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GD4.1

EDI
Early Earth: Dynamics, Geology, Chemistry and Life in the Archean Earth

Processes responsible for formation and development of the early Earth (> 2500Ma) are not well understood and strongly debated, reflecting in part the poorly preserved, altered, and incomplete nature of the geological record from this time.
In this session we encourage the presentation of new approaches and models for the development of Earth's early crust and mantle and their methods of interaction. We encourage contributions from the study of the preserved rock archive as well as geodynamic models of crustal and mantle dynamics so as to better understand the genesis and evolution of continental crust and the stabilization of cratons.
We invite abstracts from a large range of disciplines including geodynamics, geology, geochemistry, and petrology but also studies of early atmosphere, biosphere and early life relevant to this period of Earth history.

Co-organized by BG5/GMPV3/PS10
Convener: Ria FischerECSECS | Co-conveners: Peter A. Cawood, Antoine RozelECSECS, Nicholas Gardiner, Jeroen van Hunen
Presentations
| Thu, 26 May, 15:55–18:17 (CEST)
 
Room -2.91

Thu, 26 May, 15:10–16:40

Chairpersons: Ria Fischer, Jeroen van Hunen

15:55–16:02
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EGU22-10352
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On-site presentation
Nicolas D. Greber

The lithologic and chemical composition of the continental crust impacts Earth atmosphere and environment through e.g. weathering feedbacks and nutrient supply. However, despite being important for  the biological and atmospheric evolution of our planet, the question of how the lithological composition of Earth’s landmasses evolved from around 3.5 Ga to present is still a matter of considerable debate.

Here I will present a summary of the work that has been conducted by my colleagues and myself over the past five years and that improved our understanding of the chemical and lithological evolution of Earth landmasses since 3.5 Ga. Reconstructing the composition of past continents is difficult because erosion and crustal reworking may have modified the geologic record in deep time, so direct examination of the nature of igneous rocks could provide a biased perspective on the nature of the continents through time. A less biased record is likely provided by terrigenous sediments that average the composition of rocks exposed to weathering on emerged lands and we therefore use major and trace element concentrations and stable isotope compositions of shales as a proxy for the average composition of the emerged continents in the past. Applying a three-component mixing model to the sediment record shows that since 3.5 Ga, the landmasses that were subjected to erosion were dominated by felsic rocks. Furthermore, our reconstructed relative abundance of felsic, mafic and komatiitic rocks in the Archean is close to that currently observed in these ancient terrains. While our model does not suggest a strong change in the lithologic composition of Earth continents, we find a secular change in the average major and trace element concentration, with incompatible elements being more depleted and compatible elements being more enriched in the old landmasses.

How to cite: Greber, N. D.: The lithologic composition of Earth’s emerged lands reconstructed from the chemistry of terrigenous sediments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10352, https://doi.org/10.5194/egusphere-egu22-10352, 2022.

16:02–16:09
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EGU22-9527
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ECS
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Virtual presentation
Lisa Ardoin et al.

Several geochemical tracers (S, C, O, Xe) underwent irreversible global changes during the Precambrian, and in particular during the Great Oxygenation Event (GOE), between the Archean and Proterozoïc eons [1]. Xenon is of particular interest as it presents a secular isotopic evolution during the Archean that ceased around the time of the GOE. In this regard Xe is somewhat analogous to mass-independent fractionation sulfur (MIF-S) in that it can be used to categorically identify Archean atmospheric components [2]. Xe isotopes in the modern atmosphere are strongly mass-dependent fractionated (MDF-Xe), with a depletion of the light isotopes relative to the heavy ones. There was a continuous Xe isotope evolution from primitive Xe to modern Xe that ceased between 2.6 and 1.8 Ga [2] and this evolution has been attributed to coupled H+-Xe+ escape to space [3].

The purpose of this project is to document the Xe composition of the paleo-atmosphere trapped in well-dated hydrothermal quartz fluid inclusions with ages covering the Archean-Proterozoic transition to better constraint its link with the GOE.

We have measured an isotopically fractionated Xe composition of 2.0 ± 1.8 ‰ relative to modern atmosphere at 2441 ± 1.6 Ma, in quartz vein from the Seidorechka sedimentary formation (Imandra-Varzuga Greenstone belt, Russia). A slightly younger sample from the Polisarka sedimentary formation (Imandra-Varzuga Greenstone belt, Russia) of 2434 ± 6.6 Ma does not record such fractionation and is indistinguishable from the modern atmospheric composition. A temporal link between the disappearance of the Xe isotopes fractionation and the MIF-S signature at the Archean-Proterozoic transition is clearly established for the Kola Craton. The mass-dependent evolution of Xe isotopes is the witness of a cumulative atmospheric process that may have played an important role in the oxidation of the Earth's surface [3], independently of biogenic O2 production that started long before the permanent rise of O2 in the atmosphere [4].

 

[1] Catling & Zahnle, 2020, Sciences Advances 6, eaax1420. [2] Avice et al., 2018, Geochimica et Cosmochimica Acta 232, 82-100 [3] Zahnle et al., 2019, Geochimica et Cosmochimica Acta 244, 56-85. [4] Lyons et al., 2014, Nature 506, 307-315.

