Content:

GD – Geodynamics

GD1.1 – Causes of dynamic, tectonic, and compositional transitions in the Earth and rocky planets

EGU21-3315 | vPICO presentations | GD1.1

On the fate of water in the formation of rocky planets

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.

EGU21-9238 | vPICO presentations | GD1.1

On the competing roles of volatile outgassing and cumulate compaction in the solidification of magma oceans.

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.

EGU21-8785 | vPICO presentations | GD1.1

Thermodynamicsof giant planetary impacts from ab initiosimulations

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.

EGU21-12082 | vPICO presentations | GD1.1

Is Venus an analogue for Proterozoic Earth?

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.

EGU21-16263 | vPICO presentations | GD1.1

New Analog Experiment for Convergent Regime an example of planet Mercury

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.

EGU21-6258 | vPICO presentations | GD1.1

Effects of a long lived global magma ocean on mantle dynamics of the early Moon

Adrien Morison, Stephane Labrosse, Daniela Bolrao, Antoine Rozel, Maxim Ballmer, Renaud Deguen, Thierry Alboussiere, and Paul Tackley

EGU21-8013 | vPICO presentations | GD1.1

The long-term evolution of the Earth mantle with a basal magma ocean

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.

EGU21-15790 | vPICO presentations | GD1.1

On the generation of plate-like surface tectonics in whole-mantle convection models employing composite rheology 

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.

EGU21-3572 | vPICO presentations | GD1.1

Using Volcanic Geochemistry and Seismic Tomography to Refine Global Models of Mantle Temperature and Plate Thickness

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.

EGU21-540 | vPICO presentations | GD1.1

Tectonic regime variety and stability in mantle convection with strain-induced weakening

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.

EGU21-5529 | vPICO presentations | GD1.1

Basaltic mantle reservoirs from seismic inversion of reflection data 

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.

EGU21-7545 | vPICO presentations | GD1.1

Coupled dynamics of primordial and recycled heterogeneity in Earth's lower mantle, and their present-day seismic signatures

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.

EGU21-13892 | vPICO presentations | GD1.1

Coupled evolution of supercontinents and lower mantle structure

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.

EGU21-13570 | vPICO presentations | GD1.1

Implication of Mesoproterozoic (∼1.4 Ga) magmatism within Sette-Daban (Southeast Siberia)

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.

EGU21-15374 | vPICO presentations | GD1.1

Geology of non-plate tectonic regimes: detrital zircon age records and beyond

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.

EGU21-9963 | vPICO presentations | GD1.1

The first finding of Th-rich peraluminous alaskitic granite in Western Anatolia 

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.

EGU21-3461 | vPICO presentations | GD1.1

Earth's supecontinental climate control

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.

GD1.2 – Melts and volatiles in Earth and planetary interiors: from atmosphere to core, from global cycles to the micro-scale, from transport dynamics to storage to geophysical detection

EGU21-6416 | vPICO presentations | GD1.2

Sulfur isotope evidence of geochemical zonation of the Samoan mantle plume

James Dottin, Jabrane Labidi, Matthew Jackson, and James Farquhar

The radiogenic Pb isotope compositions of basalts from the Samoan hotspot suggest various mantle endmembers contribute compositionally distinct material to lavas erupted at different islands [1]. Basalts from the Samoan islands sample contributions from all of the classical mantle endmembers, including extreme EM II and high 3He/4He components, as well as dilute contributions from the HIMU, EM I, and DM components. Here, we present multiple sulfur isotope data on sulfide extracted from subaerial and submarine whole rocks associated with several Samoan volcanoes—Malumalu, Malutut, Upolu, Savaii, and Tutuila—that sample the full range of geochemical heterogeneity at Samoa and allow for an assessment of the S-isotope compositions associated with the different mantle components sampled by the Samoan hotspot. We observe variable S concentrations (10-1000 ppm) and δ34S values (-0.29‰ to +4.84‰ ± 0.3, 2σ). The variable S concentrations likely reflect weathering, sulfide segregation and degassing processes. The range in δ34S reflects mixing between the primitive mantle and recycled components, and isotope fractionations associated with degassing. The majority of samples reveal Δ33S within uncertainty of Δ33S=0 ‰ ± 0.008, suggesting Δ33S is relatively well mixed within the Samoan mantle plume. Important exceptions to this observation include: (1) a negative Δ33S (-0.018‰ ±0.008, 2σ) from a rejuvenated basalt on Upolu island (associated with a diluted EM I component) and (2) a previously documented small (but resolvable) Δ33S values (up to +0.027±0.016) associated with the Vai Trend (associated with a diluted HIMU component) [2]. The variability we observed in Δ33S is interpreted to reflect contributions of sulfur of different origins and likely multiple crustal protoliths. Δ36S vs. Δ33S relationships suggest all recycled S is of post-Archean origin. The heterogeneous S isotope values and distinct isotopic compositions associated with the various compositional trends confirms a prior hypothesis; unique crustal materials are heterogeneously delivered to the Samoan mantle plume and compositionally influence the individual groups of islands.

[1] Jackson et al. (2014), Nature; [2] Dottin et al. (2020), EPSL

How to cite: Dottin, J., Labidi, J., Jackson, M., and Farquhar, J.: Sulfur isotope evidence of geochemical zonation of the Samoan mantle plume, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6416, https://doi.org/10.5194/egusphere-egu21-6416, 2021.

EGU21-10891 | vPICO presentations | GD1.2

Defining the composition of the deep mantle and the primordial He inventory of the Afar plume

Finlay Stuart, Ugur Balci, and Jean-Alix Barrat

Basaltic rocks generated by upwelling mantle plumes display a range of trace element and isotope compositions indicative of strong heterogeneity in deep material brought to Earth surface.  Helium isotopes are an unrivalled tracer of the deep mantle in plume-derived basalts.  It is frequently difficult to identify the composition of the deep mantle component as He isotopes rarely correlate with incompatible trace element and radiogenic isotope tracers. It is supposed that this is due to the high He concentration of the deep mantle compared to degassed/enriched mantle reservoirs dominating the He in mixtures, although this is far from widely accepted.  The modern Afar plume is natural laboratory for testing the prevailing paradigm.

The 3He/4He of basalt glasses from 26°N to 11°N along the Red Sea spreading axis increases systematically from 7.9 to 15 Ra. Strong along-rift relationships between 3He/4He and incompatible trace element ratios are consistent with a binary mixture between moderately enriched shallow asthenospheric mantle in the north and plume mantle evident in basalts from the Gulf of Tadjoura, Djibouti (the Ramad enriched component of Barrat et al. 1990).  The high-3He/4He basalts have trace element-isotopic compositions that are similar, but not identical, to the high 3He/4He (22 Ra) high Ti (HT2) flood basalts erupted during the initial phase of the Afar plume volcanism (Rogers et al. in press). This suggests that the deep mantle component in the modern Afar plume has a HIMU-like composition. From the hyperbolic 3He/4He-K/Th-Rb/La mixing relationships we determine that the upwelling deep mantle has 3-5 times higher He concentration than the asthenosphere mantle beneath the northern Red Sea.

Barrat et al. 1990.  Earth and Planetary Science Letters 101, 233-247.

How to cite: Stuart, F., Balci, U., and Barrat, J.-A.: Defining the composition of the deep mantle and the primordial He inventory of the Afar plume, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10891, https://doi.org/10.5194/egusphere-egu21-10891, 2021.

EGU21-1944 | vPICO presentations | GD1.2

Behaviors of Wet Plume Controlled by Olivine-Wadsleyite Phase Transition and Water Distribution

Hyunseong Kim, Youngjun Lee, Doyoung Kim, and Changyeol Lee

Quaternary Intraplate volcanoes are sparsely distributed in Northeast Asia including Northeast China and Korean Peninsula and roles of the stagnant Pacific plate in the volcanoes have been studied. Recent geochemical studies suggest that the hydrated mantle in the mantle transition zone was incorporated in the wet plumes that were generated from the hydrated layer atop the stagnant slab, and the ascending wet plumes experienced partial melting in the shallow asthenosphere. To quantitatively evaluate the incorporation of the mantle in the transition zone into the wet plumes and their partial melting in the asthenosphere, we conducted a series of two-dimensional thermochemical numerical models by including the olivine-wadsleyite phase transition at the 410km discontinuity. The buoyancy is controlled by temperature, bound-water content and mineral phase. Viscosity reduction by the bound-water is added to the temperature-dependent viscosity. Particle tracers are used to track the incorporation of the mantle in the transition zone into the wet plumes. We vary the Clapeyron slope of the phase transition and water distributions in the mantle transition zone and hydrated layer of the stagnant slab to evaluate their effects on the behavior of the wet plumes. Results show that multiple wet plumes generated from atop the stagnant slab incorporate the hydrated mantle in the transition zone. Due to the endothermic phase transition at the 410 km discontinuity, the ascending wet plumes are retarded and laterally migrated beneath the 410 km discontinuity for several million years, and enter the overlying asthenosphere as merged large wet plumes. The ascending merged wet plumes laterally spread beneath the thermal lithosphere and experience partial melting, consistent with the interpretation based on the geochemical studies. The spacing of the merged wet plumes (~440 km) caused by the phase transition at the 410 km discontinuity is consistent with the sparse volcano distribution in Northeast China and Korean Peninsula.

How to cite: Kim, H., Lee, Y., Kim, D., and Lee, C.: Behaviors of Wet Plume Controlled by Olivine-Wadsleyite Phase Transition and Water Distribution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1944, https://doi.org/10.5194/egusphere-egu21-1944, 2021.

EGU21-8489 | vPICO presentations | GD1.2

Connecting mantle flow below passive margins and intraplate melt generation: an application to the Cameroon Volcanic Line.

Matthew Likely, Jeroen van Hunen, Linda Kirstein, Godfrey Fitton, Lara Kalnins, Jennifer Jenkins, and Ana Negredo

Approximately 90% of all magmatism on Earth can be explained through plate tectonics; the remainder is associated with intraplate volcanism. In large part, this intraplate volcanism can be attributed to mantle plumes, yet this does not represent all known examples. A number of hypotheses have been proposed to explain non-plume related intraplate volcanism. One geodynamically viable theory through the process of small-scale convection associated with lithospheric instabilities evolving into edge driven convection (EDC) in regions which possess large variations in lithospheric thickness. One such intraplate volcanic example that may be explained by this process is the Cameroon Volcanic Line, which forms a linear chain of non-age progressive volcanoes that straddle the African continental lithosphere and the Atlantic oceanic lithosphere.

In this study we compute numerical models utilising mantle convection modelling software ‘ASPECT’, to investigate the initiation, evolution and potential of melt generation as a result of EDC through geological time, applying these models to the Cameroon Volcanic Line. Our preliminary modelling results suggest that episodic intraplate melting events can indeed be generated through edge-driven convection. But in order to do so, mantle temperatures need to be higher than average to produce sufficient melt from a typical upper mantle source. We therefore investigate the possibility that more enriched mantle lithosphere, destabilised by the assembly and breakup of Pangaea, could flow into the source region of the Cameroon volcanism, allowing the production of similar quantities of melt with less elevated mantle temperatures. We present results on how lithospheric development, evolution and stability, as well as supercontinent cycles can influence intraplate volcanism.

How to cite: Likely, M., van Hunen, J., Kirstein, L., Fitton, G., Kalnins, L., Jenkins, J., and Negredo, A.: Connecting mantle flow below passive margins and intraplate melt generation: an application to the Cameroon Volcanic Line., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8489, https://doi.org/10.5194/egusphere-egu21-8489, 2021.

EGU21-2066 | vPICO presentations | GD1.2 | Highlight

Plate tectonics and volatiles: the nanorock connexion

Gautier Nicoli and Silvio Ferrero

The global geological volatile cycle (H, C, N) plays an important role in the long term self-regulation of the Earth system. However, the complex interaction between its deep, solid Earth component (i.e. crust and mantle), Earth’s fluid envelope (i.e. atmosphere and hydrosphere) and plate tectonic processes is a subject of ongoing debate. Here, we want to draw attention to how the presence of primary, pristine melt (MI) and fluid (FI) inclusions in high grade metamorphic minerals could help constrain the crustal component of the volatile cycle. We review the distribution of pristine MI and FI throughout Earth’s history, from the onset of plate tectonics at ca. 3.0 Ga to the present day. Combined with thermodynamic modelling, our compilation indicates that periods of well-established plate tectonics regimes at 0-1.2 Ga and 1.8-2.0 Ga, might be more prone to the reworking of supracrustal lithologies and the storage of volatiles at lower crustal depths. We then argue that the lower crust might constitute an important, although temporary, volatile storage unit, capable to influence the composition of the surface envelopes through the mean of weathering, crustal thickening, partial melting and crustal assimilation during volcanic activity.

Such hypothesis has implication beyond the scope of metamorphic petrology as it potentially links geodynamic mechanisms to habitable surface conditions. MI and FI in metamorphic rocks is a rich but still relatively uncharted realm. In the near future, a concerted research effort should aim to find and characterize new instances of pristine inclusions in periods of the Earth’s history currently underrepresented in the inclusion database, e.g. the Boring Billion. The merging of the messages of thousands of minuscule droplets of fluids trapped in the deepest roots of the continental plates will then eventually provide a truly comprehensive portrait of how the Earth’s evolution proceeds through the geological timescale.

 

How to cite: Nicoli, G. and Ferrero, S.: Plate tectonics and volatiles: the nanorock connexion, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2066, https://doi.org/10.5194/egusphere-egu21-2066, 2021.

EGU21-5932 | vPICO presentations | GD1.2

Eclogite xenoliths document water cycling at the lithosphere-asthenosphere boundary

Julien Reynes and Jörg Hermann

The amount of water stored as OH-defects in nominally anhydrous minerals in the deep mantle is poorly constrained and its direct quantification can only be accessed by the analysis of mantle xenoliths. While the vast majority of xenoliths are peridotites and minor pyroxenites, some very rare xenoliths found in kimberlite pipes display an eclogitic mineral assemblage. We investigated three eclogite xenoliths from the 128 m.y. old Robert Victor kimberlite from South Africa that display an assemblage of garnet and omphacite with two samples showing additional kyanite, suggesting low-pressure gabbroic rock as protolith. Thermobarometry estimations based on Fe-Mg partitioning between garnet and pyroxene gives temperatures of 1100-1250 °C. When projected on the cratonic geotherm (Griffin & O’Reilly 2007) an equilibrium depth of 200-210 km is obtained, confirming that these rocks come from the lithosphere-asthenosphere boundary. Therefore these fragments might be key witnesses to understand the deep cycling of water in the mantle.

This study focuses on the H2O quantification in the three rock-forming minerals using Fourier transform infrared spectroscopy (FTIR). Omphacite contains 50-250 ppm H2O, kyanite contains 40-60 ppm H2O and garnet of only one eclogite contains 40 ppm H2O. Garnet and omphacite with the highest OH content are enriched in Ca.

The use of advanced mapping and profiling techniques enabled the investigation of the spatial repartition of the OH component in these minerals. High-resolution mapping (5.6 µm) of kyanite reveals diffusive gain of OH at the rim of the crystal that is interpreted as hydration during interaction with the kimberlitic melt. The OH plateau in the core of kyanite must therefore have been acquired previously, suggesting that this is residual OH that has been transported by subduction to the lithosphere-asthenosphere boundary by a once hydrated gabbroic protolith. Our results have implications for the retention of hydrogen over long timescale at the lithosphere-asthenosphere boundary and suggest that the deep cycling of water has been running since Archean times.

