions) to asses formed phases as well as their gravitationally possible separation during the magma ocean stage of Mercury. The effect of and on the formation of phases is evaluated at 1 atm, similarly to a limited amount of foregone experiments conducted by [4]. We use both the acquired sulfide-phase diagram data and the information on the sulfide-silicate-melt interaction to assess mechanisms of sulfur accumulation on the surface of Mercury by gravitational separation within the magma ocean [5].   
Experiments were performed with stoichiometric mixes of pure components in graphite capsules sealed in evacuated silica tubes at ~10-5 bar. Quenched samples were prepared under anhydrous conditions, and phase compositions determined by energy-dispersive spectroscopy (binary and ternary phase diagrams) and electron probe micro-analysis (EPMA) (silicate-melt experiments).      
The solubility of FeS in oldhamite (CaS) is higher than previously reported, reaching 2.5 mol% at 1065°C. The eutectic is located at 8 ± 1 mol % CaS, significantly poorer in CaS than previously suggested [6], at 1065 ± 5 °C. Our data suggests that solid-solution compositions in the MgS-FeS binary are in accord with those reported in the only other study on this system [7]. However, we find the system to be eutectic in nature, with the eutectic point being located at 1180°C ± 2 °C and 0.3 mol% MgS. Formed liquids have been found to contain much higher concentrations of FeS than previously reported.

Our data show that Ca dissolves extensively in sulfides under graphite-saturated conditions at low pressures, which may have prevailed during crust formation on Mercury [8]. However, in silicate-melts, liquid FeS and solid niningerite (MgS) phases dominate for all investigated silicate compositions (komatiitic and basaltic compositions).  The produced solid phases are not light enough to be able to float in a Hermean magma ocean. Formed oldhamite solid solutions are small and interspersed in liquid FeS, which prohibits their effective separation of these dense phases.

 

[1]          Skinner + Luce (1971) AmMin

[2]          Nittler + Starr et al., (2011) Science

[3]          Dilner + Kjellqvist + Selleby (2016) J Phase Equilibria Diffus

[4]          Namur + Charier et al., (2016) Earth Planet. Sci. Lett

[5]          Malavergne et al. (2014) Earth Planet. Sci. Lett.

[6]          Heumann (1942) Arch Eisenhuttenwes

[7]          Andreev et al. (2006) Russ. J. Inorg. Chem.

[8]          Vander Kaaden + McCubbin (2015) J. Geophys. Res. Planets



How to cite: Pitsch, S., Sossi, P. A., Schmidt, M. W., and Liebske, C.: The behaviour of S in reduced systems and its application to Mercury, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15144, https://doi.org/10.5194/egusphere-egu23-15144, 2023.

14:14–14:16
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PICO3a.8
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EGU23-2786
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ECS
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On-site presentation
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Valentin Bonnet Gibet, Chloé Michaut, Thomas Bodin, Mark Wieczorek, and Fabien Dubuffet

The Martian crust is made up of sedimentary and volcanic rocks that are mainly mafic in composition. Nevertheless, orbital and in-situ observations have revealed the presence of felsic rocks (Payré et al, 2022), all located in the southern hemisphere, where the crust is thicker. These rocks likely formed by differentiation of a basic protolith. On Earth, this process occurs at plate boundaries and is linked to active plate tectonics. But on Mars, we have no evidence of active or ancient plate tectonics.

On one-plate planets, there exists a positive feedback mechanism on crustal growth: the crust being enriched in heat-producing elements, the lithosphere is hotter and thinner where the crust is thicker, which implies a larger melt fraction at depth and therefore a larger extraction rate and a larger crustal thickening where the crust is thicker. We proposed that this mechanism could have been at the origin of the Martian dichotomy (Bonnet Gibet et al, 2022). This mechanism further implies that regions of thicker crusts, characterized by a larger amount of heat sources, a thinner lithosphere and an increased magmatism, are also marked by higher temperatures. Here we investigate whether crustal temperatures in regions of thick crust may be maintained above the basalt solidus temperature during crust construction, which would allow for the formation of partially molten zones in the crust and hence differentiated rocks by extraction of the melt enriched in water and silica. In this scenario, felsic rock formation would be concomitant to crustal construction and dichotomy formation on Mars.

