GD1.1

Causes of dynamic, tectonic, and compositional transitions in the Earth and rocky planets

The present state of Earth and other rocky planets are an expression of dynamical and chemical processes occurring throughout their history. Plate tectonics is one of several planetary heat and mass transport regimes, and transitions into and out of this regime cannot be understood by looking at a single example. The rock-record, through geochemistry and magnetism, is used to interrogate changes in the tectono-thermal regime of Earth’s interior through time, while seismic imaging and gravity data, for instance, provide a snapshot of processes occurring in the contemporary mantle, crust and core. These classes of observations may be linked through geodynamic models, whose accuracy is underpinned by the physical properties (e.g., viscosity and density) of its constituent phases (minerals, melts and fluids). Information on the fundamental thermodynamic and physical behaviour of phases is subject to constant advance via experimental and ab-initio techniques.

This session aims to provide a holistic view of the dynamics, structure and composition of Earth, from core to atmosphere, and their evolution through time. We welcome contributions that address questions surrounding Earth’s major geological transformations and initial conditions that include, but are not limited to, study of the Hadean/Archean to better understand plate tectonic behaviour and transitions, magma ocean dynamics, oxidation of planetary interiors/atmospheres and the habitability of silicate worlds. Studies using a multidisciplinary approach are particularly encouraged.

Co-organized by GMPV4/PS3
Convener: Ingo StotzECSECS | Co-conveners: Doris Breuer, Hiu Ching Jupiter ChengECSECS, Tim JonesECSECS, Adrian Lenardic, William Moore, Simone PiliaECSECS, Paolo SossiECSECS
vPICO presentations
| Fri, 30 Apr, 15:30–17:00 (CEST)

vPICO presentations: Fri, 30 Apr

Chairpersons: Ingo Stotz, William Moore, Hiu Ching Jupiter Cheng
15:30–15:40
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EGU21-3315
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solicited
Lindy Elkins-Tanton, Jenny Suckale, and Sonia Tikoo

Rocky planets go through at least one and likely multiple magma ocean stages, produced by the giant impacts of accretion. Planetary data and models show that giant impacts do not dehydrate either the mantle or the atmosphere of their target planets. The magma ocean liquid consists of melted target material and melted impactor, and so will be dominated by silicate melt, and also contain dissolved volatiles including water, carbon, and sulfur compounds.

As the magma ocean cools and solidifies, water and other volatiles will be incorporated into the nominally anhydrous mantle phases up to their saturation limits, and will otherwise be enriched in the remaining, evolving magma ocean liquids. The water content of the resulting cumulate mantle is therefore the sum of the traces in the mineral grains, and any water in trapped interstitial liquids. That trapped liquid fraction may in fact be by far the largest contributor to the cumulate water budget.

The water and other dissolved volatiles in the evolving liquids may quickly reach the saturation limit of magmas near the surface, where pressure is low, but degassing the magma ocean is likely more difficult than has been assumed in some of our models. To degas into the atmosphere, the gases must exsolve from the liquid and form bubbles, and those bubbles must be able to rise quickly enough to avoid being dragged down by convection and re-dissolved at higher pressures. If bubbles are buoyant enough (that is, large enough) to decouple from flow and rise, then they are also dynamically unstable and liable to be torn into smaller bubbles and re-entrained. This conundrum led to the hypothesis that volatiles do not significantly degas until a high level of supersaturation is reached, and the bubbles form a buoyant layer and rise in diapirs in a continuum dynamics sense. This late degassing would have the twin effects of increasing the water content of the cumulates, and of speeding up cooling and solidification of the planet.

Once the mantle is solidified, the timeclock until the start of plate tectonics begins. Modern plate tectonics is thought to rely on water to lower the viscosity of the asthenosphere, but plate tectonics is also thought to be the process by which water is brought into the mantle. Magma ocean solidification, however, offers two relevant processes. First, following solidification the cumulate mantle is gravitationally unstable and overturns to stability, carrying water-bearing minerals from the upper mantle through the transition zone and into the lower mantle. Upon converting to lower-mantle phases, these minerals will release their excess water, since lower mantle phases have lower saturation limits, thus fluxing the upper mantle with water. Second, the mantle will be near its solidus temperature still, and thus its viscosity will be naturally low. When fluxed with excess water, the upper mantle would be expected to form a low degree melt, which if voluminous enough with rise to help form the earliest crust, and if of very low degree, will further reduce the viscosity of the asthenosphere.

How to cite: Elkins-Tanton, L., Suckale, J., and Tikoo, S.: On the fate of water in the formation of rocky planets, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3315, https://doi.org/10.5194/egusphere-egu21-3315, 2021.