 

Griffin, W. L., & O'Reilly, S. Y. (2007). Cratonic lithospheric mantle: is anything subducted?. Episodes, 30(1), 43-53.

How to cite: Reynes, J. and Hermann, J.: Eclogite xenoliths document water cycling at the lithosphere-asthenosphere boundary, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5932, https://doi.org/10.5194/egusphere-egu21-5932, 2021.

EGU21-10847 | vPICO presentations | GD1.2

The vaporization behavior of carbon and hydrogen from the early global magma ocean

Natalia Solomatova and Razvan Caracas

Estimating the fluxes and speciation of volatiles during the existence of a global magma ocean is fundamental for understanding the cooling history of the early Earth and for quantifying the volatile budget of the present day. Using first-principles molecular dynamics, we predict the vaporization rate of carbon and hydrogen at the interface between the magma ocean and the hot dense atmosphere, just after the Moon-forming impact. The concentration of carbon and the oxidation state of the melts affect the speciation of the vaporized carbon molecules (e.g., the ratio of carbon dioxide to carbon monoxide), but do not appear to affect the overall volatility of carbon. We find that carbon is rapidly devolatilized even under pressure, while hydrogen remains mostly dissolved in the melt during the devolatilization process of carbon. Thus, in the early stages of the global magma ocean, significantly more carbon than hydrogen would have been released into the atmosphere, and it is only after the atmospheric pressure decreased, that much of the hydrogen devolatilized from the melt. At temperatures of 5000 K (and above), we predict that bubbles in the magma ocean contained a significant fraction of silicate vapor, increasing with decreasing depths with the growth of the bubbles, affecting the transport and rheological properties of the magma ocean. As the temperature cooled, the silicate species condensed back into the magma ocean, leaving highly volatile atmophile species, such as CO2 and H2O, as the dominant species in the atmosphere. Due to the greenhouse nature of CO2, its concentration in the atmosphere would have had a considerable effect on the cooling rate of the early Earth.

How to cite: Solomatova, N. and Caracas, R.: The vaporization behavior of carbon and hydrogen from the early global magma ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10847, https://doi.org/10.5194/egusphere-egu21-10847, 2021.

EGU21-14371 | vPICO presentations | GD1.2

A numerical study of the nucleation, growth and settling of crystals from a turbulent convecting fluid

Vojtech Patocka, Nicola Tosi, and Enrico Calzavarini

We evaluate the equilibrium concentration of a thermally convecting suspension that is cooled from above and in which
solid crystals are self-consistently generated in the thermal boundary layer near the top. In a previous study (Patočka et
al., 2020), we investigated the settling rate of solid particles suspended in a highly vigorous (Ra = 108 , 1010, and 1012 ),
finite Prandtl number (Pr = 10, 50) convection. In this follow-up study we additionally employ the model of crystal
generation and growth of Jarvis and Woods (1994), instead of using particles with a predefined size and density that are
uniformly injected into the carrier fluid.

We perform a series of numerical experiments of particle-laden thermal convection in 2D and 3D Cartesian geometry
using the freely available code CH4 (Calzavarini, 2019). Starting from a purely liquid phase, the solid fraction gradually
grows until an equilibrium is reached in which the generation of the solid phase balances the loss of crystals due to
sedimentation at the bottom of the fluid. For a range of predefined density contrasts of the solid phase with respect to
the density of the fluid (ρpf = [0, 2]), we measure the time it takes to reach such equilibrium. Both this time and
the equilibrium concentration depend on the average settling rate of the particles and are thus non-trival to compute for
particle types that interact with the large-scale circulation of the fluid (see Patočka et al., 2020).

We apply our results to the cooling of a large volume of magma, spanning from a large magma chamber up to a
global magma ocean. Preliminary results indicate that, as long as particle re-entrainment is not a dominant process, the
separation of crystals from the fluid is an efficient process. Fractional crystallization is thus expected and the suspended
solid fraction is typically small, prohibiting phenomena in which the feedback of crystals on the fluid begins to govern the
physics of the system (e.g. Sparks et al, 1993).

References
Patočka V., Calzavarini E., and Tosi N.(2020). Settling of inertial particles in turbulent Rayleigh-Bénard convection.
Physical Review Fluids, 26(4) 883-889.

Jarvis, R. A. and Woods, A. W.(1994). The nucleation, growth and settling of crystals from a turbulently convecting
fluid. J. Fluid. Mech, 273 83-107.

Sparks, R., Huppert, H., Koyaguchi, T. et al (1993). Origin of modal and rhythmic igneous layering by sedimentation in
a convecting magma chamber. Nature, 361, 246-249.

Calzavarini, E (2019). Eulerian–Lagrangian fluid dynamics platform: The ch4-project. Software Impacts, 1, 100002.

How to cite: Patocka, V., Tosi, N., and Calzavarini, E.: A numerical study of the nucleation, growth and settling of crystals from a turbulent convecting fluid, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14371, https://doi.org/10.5194/egusphere-egu21-14371, 2021.

EGU21-12691 | vPICO presentations | GD1.2

Scaling of convection in high-Pressure ice layers of large icy moons and implications for habitability

Laëtitia Lebec, Stéphane Labrosse, Adrien Morison, and Paul Tackley

The existence of a high pressure ice layer between the silicate core and the liquid ocean in large icy moons and ocean worlds is usually seen as a barrier to habitability, preventing the compounds needed for life to flow into the ocean. More recently, three studies from Choblet et al [1] and Kalousová et al [2, 3] challenged that hypothesis and showed that, in certain conditions, exchanges were possible between the core and the ocean, allowing transport of salts toward the ocean. Here, we consider an effect not taken into account in these previous studies: the possibility of mass exchange between the ice and ocean layers by phase change. Convective stresses in the solid create a topography of the interface which can be erased by melting and freezing if flow on the liquid side is efficient. This effect is included in a convection model as a phase change boundary condition, allowing a non-zero vertical velocity at the surface of the HP ice layer, which has a significant impact on the flow dynamics and enables exchanges with the ocean by fusion and crystallization of the ice at the top interface, even without partial melting in the bulk of the ice layer. These exchanges are directly linked to the melting capacity of the ice at the interface between the HP ice layer and the core, depending on the Rayleigh number and the efficiency of convection. Then, considering this new condition at the interface between the HP ice layer and the liquid ocean, we propose a scaling of the bottom temperature and the vertical velocity. Applied to a specific celestial body, as Ganymede or Titan, it would be the first step to conclude about its habitability.

 

References:

[1] G. Choblet, G. Tobie, C. Sotin, K. Kalousová, O. Grasset (2017). Heat transport in the high-pressure ice mantle of large icy moons. Icarus, 285, 252-262

[2] K. Kalousová, C. Sotin, G. Choblet, G. Tobie, O. Grasset (2018). Two-phase convection in Ganymede’s high-pressure ice layer — Implications for its geological evolution. Icarus, 299, 133-147

[3] K.Kalousová, C. Sotin (2018). Melting in High-Pressure Ice Layers of Large Ocean Worlds—Implications for Volatiles Transport. Geophys. Res. Lett., 45, 8096-8103.

How to cite: Lebec, L., Labrosse, S., Morison, A., and Tackley, P.: Scaling of convection in high-Pressure ice layers of large icy moons and implications for habitability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12691, https://doi.org/10.5194/egusphere-egu21-12691, 2021.

EGU21-5881 | vPICO presentations | GD1.2

Magma transport beneath mid-ocean ridges

Shi Sim, Marc Spiegelman, Dave Stegman, and Cian Wilson

Melt transport beneath the lithosphere is elusive. With a distinct viscosity and density from the surrounding mantle, magmatic melt moves on a different time scale as the surrounding mantle. To resolve the temporal scale necessary to accurately capture melt transport in the mantle, the model simulations become numerically expensive quickly. Recent computational advances make possible two-phase numerical explorations to understand magma transport in the mantle. We review results from a suite of two-phase models applied to the mid-ocean ridges, where we varied half-spreading rate and intrinsic mantle permeability using new openly available models, with the goal of understanding melt focusing beneath mid-ocean ridges and its relevance to the lithosphere-asthenosphere boundary (LAB). Here, we highlight the importance of viscosities for the melt focusing mechanisms. In addition, magmatic porosity waves that are a natural consequence of these two-phase flow formulations. We show that these waves could explain long-period temporal variations in the seafloor bathymetry at the Southeast Indian Ridge.

How to cite: Sim, S., Spiegelman, M., Stegman, D., and Wilson, C.: Magma transport beneath mid-ocean ridges, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5881, https://doi.org/10.5194/egusphere-egu21-5881, 2021.

EGU21-10306 | vPICO presentations | GD1.2

Buoyancy-driven flow beneath mid-ocean ridges: the role of chemical heterogeneity

Adina E. Pusok, Richard F. Katz, Dave A. May, and Yuan Li

In the classical model, mid-ocean ridges (MOR) sit above an asthenospheric corner flow that is symmetrical about a vertical plane aligned with the ridge axis. However, geophysical observations of MORs indicate strong asymmetry in melt production and upwelling across the axis (e.g., Melt Seismic Team, 1998, Rychert et al., 2020). In order to reproduce the observed asymmetry, models of plate-driven (passive) flow require unrealistically large forcing, such as rapid asthenospheric cross-axis flow (~30 cm/yr) at high asthenospheric viscosities (~10^21 Pa.s), or temperature anomalies of >100 K beneath the MELT region in the East Pacific Rise (Toomey et al, 2002). 

Buoyancy-driven flows are known to produce symmetry-breaking behaviour in fluid systems. A small contribution from buoyancy-driven (active) flow promotes asymmetry of upwelling and melting beneath MORs (Katz, 2010). Previously, buoyancy has been modelled as a consequence of the retained melt fraction, but depletion of the residue (and heterogeneity) should be involved at a similar level. 

Here, we present new 2-D mid-ocean ridge models that incorporate density variations within the partial-melt zone due to the low density of the liquid relative to the solid (porous buoyancy), and the Fe/Mg partitioning between melt and residue (compositional buoyancy). The model is built after Katz (2010) using a new finite difference staggered grid framework for solving partial differential equations (FD-PDE) for single-/two-phase flow magma dynamics (Pusok et al., 2020). The framework uses PETSc (Balay et al., 2020) and aims to separate the user input from the discretisation of governing equations, thus allowing for extensible development and a robust framework for testing. 

Results show that compositional buoyancy beneath the ridge is negative and can partially balance porous buoyancy. Despite this, models with both chemical and porous buoyancy are susceptible to asymmetric forcing. Asymmetrical upwelling in this context is obtained for forcing that is entirely plausible. A scaling analysis is performed to determine the relative importance of the contribution of compositional and porous buoyancy to upwelling, which is followed by predictions on the crustal thickness production and asymmetry beneath the ridge axis. 

Balay et al. (2020), PETSc Users Manual, ANL-95/11-Revision 3.13.

Katz (2010), G-cubed, 11(Q0AC07), 1-29, https://doi.org/10.1029/2010GC003282

Melt Seismic Team (1998), Science, 280(5367), 1215–1218, https://doi.org/10.1126/science.280.5367.1215 

Pusok et al. (2020), EGU General Assembly 2020, EGU2020-18690 https://doi.org/10.5194/egusphere-egu2020-18690 

Rychert et al. (2020), JGR Solid Earth, 125, e2018JB016463. https://doi. org/10.1029/2018JB016463  

Toomey et al. (2002), EPSL, 200(3-4), 287-295, https://doi.org/10.1016/S0012-821X(02)00655-6

How to cite: Pusok, A. E., Katz, R. F., May, D. A., and Li, Y.: Buoyancy-driven flow beneath mid-ocean ridges: the role of chemical heterogeneity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10306, https://doi.org/10.5194/egusphere-egu21-10306, 2021.

EGU21-6191 | vPICO presentations | GD1.2

Numerically modeling routes of sequential magma pulses in the upper crust

Mara Arts, Boris Kaus, and Nicolas Berlie

EGU21-14267 | vPICO presentations | GD1.2

Effects on the solubility and the volatile release from magmatic intrusions

Sara Vulpius and Lena Noack

The process of fractional crystallization within a magma body has an influence on the solubility and thus on the associated release of volatiles. Nevertheless, this mechanism is widely neglected in the literature. Due to cooling of an intrusion, nominally anhydrous minerals precipitate from the melt. These minerals mainly incorporate elements that are compatible with their crystal lattice. Since volatiles such as H2O and CO2 behave like incompatible elements, they accumulate in the remaining melt. At a certain point, the melt is saturated and the exsolution of the volatiles initiates. The solubility is determined by several parameters like the lithostatic and the partial pressure, the temperature and the melt composition. 
In this study, we investigate the effect of these parameters as well as the impact of fractional crystallization on the solubility and the related volatile release. We focus on the exsolution of H2O and CO2 from basaltic magma bodies within the lithosphere. To determine the fate of the accumulating volatiles, we compare the density of the developing liquid phase (volatiles and residual melt) with the density of the host rock. If the host rock has a higher density, the liquid phase will ascent either directly to the surface or to shallower levels of the crust. Furthermore, we take into account the possibility that hydrous minerals (e.g., amphibole) are precipitated during fractional crystallization or due to a reaction with the surrounding rock. 

How to cite: Vulpius, S. and Noack, L.: Effects on the solubility and the volatile release from magmatic intrusions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14267, https://doi.org/10.5194/egusphere-egu21-14267, 2021.

The water exchange between the Earth’s surface and the deep interior is a prime process for the geochemical evolution of our planet and its dynamics. The degassing of water from the mantle takes place through volcanism whereas mantle regassing occurs through the subduction of H2O chemically bound to hydrous minerals. The (im)balance between degassing and regassing controls the budget of surficial liquid water over geological timescales, i.e, the long-term global sea level. Continental freeboard constraints show that the mean-sea level has remained relatively constant in the last 540 Ma (changes less than about 100 m), thus suggesting a limited imbalance. However, thermopetrological models of water fluxes at present-day subduction zones predict that regassing exceeds degassing by about 50% which, if extrapolated to the past, would have induced a drop inconsistent with the estimations of the long-term sea-level. We have made the case that these inconsistencies arise from thermodynamic predictions for the hydrated lithospheric mantle mineralogy that are poorly constrained at a high pressure (P) and temperature (T). In our study, we thus have revised the global-water flux calculations in subduction zones using petrological constraints on post-antigorite assemblages from recent laboratory experimental data on natural peridotites under high-PT conditions [e.g. Maurice et al, 2018].

We model the thermal state of all present-day mature subduction zones along with petrological modeling using the thermodynamic code Perple_X and the most updated version of the thermodynamic database of Holland and Powell [2011]. For the modeling of peridotite, we build a hybrid phase diagram that combines thermodynamic calculations at moderate PT and experimental data at high PT (> 6 GPa- 600˚C). Our updated thermopetrological model reveals that the hydrated mantle efficiently dehydrates upon the breakdown of the hydrous aluminous-phase E before reaching 250 km in all but the coldest subduction zones. Further subducting slab dehydration is expected between 300-350 km depths, regardless of its thermal state, as a result of lawsonite breakdown in the gabbroic crust. Overall, we predict that present-day global water retention in subducting plates beyond a depth of 350 km barely exceeds the estimations of mantle degassing for average thicknesses of subducting serpentinized mantle subducting at the trenches of up to 6 km. Finally, our models quantitatively support the steady-state sea level scenario over geological times.