We use a bi-hemispheric parameterized thermal evolution model with a well-mixed mantle topped by two different lithospheres (North and South) and we account for crustal extraction and magmatism in these two hemispheres. We formulate a Bayesian inverse problem in order to estimate the possible scenarios of thermal evolution that are compatible with constraints on crustal thickness and dichotomy amplitude derived from the InSight NASA mission. The solution is represented by a probability distribution representing the distribution on the model parameters and evolution scenarios. This distribution is sampled with a Markov chain Monte Carlo algorithm, and shows that a non-negligible range of scenarios allows for partial melting at the base of the Southern crust below the Highlands during the first Gyr of Mars' evolution. On the contrary, partial melting of the base of the northern crust is insignificant. Models that fit InSight constraints and allow for differentiation of a fraction of the Southern crust point to a relatively low reference viscosity (~1020 Pa.s) that can be explained by a wet mantle at the time of crust extraction.

How to cite: Bonnet Gibet, V., Michaut, C., Bodin, T., Wieczorek, M., and Dubuffet, F.: Differentiation of the Martian Highlands during its formation., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2786, https://doi.org/10.5194/egusphere-egu23-2786, 2023.

14:16–14:18
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PICO3a.9
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EGU23-11648
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ECS
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On-site presentation
Kar Wai Cheng, Maxim Ballmer, and Paul Tackley

It has been proposed that a basal magma ocean (BMO) may have existed, or even still exists, at the base of the Martian mantle [1]. One formation scenario for such a BMO involves a mantle-scale overturn just after the crystallization of the main magma ocean. In this case, the BMO would be enriched in iron and heat-producing elements (HPE), and hence gravitationally stable at the base of the mantle, with potential effects on the efficiency of mantle convection. The Insight mission has allowed geophysical investigation of the Martian interior and has indeed provided seismic evidence of a basal liquid silicate layer just above the core-mantle boundary. It is thus crucial to understand the effect of such a layer on the long-term evolution of the interior of Mars.

Here, we model thermochemical mantle convection and crust production for a Mars-sized planet in a 2D spherical annulus geometry using code StagYY.  Assuming that the top of the BMO is at ~1800 km radius, we parameterize the basal magma ocean as a ‘primordial layer’ with a low viscosity and a high effective thermal conductivity to account for the enhanced effective heat flux in a liquid layer due to turbulent flow. HPE are preferentially partitioned into the silicate liquid layer following a mass balance equation assuming an interstitial porosity.  We systematically vary BMO thickness and interstitial porosity in order to study the outcome of the different HPE distributions.  The liquid density, which is attributed by the different degrees of iron enrichment, is also examined to explore the mechanical stability and entrainment of the BMO.

We present results of our models, comparing our present-day temperature profiles with areotherms deduced from seismic observation [2,3].  We find that the interstitial porosity is an important factor that determines the thermal structure of the mantle throughout Martian evolution. A value of ~50% provides the best fit with crustal production history, crustal thickness, HPE enrichment in the crust, as well as the seismically-constrained present-day areotherm. This result suggests that the initial HPE partitioning has not been controlled by end-member fractional crystallization of the main magma ocean (for which interstitial porosity would be close to 0%), and/or that some re-equilibration occurred during subsequent overturn. Meanwhile, the BMO thickness, within the uncertainties from seismic inversion, does not strongly influence Mars thermal evolution.

 

[1] Samuel et al. (2021) doi:10.1029/2020JE006613

[2] Khan et al. (2021) doi: 10.1126/science.abf2966

[3] Duran et al. (2022) doi: 10.1016/j.pepi.2022.106851

How to cite: Cheng, K. W., Ballmer, M., and Tackley, P.: Long-term effect of a basal magma ocean on Martian mantle convection, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11648, https://doi.org/10.5194/egusphere-egu23-11648, 2023.

14:18–14:20
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PICO3a.10
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EGU23-14685
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ECS
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On-site presentation
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Julia Marleen Schmidt and Lena Noack

Upon melting inside planetary upper mantles, trace elements which are incompatible in the solid rock – such as heat producing elements or volatiles - are redistributed into the melt. If the melt is less dense than the surrounding material, the melt transports the elements towards the surface, where it enriches the crust and leaves a depleted upper mantle behind. In the case of heat producing elements, this process can affect the thermal evolution and crust production of a planet, whereas in the case of volatiles, the outgassing and atmosphere evolution can be influenced. With the help of mineral/melt partition coefficients, we are able to quantify the amount of the redistributed elements and can therefore infer the impact on the aforementioned planetary processes. Mineral/melt partition coefficients depend highly on pressure, temperature, and composition. However, due to a lack of high-pressure experiments and models, they were typically taken as constant in mantle evolution models.