15:40–15:42
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EGU21-9238
Edgar M. Parmentier, Linda Elkins-Tanton, and Christian Huber

Planetary bodies with a sufficiently energetic origin are likely to begin their evolution in largely liquid state.  Cooling and crystallization at the surface of a mostly liquid magma ocean (MO) is expected to produce a sedimented partially crystallized cumulate of melt and denser mineral grains at its base.  The rate of crystallization and cumulate sedimentation are controlled by radiation through an atmosphere devolatilized from the vigorously convecting MO.  Melt retained in the cumulate is initially isolated from the overlying MO and atmosphere; but through compaction and buoyant migration in permeable cumulates, retained melt may be discharged into the overlying MO and its dissolved volatiles contributed to the growing atmosphere. The rates of cumulate compaction and radiative cooling though the atmosphere may thus play interacting and competing roles governing the time scale of MO evolution.

We explore these effects using a thermal evolution model similar to that described by Elkins-Tanton (2008; doi.org/10.1016/j.epsl.2008.03.062).  In the current study, the top of the cumulate layer is defined by a depositional melt fraction (~50%) and temperature at which a liquid of MO composition behaves like a viscous solid. Heat flux from the MO surface is limited by radiation through a gray H2O-CO2 rich atmosphere (Abe and Matsui, 1988; doi.org/10.1175/1520-0469(1988)045<3081:EOAIGH>2.0.CO;2).  We consider Mars and Earth-like bodies with initial bulk H2O-CO2 concentrations 0.5%-0.1% and 0.05%-0.01% and vary the prescribed amount of retained melt in the cumulate from 0% (instantaneous compaction) to 50% (no compaction).  For the Mars-sized body increasing retained melt fraction over this range reduces MO freezing time by nearly one order of magnitude (from ~1 Myr to <0.1 Myr) and two orders of magnitude (from ~0.1 Myr to <0.001 Myr) for the larger and smaller volatile concentrations, respectively.  The Earth-like body shows similar behavior.

The melt fraction retained in compacting cumulate deposited at constant, prescribed sedimentation rate is determined by the rate of buoyant melt migration (Shirley 1986; doi.org/10.1086/629088).  For reasonable values of cumulate grainsize (~1 mm; Solomatov and Stevenson, 1993; doi.org/10.1029/92JE02839) and interstitial melt viscosity (~0.1 Pa-s). Cumulates in a Mars-sized, 1000 km deep MO solidifying in 0.1 Myr (cumulate thickening rate ~ 104 km/Myr) should retain melt fractions in the range of 10 to 30%, consistent with values the above thermal model shows are needed to produce this solidification rate.  Nearly an order of magnitude reduction in freezing time due to retained melt can be expected.

Ongoing work integrates the thermal evolution and migration of retained melt into a unified self-consistent model in which the variation of cumulate sedimentation rate with time is determined by the heat flux through the evolving atmosphere.  Our results thus far indicate that volatiles contained in melt retained within cumulates, rather than being added to a growing atmospheric mass, could significantly reduce the time scale of MO solidification.  Exploring this for small planetesimal-sized bodies will be particularly interesting since smaller gravity will reduce the rate of cumulate melt segregation while atmospheric escape may limit the mass of a growing atmosphere.

How to cite: Parmentier, E. M., Elkins-Tanton, L., and Huber, C.: On the competing roles of volatile outgassing and cumulate compaction in the solidification of magma oceans., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9238, https://doi.org/10.5194/egusphere-egu21-9238, 2021.

15:42–15:44
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EGU21-8785
Razvan Caracas and Sarah T. Stewart

Impacts are highly energetic phenomena. They abound in the early stages of formation of the solar system, when they actively participated to the formation of large bodies in the protoplanetary disk. Later on, when planetesimals and embryo planets formed, impacts merged smaller bodies into the large planets that we know today. Giant impacts dominated the last phase of the planetary accretion, with some of these impacts leaving traces observable even today (planets tilts, moon, missing mantle, etc). The Earth was not spared, and its most cataclysmic event also contributed to the formation of the Moon.

Here we present the theoretical tools used to explore the thermodynamics of the formation of the protolunar disk and the subsequent condensation of this disk. We show how ab initio-based molecular dynamics simulations contribute to the determination of the stability field of melts, to the equilibrium between melts and vapor and the positioning of the critical points. Together all this information helps building the liquid-vapor stability dome. Next we investigate the supercritical regime, typical of the post-impact state. We take a focused look to the transport properties, the formation of the first atmosphere, and compare the properties of the liquid state typical of magma oceans, to the super-critical state, typical of protolunar disks.

We apply this theoretical approach on pyrolite melts, as best approximants for the bulk silicate Earth. These simulations help us retrace the thermodynamic state of the protolunar disk and infer possible condensation paths for both the Earth and the moon.