 

Maurice, J., Bolfan-Casanova, N., Padrón-Navarta, J. A., Manthilake, G., Hammouda, T., Hénot, J. M., & Andrault, D. (2018). The stability of hydrous phases beyond antigorite breakdown for a magnetite-bearing natural serpentinite between 6.5 and 11 GPa. Contributions to Mineralogy and Petrology, 173(10), 86.

Holland, T. J. B., & Powell, R. (2011). An improved and extended internally consistent thermodynamic dataset for phases of petrological interest, involving a new equation of state for solids. Journal of Metamorphic Geology, 29(3), 333-383.

How to cite: Cerpa, N., Arcay, D., and Padrón-Navarta, J. A.: Limited subduction of water to mid-upper mantle depths predicted by the phase assemblages in hydrated peridotites with natural chemical composition at high-PT conditions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4015, https://doi.org/10.5194/egusphere-egu21-4015, 2021.

EGU21-12478 | vPICO presentations | GD1.2

Modelling clinopyroxene/melt partition coefficients for higher upper mantle pressures 

Julia Marleen Schmidt and Lena Noack

When partial melt occurs in the mantle, redistribution of trace elements between the solid mantle material and partial melt takes place. Partition coefficients play an important role when determining the amount of trace elements that get redistributed into the melt. Due to a lower density compared to surrounding solid rock, partial melt that was generated in the upper mantle will rise towards the surface, leaving the upper mantle depleted in incompatible trace elements and an enriched crust. Studies investigating trace element partitioning in the mantle typically rely on constant partition coefficients throughout the mantle, even though it is known that partition coefficients depend on pressure, temperature, and composition. Between the pressures of 0-15 GPa, partition coefficients vary by two orders of magnitude along both, solidus and liquidus. Since partition coefficients exhibit a parabolic relationship in an Onuma diagram, a similar variation is expected for all trace element partition coefficients that can be derived from the sodium partition coefficients.

In this study, we developed a thermodynamic model for sodium in clinopyroxene after Blundy et al. (1995). With the thermodynamic model results, we were able to deduce a P-T dependent equation for sodium partitioning that is applicable up to 12 GPa between the peridotite solidus and liquidus. Because sodium is an almost strain-free element in jadeite, it can be used as a reference to model partition coefficients for other elements, including heat producing elements like K, Th, and U. This gives us the opportunity to insert P-T dependent partition coefficient calculations of any trace element into mantle melting models, which will have a big impact on the accuracy of elemental redistribution calculations and therefore, if the partitioning of the heat producing elements is taken into account, also the evolution of the mantle and crust.

Blundy, J. et al. (1995): Sodium partitioning between clinopyroxene and silicate melts, J. Geophys. Res., 100, 15501-15515.

Schmidt, J.M. and Noack, L. (2021): Parameterizing a model of clinopyroxene/melt partition coefficients for sodium to higher upper mantle pressures (to be submitted)

How to cite: Schmidt, J. M. and Noack, L.: Modelling clinopyroxene/melt partition coefficients for higher upper mantle pressures , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12478, https://doi.org/10.5194/egusphere-egu21-12478, 2021.

EGU21-817 | vPICO presentations | GD1.2

Solid to superionic transition in iron oxide-hydroxide

Qingyang Hu, Mingqiang Hou, and Yu He

At planetary interior conditions, water ice has been proved to enter a superionic phase recently since it was predicted about 30-year ago. Hydrogen in superionic water become liquid-like, and move freely within solid oxygen lattice. Under extreme pressure and temperature conditions of Earth’s deep mantle, the solid-superionic transition can also occur readily in the pyrite-type FeO2Hx, a candidate mineral in the lower mantle and probably also in other hydrous minerals. We find that when the pressure increases beyond 73 GPa at room temperature, symmetric hydroxyl bonds are softened and the H+ (or proton) become diffusive within the vicinity of its crystallographic site. Increasing temperature under pressure, the diffusivity of hydrogen is extended beyond individual unit cell to cover the entire solid, and the electrical conductivity soars, indicating a transition to the superionic state which is characterized by freely-moving proton and solid FeO2 lattice. The superionic hydrogen will dramatically change the geophysical picture of electrical conductivity and magnetism, as well as geochemical processes of hydrogen isotopic mixing and redox equilibria at local regions of Earth’s deep interiors.

How to cite: Hu, Q., Hou, M., and He, Y.: Solid to superionic transition in iron oxide-hydroxide, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-817, https://doi.org/10.5194/egusphere-egu21-817, 2021.

EGU21-13234 | vPICO presentations | GD1.2

Geological CO2 contributions quantified by high-temporal resolution carbon stable isotope monitoring in a salt mine

Alexander H. Frank, Robert van Geldern, Anssi Myrttinen, Martin Zimmer, Johannes A. C. Barth, and Bettina Strauch

CO2 emissions from geological sources have been recognized as an important input to the global carbon cycle. In regions without active volcanism, mines provide an extraordinary opportunity to observe dynamics of geogenic degassing close to its source.

High temporal resolution of stable carbon isotopes allows to outline temporal and interdependent dynamics of geogenic CO2 contributions. We present data from an active underground salt mine in central Germany that were collected on site with a field-deployed laser isotope spectrometer.

Throughout the 34-day measurement period, total CO2 concentrations varied between 805 ppmV (5th percentile) and 1370 ppmV (95th percentile). With a 400 ppm atmospheric background concentration, an isotope mixing model enabled the separation of geogenic (16–27 %) from highly dynamic contributions from anthropogenic CO2-sources (21–54 %). The geogenic fraction was inversely correlated to established CO2 concentrations that were driven by anthropogenic CO2 emissions within the mine. This indicates gradient-driven diffusion along microcracks.

Read more about this work in our open access publication in Scientific Reports at: http://rdcu.be/cblTz

How to cite: Frank, A. H., van Geldern, R., Myrttinen, A., Zimmer, M., Barth, J. A. C., and Strauch, B.: Geological CO2 contributions quantified by high-temporal resolution carbon stable isotope monitoring in a salt mine, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13234, https://doi.org/10.5194/egusphere-egu21-13234, 2021.

GD1.4 – Early Earth: Dynamics, Geology, Chemistry and Life in the Archean Earth

EGU21-560 | vPICO presentations | GD1.4

Plate tectonics: what, where, why, and when?

Richard Palin and M. Santosh

The theory of plate tectonics is widely accepted by scientists and provides a robust framework with which to describe and predict the behavior of Earth’s rigid outer shell – the lithosphere – in space and time. Expressions of plate tectonic interactions at the Earth’s surface also provide critical insight into the machinations of our planet’s inaccessible interior, and allow postulation about the geological characteristics of other rocky bodies in our solar system and beyond. Formalization of this paradigm occurred at a landmark Penrose conference in 1969, representing the culmination of centuries of study, and our understanding of the “what”, “where”, “why”, and “when” of plate tectonics on Earth has continued to improve since. Here, we summarize the major discoveries that have been made in these fields and present a modern-day holistic model for the geodynamic evolution of the Earth that best accommodates key lines of evidence for its changes over time. Plate tectonics probably began at a global scale during the Mesoarchean (c. 2.9–3.0 Ga), with firm evidence for subduction in older geological terranes accounted for by isolated plate tectonic ‘microcells’ that initiated at the heads of mantle plumes. Such early subduction likely operated at shallow angles and was short-lived, owing to the buoyancy and low rigidity of hotter oceanic lithosphere. A transitional period during the Neoarchean and Paleoproterozoic/Mesoproterozoic was characterized by continued secular cooling of the Earth’s mantle, which reduced the buoyancy of oceanic lithosphere and increased its strength, allowing the angle of subduction at convergent plate margins to gradually steepen. The appearance of rocks during the Neoproterozoic (c. 0.8–0.9 Ga) diagnostic of subduction do not mark the onset of plate tectonics, but simply record the beginning of modern-style cold, deep, and steep subduction that is an end-member state of an earlier, hotter, mobile lid regime

How to cite: Palin, R. and Santosh, M.: Plate tectonics: what, where, why, and when?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-560, https://doi.org/10.5194/egusphere-egu21-560, 2021.

EGU21-8809 | vPICO presentations | GD1.4

Emergence of plate tectonic during the Archean: insights from 3D numerical modelling.

Andrea Piccolo, Boris Kaus, Richard White, Nicolas Arndt, and Nicolas Riel

In the plate tectonic convection regime, the external lid is subdivided into discrete plates that move independently. Although it is known that the system of plates is mainly dominated by slab-pull forces, it is not yet clear how, when and why plate tectonics became the dominant geodynamic process in our planet. It could have started during the Meso-Archean (3.0-2.9 Ga). However, it is difficult to conceive a subduction driven system at the high mantle potential temperatures (Tp) that are thought to have existed around that time, because Tp controls the thickness and the strength of the compositional lithosphere making subduction unlikely. In recent years, however, a credible solution to the problem of subduction initiation during the Archean has been advanced, invoking a plume-induced subduction mechanism[1] that seems able to generate plate-tectonic like behaviour to first order. However, it has not yet been demonstrated how these tectonic processes interact with each other, and whether they are able to eventually propagate to larger scale subduction zones.

The Archean Eon was characterized by a high Tp[2], which generates weaker plates, and a thick and chemically buoyant lithosphere. In these conditions, slab pull forces are inefficient, and most likely unable to be transmitted within the plate. Therefore, plume-related proto-plate tectonic cells may not have been able to interact with each other or showed a different interaction as a function of mantle potential temperature and composition of the lithosphere. Moreover, due to secular change of Tp, the dynamics may change with time. In order to understand the complex interaction between these tectonic seeds it is necessary to undertake large scale 3D numerical simulations, incorporating the most relevant phase transitions and able to handle complex constitutive rheological model.

Here, we investigate the effects of the composition and Tp independently to understand the potential implications of the interaction of plume-induced subduction initiation. We employ a finite difference visco-elasto-plastic thermal petrological code using a large-scale domain (10000 x 10000 x 1000 km along x, y and z directions) and incorporating the most relevant petrological phase transitions. We prescribed two oceanic plateaus bounded by subduction zones and we let the negative buoyancy and plume-push forces evolve spontaneously. The paramount question that we aim to answer is whether these configurations allow the generation of stable plate boundaries. The models will also investigate whether the presence of continental terrain helps to generate plate-like features and whether the processes are strong enough to generate new continental terrains or assemble them

.

 

[1]       T. V. Gerya, R. J. Stern, M. Baes, S. V. Sobolev, and S. A. Whattam, “Plate tectonics on the Earth triggered by plume-induced subduction initiation,” Nature, vol. 527, no. 7577, pp. 221–225, 2015.

[2]       C. T. Herzberg, K. C. Condie, and J. Korenaga, “Thermal history of the Earth and its petrological expression,” Earth Planet. Sci. Lett., vol. 292, no. 1–2, pp. 79–88, 2010.

[3]       R. M. Palin, M. Santosh, W. Cao, S.-S. Li, D. Hernández-Uribe, and A. Parsons, “Secular metamorphic change and the onset of plate tectonics,” Earth-Science Rev., p. 103172, 2020.

How to cite: Piccolo, A., Kaus, B., White, R., Arndt, N., and Riel, N.: Emergence of plate tectonic during the Archean: insights from 3D numerical modelling., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8809, https://doi.org/10.5194/egusphere-egu21-8809, 2021.

EGU21-983 | vPICO presentations | GD1.4

What led to episodic subduction during the Archean?

Prasanna Gunawardana, Gabriele Morra, Priyadarshi Chowdhury, and Peter Cawood

The tectonic regime of the early Earth is crucial to understand how interior and exterior elements of the Earth interacted to make our planet habitable (Cawood et al., 2018). Our understanding of the processes involved is far from complete, particularly about how the switch between non-plate tectonic and plate tectonic regimes may have happened during the Archean. In this study, we investigate how Archean subduction events (albeit isolated and intermittent) may have evolved within/from a stagnant-lid regime. We perform 2D numerical modelling of mantle convection (using Underworld2) under a range of conditions appropriate for the early-to-mid Archean Earth including hotter mantle potential temperature and internal heat production. Using the models, we evaluate how the mantle temperature and viscosity, buoyancy force, surface heat flow and surface velocity may have evolved over a duration of ~800-1000 million years.

Our models indicate that lithospheric drips are an efficient way of releasing a large amount of heat from the Earth’s surface over a short period of time. Repeated occurrences of dripping events result in average mantle temperature gradually decreasing. Concomitant with this thermal evolution, the drip dimensions grew to form large, symmetrical drips as well as occasional, asymmetric subduction type events. The subduction events lead to large-scale resurfacing of the lithosphere. We surmise that the decreasing of average mantle temperature: (1) increases the temperature dependent viscosity of the mantle, and 2) decreases the buoyancy forces of mantle convection. Both these factors lower the convective vigour and increases the lithospheric (the upper thermal boundary layer) thickness via decreasing the effective Rayleigh number. These changes in the lithosphere-asthenosphere system facilitate the transition from a dripping dominated regime to a mix of large-dripping and intermittent subduction regime over a period of ~1 billon years. This change in tectonic setting is predicted to alter surface velocity patterns, surface heat flux and production rate of felsic magmas, which allows the modelling results can be tested against the rock record.

Reference

Cawood, P. A., Hawkesworth, C. J., Pisarevsky, S. A., Dhuime, B., Capitanio, F. A., and Nebel, O., 2018, Geological archive of the onset of plate tectonics: Philosophical transactions. Series A, Mathematical, physical, and engineering sciences, v. 376, no. 2132.

How to cite: Gunawardana, P., Morra, G., Chowdhury, P., and Cawood, P.: What led to episodic subduction during the Archean?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-983, https://doi.org/10.5194/egusphere-egu21-983, 2021.

EGU21-5386 | vPICO presentations | GD1.4

Numerical Insights into the Formation and Stability of Cratons

Charitra Jain, Antoine Rozel, Emily Chin, and Jeroen van Hunen
Geophysical, geochemical, and geological investigations have attributed the stable behaviour of Earth's continents to the presence of strong and viscous cratons underlying the continental crust. The cratons are underlain by thick and cold mantle keels, which are composed of melt-depleted and low density peridotite residues [1]. Progressive melt extraction increases the magnesium number Mg# in the residual peridotite, thereby making the roots of cratons chemically buoyant [2, 3] and counteracting their negative thermal buoyancy. Recent global models have shown the production of Archean continental crust by two-step mantle differentiation, however this primordial crust gets recycled and no stable continents form [4]. This points to the missing ingredient of cratonic lithosphere in these models, which could act as a stable basement for the crustal material to accumulate on and may also help with the transition of global regime from "vertical tectonics'' to "horizontal tectonics''. Based on the bulk FeO and MgO content of the residual peridotites, it has been proposed that cratonic mantle formed by hot shallow melting with mantle potential temperature, which was higher by 200-300 °C than present-day [5]. We introduce Fe-Mg partitioning between mantle peridotite and melt to track the Mg# variation through melting, and parametrise craton formation using the corresponding P-T formation conditions. Using self-consistent global convection models, we show the dynamic formation of cratons as a result of naturally occurring lateral compression and thickening of the lithosphere, which has been suggested by geochemical and petrological data. To allow for the material to compact and thicken, but prevent it from collapsing under its own weight, a combination of lithospheric strength, plastic yielding, dehydration strengthening, and depletion-induced density reduction of the depleted mantle material is necessary.
 