In this study, we developed a 1D interior evolution model and included a pressure, temperature, and melt composition dependent mineral/melt partition coefficient model that is applicable for higher pressures (Schmidt & Noack, 2021). We apply the model to the five planetary bodies Mercury, Venus, Earth, Moon, and Mars and show that the planet size has a significant effect on the partition coefficients and therefore on the redistribution of heat producing elements and volatiles. This makes most partition coefficients based on low-pressure experiments with an Earth-based composition quite inaccurate in interior evolution models. We quantify the resulting effects on the thermal evolution, crust production, and outgassing rate. Additionally, we vary other starting parameters and compare how this affects the amount of the elements that were redistributed into the crust or outgassed into the atmosphere. These findings help us to understand the effect of depth-dependent redistribution for different types of rocky planets and might be relevant for a wide range of mantle evolution models which include mantle melting and trace element redistribution.

Schmidt, J.M. and Noack, L. (2021): Clinopyroxene/Melt Partitioning: Models for Higher Upper Mantle Pressures Applied to Sodium and Potassium, SysMea, 13(3&4), 125-136.

How to cite: Schmidt, J. M. and Noack, L.: Planet size controls the redistribution of heat producing elements and volatiles from mantle to crust, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14685, https://doi.org/10.5194/egusphere-egu23-14685, 2023.

14:20–14:22
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PICO3a.11
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EGU23-3076
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ECS
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On-site presentation
Sheng Shang and Yanhao Lin

Zircon is an important silicate mineral to help understand the evolution of geochemistry and genesis of magma in early planets. The composition of evolved magma can be deduced from the concentrations of elements in zircon and their partition coefficients between zircon and silicate melt. Although the phosphorus (P) contents range from ~100 to ~100000 ppm in extraterrestrial zircon, the effects of P on REE partition coefficients between zircon and silicate melt are still debated. Here we have studied the effect of P contents on the partition coefficients of elements between zircon and silicate melt using high-temperature experiment. With the increase of phosphorus content, the partition coefficients of alkaline elements and Al between zircon and silicate melt show a negative and positive trend, respectively, and there is no effect on itself and Ti. It is worth mentioning that phosphorus content has a negligible effect on REE partitioning, indicating that the REE partition coefficients in this study can be applied to extraterrestrial zircon even with varying P concentrations. After filtering out altered zircon and combining the experimentally updated partition coefficients of REE, the characteristic of evolved melt equilibrated with early protogenetic zircon can thus be yielded and then help to understand early magmatism on the planets. 

How to cite: Shang, S. and Lin, Y.: Experimentally revisiting the REE partition coefficients between zircon and silicate melt, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3076, https://doi.org/10.5194/egusphere-egu23-3076, 2023.

14:22–15:45
Chairpersons: Lena Noack, Paolo Sossi
Earth and exoplanets
16:15–16:17
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PICO3a.1
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EGU23-10404
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On-site presentation
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Zheng-Xiang Li, Yebo Liu, and Richard Ernst

An advanced understanding of how tectonic plates have moved since deep time is essential for understanding how Earth’s geodynamic system has evolved and interacted with the plate tectonic system, i.e., the longstanding question of what “drives” plate tectonics. In this work, we take advantage of the rapidly improving database and knowledge about the Precambrian world, and the conceptual breakthroughs both regarding the presence of a supercontinent cycle and possible dynamic coupling between the supercontinent cycle and mantle dynamics, to establish a full-plate global reconstruction back to 2000 Ma. We utilise a variety of global geotectonic databases to constrain our reconstruction, and use palaeomagnetically recorded true polar wander events and global plume records to help evaluate competing geodynamic models regarding the origin and evolution of first-order mantle structures, and provide new constraints on the absolute longitude of continents and supercontinents. After revising the configuration and life span of both supercontinents Nuna (1600–1300 Ma) and Rodinia (900–720 Ma), we present here a 2000–540 Ma animation featuring the rapid assembly of large cratons and supercratons (or megacontinents) between 2000 Ma and 1800 Ma after billion years of dominance by many small cratons, that kick started the ensuing Nuna and Rodinia supercontinent cycles and the emergence of hemisphere-scale (long-wavelength) degree-1/degree-2 mantle structures. We further use the geodynamically-defined type-1 and type-2 inertia interchange true polar wander (IITPW) events, which likely occurred during Nuna (type-1) and Rodinia (type-2) times as shown by the palaeomagnetic record, to argue that Nuna assembled at about the same longitude as the latest supercontinent Pangea (320–170 Ma), whereas Rodinia formed through introversion assembly over the legacy Nuna subduction girdle either ca. 90° to the west (our preferred model) or to the east before the migrated subduction girdle surround it generated its own degree-2 mantle structure. Our interpretation is broadly consistent with the global LIP record. Using TPW and LIP observations and geodynamic model predictions, we further argue that the Phanerozoic supercontinent Pangaea assembled through extroversion on a legacy Rodinia subduction girdle with a geographic centre at around 0°E longitude before the formation of its own degree-2 mantle structure, the legacy of which is still present in present-day mantle.   