 

RC acknowledges support from the European Research Council under EU Horizon 2020 research and innovation program (grant agreement 681818 – IMPACT) and access to supercomputing facilities via the eDARI gen6368 grants, the PRACE RA4947 grant, and the Uninet2 NN9697K grant. STS was supported by NASA grants NNX15AH54G and 80NSSC18K0828; DOE-NNSA grants DE-NA0003842 and DE-NA0003904.

How to cite: Caracas, R. and Stewart, S. T.: Thermodynamicsof giant planetary impacts from ab initiosimulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8785, https://doi.org/10.5194/egusphere-egu21-8785, 2021.

15:44–15:46
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EGU21-12082
Richard Ghail

Venus is our most Earth-like twin, from a geological standpoint, but lacks Earth-like plate tectonics. Its lower mean density implies a smaller core and relatively large mantle, which combined with the inhibited cooling effected by its high surface temperature, suggests that Venus today may be at an earlier evolutionary stage than Earth. Geologically, a global network of rifts and corona chains (e.g. Parga Chasma) indicate subsurface, plate tectonic-like, spreading ridges below a crustal detachment layer, but there are no obvious corresponding subduction zones. Subduction has been inferred locally at a few large corona (e.g. Artemis) but only in relation to specific plumes, not global plate tectonics. Elsewhere there is evidence for numerous large igneous provinces and perhaps an even larger Overturn Upwelling Zones (OUZO) event at Lada Terra. These features suggest a planet in transition from an Archaean-like regime dominated by instability and overturns, towards a more stable plate tectonic regime: i.e. a planet analogous to the early Proterozoic Earth.

How to cite: Ghail, R.: Is Venus an analogue for Proterozoic Earth?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12082, https://doi.org/10.5194/egusphere-egu21-12082, 2021.

15:46–15:48
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EGU21-16263
Sinan Özeren, A. M. Celal Şengör, Dursun Acar, M. Nazmi Postacıoğlu, Christian Klimczak, Paul K. Byrne, and Tayfun Öner

We conduct a series of experiments to understand the nature of thrust faulting as a result of global thermal contraction in planetary bodies such as Mercury. The spatial scales and patterns of faulting due to contraction are still not very well understood. However, the problem is complicated even for the homogeneous case where the crustal thickness and material properties do not vary spatially. Previous research showed that the thrust faulting patterns are non-random and are arranged in long systems. This is probably due to the regional-scale stress patterns interacting with each other, leading to the creation of coherent structures. We first conduct 1-Axis experiments where we simulate the contraction of the substratum using an elastic ribbon. On top of this we place the material for which the friction, cohesion and thickness can be controlled for each experiment. The shared interface between the frictional-cohesive material and the shortening elastic substratum dictates undulations and finally the generation of slip planes in the upper layer. We discuss the spatial distribution of these patterns spatially. We then speculate the interaction of such patterns on a 2D plane.

 
 

How to cite: Özeren, S., Şengör, A. M. C., Acar, D., Postacıoğlu, M. N., Klimczak, C., Byrne, P. K., and Öner, T.: New Analog Experiment for Convergent Regime an example of planet Mercury, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16263, https://doi.org/10.5194/egusphere-egu21-16263, 2021.

15:48–15:50
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EGU21-6258
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ECS
Adrien Morison, Stephane Labrosse, Daniela Bolrao, Antoine Rozel, Maxim Ballmer, Renaud Deguen, Thierry Alboussiere, and Paul Tackley

The light plagioclase-enriched crust as well as the KREEP layer at the surface of the Moon are believed to be remnants of the bottom-up crystallization of a global Lunar Magma Ocean.  In such a setup, the primitive Lunar solid mantle is coated by a liquid magma ocean of similar composition. We propose here to study the dynamic and evolution of the primitive Lunar solid mantle, accounting for the presence of the Lunar Magma Ocean.

We solve numerically the equations of solid-state convection in the solid part of the mantle.  This model is coupled to 1D models of crystallization of the magma oceans to self-consistently compute the thickening of the solid part as heat is evacuated from the mantle.  We take into account fractional crystallization at the freezing front.

Moreover, the boundaries between the solid and the magma oceans are phase-change interfaces.  Convecting matter in the solid arriving near the boundary or getting away from it forms a topography which can be erased by melting or freezing.  Hence, provided the melting and freezing occurs rapidly compared to the time needed to build the topographies by viscous forces, dynamical exchange of matter can occur between the solid mantle and the magma oceans.  We take this effect into account in our model with a boundary condition applied to the solid.

We find that the boundary condition allowing matter to cross the interfaces between the solid and the magma oceans greatly affects the convection patterns in the solid as well as its heat flux.  Larger-scale convection patterns are selected compared to the classical case with non-penetrative boundary conditions; and the heat transfert in the solid is more efficient with these boundary conditions.  This affects the long term thermal evolution of the mantle as well as the shape of chemical heterogeneities that can be built by fractional crystallization of magma oceans.