How to cite: Ardoin, L., Broadley, M., Almayrac, M., Avice, G., Byrne, D., Tarantola, A., Lepland, A., Saito, T., Komiya, T., Shibuya, T., and Marty, B.: The end of the atmospheric xenon Archean’s evolution: a study of the Great Oxygenation Event period, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9527, https://doi.org/10.5194/egusphere-egu22-9527, 2022.

16:09–16:16
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EGU22-13571
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On-site presentation
Juan García de la Concepción and Pablo Marcos-Arenal

Photosynthesis, the metabolic route for conversion of solar to chemical energy, could be present in any planetary system provided with the only three required ingredients: a light source, water, and carbon dioxide.

The ExoPhot project aims to study the relation between photosynthetic systems and exoplanet conditions around different types of stars (i.e. stellar spectral types) by focusing on two aspects: Assessing the photosynthetic fitness of a variety of photopigments (either real or theoretical) as a function of stellar spectral type, star-exoplanet separation, and planet atmosphere composition; and delineating a range of stellar, exoplanet and atmospheric parameters for which photosynthetic activity might be feasible. In order to tackle this goals, this project is studying the evolutionary steps that led to the highly evolved chlorophylls and analogues, and assessing the feasibility or likelihood to trigger photosynthetic activity in an exoplanetary system.

Based on the Darwinian theory of common ancestors, the first (photosynthetic) organism should have had simple oligopeptides, oligonucleotides and alkyl amphiphilic hydrocarbons as primeval membranes. Therefore, it should have had simple pigments. We propose that there could exist geochemical conditions allowing the abiotic formation of a simple pigment which might become sufficiently abundant in the environment of an exoplanet. Besides, we show that the proposed pigment could also be a precursor of the more evolved pigments known today on Earth by proposing, for the first time, an abiotic chemical route leading to tetrapyrroles not involving pyrrole derivatives.

 

Juan García de la Concepcióna,* Pablo Marcos-Arenala, Luis Cerdánb, Mercedes Burillo-Villalobosc, Nuria Fonseca-Bonillaa,María-Ángeles López-Cayuelad, José A. Caballeroe, and Felipe Gómez Gómeza

aCentro de Astrobiología (CSIC-INTA), Ctra. de Ajalvir km. 4, Torrejón de Ardoz, 28850 Madrid, Spain; bInstituto de Ciencia Molecular (ICMoL), Universidad de Valencia, 46071 Valencia, Spain.;cInstituto Nacional de Técnica Aeroespacial, 28850 Torrejón de Ardoz, Madrid, Spain.; dÁrea de Investigación e Instrumentación Atmosférica,Instituto Nacional de Técnica Aeroespacial, 28850 Torrejón de Ardoz, Madrid, Spain.; eCentro de Astrobiología (CSIC-INTA), ESAC, camino bajo del castillo, 28691 Villanueva de la Cañada, Madrid, Spain

How to cite: García de la Concepción, J. and Marcos-Arenal, P.: ExoPhot: Phot0, a plausible primeval pigment on Earth and rocky exoplanets, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13571, https://doi.org/10.5194/egusphere-egu22-13571, 2022.

16:16–16:23
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EGU22-10741
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ECS
Diverse hydrothermal and sedimentary habitats in the 3.5 Ga Dresser Formation, Pilbara Craton, Western Australia
(withdrawn)
Michaela J Dobson et al.
16:23–16:30
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EGU22-955
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ECS
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Virtual presentation
Julie Andréa Ngwal Ghoubou Ikouanga et al.

Biogenicity and taphonomy of the early life fossil records are debated as most of the previous studies focussed mainly on isotopes geochemistry. The non-metamorphosed Paleoproterozoic (~2.1 Ga) sedimentary succession of the Francevillian Basin (Gabon) contains the oldest complex multicellular organisms embedded in black shale facies. Several studies have confirmed the biogenicity of these soft-bodied organisms. Here, we used multi-proxy techniques to show that the preservation of these macro-organisms happened in a close system that limits interaction with their host rocks, which leads to their good preservations. The macro-organisms are present in different shapes and sizes: lobate (L), elongate (E), tubular (T), segmented (S), and circular (C), and are often associated with bacterial mats. Except for the C form, most of the other specimens are pyritized. Sulfur isotopes data confirms that pyritization occurred by bacterial sulfato-reduction during early diagenesis. We compare the clay mineral assemblages between the pyritized specimens and the late-diagenetically formed pure pyritized concretions in the sediments because the early pyritization process could not explain the taphonomic preservation alone. Our clay mineralogical data show that the specimens are dominated mainly by randomly mixed layer Illite-smectite (IS MLMs), illite, and chlorite relative to the host rocks. The abundance of IS MLMs indicates incomplete illitization of smectite, potassium deficiency, and limited mineral reactions in a semi-close local chemical system within the fossils.  In addition, the authigenic chlorites are more iron-rich and show vermicular habitus. By contrast, the pyritized concretions mainly consist of well-crystallized illite and less iron-rich chlorite, while the smectite phases are absent. These results confirmed that the diagenetic reaction is controlled by interaction with an open late diagenetic system. We concluded that taphonomic preservation of the ancient fossil record resulted from the early diagenetic growth of pyrite crystals during bacterial sulfato reduction in the fossils, which creates a semi-closed system that drastically reduced fluid-rock interactions with the host sediments.