How to cite: Li, Z.-X., Liu, Y., and Ernst, R.: A geodynamic framework for 2 billion years of tectonic evolution: From cratonic amalgamation to the age of supercontinent cycle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10404, https://doi.org/10.5194/egusphere-egu23-10404, 2023.

16:17–16:19
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PICO3a.2
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EGU23-4755
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ECS
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On-site presentation
Charitra Jain and Stephan Sobolev

In this work, we test the hypothesis of surface-erosion controlled plate tectonics preceded by plume-induced retreating subduction tectonic regime on Earth proposed by Sobolev and Brown (2019) using 2D global compressible convection models. To simulate the effect of increased sediment supply as a result of surface erosion after the emergence of continents in the late Archean and after the Neoproterozoic "snowball Earth" glaciation, we decrease the effective frictional strength of the oceanic lithosphere in models spanning the age of the Earth. These StagYY models self-consistently generate oceanic and continental crust while considering both plutonic and volcanic magmatism (Jain et al., 2019). Pressure-, temperature-, and composition-dependent water solubility maps calculated with Perplex (Connolly, 2009) are also utilised, which control the ingassing and outgassing of water between the mantle and surface (Jain et al., 2022). The core cools with time and different initial mantle potential temperature values are tested within the range of 1750-1900 K (Herzberg et al., 2010; Aulbach and Arndt, 2019).

Models that consider a more realistic upper mantle rheology (diffusion creep and dislocation creep proxy) show higher recycling of denser basaltic-eclogitic (oceanic) crust, efficient cooling of the planet, and higher mobilities (ratio of surface to mantle rms velocities) (Tackley (2000); Lourenço et al. (2020)). These models exhibit intermittent episodes of long-lasting mobile-lid regime and short-lived plutonic-squishy-lid regime in the Hadean and the early Archean accompanied by extensive subduction leading to rapid production and recycling of the continental crust. Models that consider adaptive frictional strength (to mimic sedimentation post glaciation and continental emergence) predict the transition to continuous plate tectonics in the late Archean, reproduce features of supercontinent cycles, and appear to be consistent with cooling history of the Earth inferred from petrological observations (Herzberg et al., 2010). 

The thermo-compositional evolution can vary between models due to the inherent randomness arising from the initial thermal perturbations and the initial positions of the tracers/particles. Accordingly, we intend to run multiple instances of every model considered in our parameter space to present statistically robust results. We also aim to test more realistic models where the lithospheric frictional strength adapts with the surface topography.

How to cite: Jain, C. and Sobolev, S.: Exploring the interplay between continent formation, surface erosion, and the evolution of plate tectonics on Earth, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4755, https://doi.org/10.5194/egusphere-egu23-4755, 2023.

16:19–16:21
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PICO3a.3
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EGU23-8965
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ECS
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On-site presentation
Jiacheng Tian and Paul Tackley

The short-lived isotope systems, including 146Sm-142Nd (half-life = 103 Ma) and 182Hf-182W (half-life = 8.9 Ma), provide evidence for mantle differentiation events in early Earth, as both the daughter nuclides are more incompatible than the parent nuclides. For the 146Sm-142Nd system, both positive and negative μ142Nd measurements are observed in Hadean-Archean mantle-derived rocks, which possibly indicates a major differentiation event of the silicate Earth before the extinction of 146Sm (e.g., Boyet and Carlson, 2005, Science). The diminishing trend of μ142Nd between Hadean and Archean, on the other hand, suggests continuous mantle mixing during this period. However, for the 182Hf-182W system, Hadean-Archean mantle-derived rocks often show positive μ182W anomalies followed by a decline in at 2.5~3.0 Ga ago without a mixing trend (e.g., Carlson et al., 2019, Chem. Geol.). Also, μ142Nd and μ182W often show no or negative correlation in Hadean-Archean mantle derived rocks (e.g., Rizo et al. 2016, Geochim. Cosmochim. Acta), which requires a mechanism to decouple these two isotopic systems.