How to cite: Morison, A., Labrosse, S., Bolrao, D., Rozel, A., Ballmer, M., Deguen, R., Alboussiere, T., and Tackley, P.: Effects of a long lived global magma ocean on mantle dynamics of the early Moon, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6258, https://doi.org/10.5194/egusphere-egu21-6258, 2021.

15:50–16:00
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EGU21-8013
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solicited
Stephane Labrosse, Adrien Morison, Daniela Bolrão, Antoine Rozel, Maxim Ballmer, Renaud Deguen, Thierry Alboussière, and Paul Tackley

The early evolution of the Earth was likely affected by a large scale magma ocean, in particular in the aftermath of the giant impact that formed the Moon. The exact structure and dynamics of the Earth following that event is unknown but several possible scenarios feature the existence of a basal magma ocean (BMO), whose last remaining drops may explain the current seismically detected ultra low velocity zones. The presence of a BMO covering the core carries many implications for the dynamics and evolution of the overlying solid mantle. The phase equilibrium between the magma and the solid mantle allows matter to flow through the boundary by melting and freezing. In practice, convective stresses in the solid create a topography of the interface which displaces the equilibrium. Heat and solute transfer in the liquid acts to erase this topography and, if this process is faster than that the producing topography, the boundary appears effectively permeable to flow. This leads to convective motions much faster than in usual mantle convection. We developed a mantle convection model coupled to a model for the thermal and compositional evolution of the BMO and the core that takes into account the phase equilibrium at the bottom of the solid mantle. It also includes the fractional crystallisation at the interface and net freezing of the magma ocean. Early in the history, convection in the mantle is very fast and dominated by down-welling currents. As fractional crystallisation proceeds, the magma ocean gets enriched in FeO which makes the cumulate to also get richer. Eventually, it becomes too dense to get entrained by mantle convection and starts to pile up at the bottom of the mantle, which inhibits direct mass flow through the phase change boundary. This allows a thermal boundary layer and hot plumes to develop.

This model therefore allows to explain the present existence of both residual partial melt and large scale compositional variations in the lower mantle, as evidenced by seismic velocity anomalies. It also predicts a regime change between early mantle convection dominated by down-welling flow to the onset of hot plumes in the more recent past.

How to cite: Labrosse, S., Morison, A., Bolrão, D., Rozel, A., Ballmer, M., Deguen, R., Alboussière, T., and Tackley, P.: The long-term evolution of the Earth mantle with a basal magma ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8013, https://doi.org/10.5194/egusphere-egu21-8013, 2021.

16:00–16:02
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EGU21-15790
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ECS
Maelis Arnould, Tobias Rolf, and Antonio Manjón-Cabeza Córdoba

Earth’s lithospheric behavior is tied to the properties and dynamics of mantle flow. In particular, upper mantle rheology controls the coupling between the lithosphere and the asthenosphere, and therefore partly dictates Earth’s tectonic behavior. It is thus important to gain insight into how Earth’s upper mantle deforms in order to understand the evolution of plate tectonics. The presence of seismic anisotropy in the uppermost mantle suggests the existence of mineral lattice-preferred orientation (LPO) caused by the asthenospheric flow. Together with laboratory experiments of mantle rock deformation, this indicates that Earth’s uppermost mantle can deform in a non-Newtonian way, through dislocation creep. Although such a deformation mechanism can significantly impact both mantle flow and the surface tectonic behavior, most numerical studies of whole-mantle convection use a viscoplastic rheology involving diffusion creep as the only deformation mechanism in the mantle.

Here, we investigate the effects of using a composite rheology (with both diffusion and dislocation creep) on the surface tectonic behavior in 2D-cartesian whole-mantle convection models that self-consistently generate plate-like tectonics. We vary the proportion of dislocation creep in the mantle by imposing different temperature- and depth-dependent transitional stresses between diffusion and dislocation creep. Using different yield stresses, we investigate how the amount of dislocation creep affects the planform of convection and promotes surface plate-like or stagnant-lid behavior. In particular, we show that for a given yield stress promoting plate-like behavior in diffusion-creep-only models, a progressive increase in the amount of dislocation creep affects the shape and dynamics of slabs, eventually leading to stagnant-lid convection. We discuss the spatio-temporal distribution of dislocation creep in the mantle in light of the observed geometry of slabs and the spatial distribution of seismic anisotropy in Earth’s upper-mantle.

How to cite: Arnould, M., Rolf, T., and Manjón-Cabeza Córdoba, A.: On the generation of plate-like surface tectonics in whole-mantle convection models employing composite rheology , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15790, https://doi.org/10.5194/egusphere-egu21-15790, 2021.