 [1] Boyd, F. R. High-and low-temperature garnet peridotite xenoliths and their possible relation to the lithosphere- asthenosphere boundary beneath Africa. In Nixon, P. H. (ed.) Mantle Xenolith, 403–412 (John Wiley & Sons Ltd., 1987).
[2] Jordan, T. H. Mineralogies, densities and seismic velocities of garnet lherzolites and their geophysical implications. In The Mantle Sample: Inclusion in Kimberlites and Other Volcanics, 1–14 (American Geophysical Union, Washington, D. C., 1979).
[3] Schutt, D. L. & Lesher, C. E. Effects of melt depletion on the density and seismic velocity of garnet and spinel lherzolite. Journal of Geophysical Research 111 (2006).
[4] Jain, C., Rozel, A. B., Tackley, P. J., Sanan, P. & Gerya, T. V. Growing primordial continental crust self-consistently in global mantle convection models. Gondwana Research 73, 96–122 (2019).
[5] Lee, C.-T. A. & Chin, E. J. Calculating melting temperatures and pressures of peridotite protoliths: Implications for the origin of cratonic mantle. Earth and Planetary Science Letters 403, 273–286 (2014)

How to cite: Jain, C., Rozel, A., Chin, E., and van Hunen, J.: Numerical Insights into the Formation and Stability of Cratons, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5386, https://doi.org/10.5194/egusphere-egu21-5386, 2021.

EGU21-7477 | vPICO presentations | GD1.4

How did the Archean crust evolve? Insights from the structure and petrology of the Lewisian of Scotland

Sophie Miocevich, Alex Copley, and Owen Weller

High-grade Archean gneiss terranes expose mid to lower crustal rocks and are generally dominated by tonalite-trondhjemite-granodiorite (TTG) gneisses. Occurrences of mafic-ultramafic bodies and garnet-bearing felsic gneisses within these environments have been interpreted as supracrustal or near-surface rocks requiring a tectonic process involving mass transfer from the near-surface to the mid-crust. However, there is significant uncertainty regarding the nature of this mass transfer, with suggestions including a range of uniformitarian and non-uniformitarian scenarios.  One non-uniformitarian scenario, ‘sagduction’, has been proposed as a possible mechanism (Johnson et al., 2016, and references therein), although the dynamics of sagduction are still relatively unexplored.

This study focuses on mafic, ultramafic and garnet-bearing felsic gneiss bodies in the central region in the Lewisian Gneiss Complex of northwest Scotland as test cases to investigate the behaviour of possibly supracrustal rocks in a mid-crustal environment. Existing datasets of TTGs (Johnson et al., 2016), mafic gneisses (Feisel et al., 2018) and ultramafic gneisses (Guice et al., 2018) from across the central region were utilised in addition to felsic and mafic gneiss samples obtained in this study from the ~10 km2 Cnoc an t-Sidhean (CAS) suite. The CAS suite is the largest reported supracrustal in the Lewisian, and dominantly comprises garnet-biotite felsic gneiss assemblages and an associated two-pyroxene mafic gneiss. Field mapping was undertaken to collect samples representative of the observed heterogeneity of the suite, and to assess field associations between possible supracrustals and surrounding TTGs. Phase equilibria modelling was conducted on all lithologies to ascertain peak pressure-temperature (P-T) conditions, and to calculate the density of the modelled rocks at peak conditions.

The results obtained in this study indicate peak metamorphic conditions of 950 ± 50 °C and 9 ± 1 kbar for the CAS suite, consistent with the central region of the Lewisian Complex (Feisel et al., 2018). Density contrasts at mid-crustal conditions of 0.12–0.56 gcm-3 were calculated between TTGs and the other lithologies and used to estimate the buoyancy force that drives density-driven segregation. This allowed us to investigate the rates of vertical motion that result from density contrasts, as a function of the effective viscosity during metamorphism. Independent viscosity estimates were attained using mineral flow-laws and our estimated P-T conditions, and from examination of modern-day regions of crustal flow. We were therefore able to estimate the conditions under which sagduction could have been a viable mechanism for crustal evolution in the Lewisian and similar high-grade metamorphic terranes. We conclude that sagduction was unlikely to have operated in the Lewisian under the dry conditions implied by preserved mineral assemblages.

 

 

Feisel, Y., et al. 2018. New constraints on granulite facies metamorphism and melt production in the Lewisian Complex, northwest Scotland. Journal of Metamorphic Geology. 36, 799-819

Guice, G.L., et al. 2018. Assessing the Validity of Negative High Field Strength-Element Anomalies as a Proxy for Archaean Subduction: Evidence from the Ben Strome Complex, NW Scotland. Geosciences, 8, 338.

Johnson, T.E., et al. 2016. Subduction or sagduction? Ambiguity in constraining the origin of ultramafic–mafic bodies in the Archean crust of NW Scotland. Precambrian Research, 283, 89-105.

How to cite: Miocevich, S., Copley, A., and Weller, O.: How did the Archean crust evolve? Insights from the structure and petrology of the Lewisian of Scotland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7477, https://doi.org/10.5194/egusphere-egu21-7477, 2021.

EGU21-15688 | vPICO presentations | GD1.4

Zircon U-Pb and Lu-Hf record from the Archean Lewisian Gneiss Complex, NW Scotland

Lanita Gutieva, Annika Dziggel, Silvia Volante, and Tim Johnson

The Lewisian Gneiss Complex (LGC) in NW Scotland, a classic example of Archean lower crust, is mostly composed of deformed and metamorphosed tonalite–trondhjemite–granodiorite (TTG) gneisses, gneissose granite sheets, and subordinate mafic, ultramafic, and metasedimentary lithologies. It has been traditionally subdivided into three regions that are interpreted to record discrete ages and metamorphic histories, and which are separated by crustal-scale shear zones. A smear of concordant U–Pb zircon ages from the granulite-facies central region has been interpreted to record metamorphic resetting of earlier magmatic and granulite facies metamorphic ages during a subsequent high-temperature metamorphic event. Here, we present U–Pb and Hf isotope data collected via laser-ablation split-stream (LASS) analyses of zircon cores from twenty-seven felsic meta-igneous rocks from the northern, southern, and central regions of the LGC, as well as U–Pb data from zircon rims within most of those samples.

In samples from the northern and southern regions, the crystallization age (i.e., from zircon cores) was calculated from the upper-intercept age, yielding age range of 2.82-2.63 Ga for the northern, and 3.11–2.63 Ga for the southern region. Zircons in these samples generally have thin or no rims, suggesting an absence of a prolonged high-grade (granulite facies) metamorphic event in those regions. In the central region, zircon cores yield U–Pb crystallization ages between ca. 3.0 Ga and 2.7 Ga, while zircon rims define a continuous spread of ages from ca. 2.8 to 2.4 Ga. Overall, the central region exhibits a continuous and overlapping smear of zircon core and rim ages, suggesting a protracted thermal event in which high-ultrahigh temperature conditions were maintained for >200 m.y., and that discrete magmatic and metamorphic ‘events’ are difficult to identify. Nevertheless, an estimation of the crystallization age of each sample is crucial for interpreting their Lu–Hf isotopic signature. Zircon cores from the tonalite–trondhjemite gneisses have broadly chondritic compositions with a range of calculated mean initial εHf of +2.5 to –1.2, potentially reflecting a mixture of juvenile material and reworked crust, with one outlier at εHfi = +4.5 perhaps indicating a renewed influx of juvenile magma. Granite gneisses also have near-chondritic values, although the range is larger and the two youngest granite gneisses have slightly sub-chondritic εHfi (–1.5 and –2.5), which indicates that pre-existing crust was involved in their formation. Since there is no significant difference in the Hf isotopic composition between rocks from the three regions, or between the TTG and granite gneisses, we suggest that the broadly chondritic εHfi in most of our samples reflects mixing of both depleted mantle and evolved crust during their generation. Despite the similarity of the U-Pb and εHf data from the three regions, the data do not allow to unambiguously discriminate whether the LGC is composed of different levels of a once continuous Archean continent or discrete microcontinents that were amalgamated in the late Archean to Paleoproterozoic.

How to cite: Gutieva, L., Dziggel, A., Volante, S., and Johnson, T.: Zircon U-Pb and Lu-Hf record from the Archean Lewisian Gneiss Complex, NW Scotland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15688, https://doi.org/10.5194/egusphere-egu21-15688, 2021.

EGU21-2468 | vPICO presentations | GD1.4

P-T-fluid conditions of mineral equilibria in garnet-biotite crustal xenoliths from the Yubileinaya and Sytykanskaya kimberlite pipes, Yakutian kimberlite province.

Natalia Seliutina, Oleg Safonov, Vasiliy Yapaskurt, Dmitry Varlamov, Igor Sharygin, and Konstantin Konstantinov

This study provides the results of research of the garnet-biotite crustal xenoliths from the Yubileinaya (372±4.8 Ma) and Sytykanskaya (363±13 Ma) kimberlite pipes of the Alakit-Markhinsky field (Siberian craton). Isotopic evidence on zircons from similar crustal xenoliths (Grt+Bt+Pl+Kfs+Qtz±Scp) showed Archean Hf model ages (TDM = 3.13-2.5 Ga) and thus indicated that most of the lower and middle crust beneath the Markha terrane was produced in the Archean time (Shatsky et al., 2016).

The xenoliths are represented by the assemblage Grt+Bt+Pl+Kfs±Opx. Quartz is present only as rare inclusions in garnets. The rocks are coarse-grained, slightly foliated with garnets porphyroblasts of up to 5 cm in size. A spectacular feature of the rocks is an abundance of K-feldspar. Garnet grains are almost compositionally homogeneous, although they show a rimward decrease of the Mg and Ca contents indicating exchange reactions during cooling. Biotites are characterized by high F increasing from 1.5 wt.% in cores up to 2.2 wt.% in rims, as well as TiO2 up to 7.8 wt.%, which is typical for high-grade rocks. Orthopyroxene (up to 5.5 wt. % Al2O3) relics are preserved both as inclusions in garnet and as individual grains in the rock matrix. Plagioclase occurs both as separate grains and as lamellae in potassium feldspar.

The bulk chemical compositions correspond to a metagraywacke. The REE spectra in these rocks are rather flat with slight enrichment in LREE. All the studied rocks are characterized by a distinct negative Eu anomaly (Eu/Eu* = 0.31-0.45).

Calculations using the PERPLEX software version 6.7.6 (Connolly, 2005) for Mg and Ca in Grt, Mg in Bt, and Ca in Pl indicated temperatures 630-730°C and pressures 5.8-7.2 kbar for the rocks. However, equilibria involving Al2O3 in orthopyroxene corresponds to temperatures of 750-800oС at a similar pressure. It indicates that metamorphism of the garnet-biotite rocks reached higher temperatures, but they were actively modified later during cooling and insignificant decompression (by about 1 kbar). Calculations using the TWQ software version 2.3 (Berman, 2007) indicate consistent temperatures 610-680°C for the garnet-orthopyroxene and 640-690oC for garnet-biotite Mg-Fe exchange equilibria. Calculations using the Grs+2Prp+Kfs+H2O=Phl+3En+3An equilibrium demonstrated water activity below 0.1. Such low water activity could indicate an influence of highly concentrated alkaline Cl-F-bearing brines. This assumption is confirmed by extensive development of potassium feldspar, absence of quartz in the matrix, and elevated Cl contents of biotite, 0.1-0.3 wt. % at high #Mg (>0.7) and F content.

The study is supported by the Russian Science Foundation project 18-17-00206.

 References

Berman, R. G. (2007). winTWQ (version 2.3): a software package for performing internally-consistent thermobarometric calculations. Geological survey of Canada, open file, 5462, 41.

Connolly, T. M., & Begg, C. E. (2005). Database systems: a practical approach to design, implementation, and management. Pearson Education.

Shatsky, V. S., Malkovets, V. G., Belousova, E. A., ... & O’Reilly, S. Y. (2016). Tectonothermal evolution of the continental crust beneath the Yakutian diamondiferous province (Siberian craton): U–Pb and Hf isotopic evidence on zircons from crustal xenoliths of kimberlite pipes. Precambrian Research, 282, 1-20.

How to cite: Seliutina, N., Safonov, O., Yapaskurt, V., Varlamov, D., Sharygin, I., and Konstantinov, K.: P-T-fluid conditions of mineral equilibria in garnet-biotite crustal xenoliths from the Yubileinaya and Sytykanskaya kimberlite pipes, Yakutian kimberlite province., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2468, https://doi.org/10.5194/egusphere-egu21-2468, 2021.

The Banded Iron Formation (BIF) in Bundelkhand craton (BuC) occurred as supracrustals associated with TTG’s, amphibolites, calcsilicate rocks, and quartzite within the east-west trending Bundelkhand tectonic zone (BTZ). The BIFs near Mauranipur do not show any prominent iron-rich and silica-rich layer band and are composed of garnet, amphibole, quartz, and magnetite. The volumetrically dominant monoclinic-amphiboles are grunerite in composition. XMg of grunerite varies between 0.39-0.37. The garnets are Mn-rich, the XSpss of garnet ranges from 0.26-0.20, XPyp and XGrs vary between 0.10-0.06 and 0.07-0.05, respectively. P-T pseudosection analysis indicates that by destabilizing iron-silicate hydroxide phases through a series of dehydration and decarbonation reactions, amphibole and garnet stabilized in BIF at temperature 400-450°C and pressure 0.1-0.2 GPa.

Massive type BIFs have monazite grains that vary from 10 to 50 µm in size, yield three distinct U-Th-Pbtotal age clusters. 10-20 µm sized monazite grains yield the oldest age, 3098±95 Ma. 2478±37 Ma average age is obtained from the second group, which is relatively larger and volumetrically predominant. The third age group of Monaiztes gives an age of 2088±110 Ma. ~3100 Ma monazite suggests the older supracrustal rocks of Bundelkhand craton, similar to those obtained from Singhbhum and the Dharwar craton. The 2478±37 Ma age is constrained as the timing of metamorphism and stabilization of BuC. The third age group, 2088±110 Ma probably associated with renewed hydrothermal activities, leading to rifting and emplacement of mafic dykes in BuC.

How to cite: Raza, M. B., Nasipuri, P., and Hifzurrahman, : Mineral chemistry, P-T pseudosection and in-situ U-Th-Pbtotal monazite geochronology of Banded Iron Formation from Bundelkhand craton North-Central India, and its geodynamic significance, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9812, https://doi.org/10.5194/egusphere-egu21-9812, 2021.

EGU21-4701 | vPICO presentations | GD1.4

The emergence of subaerial crust and onset of weathering 3.7 billion years ago

Desiree Roerdink, Yuval Ronen, Harald Strauss, and Paul Mason

Reconstructing the emergence and weathering of continental crust in the Archean is crucial for our understanding of early ocean chemistry, biosphere evolution and the onset of plate tectonics. However, considerable disagreement exists between the various elemental and isotopic proxies that have been used to trace crustal input into marine sediments, and data are scarce prior to 3 billion years ago. Here we show that chemical weathering modified the Sr isotopic composition of Archean seawater as recorded in 3.52 to 3.20 Ga stratiform marine-hydrothermal barite deposits from three different cratons. We use a combination of barite crystal morphology, oxygen, multiple sulfur and strontium isotope data to select barite samples with the most seawater-like isotopic compositions, and subsequently use these in a hydrothermal mixing model to calculate a plausible seawater Sr isotope evolution trend from measured 87Sr/86Sr data. From modeled mixing ratios between seawater and hydrothermal fluids required for barite precipitation and comparison of 87Sr/86Sr in theoretical seawater-hydrothermal fluid mixtures with those recorded in the barite, we obtain a novel seawater Sr isotope evolution trend for Paleoarchean seawater that is much more radiogenic than the curve previously determined from carbonate rocks. Our findings require the presence and weathering of subaerial and evolved (high Rb/Sr) crust from 3.7 ± 0.1 Ga onwards, and demonstrate that crustal weathering affected the chemistry of the oceans 500 million years earlier than previously thought.