In this study, we implement both 182Hf-182W and 146Sm-142Nd system in a global thermochemical geodynamic model, StagYY (Tackley, 2008, PEPI), to track the evolution of these isotope systems through Earth’s mantle evolution. Based on the particle-in-cell method, the geodynamic model incorporates melting and magmatic crust production that allow us to track both fractionation (by melting and crustal production) and mixing (through mantle convection) of trace elements through time. We discuss in detail how (1) the ‘basalt barrier’ at the base of the mantle transition zone (Davies, 2008 EPSL), (2) crustal delamination from intrusive magmatism, or plutonic-squishy-lid tectonics (Lourenco et al., Nat. Geo. 2018; GCubed 2020), and (3) late accretion could affect the tectonics of early Earth, and the preservation of geochemical heterogeneities and decoupling of two isotopic systems in the mantle through time.

 

How to cite: Tian, J. and Tackley, P.: Long-term preservation of geochemical heterogeneities in early Earth: tracking short-lived isotopes in geodynamic models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8965, https://doi.org/10.5194/egusphere-egu23-8965, 2023.

16:21–16:23
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PICO3a.4
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EGU23-10101
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ECS
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On-site presentation
Matteo Desiderio, Anna J. P. Gülcher, and Maxim D. Ballmer

Our understanding of the compositional structure of Earth's mantle is still incomplete. Heterogeneity in the lower mantle, documented by both geochemical and geophysical observations, has not yet been explained within a definitive geodynamic framework. Moreover, the origin, geometry and interaction of such heterogeneities remain controversial. In the “marble cake” mantle hypothesis, slabs of basaltic Recycled Oceanic Crust (ROC) are subducted and deformed but never fully homogenized in the convecting mantle. Conversely, MgSiO3-rich primordial material may resist convective entrainment due to its intrinsic strength, leading to a “plum pudding” mantle. While previous geodynamic studies have successfully reproduced these regimes of mantle convection in numerical models, the effects of the physical properties of ROC on mantle dynamics have not yet been fully explored. Furthermore, predictions from numerical models need to be tested against geophysical observations. However, current imaging techniques may be unable to discriminate between these two end members, due to limited resolution in the lower mantle.

Here, we model mantle convection in a 2D spherical-annulus geometry with the finite-volume code StagYY. We investigate the style of heterogeneity preservation as a function of the intrinsic density and strength (viscosity) of basalt at lower-mantle conditions. Additionally, we use the thermodynamic code Perple_X and the spectral-element code AxiSEM to compute, respectively, seismic velocities and synthetic seismograms from the predictions of our models.

Our results fall between two end-member regimes of mantle convection: low-density basalt leads to a well-mixed, "marble cake"-like mantle, while dense basalt aids the preservation of primordial blobs at mid-mantle depths as in a "plum pudding". Intrinsically viscous basalt also promotes the preservation of primordial material. These trends are well explained by lower convective vigour of the mantle as intrinsically dense (and viscous) piles of basalt shield the core. In order to test these results, we translate the predicted compositional, temperature and pressure fields to seismic velocities for two opposite end-member cases. These two synthetic velocity maps are first analysed and compared in terms of their respective radial correlation matrices and spherical harmonic spectra. Then, we use AxiSEM to simulate wave propagation through the two velocity models. Finally, we discriminate between the two end-members by comparing statistical properties of the corresponding ensembles of synthetic seismograms. Our results highlight how the interplay between primordial and recycled heterogeneities shape the evolution of the thermal and compositional structure of the lower mantle. Furthermore, they provide a framework for relating the style of heterogeneity preservation in the Earth's lower mantle with specific features of the seismic waveforms.

How to cite: Desiderio, M., Gülcher, A. J. P., and Ballmer, M. D.: The Heterogeneous Earth Mantle: Numerical Models of Mantle Convection and their Synthetic Seismic Signature, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10101, https://doi.org/10.5194/egusphere-egu23-10101, 2023.

16:23–16:25
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PICO3a.5
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EGU23-12628
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ECS
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On-site presentation
Alexandre Janin, Nicolas Coltice, Julien Tierny, and Nicolas Chamot-Rooke

The rigid surface of the Earth is divided into a jigsaw puzzle of about 50 tectonic plates separated by boundaries. Nowadays, three-dimensional spherical mantle modelling manages to produce self-consistently a stiff surface fragmented into several rigid caps that exhibit a plate-like behaviour. It thus becomes possible to analyse the dynamics of these models through the prism of plate tectonics theory and compare it to plate reconstruction models for the Earth. Such an analysis requires a robust method to automatically detect plates and their boundaries from continuous geophysical fields. The method should further recognize diffuse plate boundaries, as observed on Earth and reproduced in mantle convection models. We propose here a method to automatically detect and track plates through time, based on a trans-disciplinary approach combining a geodynamical and kinematic analysis with applied mathematics and computer sciences. This analysis is performed using the free and open-source software Paraview and the open-source software platform TTK (Topology ToolKit) designed for an efficient topological analysis of scalar fields. We apply our method to a three-dimensional spherical mantle convection model generating Earth-like plate tectonics at its surface. Our results show that, as for the Earth, the motion of modelled plates is stable over million-years-long periods separated by abrupt reorganizations occurring in less than 5 Myrs. The full plate-motion analysis over 262 Myrs in the model allows us to discuss the spatial extent of kinematic changes and shows that a plate reorganization can have regional to global effects on the plate network.