16:02–16:04
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EGU21-3572
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ECS
Patrick Ball, Nicky White, John Maclennan, and Simon Stephenson

The thermochemical structure of lithospheric and asthenospheric mantle exert primary controls on surface topography and volcanic activity. Volcanic rock compositions and mantle seismic velocities provide indirect observations of this structure. Here, we compile and analyze a global database of the distribution and composition of Neogene-Quaternary intraplate volcanic rocks. By integrating this database with seismic tomographic models, we show that intraplate volcanism is concentrated in regions characterized by slow upper mantle shear-wave velocities and by thin lithosphere (i.e. < 100 km). We observe a negative correlation between shear-wave velocities at depths of 125-175 km and melt fractions inferred from volcanic rock compositions. Furthermore, mantle temperature and lithospheric thickness estimates obtained by geochemical modeling broadly agree with values determined from tomographic models that have been converted into temperature. Intraplate volcanism often occurs in regions where uplifted (but undeformed) marine sedimentary rocks are exposed. Regional elevation of these rocks can be generated by a combination of hotter asthenosphere and lithospheric thinning. Therefore, the distribution and composition of intraplate volcanic rocks through geologic time will help to probe past mantle conditions and surface processes.

How to cite: Ball, P., White, N., Maclennan, J., and Stephenson, S.: Using Volcanic Geochemistry and Seismic Tomography to Refine Global Models of Mantle Temperature and Plate Thickness, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3572, https://doi.org/10.5194/egusphere-egu21-3572, 2021.

16:04–16:06
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EGU21-540
Tobias Rolf and Maëlis Arnould

Earth's tectonic evolution and its link to global mantle dynamics are controlled by the pre-existing structure of the lithosphere which guides how strain localizes and causes the necessary weakness to (re-)activate plate boundaries. Recent models of global-scale mantle convection have self-consistently reproduced Earth-like tectonic regimes, consistent with several aspects of today’s observed tectonics. In many cases these models ignore the memory on pre-existing deformation though. Here, a mantle convection model is advanced to include the associated rheological inheritance via a parameterization of strain-induced plastic (brittle) weakening. Based on more than 180 simulations in a wide 2D cartesian box, the control of strain-induced weakening on the resulting tectonic regime is demonstrated. Strain-induced brittle weakening impacts the stability fields of the different tectonic regimes observed, but to first order it does not generate new tectonic regimes or change the dynamics of a given regime (e.g., its characteristic surface mobility). A time-dependent plate-like regime similar to Earth's becomes more feasible with decreasing critical strain at (and above) which maximum weakening is observed. It is less feasible with increasing temperature-dependence of the healing rate, but remains a possibility at small critical strain. While the critical yield stress that still allows for plate-like behavior is apparently larger with strain-induced weakening considered, the effective shift (incorporating the yield stress reduction due to strain weakening) is relatively small and only about 10% under the tested conditions. Strain accumulation in stable continental lithosphere is generally small because of the necessity of high rheological strength. This holds true even for continental collision events, although at least some strain is accumulated and preserved following such events in the immediate proximity of the colliding continental margins.

 

How to cite: Rolf, T. and Arnould, M.: Tectonic regime variety and stability in mantle convection with strain-induced weakening, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-540, https://doi.org/10.5194/egusphere-egu21-540, 2021.

16:06–16:08
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EGU21-5529
Benoit Tauzin, Lauren Waszek, Jun Yan, Maxim Ballmer, Nick Schmerr, Juan Carlos Afonso, and Thomas Bodin

Convective stirring of chemical heterogeneities introduced through oceanic plate subduction results in the marble cake model of mantle composition. A convenient description invokes a chemically unequilibrated mixture of oceanic basaltic crust and harzburgitic lithosphere. Such a composition is required to explain joint observations of shear and compressional waves reflected underneath transition zone (TZ) discontinuities1. The formation of basaltic reservoirs at TZ depth results from complex interaction between phase-change induced chemical segregation, subducted slab downward entrainment, and plume upward advection. However, the dominant mechanism to create and maintain the reservoirs is debated, because both present-day reservoir location and the amount of basalt in these reservoirs are unconstrained. Here, Bayesian inversion of SS- and PP-precursors reflection data indicates that the TZ comprises a global average basalt fraction f = 0.32 ± 0.11. We find the most enriched basaltic reservoirs (f = 0.5-0.6) are associated with recent subduction in the circum-Pacific region. We investigate the efficiency of plate subduction to maintain such reservoirs using global-scale thermochemical  convection models2.

[1] Waszek, L., Tauzin, B., Schmerr, N.C., Ballmer, M., & Afonso, J.C. (in review). A poorly mixed mantle and its thermal state inferred from seismic waves.