How to cite: Roerdink, D., Ronen, Y., Strauss, H., and Mason, P.: The emergence of subaerial crust and onset of weathering 3.7 billion years ago, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4701, https://doi.org/10.5194/egusphere-egu21-4701, 2021.

EGU21-14443 | vPICO presentations | GD1.4

Effect of Hydrocarbon Haze on Marine Primary Production in the Early Earth System

Yasuto Watanabe, Eiichi Tajika, Kazumi Ozaki, and Peng Hong

During the Archean (4.0–2.5 Ga), atmospheric oxygen levels would have been much lower than the present value (pO2 < ~10–5 PAL) [1], and the majority of the primary production would have been carried by anoxygenic photosynthetic bacteria. In a sufficiently reducing atmosphere (CH4/CO2 > ~0.2) [2], the layer of hydrocarbon haze could be formed in the upper atmosphere, possibly affecting the climate. Because haze particles significantly absorb the solar UV flux, the formation of hydrocarbon haze could affect the marine microbial ecosystem via the change in the production rate of electron donors (H2 and CO). However, how the formation of hydrocarbon haze affects the global activity of the marine microbial ecosystem remains unclear. Here, we employ a novel carbon cycle model in which a one-dimensional photochemical model “Atmos” [2], a marine microbial ecosystem model, and the carbonate-silicate geochemical cycle model are coupled. We assessed the effect of the formation of hydrocarbon haze on marine microbial ecosystems assuming completely anoxic conditions (pO2 < ~10–10 PAL) in the middle Archean and assuming mildly oxidized conditions (pO2 > 10–10 PAL) in the late Archean.

We found that, under the completely anoxic condition, haze formation works as a negative feedback for the oceanic biological activity. This is because the formation rate of electron donors (H2 and CO) in the atmosphere decreases with the progress of haze formation, so that the changes in the biogenic methane flux and the haze formation rate are suppressed. More specifically, the decrease in the formation rate of electron donors is caused by the decrease in the photo-dissociation rate of CO2 because of UV-shielding due to haze particles, and also by removal of C- and H-atom, which are supposed to be converted to CO and H2 if the haze is not formed, due to rainout of haze particles. 

We also found that, under the mildly oxidized condition, there are multiple equilibrium climate states that have a different haze thickness. The solution with thicker haze layer is similar to the completely anoxic condition, however, the other solution with the thinner haze layer is unique to the mildly oxidized condition. In this new equilibrium state, the formation rate of electron donors further decreases with the progress of haze formation because of the decrease in the photo-dissociation rate of formaldehyde. Thus, this mechanism works as a strong negative feedback for ocean biological activity and haze thickness, keeping the haze thickness thinner than the completely anoxic condition. We show that, as a result of this negative feedback, climate with the thinner haze could be stably achieved under the mildly oxidized condition. This result is consistent with a geological record which suggests possible transient formation of the haze in the Late Archean [3]. We suggest that haze formation is a vital process in understanding the biological activity and climate stability on terrestrial Earth-like planets.

[1] Lyons et al. (2014). Nature 506, 307-315. [2] Arney et al. (2016). Astrobiology 16(11), 873-899. [3] Izon et al. (2017). PNAS 114(13), E2571-E2579.

How to cite: Watanabe, Y., Tajika, E., Ozaki, K., and Hong, P.: Effect of Hydrocarbon Haze on Marine Primary Production in the Early Earth System, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14443, https://doi.org/10.5194/egusphere-egu21-14443, 2021.

EGU21-13722 | vPICO presentations | GD1.4

The Isua (Greenland) relict stromatolites cannot be confidently interpreted as original sedimentary structures

Mike Zawaski, Nigel Kelley, Phil (Omero) Orlandini, Claire Nichols, Abigail Allwood, and stephen Mojzsis

The biogenicity of proposed stromatolite structures from Eoarchean (ca. 3.71 Ga) rocks of the Isua Supracrustal Belt (ISB) in West Greenland is under debate. Our 2020 publication argues against biogenicity for the proposed stromatolites. The subsequent Comment to our work challenged some of our fundamental arguments for a tectonic origin to the structures. This Comment has been an opportunity for us to elaborate on these structures and further refine and solidify our initial conclusion that they represent the expected outcome of the tectonic deformation displayed in the ISB. This dialogue between groups is essential as the consequence of these structures being biogenic would move the date for complex microbial communities 200 million years closer to Earth's formation, to a time when Earth’s surface would have been even less habitable. Here we reexamine our four key observations that support our tectonic origin. First, we report detailed field characterization and structural analysis to show that the structures are linear inverted ridges aligned with azimuths of local and regional fold axes and parallel to linear structures; they were never primary linear, deformation-parallel stromatolites or deformed conical stromatolites. Second, our combined major element (e.g., Ca, Mg, Si) scanning μXRF maps fail to reveal internal laminations for the cores of these structures, but other authors argue layers are present. In the instance where layers appear to be preserved, we argue that an amorphous core is still present.  Also, layering on its own is inconclusive of a biogenic origin as relict internal laminations could be preserved. Third, the gross morphology of these structures being nearly identical in morphology and dimensions to clearly tectonic structures only tens of meters away is a more reliable indicator of a tectonic versus biogenic origin than internal laminations. Lastly, discontinuous field relationships and absence of primary sedimentary structures that could serve as way-up indicators preclude confident assignment of these outcrops as being structurally overturned, as originally argued. Collectively, our results reinforce that the Isua structures are the expected result of a tectonic fabric that preserves no fine-scale primary sedimentary structures and were probably never stromatolites.

How to cite: Zawaski, M., Kelley, N., Orlandini, P. (., Nichols, C., Allwood, A., and Mojzsis, S.: The Isua (Greenland) relict stromatolites cannot be confidently interpreted as original sedimentary structures, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13722, https://doi.org/10.5194/egusphere-egu21-13722, 2021.

GD1.6 – Multi-disciplinary perspectives on plume-plate interactions and geodynamic influences on topography

EGU21-13169 | vPICO presentations | GD1.6 | Highlight

Characterizing plume-plate interactions at ocean hotspots from the vertical motion history of volcanic ocean islands

Kimberly Huppert, J. Taylor Perron, Leigh Royden, and Michael Toomey

Geologic evidence of island uplift and subsidence can provide important observational constraints on the rheology, thermal evolution, and dynamics of the lithosphere and mantle – all of which have implications for understanding Earth’s heat budget, the styles of deformation that develop at plate boundaries, and the surface expression of mantle convection. Hotspot ocean islands, like the Hawaiian Islands, result from mantle plumes, which may originate as deep as the core-mantle boundary. They often host paleoshorelines, which preserve a geologic record of surface deformation, and they can also be situated far from complex plate boundaries that obscure evidence of dynamic topography – long wavelength, low amplitude topography resulting from mantle flow. Ocean islands therefore provide a unique window to deep earth processes operating today and in the geologic past.

We examine the relative contribution of lithosphere and mantle processes to surface deflection at ocean hotspots. The seafloor surrounding ocean hotspots is typically 0.5 - 2 km shallower than expected for its age over areas hundreds to >1000 km wide, but the processes generating these bathymetric swells are uncertain. Swells may result from reheating and thinning of the lithosphere and the isostatic effect of replacing colder, denser lithosphere with hotter, less dense upper mantle. Alternately, they may be supported by upward flow of ascending mantle plumes and/or hot, buoyant plume material ponded beneath the lithosphere. Because these two end-member models predict different patterns of seafloor and island subsidence, swell morphology and the geologic record of island drowning may reveal which of these mechanisms dominates the process of swell uplift. We examine swell bathymetry and island drowning at 14 hotspots and find a correspondence between island lifespan and residence time atop swell bathymetry, implying that islands drown as tectonic plate motion transports them past mantle sources of uplift. This correspondence argues strongly for dynamic uplift of the lithosphere at ocean hotspots. Our results also explain global variations in island lifespan on fast- and slow-moving tectonic plates (e.g. drowned islands in the Galápagos <4 Myr old versus islands >20 Myr old above sea level in the Canary Islands), which strongly influence island topography, biodiversity, and climate.

Over shorter timescales, paleoshorelines on hotspot ocean islands may constrain transient changes in local swell morphology. Accounting for flexural isostatic adjustment of the lithosphere to volcanic loading, we also examine patterns in the residual deflection of paleoshorelines across the Hawaiian Islands that might correspond to non-steady state behavior of the Hawaiian plume. Together, these analyses highlight the unique constraints that island paleoshorelines and topo-bathymetry can place on plume-plate interactions at ocean hotspots.

How to cite: Huppert, K., Perron, J. T., Royden, L., and Toomey, M.: Characterizing plume-plate interactions at ocean hotspots from the vertical motion history of volcanic ocean islands, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13169, https://doi.org/10.5194/egusphere-egu21-13169, 2021.

EGU21-13277 | vPICO presentations | GD1.6

Plume-induced subduction and plate fracturation, deep mantle overturn, and the onset of plate tectonics.

Anne Davaille

EGU21-546 | vPICO presentations | GD1.6

A record of plume-induced plate rotation triggering seafloor spreading and subduction initiation

Douwe J. J. van Hinsbergen, Bernhard Steinberger, Carl Guilmette, Marco Maffione, Derya Gürer, Kalijn Peters, Alexis Plunder, Peter McPhee, Carmen Gaina, Eldert Advokaat, Reinoud Vissers, and Wim Spakman

The formation of a global network of plate boundaries surrounding a mosaic of lithospheric fragments was a key step in the emergence of Earth’s plate tectonics. So far, propositions for plate boundary formation are regional in nature but how plate boundaries are being created over 1000s of km in short periods of geological time remains elusive. Here, we show from geological observations that a >12,000 km long plate boundary formed between the Indian and African plates around 105 Ma with subduction segments from the eastern Mediterranean region to a newly established India-Africa rotation pole in the west-Indian ocean where it transitioned into a ridge between India and Madagascar. We find no plate tectonics-related potential triggers of this plate rotation and identify coeval mantle plume rise below Madagascar-India as the only viable driver. For this, we provide a proof of concept by torque balance modeling revealing that the Indian and African cratonic keels were important in determining plate rotation and subduction initiation in response to the spreading plume head. Our results show that plumes may provide a non-plate-tectonic mechanism for large plate rotation initiating divergent and convergent plate boundaries far away from the plume head that may even be an underlying cause of the emergence of modern plate tectonics.

How to cite: van Hinsbergen, D. J. J., Steinberger, B., Guilmette, C., Maffione, M., Gürer, D., Peters, K., Plunder, A., McPhee, P., Gaina, C., Advokaat, E., Vissers, R., and Spakman, W.: A record of plume-induced plate rotation triggering seafloor spreading and subduction initiation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-546, https://doi.org/10.5194/egusphere-egu21-546, 2021.

EGU21-7763 | vPICO presentations | GD1.6

Effect of plate motion on plume-induced subduction initiation

Marzieh Baes, Stephan Sobolev, Taras Gerya, Robert Stern, and Sascha Brune

Subduction zones are key components of plate tectonics and plate tectonics could not begin until the first subduction zone formed. Plume-induced subduction initiation, which has been proposed as triggering the beginning of plate tectonics (Gerya et al., 2015), is one of the few scenarios that can break the lithosphere and recycle a stagnant lid without requiring any pre-existing weak zones. So far, two natural examples of plume-induced subduction initiation have been recognized. The first was found in southern and western margins of the Caribbean Plate (Whattam and Stern 2014). Initiation of the Cascadia subduction zone in Eocene times has been proposed to be the second example of plume-induced subduction initiation (Stern and Dumitru, 2019).

The focus of previous studies was to inspect plume-lithosphere interaction either for the case of stationary lithosphere (e.g., Gerya et al., 2015) or for moving lithosphere without considering the effect of lithospheric magmatic weakening above the plume head (e.g., Moore et al., 1998). In present study we investigate the response of moving oceanic lithosphere to the arrival of a rising mantle plume head including the effect of magmatic lithospheric weakening. We used 3D numerical thermo-mechanical modeling. Using I3ELVIS code, which is based on finite difference staggered grid and marker-in-cell with an efficient OpenMP multigrid solver (Gerya, 2010), we show that plate motion may affect the plume-induced subduction initiation only if a moderate size plume head (with a radius of 140 km in our experiments) impinges on a young but subductable lithosphere (with the age of 20 Myr). Outcomes indicate that lithospheric strength and plume buoyancy are key parameters in penetration of the plume and subduction initiation and that plate speed has a minor effect. We propose that eastward motion of the Farallon plate in Late Cretaceous time could play a key role in forming new subduction zones along the western and southern margin of the Caribbean plate.

 

References:

Gerya, T., 2010, Introduction to Numerical Geodynamic Modelling.. Cambridge University Press.

Gerya, T.V., Stern, R.J., Baes, M., Sobolev, S.V. and Whattam, S.A., 2015. Plume-induced subduction initiation triggered Plate Tectonics on Earth. Nature, 527, 221–225.

Moore, W. B., Schubert, G. and Tackley, P., 1998, Three-dimensional simulations of plume-lithosphere interaction at the Hawaiian swell. Science, 279, 1008-1011.

Stern, R.J., and Dumitru, T.A., 2019, Eocene initiation of the Cascadia subduction zone: A second example of plume-induced subduction initiation? Geosphere, v. 15, 659-681.

Whattam, S.A. and Stern, R.J., 2014. Late Cretaceous plume-induced subduction initiation along the southern margin of the Caribbean and NW South America: The first documented example with implications for the onset of plate tectonics. Gondwana Research, 27, doi: 10.1016/j.gr.2014.07.011.

How to cite: Baes, M., Sobolev, S., Gerya, T., Stern, R., and Brune, S.: Effect of plate motion on plume-induced subduction initiation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7763, https://doi.org/10.5194/egusphere-egu21-7763, 2021.

EGU21-3726 | vPICO presentations | GD1.6

Interaction of the Indian craton with the Reunion plume

Jyotirmoy Paul and Attreyee Ghosh

One of the fundamental characteristics of cratons is the presence of thick lithosphere of more than 200 km, whereas any standard non-cratonic lithosphere thickness is about 100 km thick. The thickness of Indian craton has remained quite controversial. Under the Indian plate, most seismic studies fail to recognise a thick lithosphere; however, a few studies using other geophysical methods (e.g., magnetotellurics) argue for a thick Indian craton. In the last 30 years, more than ten research articles estimated the thickness of the Indian craton that varied from less than 100 km to 260 km. Such controversy arose primarily because of the Reunion plume and Indian craton interaction at ~65 Ma. Some studies suggested that due to the Reunion plume's eruption underneath the Indian craton, the thick lithosphere of the Indian craton was thinned down. This thin lithosphere is attributed as one of the primary reasons for the acceleration of the Indian plate since 65 Ma. On the other hand, several studies advocated that the Reunion plume did not affect the thickness of the Indian craton. Still now, no study has actually investigated the nature of plume-craton interaction under the Indian plate and how the craton was deformed in the presence of a plume. In this study, we develop time-dependent global mantle convection models using CitcomS to understand the evolution of Indian craton for the last 100 Ma. The models are initiated at 100 Ma and are driven forward  up to the present day using reconstructed plate velocities at every 1 Myr interval. Our results show that it is possible to thin down the thicker cratonic lithosphere due to the eruption of the Reunion plume. We also observe that the plume could get bifurcated due to the craton, and eruptions could occur on both the eastern and western parts of the Indian continental lithosphere.