How to cite: Janin, A., Coltice, N., Tierny, J., and Chamot-Rooke, N.: The automatic detection of tectonic plates in 3D mantle convection models and plate motion changes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12628, https://doi.org/10.5194/egusphere-egu23-12628, 2023.

16:25–16:27
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PICO3a.6
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EGU23-7732
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On-site presentation
Hongzhan Fei, Ulrich Faul, Maxim Ballmer, Nicolas Walte, and Tomoo Katsura

The absence of seismic anisotropy in most regions of the lower mantle suggests that diffusion creep may be the dominant mechanism in the lower mantle. Because the diffusion-creep rate is inversely proportional to the 2~3 power of grain size, knowledge of the grain-growth kinetics is crucial for studying lower-mantle dynamics. For these reasons, this study determined the grain-growth kinetics of bridgmanite at a pressure of 27 GPa using advanced multi-anvil techniques.

We first measured the grain sizes of bridgmanite in an olivine bulk composition with various annealing durations at 2200 K. The results were fitted to an equation dnd0n = kt, where d and d0 are the final and initial grain sizes, respectively, n is the grain-size exponent, t is the annealing duration, and k is the growth-rate constant. This fitting yielded n = 5.2 ± 0.3, which is much smaller than given by a previous study [Yamazaki et al., 1996], n = 10.6 ± 1.1. This discrepancy may be because Yamazaki et al.’s [1996] olivine starting material may have contained adhesive water, which enhanced grain growth at the beginning of annealing. We then conducted runs at various temperatures, yielding the activation energy of 260 ± 20 kJ/mol. These results suggest that the bridgmanite grain sizes over 0.1 – 1 Gyr should have grain sizes of 150-230 μm, which is one order of magnitude larger than Yamazaki et al.’s [2006] estimation. Consequently, the lower mantle should be much harder than previously considered.

Furthermore, we measured the grain-growth kinetics as a function of the fraction of coexisting ferropericlase. Although the grain-growth kinetics is almost independent of the ferropericlase fraction down to 20 vol.%, it rapidly increases with decreasing ferropericlase fraction at lower fractions. Over 0.1~4.5 Gyr, the bridgmanite grain sizes in pure-bridgmanite rock should be 2 ~ 3 orders of magnitude larger than those coexisting with 20 vol.% of ferropericlase. These results suggest that pure-bridgmanite rock has 4 ~ 9 orders of magnitude lower flow rates than pyrolite if the diffusion creep is dominant. Since the diffusion creep rate in pure-bridgmanite rock is so low, the dislocation creep should dominate in pure-bridgmanite rock. We estimated that the pure-bridgmanite rock should have 1 ~ 2.5 orders of magnitude more viscous than pyrolite if the stress condition is 0.1~0.5 MPa in the lower mantle. This variation may interpret the viscosity variation in the lower mantle inferred from the geoid analysis [Rudolph et al., 2015], subduction speed [van der Meer et al., 2018], and plume morphology [French & Romaniwicz, 2016].

How to cite: Fei, H., Faul, U., Ballmer, M., Walte, N., and Katsura, T.: Grain growth kinetics of bridgmanite under topmost lower-mantle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7732, https://doi.org/10.5194/egusphere-egu23-7732, 2023.

16:27–16:29
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PICO3a.7
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EGU23-9100
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ECS
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On-site presentation
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Jyotirmoy Paul, Gregor Golabek, Antoine Rozel, Paul Tackley, Tomo Katsura, and Hongzhan Fei