[2] Yan, J., Ballmer, M. D., & Tackley, P. J. (2020). The evolution and distribution of recycled oceanic crust in the Earth's mantle: Insight from geodynamic models. Earth and Planetary Science Letters, 537, 116171.

How to cite: Tauzin, B., Waszek, L., Yan, J., Ballmer, M., Schmerr, N., Afonso, J. C., and Bodin, T.: Basaltic mantle reservoirs from seismic inversion of reflection data , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5529, https://doi.org/10.5194/egusphere-egu21-5529, 2021.

16:08–16:10
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EGU21-7545
Anna J. P. Gülcher, Maxim D. Ballmer, and Paul J. Tackley

The nature of compositional heterogeneity in Earth’s lower mantle is a long-standing puzzle that can inform about the thermochemical evolution and dynamics of our planet. On relatively small scales (<1km), streaks of recycled oceanic crust (ROC) and lithosphere are distributed and stirred throughout the mantle, creating a “marble cake” mantle. On larger scales (10s-100s of km), compositional heterogeneity may be preserved by delayed mixing of this marble cake with either intrinsically-dense or -strong materials of e.g. primordial origin. Intrinsically-dense materials may accumulate as piles at the core-mantle boundary, while intrinsically viscous (e.g., enhanced in the strong mineral MgSiObridgmanite) may survive as blobs in the mid-mantle for large timescales (i.e., as plums in the mantle “plum pudding”). So far, only few, if any, studies have quantified mantle dynamics in the presence of different types of heterogeneity with distinct physical properties.

Here, we use 2D numerical models of global-scale mantle convection to investigate the coupled evolution and mixing of (intrinsically-dense) recycled and (intrinsically-strong) primordial material. We explore the effects of ancient compositional layering of the mantle, as motivated by magma-ocean solidification studies, and the physical parameters of the primordial material. Over a wide parameter range, primordial and recycled heterogeneity is predicted to coexist with each other. Primordial material usually survives as mid-to-large scale blobs in the mid-mantle, and this preservation is largely independent on the initial primordial-material volume. In turn, recycled oceanic crust (ROC) persists as piles at the base of the mantle and as small streaks everywhere else. The robust coexistence between recycled and primordial materials in the models indicate that the modern mantle may be in a hybrid state between the “marble cake” and “plum pudding” styles.

Finally, we put our model predictions in context with geochemical studies on early Earth dynamics as well as seismic discoveries of present-day lower-mantle heterogeneity. For the latter, we calculate synthetic seismic velocities from output model fields, and compare these synthetics to tomography models, taking into account the limited resolution of seismic tomography. Because of the competing effects of compositional and thermal anomalies on S-wave velocities, it is difficult to identify mid-mantle bridgmanitic domains in seismic tomography images. This result suggests that, if present, bridgmanitic domains in the mid-mantle may be “hidden” from seismic tomographic studies, and other approaches are needed to establish the presence/absence of these domains in the present-day deep Earth.

How to cite: Gülcher, A. J. P., Ballmer, M. D., and Tackley, P. J.: Coupled dynamics of primordial and recycled heterogeneity in Earth's lower mantle, and their present-day seismic signatures, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7545, https://doi.org/10.5194/egusphere-egu21-7545, 2021.

16:10–16:12
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EGU21-13892
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ECS
Xianzhi Cao, Nicolas Flament, Ömer Bodur, and Dietmar Müller

The relationships between plate motions and basal mantle structure remain poorly understood, with some models implying that the basal mantle structure has remained stable over time, while others suggest that it could be shaped by the aggregation and dispersal of supercontinents. Here we investigate the plate-basal mantle relationship through 1) building a series of end-member plate tectonic models over one billion years, and 2) creating mantle flow models assimilated by those plate models. To achieve that, we build synthetic plate tectonic models dating from 1 Ga to 250 Ma that we connect to an existing palaeogeographical plate reconstruction from 250 Ma to create a relative plate motion model for the last 1 Gyr, in which supercontinent breakup and reassembly occur via introversion. We consider three distinct reference frames that result in different net lithospheric rotation. We find that the flow models predict a dominant degree-2 lower mantle structure most of the time and that they are in first-order agreement (~70% spatial match) with tomographic models. Model thermochemical structures at the base of the mantle may split into smaller structures when slabs sink onto them, and smaller basal structures may merge into larger ones as a result of slab pushing. The basal thermochemical structure under the superocean is large and continuous, whereas the basal thermochemical structure under the supercontinent is smaller and progressively assembles during and shortly after supercontinent assembly. In the models, plumes also develop preferentially along the edge of the basal thermochemical structures and tend to migrate towards the interior of basal structures over time as they interact with the slabs. Lone plumes can also form away from the main thermochemical structures, often within a small network of sinking slabs. Lone plumes may migrate between basal structures. We analyse the relationship between imposed tectonic velocities and deep mantle flow, and find that at spherical harmonic degree 2, the maxima of lower mantle radial flow and temperature follow the motion path of the maxima of surface divergence. It may take ~160-240 Myr for lower mantle structure to reflect plate motion changes when the lower mantle is reorganised by slabs sinking onto basal thermochemical structures, and/or when slabs stagnate in the transition zone before sinking to the lower mantle. Basal thermochemical structures move at less than 0.6 °/Myr in our models with a temporal average of 0.16 °/Myr when there is no net lithospheric rotation, and between 0.20-0.23 °/Myr when net lithospheric rotation exists and is induced to the lower mantle. Our results suggest that basal thermochemical structures are not stationary, but rather linked to global plate motions and plate boundary reconfigurations, reflecting the dynamic nature of the co-evolving plate-mantle system.