How to cite: Paul, J. and Ghosh, A.: Interaction of the Indian craton with the Reunion plume, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3726, https://doi.org/10.5194/egusphere-egu21-3726, 2021.

EGU21-8496 | vPICO presentations | GD1.6 | Highlight

No signal of a plume push force in Indo-Atlantic plate speeds before, during, or after Deccan plume arrival

Graeme Eagles, Lucía Pérez Díaz, and Karin Sigloch

Observations of the apparent links between plate speeds and the global distribution of plate boundary types have led to the suggestion that subduction may provide the largest component in the balance of torques maintaining plate motions. This would imply that plate speeds should not exceed the sinking rates of slabs into the upper mantle. Instances of this ‘speed limit’ having been broken may thus hint at the existence of driving mechanisms additional to those resulting from plate boundary forces. The arrival and emplacement of the Deccan-Réunion mantle plume beneath the Indian-African plate boundary in the 67-62 Ma period has been discussed in terms of one such additional driving mechanism, leading to the establishment of “plume-push” hypothesis, which in recent years has gained significant traction. We challenge the model-based observations that form the principal evidence in favour of plume-push: a late Cretaceous pulse of anticorrelating accelerations and decelerations in seafloor spreading rates around the African and Indian plates. Using existing and newly-calculated high-resolution models of plate motion, we instead document an increase in divergence rates at 67-64 Ma. Because of its ubiquity, we consider this increase to be the artefact of a timescale error affecting chrons 29-28. Corrected for this artefact, the evolution of plate speeds resembles a smooth continuation of pre-existing late Cretaceous trends, consistent with the idea that the arrival of the Réunion plume did not substantially affect the existing balance of plate boundary forces on the Indian and African plates. 

How to cite: Eagles, G., Pérez Díaz, L., and Sigloch, K.: No signal of a plume push force in Indo-Atlantic plate speeds before, during, or after Deccan plume arrival, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8496, https://doi.org/10.5194/egusphere-egu21-8496, 2021.

EGU21-4669 | vPICO presentations | GD1.6

Continental Hotspots Tracks from Analysis of GOCE Gravity Gradients Data

Marianne Greff-Lefftz, Isabelle Panet, and Jean Besse

Hotspots are thermal instabilities that originate in the mantle and manifest themselves on the surface by volcanism, continental breaks or "traces" observed in the oceans. Theirs effects under the continents are still debated: in addition to a phase of activity associated with surface volcanism, a residual thermal anomaly could persist durably under the lithosphere along the trajectory of the hotspot.
For a simple model of thermal anomaly (parallelogram aligned in a fixed direction), we compute the perturbations of the geoid, of the gravity vector and of the associated gravity gradients. We show that in a coordinate system aligned with the parallelogram, gravity gradients have a characteristic signal with an order of magnitude of a few hundred mEotvos, well above the current data detection level. Thus for four real cases: in North Africa (with the Hoggar, Tibesti, Darfur and Cameroon hotspots), in Greenland (Iceland and Jan Mayen), in Australia (Cosgrove) and in Europe (Eifel), we calculate the paleo-positions of the hotspots during the last 100 Ma in a reference frame linked to the lithospheric plates, and we build maps of gravity gradients at different altitudes filtered at the spatial scale of a few hundred kilometers (scale of the hotspot) and oriented along the direction of the trajectory.
We clearly find signals aligned in the direction of the movement of the plates on spatial scales of a few hundred kilometers.
This signal is sometimes correlated with the topography and it is difficult to separate the sources resulting from volcanic edifices and their associated isostatic crustal roots from that induced by residual thermal anomaly. These results show that gradiometric data are able to detect and follow the tracks of hotspots in the continental lithosphere, during at least a few tens of millions of years, providing new clues to constrain their trajectory and improve reference frame tied to the mantle.

How to cite: Greff-Lefftz, M., Panet, I., and Besse, J.: Continental Hotspots Tracks from Analysis of GOCE Gravity Gradients Data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4669, https://doi.org/10.5194/egusphere-egu21-4669, 2021.

EGU21-6857 | vPICO presentations | GD1.6

Intraplate volcanism triggered by bursts in slab flux

Ben Mather, Dietmar Muller, Maria Seton, Saskia Ruttor, Oliver Nebel, and Nick Mortimer

Long-lived, widespread intraplate volcanism without age progression is one of the most controversial features of plate tectonics. The eastern margin of Australia and Zealandia has experienced extensive mafic volcanism over the last 100 million years. A plume origin has been proposed for three distinct chains of volcanoes, however, the majority of eruptions exhibit no clear age progression. Previously proposed edge-driven convection, asthenospheric shear, and lithospheric detachment fail to explain the non age-progressive eruptions across the ~5000 km wide intraplate volcanic province from Eastern Australia to Zealandia. We model the subducted slab volume over 100 million years and find that slab flux drives volcanic eruption frequency, indicating stimulation of an enriched mantle transition zone reservoir. Volcanic isotope geochemistry allows us to distinguish a HIMU reservoir (>1 Ga old) in the slab-poor south, from a northern EM1/EM2 reservoir, reflecting a more recent voluminous influx of oceanic lithosphere into the mantle transition zone. We provide a unified theory linking plate boundary and slab volume reconstructions to upper mantle reservoirs and intraplate volcano geochemistry.

How to cite: Mather, B., Muller, D., Seton, M., Ruttor, S., Nebel, O., and Mortimer, N.: Intraplate volcanism triggered by bursts in slab flux, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6857, https://doi.org/10.5194/egusphere-egu21-6857, 2021.

EGU21-7279 | vPICO presentations | GD1.6 | Highlight

Late Cretaceous to Paleogene exhumation in Central Europe – localized inversion vs. large-scale domal uplift

Hilmar von Eynatten, Jonas Kley, and István Dunkl

Large parts of Central Europe have experienced exhumation in Late Cretaceous to Paleogene time. Previous studies mainly focused on thrusted basement uplifts to unravel magnitude, processes and timing of exhumation. In this study we present a comprehensive thermochronological dataset from mostly Permo-Triassic strata exposed adjacent to and between the major basement uplifts in central Germany, comprising an area of at least some 250-300 km across. Results of apatite fission track and (U-Th)/He analyses from >100 new samples reveal that (i) km-scale exhumation affected the entire region, suggesting long-wavelength domal uplift, (ii) thrusting of basement blocks like the Harz Mountains and the Thuringian Forest focused in the Late Cretaceous (about 90-70 Ma) while superimposed domal uplift of central Germany appears slightly younger (about 75-55 Ma), and (iii) large parts of the domal uplift experienced removal of 3 to 4 km of Mesozoic strata. Using spatial extent, magnitude and timing as constraints we find that thrusting and crustal thickening alone can account for no more than half of the domal uplift. Most likely, dynamic topography caused by upwelling asthenosphere has contributed significantly to the observed pattern of exhumation in central Germany.

How to cite: von Eynatten, H., Kley, J., and Dunkl, I.: Late Cretaceous to Paleogene exhumation in Central Europe – localized inversion vs. large-scale domal uplift, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7279, https://doi.org/10.5194/egusphere-egu21-7279, 2021.

EGU21-8199 | vPICO presentations | GD1.6

The post-Caledonian thermo-tectonic evolution of Fennoscandia

Peter Japsen, Paul F. Green, Johan M. Bonow, James A. Chalmers, Ian Duddy, and Ilmo Kokkunen

EGU21-10188 | vPICO presentations | GD1.6

The preserved plume conduits of the Caribbean Large Igneous Plateau and their relation with the Galápagos hotspot back to 90 Ma

Angela Maria Gomez Garcia, Eline Le Breton, Magdalena Scheck-Wenderoth, Gaspar Monsalve, and Denis Anikiev

Remnants of the Caribbean Large Igneous Plateau (C-LIP) are found as thickened zones of oceanic crust in the Caribbean Sea, that formed during strong pulses of magmatic activity around 90 Ma. Previous studies have proposed the Galápagos hotspot as the origin of the thermal anomaly responsible for the development of this igneous province. Particularly, geochemical signature relates accreted C-LIP fragments along northern South America with the well-known hotspot material.

In this research, we use 3D lithospheric-scale structural and density models of the Caribbean region, in which up-to-date geophysical datasets (i.e.: tomographic data, Moho depths, sedimentary thickness, and bathymetry) have been integrated. Based on the gravity residuals (modelled minus observed EIGEN6C-4 dataset), we reconstruct density heterogeneities both in the crust and the uppermost oceanic mantle (< 50km).

Our results suggest the presence of two positive mantle density anomalies in the Colombian and the Venezuelan basins, interpreted as the preserved plume material which migrated together with the Proto-Caribbean plate from the east Pacific. Such bodies have never been identified before, but a positive density trend is also observed in the mantle tomography, at least down to 75 km depth.

Using recently published regional plate kinematic models and absolute reference frames, we test the hypothesis of the C-LIP origin in the Galápagos hotspot. However, misfits of up to ~3000 km between the present hotspot location and the mantle anomalies, reconstructed back to 90 Ma, is observed, as other authors reported in the past.

Therefore, we discuss possible sources of error responsible for this offset and pose two possible interpretations: 1. The Galápagos hotspot migrated (~1200-3000 km) westward while the Proto-Caribbean moved to the northeast, or 2. The C-LIP was formed by a different plume, which – if considered fixed - would be nowadays located below the South American continent.

How to cite: Gomez Garcia, A. M., Le Breton, E., Scheck-Wenderoth, M., Monsalve, G., and Anikiev, D.: The preserved plume conduits of the Caribbean Large Igneous Plateau and their relation with the Galápagos hotspot back to 90 Ma, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10188, https://doi.org/10.5194/egusphere-egu21-10188, 2021.

EGU21-10964 | vPICO presentations | GD1.6

Multiphase inversion in the Baltic sector of the North German Basin: Influence of Africa-Iberia-Europe convergence during the Late Cretaceous and Cenozoic

Niklas Ahlrichs, Vera Noack, Christian Hübscher, and Elisabeth Seidel

Within the DFG project StrucFlow, we investigate the multiphase character of Late Cretaceous to Cenozoic inversion in the Baltic sector of the North German Basin based on seismic interpretation. Our analysis rests upon modern high-resolution seismic profiles in combination with data from older seismic surveys and borehole information. The resulting seismic database consists of a dense profile network with a total length of some 10.000 km. This unprecedented seismic grid allows for a detailed tectono-stratigraphic interpretation of Cretaceous and Paleogene deposits in the Baltic sector of the North German Basin. Here, basin inversion began in the Coniacian and Santonian with uplift of the Grimmen High and minor reactivation of Zechstein salt structures. Crestal faults were formed or reactivated above salt pillows in the Bays of Mecklenburg and Kiel. The onset of inversion was contemporaneous with other adjacent basins and is likewise associated with building up intraplate stress within the European foreland related to the beginning Africa-Iberia-Europe convergence. Time-isopach maps of Paleocene deposits in the study area show a slight decrease in thickness to the west. This contrasts the prevailing trend of increasing thickness towards the southwest directed basin center and indicates a changed depositional environment. In the outer eastern Glückstadt Graben, increased thicknesses and diverging strata of late Eocene and Oligocene units indicate significant remobilization of salt structures during this time. Preexisting Triassic faults above the salt pillows “Schleimünde” and “Kieler Bucht” at the eastern border of the Glückstadt Graben were reactivated and form a north-south trending crestal graben filled with Paleogene sediments. This phase of salt remobilization is contemporaneous with the reintroduction of intraplate stress triggered by the Alpine and Pyrenean orogenies in the late Eocene. In the eastern Bay of Kiel and in the Bay of Mecklenburg, Late Eocene and younger sediments are largely absent due to Neogene uplift and erosion. Deepening of rim-synclines and synchronous infill of Paleogene strata give evidence for commencing salt pillow growth. Crestal faults pierce the Paleocene and Eocene strata, indicating salt movement at least during the later Eocene. This phase of salt movement occurred contemporaneously with salt remobilization in the Glückstadt Graben, initiation of the European Cenozoic Rift System and increased activity in the Alpine realm in the Late Eocene to Oligocene. We conclude that the rise of salt pillows since the Eocene significantly exceeds the growth during late Cretaceous to Paleocene inversion phase at the northeastern North German Basin.

How to cite: Ahlrichs, N., Noack, V., Hübscher, C., and Seidel, E.: Multiphase inversion in the Baltic sector of the North German Basin: Influence of Africa-Iberia-Europe convergence during the Late Cretaceous and Cenozoic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10964, https://doi.org/10.5194/egusphere-egu21-10964, 2021.

EGU21-11155 | vPICO presentations | GD1.6

Crustal architecture of Amsterdam-St. Paul Island from an integrated geophysical approach

Pankaj Kumar, Pratyush Anand, Dibakar Ghosal, and Pabitra Singha

The Amsterdam-St. Paul (ASP) island complex is a manifestation of interaction between the South-East Indian Ridge (SEIR) and the ASP mantle plume, which was formed ~10 Ma. Very few geophysical studies have been conducted over the ASP island complex and therefore we have limited information about the island so far. We performed an integrated geophysical approach using gravity, magnetic study along with the joint inversion of Ps receiver function and Rayleigh wave group velocity dispersion curve to determine the crustal architecture and Moho variation in the region. The result of integrated gravity-magnetic modeling revealed that the island complex is associated with three crustal layers beneath the sedimentary strata. Inversion of Rayleigh wave group velocity dispersion curve accounts for vertical shear wave velocity average which supported the layered velocity profile. The results revealed that magnetic material (Mid oceanic ridge basalt/Flood basalt) has carpeted the entire island causing high magnetic anomaly of -1000 to 1500 nT, which is generated by gradual accumulation of a thick pile of magnetic material of normal as well as reverse polarity. The results by integrated Gravity-magnetic model suggest that crust beneath the island is suggested to be highly affected by volcanic activity (Mantle Plume/Ridge) and is underlain by high-density underplated material. The results further suggest that SEIR has less role for the outpoured magmatic activity. Integrated Gravity-magnetic model show that Moho is variable beneath the island complex and lies in the range of ~12-17 km. Further results by joint inversion of Ps receiver function and Rayleigh wave group velocity dispersion curve for the station (AIS : Nouvelle Amsterdam - TAAF, France) suggest Moho depth of ~14 km beneath the Amsterdam island and is well in agreement with the gravity-magnetic studies. The result clearly indicates that ASP island complex is highly affected by the ASP plume activity and was evolved during the ridge-plume interaction.

How to cite: Kumar, P., Anand, P., Ghosal, D., and Singha, P.: Crustal architecture of Amsterdam-St. Paul Island from an integrated geophysical approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11155, https://doi.org/10.5194/egusphere-egu21-11155, 2021.

EGU21-12123 | vPICO presentations | GD1.6

Suitability of serpentinized rock material as mineral filler in polymer matrices

Eva Wegerer, Nicolai Aust, and Anton Mayer

Mineral fillers can significantly affect the application properties of plastic materials. The structural and chemical properties of phyllosilicates provide the conditions to change the properties of polymeric material, e.g. flexural and tensile strength or thermal properties, according to the required application. Mineral fillers frequently used are clay minerals with a two-layer structure (serpentine-kaolin group) or three-layer structure (talc-pyrophyllite group, mica group, smectite group). The mineral fillers can be directly used or after surface modification, depending on the polar nature of the polymer. Polymers containing polar groups (hydrophilic polymers) are water-soluble, like polyvinyl alcohols and polysaccharides or can form hydrogen bonds, like polyamides, polyesters and polyvinyl fluorides. Hydrophobic (non-polar) polymers show an absence of polar groups (e.g. polyethylene, polypropylene) or mutual cancelling electrical dipole moments (e.g. polytetrafluorethylene). Minerals with a hydrophilic surface are directly applicable with polar polymers. For the application with non-polar polymers their surface require hydrophobization, whereas non-polar two-layer silicates are directly applicable with these polymers.