Grain-size evolution is a crucial controlling factor for the lower mantle rheology. Notably, one order of grain size change can produce a viscosity change of the order of 100-1000 times. As diffusion creep dominates in the lower mantle, grain growth of lower mantle mineral assemblages, e.g., bridgmanite and ferropericlase, increase viscosity considerably. It has been quite challenging to constrain the grain-size evolution parameters for lower mantle mineral assemblages until recently; a new high-pressure experimental study (27 GPa, cf. Fei et al, 2021, EPSL) parameterised them. The experimental data found a slower grain growth of bridgmanite-ferropericlase phases than of the upper mantle mineral phases, e.g., olivine and spinel. Using the most updated knowledge of grain-size evolution, we develop 2-D spherical annulus numerical models of self-consistent mantle convection using the finite volume code StagYY and explore how grain-size evolution affects the lower mantle dynamics. We test our models with different heterogeneous grain size evolution and composite rheology that evolve self-consistently for 4.5 billion years. Our preliminary models show the self-consistent formation of thermochemical piles at the base of the core-mantle boundary where the grain size is maximum (~3 times than the surroundings). Even though the bridgmanite-ferropericlase grain growth is slower, a slight increase in the grain size of thermochemical piles can make them ~100-1000 times viscous, subsequently helping them to achieve morphological stability over billion years. In some of our models, we find sweeping stability of the piles for ~500 million years. 

How to cite: Paul, J., Golabek, G., Rozel, A., Tackley, P., Katsura, T., and Fei, H.: Effect of grain-size evolution on the lower mantle dynamics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9100, https://doi.org/10.5194/egusphere-egu23-9100, 2023.

16:29–16:31
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PICO3a.8
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EGU23-3622
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ECS
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On-site presentation
Diogo Louro Lourenço, Michael Manga, and Paul Tackley

A tectonic regime is the surface expression of interior dynamics in a planet. With the help of numerical models, different tectonic regimes have been proposed. Some of these are: (1) plate tectonics or mobile lid, (2) stagnant lid, (3) episodic lid, (4) plutonic-squishy lid, (5) and heat pipe (e.g., Lourenço et al., G3 2020). Over time, a tectonic regime shapes the surface of a planet, including its surface topography. Using the numerical models, we can compute the topographies associated with different tectonic regimes including spatial and temporal measures of variations. In this study, we compute statistics for the topography formed by different tectonic regimes in numerical models and compare with the statistics of observed topography of different terrestrial planets, with the aim of linking a planet to a tectonic regime at the present-day. Venus’ topography is better matched by topography distributions obtained for plutonic-squishy lid models than those for stagnant- or episodic-lid models, while Earth’s oceanic topography is best matched by mobile-lid models.

How to cite: Louro Lourenço, D., Manga, M., and Tackley, P.: Topographic signatures and statistics of different tectonic regimes and application to terrestrial planets, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3622, https://doi.org/10.5194/egusphere-egu23-3622, 2023.

16:31–16:33
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PICO3a.9
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EGU23-7777
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ECS
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On-site presentation
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Rob Spaargaren, Maxim Ballmer, Stephen Mojzsis, and Paul Tackley

New terrestrial exoplanets are being discovered at an ever faster pace, and each discovery leads to a widening of our understanding of planetary diversity. A key aspect in the quest to better quantify terrestrial planet diversity is to gain information on plausible bulk compositions, as this physical-chemical quantity determines the planet's structure, which in turn controls physical properties of the its layers (core, mantle, crust, atmosphere). Recent insights in the expected range of bulk planet compositions allow us to investigate how this fundamental parameter affects the evolution of the planetary interior and surface, and consequently to guide next-generation ground- and space-based telescopic observations of exoplanet properties, such as atmospheric composition.

Here, we first simulate mantle mineralogies for exoplanets with various bulk compositions, using a Gibbs energy minimization algorithm, Perple_X. Using mineralogy and resulting physical properties, we employ a 2D global-scale model of thermochemical mantle convection to investigate the variations between Earth-sized exoplanets of different compositions in terms of interior evolution. We include the effects of composition on planet structure, mantle physical properties, and mantle melting. We investigate how composition affects thermal evolution, and whether it has an effect on the propensity of a planet towards plate tectonics-like behaviour.

In general, Earth tends to have an average composition for most elements, except for iron, which it is relatively rich in, and therefore it has an above average core size. Our preliminary results show that core size (and thus iron abundance) affects convective vigor, and thus thermal evolution of the interior. We further find major differences for planets with different ratios of Mg-silicates, as these minerals control mantle viscosity, and thereby thermal evolution. Planets with lower Mg/Si than Earth will have a significantly stronger mantle, impeding cooling on planetary lifetimes, while planets with much higher Mg/Si have weaker upper mantles, impacting surface mobility. Stellar Mg/Si is a good indicator of the relative abundances of these minerals, and can be an important source of information. Therefore, the host stellar abundances seem to be an indicator of rocky planet properties, and can be used in the target selection for future missions.