How to cite: Cao, X., Flament, N., Bodur, Ö., and Müller, D.: Coupled evolution of supercontinents and lower mantle structure, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13892, https://doi.org/10.5194/egusphere-egu21-13892, 2021.

16:12–16:14
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EGU21-13570
Sergey Malyshev, Andrey Khudoley, Alexei Ivanov, Vadim Kamenetsky, and Maya Kamenetsky

Numerous Mesoproterozoic mafic dyke swarms are known in Siberia. The main intrusions are concentrated in the northern part of the platform and in Sette-Daban (southeastern part of Siberia), and single intrusions are known on all the outcrops of the crystalline basement in the southern part. The largest dyke swarms are located on the Anabar shield and Sette-Daban (with ~1500 Ma and 1000-950 Ma, respectively [1,2]). In the period 1400-1300 Ma, single intrusions are known: 1382 ± 2 Ma [3] on the Anabar shield, 1385 ± 30 Ma [4] on the Udzha uplift, Listvyanka and Goloustnaya dykes in the south of the platform –  1350 ± 6 Ma [5] and 1338 ± 3 Ma [6], respectively. Also, there is the north-trending dolerite dyke at Sette-Daban, which cuts the Lower Riphean sediments of the Uchur Group. The age of this dike was estimated as 1339 ± 59 Ma employing Sm-Nd isochrone [7]. We report here a new U-Pb age on apatite, Nd isotopy, and geochemistry for this dolerite dyke.

A typical apatite grain used for the U-Pb dating. On the  Tera-Wasserburg diagram, the regression line intercepts in the lower part the concordia line at 1419 ± 15 Ma. The chemical composition of this dyke corresponds to subalkaline basalts (SiO2 = 45.6, Na2O+K2O = 3.9 wt%). The rocks correspond (Mg# = 61) to the calc-alkaline series (FeO*/MgO = 1.1) with a low content of TiО2 (1.25 wt %). A clear negative Nb-Ta anomaly on the multielement diagram suggests an IAB affinity. Incompatible element ratios such as Th/Yb, Nb/Th, Nb/Yb, Zr/Nb also suggest that these dolerites are close to arc-related basalts in composition. Eps(Nd) calculated to the initial value at 1400 Ma shows a slightly negative value -0.2, which is considered as mantle source with contribution from the enriched source.

Geochemical and Nd isotopy characteristics show the affinity of the Sette-Daban dyke with low-Ti series of the Phanerozoic flood basalt provinces (e.g. Karoo, Siberian traps, etc. [8,9]) with the suggestion that these dolerites were generated from a metasomatized subcontinental lithospheric mantle source. Assuming geochemical characteristics and new U-Pb age of the dolerite we propose flood basalt province in the southeast Siberia in Mezoproterozoic (~1400 Ma).

The research was supported by the Russian Science Foundation grant (19-77-10048).

 

How to cite: Malyshev, S., Khudoley, A., Ivanov, A., Kamenetsky, V., and Kamenetsky, M.: Implication of Mesoproterozoic (∼1.4 Ga) magmatism within Sette-Daban (Southeast Siberia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13570, https://doi.org/10.5194/egusphere-egu21-13570, 2021.