Serpentinized rock material is investigated with regard to its suitability as a polymer filler and its influence on the performance characteristics of various polymers. The samples origin from the Kraubath Ultramafic Massif, which represents part of an Early Paleozoic ophiolite, at the basement of the Austro-Alpine. The Kraubath complex is dominated by metamorphosed dunites and harzburgites, which origin from fractionation processes of the primary peridotite magma. Hydrothermal alteration led to a partly or entirely serpentinization of the ultramafic rocks. The serpentinization process of dunite, ortho-pyroxenite and harzburgite transformed Mg-containing silicates, like olivine and pyroxene to serpentine group minerals. Rock material with a high grade of serpentinization offers favourable conditions for the application as mineral filler.

The qualitative and quantitative XRD-analyses reveal a predominant occurrence of the antigorite. Further serpentine group minerals, like lizardite, occur in small amounts. Talc represents the second largest mineral phase. The rock material contains a few percentage of amphibole, chlorite, olivine (forsterite) and less than two percent of chromite and bronzite. In the two-layer structure of the main component antigorite, the charge of the tetrahedral layer is compensated by the charge of the octahedral layer. The three-layer structure of talc is electrostatically neutral, with no interlayer material. Therefore, serpentine minerals and talc are suitable for the application as mineral fillers in non-polar polymers, like polypropylene. Both influence the mechanical and tribological properties of polymers. Serpentine improves elasticity, tensile strength, stress at break, elongation at break, the mass wear rate and the coefficient of friction of the polymer but reduces the impact strength. Talc positively influences rigidity, shrinkage, creep properties, heat distortion under load and the coefficient of linear thermal expansion, however reduces toughness, long thermal ageing, impact strength and tensile strength. The further mineral phases are not considered to affect the application properties negatively. Regarding tensile strength and elasticity the ratio of serpentine to talc can influence the increase and decrease of these properties in non-polar polymers. The applicability of the practical implementation is investigated with nanoparticles of the serpentinized rock material in combination with polypropylene in varying proportions.

How to cite: Wegerer, E., Aust, N., and Mayer, A.: Suitability of serpentinized rock material as mineral filler in polymer matrices, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12123, https://doi.org/10.5194/egusphere-egu21-12123, 2021.

EGU21-12348 | vPICO presentations | GD1.6

Cenozoic mountain building of Western Europe controlled by continental lithosphere evolution

Frédéric Mouthereau and Paul Angrand

The heterogeneous continental lithosphere of Europe inherits billion of years of tectonic evolution, mineral transformation and magmatic addition. Though there is now an extensive body of studies on the long-term geological, geochronological and geochemical evolution of the continental crust and lithospheric mantle available in Europe, yet this knowledge has not been linked to the understanding of tectonic evolution of Cenozoic Alpine mountain building. In this aim, we review geophysical, geological, petrographical, geochemical, and thermochronological constraints to infer a kinematically coherent time-integrated tectonic model for the evolution of mountain building in Western Europe, along a 4000 km long lithospheric transect from Africa to the East European Craton. We show that the key drivers of plate-scale processes related to  Alpine orogenic and topographic evolution reflect three main ingredients : 1) a protracted magmatic and tectono-thermal transformation of Africa (Gondwana) and North Europea (Baltica) cratonic mantle lithosphere since the Neoproterozoic, 2) an overall limited Mesozoic Tethyan extension of the weak Variscan lithosphere characterized by the lack of wide, thermally relaxed, oceanic lithosphere, 3) a relatively slow Cenozoic convergence between Africa and Europe, preserving initial stages of distributed tectonic inversion of rifted continental blocks throughout Europe, and partial subduction and delamination in the Mediterranean region of the most evolved lithospheric domains. 

How to cite: Mouthereau, F. and Angrand, P.: Cenozoic mountain building of Western Europe controlled by continental lithosphere evolution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12348, https://doi.org/10.5194/egusphere-egu21-12348, 2021.

EGU21-13087 | vPICO presentations | GD1.6

Intraplate deformations, topographic evolution and sediment production of Western Europe from 40 to 5 Myrs

Francois Guillocheau and Cécile Robin

Western Europe experienced a major rift system initiated during Bartonian times (41 Ma). This evolution is coeval with long wavelength deformations (several hundreds of kilometers) that control the topography and the sediment production beyond the rift. The climate during this time interval was first increasingly arid and then wetter.

This study is based on both landform and sediment analysis of southern England, France, Belgium and western Germany. The landforms are mainly large pediments, dated by the intersection with sediments deposited in low to high subsiding areas and volcanism. A set of paleogeographic maps with paleotopographic reconstructions, is used to constrain the uplifting and subsiding areas, their wavelength and the critical periods of intraplate deformations.

The main periods of deformations and sedimentary systems changes area as follow.

41Myrs (base Bartonian) was the beginning of a major tilting of Western Europe with subsidence of France and uplift of the Brabant/Ardennes/Rhenish Massif. Even a dense network of basement faults was reactivated, biochemical sedimentation prevailed.

35-31Myrs (Late Priabonian-Early Rupelian) initiated a period of general subsidence even along the Ardennes/Rhenish Massif and the French Massif Central. Two major marine floodings are recorded, with a differential preservation according to the balance between deformation and eustasy.

27-25Myrs (Chattian) was a period of uplift of Western Europe except the Aquitaine Basin, followed by a relaxation favoring eustatic floodings in (very) low subsiding domains. Chattian siliciclastic deposits are preserved as lowstand wedges in the surrounded basins (North Sea, Atlantic Margin).

14-11Myrs (Serravallian-Early Tortonian) initiated the overall uplift of Western Europe, still operating today. This is the beginning of a period of major denudation in southern England, Western Germany (SW Germany flat - “Stufenland”) and along the southern limb of the Franch Massif Central.

The causes and the consequences in term of sediment production are discussed.

How to cite: Guillocheau, F. and Robin, C.: Intraplate deformations, topographic evolution and sediment production of Western Europe from 40 to 5 Myrs, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13087, https://doi.org/10.5194/egusphere-egu21-13087, 2021.

The evaporitic Haselgebirge Formation hosts in many places small occurrences of basaltic rocks. The geochemistry of these basalts can potentially provide information about the tectonic setting of the Haselgebirge Formation and the evolution of the Meliata ocean, respectively. We present here 70 new XRF analyses of these basaltic rocks from various localities (Pfennigwiese, Annaberg, Wienern, Hallstatt, Moosegg, Lammertal) and compare the results with previous data from local studies (GRUBER et al., 1991; KIRCHNER 1979; KIRCHNER 1980a; KIRCHNER 1980b; KRALIK et al, 1984; LEITNER et al., 2017; SCHORN et al., 2013; ZIEGLER, 2014; ZIRKL, 1957). Based on the concentrations of immobile trace elements (Zr, Nb, Y, Ti), a predominance of MORB-like compositions is observed for the Lower Austrian occurrences and for the locality Wienern (Grundlsee). On contrast, basalts from the localities Lammertal, Moosegg and Hallstatt have predominantly within-plate-type compositions.

We discuss this striking regional (east-west) difference of basalt types in terms of existing palinspastic models for the Haselgebirge formation (LEITNER et al., 2017; STAMPFLI & BOREL, 2002; McCANN et al., 2006).

 

GRUBER, P., FAUPL, P., KOLLER, F. (1991) Mitt. Österr. Miner. Ges., 84, 77-100.

KIRCHNER, E. (1979) Tschermaks Min. Petr. Mitt. 26, 149-162.

KIRCHNER, E. (1980a) Mitt. Österr. Miner. Ges.71/72, 385-396.

KIRCHNER, E. (1980b) Verh. Geol. Bundesanstalt 1980, 249-279.

KRALIK, M., KOLLER, F., POBER, E. (1984) Mitt. Österr. Miner. Ges., 77, 37-55.

LEITNER, C., WIESMAIER, S., KÖSTER, M.H., GILG, H.A, FINGER, F, NEUBAUER, F. (2017) GSA Bulletin 129, 1537-1553.

McCANN, T., PASCAL, C., TIMMERMAN, M.J., KRZYWIEC, P., LÓPEZ-GÓMEZ, J., WETZEL, L., KRAWCZYK, C.M., RIEKE, H., LAMARCH, J. (2006) Mem. Geol. Soc. London, 32, 355-388.

SCHORN A, NEUBAUER F, GENSER J, BERNROIDER M (2013) Tectonophysics 583, 28-48.

STAMPFLI G.M., BOREL G.D. (2002) Earth Planet. Sci. Lett. 196, 17-33.

ZIEGLER, T. (2014) Unpubl. MSc thesis University of Salzburg, p. 174.

ZIRKL, E.J. (1957) Jb. Geol. Bundesanstalt 100, 10-137-177.

How to cite: Leitner, C.: Two different basalt provinces (MORB vs. WPB) in the evaporitic Permian Haselgebirge Formation (Eastern Alps, Austria) and possible tectonic implications, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13129, https://doi.org/10.5194/egusphere-egu21-13129, 2021.

EGU21-13383 | vPICO presentations | GD1.6

Late Cretaceous – Cenozoic history of the transition zone between the East European Craton and the Paleozoic Platform, Polish sector of the Baltic Sea, revealed by new offshore regional seismic reflection data (BALTEC project)

Piotr Krzywiec, Łukasz Słonka, Quang Nguyen, Michał Malinowski, Mateusz Kufrasa, Aleksandra Stachowska, Christian Huebscher, and Regina Kramarska

In 2016, approximately 850 km of high-resolution multichannel seismic reflection data of the BALTEC survey have been acquired offshore Poland within the transition zone between the East European Craton and the Paleozoic Platform. Data processing, focused on removal of multiples, strongly overprinting geological information at shallower intervals, included SRME, TAU-P domain deconvolution, high resolution parabolic Radon demultiple and SWDM (Shallow Water De-Multiple). Entire dataset was Kirchhoff pre-stack time migrated. Additionally, legacy shallow high-resolution multichannel seismic reflection data acquired in this zone in 1997 was also used. All this data provided new information on various aspects of the Phanerozoic evolution of this area, including Late Cretaceous to Cenozoic tectonics and sedimentation. This phase of geological evolution could be until now hardly resolved by analysis of industry seismic data as, due to limited shallow seismic imaging and very strong overprint of multiples, essentially no information could have been retrieved from this data for first 200-300 m. Western part of the BALTEC dataset is located above the offshore segment of the Mid-Polish Swell (MPS) – large anticlinorium formed due to inversion of the axial part of the Polish Basin. BALTEC seismic data proved that Late Cretaceous inversion of the Koszalin – Chojnice fault zone located along the NE border of the MPS was thick-skinned in nature and was associated with substantial syn-inversion sedimentation. Subtle thickness variations and progressive unconformities imaged by BALTEC seismic data within the Upper Cretaceous succession in vicinity of the Kamień-Adler and the Trzebiatów fault zones located within the MPS documented complex interplay of Late Cretaceous basin inversion, erosion and re-deposition. Precambrian basement of the Eastern, cratonic part of the study area is overlain by Cambro-Silurian sedimentary cover. It is dissected by a system of steep, mostly reverse faults rooted in most cases in the deep basement. This fault system has been regarded so far as having been formed mostly in Paleozoic times, due to the Caledonian orogeny. As a consequence, Upper Cretaceous succession, locally present in this area, has been vaguely defined as a post-tectonic cover, locally onlapping uplifted Paleozoic blocks. New seismic data, because of its reliable imaging of the shallowest substratum, confirmed that at least some of these deeply-rooted faults were active as a reverse faults in latest Cretaceous – earliest Paleogene. Consequently, it can be unequivocally proved that large offshore blocks of Silurian and older rocks presently located directly beneath the Cenozoic veneer must have been at least partly covered by the Upper Cretaceous succession; then, they were uplifted during the widespread inversion that affected most of Europe. Ensuing regional erosion might have at least partly provided sediments that formed Upper Cretaceous progradational wedges recently imaged within the onshore Baltic Basin by high-end PolandSPAN regional seismic data. New seismic data imaged also Paleogene and younger post-inversion cover. All these results prove that Late Cretaceous tectonics substantially affected large areas located much farther towards the East than previously assumed.

This study was funded by the Polish National Science Centre (NCN) grant no UMO-2017/27/B/ST10/02316.

How to cite: Krzywiec, P., Słonka, Ł., Nguyen, Q., Malinowski, M., Kufrasa, M., Stachowska, A., Huebscher, C., and Kramarska, R.: Late Cretaceous – Cenozoic history of the transition zone between the East European Craton and the Paleozoic Platform, Polish sector of the Baltic Sea, revealed by new offshore regional seismic reflection data (BALTEC project), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13383, https://doi.org/10.5194/egusphere-egu21-13383, 2021.

The Emeishan flood basalts are part of an important large igneous province along the western margin of the Yangtze Block, Southwest China. The western Guangxi region in southwestern China is geologically a part of the Yangtze Block. Mafic rocks, comprising mainly lavas and dykes in western Guangxi belong to the outer part of the ~260 Ma Emeishan Large Igneous Province (ELIP). Here we present a systematic study of platinum-group elements (PGEs) combined with the LA-ICP-MS zircon U–Pb age, whole-rock geochemical and isotopic data of the lavas and dykes in the Longlin area of outer zone of ELIP to constraints on their origin. On the basis of petrography and major elements characteristics, mafic lavas and dykes display an enrichment of LREE, LILE, HFSE, high (87Sr/86Sr)i ratios (0.704227~0.705754), low εNd(t) values(0.42~0.99), high εHf(t) values(5.19~6.04), they are similar to those of Permian Emeishan high-Ti basalts and Ocean island basalts (OIB) features. The Longlin mafic rocks was formed in the Late Permian with the zircon U-Pb dated age of 256.3± 1.7 Ma. The age of the Longlin mafic rocks is close to the formation age of the ELIP large-scale magmatism, suggesting that these lavas and dykes probably belongs to part of the ELIP large-scale magmatism. The Longlin mafic rocks have low total PGE contents ranging from 1.56×10-9 to 2.28×10-9, with Os, Ir, Ru, Rh, Pt and Pd contents of 0.040~0.076, 0.046~0.076, 0.027~0.079, 0.037~0.056, 0.6374~1.053 and 0.715~1.021ppb, respectively. They show left-leaning primitive mantle-normalized PGE patterns with depletion in Iridium group(IPGE) and enrichment in Palladium group, which also have lower contents than mafic rocks from the inner zone of the ELIP, suggesting that a low degree of partial melting of the mantle source plays an important role. The Longlin mafic rocks exhibit a marked increase in Cu/Pd ratios (>105,84655 to 174785) albeit with a narrow range of lower Pd/Ir ratios (<50,13.4 to 18.7), different from the PGE-enriched basalts of the Siberian Traps, Emeishan Large Igneous Province (ELIP), East Greenland CFBs and Deccan Traps, indicating that their parent magmas was significantly depleted in chalcophile elements. Calculations based on the available trace element geochemistry reveal that the basalts were originated by low degree of partial melting(<5%),with sulfides remain in the mantle during partial melting. Sulfide segregation could not happen during the evolution of the Longlin mafic rocks, due to the fact that neither significant fractional crystallization nor crustal contamination has been involved in their formation. Overall, mafic rocks from the outer zone of the ELIP show lower PGE contents than those in the inner zones, we find that the PGE contents in igneous rocks are related with the degrees of partial melting in the mantle source and the removal of sulfides before their emplacement.