How to cite: Spaargaren, R., Ballmer, M., Mojzsis, S., and Tackley, P.: Exploring the effects of terrestrial exoplanet bulk composition on long-term planetary evolution, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7777, https://doi.org/10.5194/egusphere-egu23-7777, 2023.

16:33–16:35
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PICO3a.10
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EGU23-16119
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On-site presentation
Paul Tackley

Terrestrial exoplanets, ranging in size up to approximately twice Earth-size (10 Earth masses), may have a range of characteristics that are not found in solar system planets, including but not limited to: larger size, different bulk composition (possibly resulting in being core-less), being tidally-locked to their host star, and being covered by water layers. Larger size has been proposed to result in sluggish deep-mantle convection and also (for stagnant-lid exoplanets) lower magmatism and outgassing, but internal differentiation is still expected to take place. Different bulk composition may lead to different viscosity (among other physical properties), modified melting behaviour and different core size (including the possibility of having no core). Tidally-locked exoplanets likely have hemispherical tectonics and internal structures, but the asymmetry would be reduced if they are continuously reorienting due to true polar wander. We are pursuing a range of studies investigating most of these different aspects using thermo-chemical convection models that include self-consistent lithospheric dynamics, partial melting and crustal production, using the code StagYY. Some of these studies are presented elsewhere at this meeting; this presentation will focus on additional interesting results.

How to cite: Tackley, P.: Studies of terrestrial exoplanet thermo-chemical-magmatic mantle and lithosphere dynamics and evolution, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16119, https://doi.org/10.5194/egusphere-egu23-16119, 2023.

16:35–16:37
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PICO3a.11
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EGU23-723
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ECS
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Virtual presentation
Mohamed Ismail and Maxim Ballmer

     Constraining thermo-chemical evolution for the interior of terrestrial planets is substantial to understanding their evolutionary path. Thermo-chemical processes is controlled by stages of large-scale melting, or magma oceans (MO), due to the energy released during accretion, differentiation, radioactive decay of heat-producing elements and crystallization of the melt. Previous work shows that one of the product of considering fractional crystallization (FC) for  MO is a FeO-enriched molten layer or basal magma ocean (BMO) which is stabilized at the core-mantle boundary for a few billion years. The BMO is expected to freeze by FC because it cools very slowly. FC always yield a highly iron-enriched BMO and last stage cumulates. Other crystallization mode could be dominated and has not yet been systemically explored – at least for the Earth-like planets.

To explore the fate of the BMO cumulates in the convecting mantle, we explore 2D geodynamic models with a moving-boundary approach. Flow in the mantle is explicitly solved, but the thermal evolution and related crystallization of the BMO are parameterized. The composition of the crystallizing cumulates is self-consistently calculated  in the FeO-MgO-SiO2 ternary system according to Boukaré et al. (2015). In some cases, we also consider the effects of Al2O3 on the cumulate density profile. We then investigate the  entrainment and mixing of BMO cumulates by solid-state mantle convection over billions of years as a function of BMO initial composition and volume, BMO crystallization timescales, distribution of internal heat sources, and mantle rheological parameters (Ra# and activation energy), . We varied the initial composition of BMO by manipulating the molar fraction of FeO, MgO, and SiO -based on published experiments- to model different BMO-compositions: Pyrolitic composition, After 50% crystallization of Pyrolitic composition Boukaré et al. (2015), After 50% crystallization of Pyrolitic composition Caracas et al. (2019), and Archean Basalt.

For all our model cases, we find that most of the cumulates (first ~90% by mass) are efficiently entrained and mixed through the mantle. However, the final ~9% of the cumulates are too dense to be entrained by solid-state mantle convection, and rather remain at the base of the mantle as a strongly FeO-enriched solid layer. We conclude that this inevitable outcome of BMO FC – at least for Earth - leads to inconsistent evolutionary path comparing to recent geophysical constraints. FC substantially change the compositional, thermal, and geometrical properties for the lower mantle structures.  An alternative mode of crystallization may be driven by an efficient reaction between a highly-enriched last-stage BMO with the overlying mantle due to chemical disequilibrium. 

How to cite: Ismail, M. and Ballmer, M.: The Consequences of Fractional Crystallization for Basal Magma Ocean on the Long-term Planetary Evolution, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-723, https://doi.org/10.5194/egusphere-egu23-723, 2023.

16:37–16:39
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PICO3a.12
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EGU23-14341
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On-site presentation
On the Origin and Stability of Thermochemical Heterogeneities
(withdrawn)
Claudia Stein, Carolin Zander, and Ulrich Hansen
16:39–18:00