16:14–16:16
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EGU21-15374
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ECS
Jiawei Zuo, Alexander Webb, Ryan McKenzie, Tim Johnson, and Christopher Kirkland

Studying the early evolution of terrestrial bodies in our solar systems is challenging. In part, this is because preserved early records are poorly preserved (e.g., Hadean rocks on Earth) and/or hard to access (e.g., rocks on Mars and Venus). Another commonly underappreciated factor is that the testable predictions for the diverse proposed tectonic regimes for early terrestrial bodies are currently underexplored. A better understanding of the consequences of different tectonic regimes can enhance our ability to constrain the early evolution of terrestrial bodies, including the timing of plate tectonic initiation on Earth. In this contribution, we use the example of detrital zircon geochronology to show how first-order predictions for various tectonic modes can be made based on their basic kinematics via relatively simple tools, and how these predictions can provide 1) abundant additional interpretive probabilities for common datasets, and 2) potentially significant implications for the tectonics of early Earth. Using simple Monte Carlo methods with MATLAB codes, we simulated detrital zircon age predictions for basins predicted by heat-pipe tectonics and cold stagnant-lid tectonics based on their relevant numerical models and/or evolutionary diagrams. We show that the first-order predictions for detrital zircon age patterns can be generated by focusing on simulating key mechanisms (e.g., volcanic resurfacing) that control the detrital zircon age characteristics of these two tectonic regimes. Such simulations can be done by simple codes based on a few parameters reflecting basic kinematics of relevant tectonic regimes. We find that differences between new detrital zircon age predictions and those of plate tectonic settings permit better tectonic discrimination via a globally compiled Archean detrital zircon age dataset. The results indicate a transition from heat-pipe tectonics to plate tectonics within the ca. 3.4-3.2 Ga period. Beyond detrital zircon age patterns, we also summarized other possible categories of first-order predictions for non-plate tectonic models, including metamorphic patterns, structural patterns, and crustal thicknesses. Relevant predictions of these categories are variably explored and can potentially be easily modeled or conceptualized via geological tools.

How to cite: Zuo, J., Webb, A., McKenzie, R., Johnson, T., and Kirkland, C.: Geology of non-plate tectonic regimes: detrital zircon age records and beyond, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15374, https://doi.org/10.5194/egusphere-egu21-15374, 2021.

16:16–16:18
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EGU21-9963
Ömer Kamacı, Ali Tugcan Ünlüer, Alp Ünal, Zeynep Doner, Şafak Altunkaynak, and Mustafa Kumral

Peraluminous alaskites are a common phenomenon in the migmatitic domes with anatectic cores. They are geochemically unique in terms of the U-Th mineralization and present critical significance in order to better understand the orogenic crustal processes. Western Anatolia was an orogenic welt in the latest Eocene, following the continental collision between Sakarya Continent and Tauride-Anatolide platform along the Izmir-Ankara-Erzincan suture zone. Çataldağ metamorphic core complex (ÇMCC) is located on the immediate north of the Izmir-Ankara-Erzincan suture zone, in Sakarya Continent. ÇMCC consists of Eo-Oligocene peraluminous anatectic leucogranites, corresponding to the partial melts of the young orogenic crust with a thickness of ≥50 km. Some of these leucogranites can be classified as alaskitic granite due to the presence of high Th content, from 12.5 to 113 ppm and relatively high ionizing radiation dose, up to 0.35 μsv/h. These alaskitic granites made up of quartz (30-35%) + plagioclase (25-30%) + K-feldspar (20-22%) + muscovite (5%) + biotite (5-3%) + monazite (≤1%) ± garnet. Th content in the alaskitic granites increases with increasing degrees of partial melting. Th enrichment in Çataldağ alaskitic granites is possibly hosted by monazite with high saturation temperature (≥770°C). Th-rich alaskitic granites in ÇMCC were derived from the partial melting of the Tauride-Anatolide Platform (Pan-African crust) underthrusted beneath the Sakarya Continent.

How to cite: Kamacı, Ö., Ünlüer, A. T., Ünal, A., Doner, Z., Altunkaynak, Ş., and Kumral, M.: The first finding of Th-rich peraluminous alaskitic granite in Western Anatolia , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9963, https://doi.org/10.5194/egusphere-egu21-9963, 2021.

16:18–16:23
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EGU21-3461
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solicited
A. Mark Jellinek, Adrian Lenardic, and Raymond Pierrehumbert

Supercontinent assembly and breakup can influence the rate and global extent to which insulated and relatively warm subcontinental mantle is mixed globally, potentially introducing lateral oceanic-continental mantle temperature variations that regulate volcanic and weathering controls on Earth's long-term carbon cycle for a few hundred million years. In this talk we explore some remarkable consequences of this class of mantle climate control consistent with varied observational constraints. Whereas the relatively unchanging and ice sheet-free climate of the Nuna supercontinental epoch (1.8–1.3 Ga) is an expected consequence of thorough mantle thermal mixing, the extreme cooling-warming climate variability of the Neoproterozoic Rodinia episode (1–0.63 Ga), marked by discontinuous periods of global glaciation (snowball Earth), is a predicted effect of protracted subcontinental mantle thermal isolation.

How to cite: Jellinek, A. M., Lenardic, A., and Pierrehumbert, R.: Earth's supecontinental climate control, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3461, https://doi.org/10.5194/egusphere-egu21-3461, 2021.

16:23–17:00