This study was financially supported by the Guangxi Natural Science Foundation for Distinguished Young Scholars (2018GXNSFFA281009) and the Fifth Bagui Scholar Innovation Project of Guangxi Province (to XU Ji-feng).

How to cite: Zhao, B., Liu, X., Li, Z., Huang, W., and Zhao, C.: Geochronology, isotopic and Platinum-group elemental geochemistry of lavas and dykes from the western Guangxi in outer zone of Emeishan mantle plume, SW China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13784, https://doi.org/10.5194/egusphere-egu21-13784, 2021.

EGU21-14385 | vPICO presentations | GD1.6 | Highlight

ScanArray - Seismological study of the connection between topographic change and deep structure in Fennoscandia

Hans Thybo, Nevra Bulut, Michael Grund, Alexandra Mauerberger, Anna Makushkina, Irina Artemieva, Niels Balling, Olafur Gudmundsson, Valerie Maupin, Lars Ottemøller, Joachim Ritter, and Frederik Tilmann

The Baltic Shield is located in northern Europe. It was formed by amalgamation of a series of terranes and microcontinents during the Archean to the Paleoproterozoic, followed by significant modification in Neoproterozoic to Paleozoic time. The Baltic Shield includes a high mountain range, the Scandes, along its western North Atlantic coast, despite being a stable craton located far from any active plate boundary.

The ScanArray international collaborative program has acquired broad band seismological data at 192 locations in the Baltic Shield during the period between 2012 and 2017. The main objective of the program is to provide seismological constraints on the structure of the lithospheric crust and mantle as well as the sublithospheric upper mantle. The new information will be applied to studies of how the lithospheric and deep structure affects observed fast topographic change and geological-tectonic evolution of the region. The recordings are of very high quality and are used for analysis by suite of methods, including P- and S-wave receiver functions for the crust and upper mantle, surface wave and ambient noise inversion for seismic velocity, body wave P- and S- wave tomography for upper mantle velocity structure, and shear-wave splitting measurements for obtaining bulk anisotropy of the upper and lower mantle. Here we provide a short overview of the data acquisition and initial analysis of the new data with focus on parameters that constrain the fast topographic change in the Scandes.

 

How to cite: Thybo, H., Bulut, N., Grund, M., Mauerberger, A., Makushkina, A., Artemieva, I., Balling, N., Gudmundsson, O., Maupin, V., Ottemøller, L., Ritter, J., and Tilmann, F.: ScanArray - Seismological study of the connection between topographic change and deep structure in Fennoscandia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14385, https://doi.org/10.5194/egusphere-egu21-14385, 2021.

GD2.1 – Planetary core structure, dynamics and evolution: observations, models, experiments

Thermal conductivity of Earth materials under relevant high pressure-temperature conditions is crucial to determine the temperature profile in Earth’s interior, which further influences its thermo-chemical evolution and structures as well as geodynamics. In Earth’s core, iron (Fe) is the major constituent along with some candidate light elements, for instance, silicon (Si), carbon (C), sulfur (S), etc. Core’s thermal conductivity plays a key role in affecting its thermal evolution history and the magnitude of thermal and compositional sources required to operate a geodynamo. Precise and direct measurements of the thermal conductivity of Earth’s core materials under extreme conditions, however, have been very challenging due to the difficulty of experimental methods. Recently we have combined time-resolved optical techniques with high-pressure diamond cells to precisely measure the thermal conductivity of core materials, including pure Fe and Fe-Si and Fe-C alloys, etc. We found that the alloying effect by these candidate light elements results in a relatively low thermal conductivity compared to the pure Fe. Combined with thermal evolution models, our new data suggest a low minimum heat flow across the core-mantle boundary than previously expected, and therefore less thermal energy needed to run the geodynamo. In addition, the age of the inner core is constrained to be older than about two billion years.

How to cite: Hsieh, W.-P.: Low thermal conductivity of Earth’s core with implications for the geodynamo and the age of inner core, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1462, https://doi.org/10.5194/egusphere-egu21-1462, 2021.

EGU21-10510 | vPICO presentations | GD2.1

A constraint to thermal conductivity of Earth’s core and CMB heat flow by assessment on a stable region of Earth’s core

Takashi Nakagawa, Shin-ichi Takehiro, and Youhei Sasaki

It is still controversial for an emergence of a stable region at the top of Earth’s core in theoretical modeling because both thermal conductivity of Earth’s core and heat flow across the core-mantle boundary (CMB) have not been clearly constrained from mineral physics and geophysical observations, ranging 20 to 220 W/m/K for the thermal conductivity (denoted as ) and 5 to 20 TW for the present-day CMB heat flow (denoted as QPCMB). In this study, in order to resolve these uncertainties, we try to constrain the values of thermal conductivity of Earth’s core and the present-day CMB heat flow by requiring continuous generation of geomagnetic field in addition to existence of a stable region at the top of present Earth’s core using a one-dimensional thermal and compositional evolution model.  

Numerical experiments for various values of  and QPCMB show that the solutions satisfying both long-term magnetic field generation and emergence of a stable region is possible only when  is larger than 40 W/m/K and QPCMB is less than 18.5 TW. The specific required value of depends on QPCMB. If the expected CMB heat flow would be as large value as 17.5 TW, which is suggested by the recent studies on the core evolution theory (e.g., Labrosse, 2015),  should be a high value such as about 212 W/m/K to satisfy our requirements. The thickness of an expected stable region would be about 30 km in this case. In contrast, when QPCMB is as small as that derived from numerical mantle convection models (e.g., 10 TW; Nakagawa and Tackley, 2010), the required value of  decreases to 110 W/m/K. In this case, a stable region extends about 75 km thickness below CMB.

If the requirements assumed in this study is confirmed by certain geophysical observations and/or QPCMB can be restricted more precisely with some methods, our assessment scheme would be useful for evaluations of the radial convective structure of Earth’s core and for further constraint of the value of thermal conductivity of Earth’s core.

How to cite: Nakagawa, T., Takehiro, S., and Sasaki, Y.: A constraint to thermal conductivity of Earth’s core and CMB heat flow by assessment on a stable region of Earth’s core, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10510, https://doi.org/10.5194/egusphere-egu21-10510, 2021.

EGU21-7896 | vPICO presentations | GD2.1

Anisotropic diffusivities' effects in rotating magnetoconvection and geodynamo problems

Enrico Filippi and Jozef Brestenský

There are many examples which show how the anisotropic diffusive coefficients crucially influence geophysical and astrophysical flows and in particular flows in the Earth’s outer core. Thus, many models concerning rotating magnetoconvection with anisotropy in the viscosity, thermal and magnetic diffusivities have been developed.  

Different models correspond to different cases of anisotropic diffusivities. For example, we consider several anisotropic models: one with anisotropy in all diffusivities and other models with various combinations of anisotropic and isotropic diffusivities.  

Firstly, all kind of anisotropies are reminded and described. Then, a thorough comparison of these anisotropies, especially of the physical differences among them is done. All physical systems with the above mentioned anisotropies are prone to the occurrence of convection and other instabilities. We show how different types of anisotropy cause a different convection and a different balance among the main forces in the Earth’s Outer Core (Magnetic, Archimedean, Coriolis).  

As usually, to study instabilities in such systems, we use analysis in term of normal modes and search for preferred modes. In all our models, only marginal modes with zero growth rate have so far been studied. Now, we present the bravest modes, i.e. the ones with maximum growth rate. The comparison of the modes dependent on basic input parameters - Prandtl numbers, anisotropic parameter, Ekman and Elsasser numbers - is made mainly for values corresponding to the Earth’s outer core. In all our models the anisotropic diffusive coefficients are represented as diagonal tensors with two equal components different from the third one giving the chance to define simply the anisotropic parameter.  

We stress how magnetoconvection problems with the anisotropy included, became more and more important among the geodynamo problems in the last years; indeed, the origin of flows necessary for dynamo action, as studied in magnetoconvection with resulting instabilities, is important, as well as the problem of the origin of magnetic fields.  

How to cite: Filippi, E. and Brestenský, J.: Anisotropic diffusivities' effects in rotating magnetoconvection and geodynamo problems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7896, https://doi.org/10.5194/egusphere-egu21-7896, 2021.

EGU21-4705 | vPICO presentations | GD2.1

Experimental investigation of precession driven flows in a triaxial ellipsoid

Fabian Burmann and Jerome Noir

Precession driven flows are relevant for geo- and astrophysical fluid dynamics as well as industrial applications. In the context of planetary core dynamics, they are attributed to the generation of magnetic fields and/or anomalous dissipation. While precession driven flows have been frequently studied in a cylindrical, spherical or spheroidal container shape, the geometry of a triaxial ellipsoid, representing the geophysical case of core mantle boundary deformation in a tidally locked planet, has received less attention.

Here, we present results from an experimental study in a triaxial ellipsoid. The main focus of our study is on the base flow of uniform vorticity and we report a good agreement between experimental data and existing semi-analytical models. The amplitude of the time averaged uniform vorticity displays a hysteresis loop as a function of the precession forcing and we demonstrate that this observation depends on the ellipticity of the container. Our study also comprises experiments where the boundary layer is expected to be in a turbulent state. Therefore, we discuss the applicability of an effective damping coefficient in the semi-analytical models to account for the dissipation in a turbulent boundary layer. 

How to cite: Burmann, F. and Noir, J.: Experimental investigation of precession driven flows in a triaxial ellipsoid, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4705, https://doi.org/10.5194/egusphere-egu21-4705, 2021.

EGU21-13699 | vPICO presentations | GD2.1

Mean zonal flow driven by precession in planetary cores: numerical simulations with a semi-lagrangian scheme

Nathanael Schaeffer and David Cébron

We revisit the generation of mean zonal flows in fluid planetary interiors subjected to precession.
The main effect of precession on a (nearly) spherical fluid envelope is to make the fluid rotate along an axis tilted with respect to the rotation axis of the solid mantle. This is the so-called "spin-over" response of the fluid.
also shows that a steady shear flow develops on top of the spin-over mode due to non-linear effects in the boundary layer equation.
This mean zonal shear flow has been studied theoretically and numerically by .

With faster computers and more efficient codes, we compute this flow down to very low viscosity and compare with the inviscid theory of Busse (1968).
In addition we investigate the width and the intensity of the detached shear layer, which is controlled by viscosity and therefore not present in the theory.

We also use this problem as a benchmark to assess the benefits of using a semi-lagrangian numerical scheme, where solid-body rotation is treated exactly.

How to cite: Schaeffer, N. and Cébron, D.: Mean zonal flow driven by precession in planetary cores: numerical simulations with a semi-lagrangian scheme, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13699, https://doi.org/10.5194/egusphere-egu21-13699, 2021.

EGU21-2176 | vPICO presentations | GD2.1

Fast (non-)diffusive Quasi-Geostrophic Magneto-Coriolis Modes in the Earth's core

Felix Gerick, Dominique Jault, and Jerome Noir

 Fast changes of Earth's magnetic field could be explained by inviscid and diffusion-less quasi-geostrophic (QG) Magneto-Coriolis modes. We present a hybrid QG model with columnar flows and three-dimensional magnetic fields and find modes with periods of a few years at parameters relevant to Earth's core. These fast Magneto-Coriolis modes show strong focusing of their kinetic and magnetic energy in the equatorial region, while maintaining a relatively large spatial structure along the azimuthal direction. Their properties agree with some of the observations and inferred core flows. We find additionally, in contrast to what has been assumed previously, that these modes are not affected significantly by magnetic diffusion. The model opens a new way of inverting geomagnetic observations to the flow and magnetic field deep within the Earth's outer core.

How to cite: Gerick, F., Jault, D., and Noir, J.: Fast (non-)diffusive Quasi-Geostrophic Magneto-Coriolis Modes in the Earth's core, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2176, https://doi.org/10.5194/egusphere-egu21-2176, 2021.

EGU21-11936 | vPICO presentations | GD2.1

Magneto-inertial waves and planetary rotation

Jérémy Rekier, Santiago Triana, and Véronique Dehant

Magnetic fields inside planetary objects can influence their rotation. This is true, in particular, of terrestrial objects with a metallic liquid core and a self-sustained dynamo such as the Earth, Mercury, Ganymede, etc. and also, to a lesser extent, of objects that don’t have a dynamo but are embedded in the magnetic field of their parent body like Jupiter’s moon, Io.
In these objects, angular momentum is transfered through the electromagnetic torques at the Core-Mantle Boundary (CMB) [1]. In the Earth, these have the potential to produce a strong modulation in the length of day at the decadal and interannual timescales [2]. They also affect the periods and amplitudes of nutation [3] and polar motion [4]. 
The intensity of these torques depends primarily on the value of the electric conductivity at the base of the mantle, a close study and detailed modelling of their role in planetary rotation can thus teach us a lot about the physical processes taking place near the CMB.

In the study of the Earth’s length of day variations, the interplay between rotation and the internal magnetic field arrises from the excitation of torsional oscillations inside the Earth’s core [5]. These oscillations are traditionally modelled based on a series of assumptions such as that of Quasi-Geostrophicity (QG) of the flow inside the core [6]. On the other hand, the effect of the magnetic field on nutations and polar motion is traditionally treated as an additional coupling at the CMB [1]. In such model, the core flow is assumed to have a uniform vorticity and its pattern is kept unaffected by the magnetic field. 

In the present work, we follow a different approach based on the study of magneto-inertial waves. When coupled to gravity through the effect of density stratification, these waves are known to play a crucial role in the oscillations of stars known as magneto-gravito-inertial modes [7]. The same kind of coupling inside the Earth’s core gives rise to the so-called MAC waves which are directly and conceptually related to the aforementioned torsional oscillations [8]. 

We present our preliminary results on the computation of magneto-inertial waves in a freely rotating planetary model with a partially conducting mantle. We show how these waves can alter the frequencies of the free rotational modes identified as the Free Core Nutation (FCN) and Chandler Wobble (CW). We analyse how these results compare to those based on the QG hypothesis and how these are modified when viscosity and density stratification are taken into account. 

[1] Dehant, V. et al. Geodesy and Geodynamics 8, 389–395 (2017). doi:10.1016/j.geog.2017.04.005
[2] Holme, R. et al. Nature 499, 202–204 (2013). doi:10.1038/nature12282
[3] Dumberry, M. et al. Geophys. J. Int. 191, 530–544 (2012). doi:10.1111/j.1365-246X.2012.05625.x
[4] Kuang, W. et al. Geod. Geodyn. 10, 356–362 (2019). doi:10.1016/j.geog.2019.06.003
[5] Jault, D. et al. Nature 333, 353–356 (1988). doi:10.1038/333353a0
[6] Gerick, F. et al. Geophys. Res. Lett. (2020). doi:10.1029/2020gl090803
[7] Mathis, S. et al. EAS Publications Series 62 323-362 (2013). doi: 10.1051/eas/1362010
[8] Buffett, B. et al. Geophys. J. Int. 204, 1789–1800 (2016). doi:10.1093/gji/ggv552

How to cite: Rekier, J., Triana, S., and Dehant, V.: Magneto-inertial waves and planetary rotation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11936, https://doi.org/10.5194/egusphere-egu21-11936, 2021.