How to cite: Ngwal Ghoubou Ikouanga, J. A., Fontaine, C., M. Bankole, O., Laforest, C., Riboulleau, A., Trentesaux, A., Boissard, C., Somogyi, A., Meunier, A., and El Albani, A.: Taphonomy of early life: Role of organic and mineral interactions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-955, https://doi.org/10.5194/egusphere-egu22-955, 2022.

Thu, 26 May, 17:00–18:30

Chairpersons: Ria Fischer, Jeroen van Hunen

17:00–17:07
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EGU22-3842
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ECS
Evidence for Hadean mantle differentiation preserved in 142,143Nd isotope constraints of granitoid rocks from the western Dharwar Craton, India
(withdrawn)
Arathy Ravindran et al.
17:07–17:14
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EGU22-12521
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Virtual presentation
Paul Tackley and Xavier Borgeat

The timing of the onset of plate tectonics on Earth remains a topic of strong debate, as does the tectonic mode that preceded modern plate tectonics. Understanding possible tectonic modes and transitions between them is also important for other terrestrial planets such as Venus and rocky exoplanets. Recent two-dimensional modelling studies have demonstrated that impacts can initiate subduction during the early stages of terrestrial planet evolution - the Hadean and Eoarchean in Earth’s case (O’Neill et al. 2017). Here, we perform three-dimensional simulations of the influence of ongoing multiple impacts on early Earth tectonics and its effect on the distribution of compositional heterogeneity in the mantle, including the distribution of impactor material. We compare two-dimensional and three-dimensional simulations to determine when geometry is important. Results show that impacts can induce subduction in both 2-D and 3-D and thus have a great influence on the tectonic regime. The effect is particularly strong in cases that otherwise display stagnant-lid tectonics: impacts can shift them to having a plate-like regime. In such cases, however, plate-like behaviour is temporary: as the impactor flux decreases the system returns to what it was without impacts. Impacts result in both greater production of oceanic crust and greater recycling of it, increasing the build-up of subducted crust above the core-mantle boundary and in the transition zone. Impactor material is mainly located in the upper mantle, at least at the end of the modelled 500 million year period. This is modified when impactors are differentiated into metal and silicate: the dense metal blobs sink to the CMB. In 2-D simulations, in contrast to 3-D simulations, impacts are less frequent but each has a larger effect on surface mobility, making the simulations more stochastic. These stronger 2-D subduction events can mix both recycled basalt and impactor material into the lower mantle. These results thus demonstrate that impacts can make a first-order difference to the early tectonics and mantle mixing of Earth and other large terrestrial planets, and that three-dimensional simulations are important so that effects are not over- or under-predicted.

How to cite: Tackley, P. and Borgeat, X.: Hadean/Eoarchean plate tectonics and mantle mixing induced by impacts: A three-dimensional study , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12521, https://doi.org/10.5194/egusphere-egu22-12521, 2022.

17:14–17:21
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EGU22-8653
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ECS
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Highlight
Timothy Gray et al.

The Earth’s lithosphere, atmosphere, and biosphere interact with one another primarily at the surface of our planet, with the lithospheric coupling arising primarily from large-scale, long-period topographic evolution driven by deep mantle processes. Global numerical modelling of mantle convection in 3D with mobile continents in a modern plate tectonic regime has been previously demonstrated (Coltice et al., 2019). Improvements on such models can provide a useful tool for investigating the effects of large scale and long term changes in Earth’s tectonic regime on the surface.

We present preliminary results in 2D spherical geometry using newly implemented additions to the existing mantle convection code StagYY (Tackley, 2008). A free surface representation using a marker chain enables higher surface resolution and the possibility of future implementation of surface processes on a global scale (Duretz et al., 2016). Initial conditions based on previous work on self-consistent continent generation enables modelling of continents with realistic rheology and structure (Jain et al., 2019).

The successful development of these tools enables further study of the evolution of the surface as a result of tectonic changes. A key goal is the modelling of the transition from a pre-plate tectonic regime to modern plate tectonics, as may have occurred in the Neoproterozoic (Stern, 2018). The tectonic changes of this period were also associated with other radical changes in the atmosphere and biosphere, such as the Cryogenian glaciations, and the Cambrian explosion. Models of topographic evolution may be used in conjunction with climate models or models of biological evolution to study the coupling between these systems as a part of the emerging field known as Biogeodynamics (Gerya et al., 2020).

How to cite: Gray, T., Tackley, P., Gerya, T., and Stern, R. J.: Global scale numerical modelling of the transition to modern day plate tectonics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8653, https://doi.org/10.5194/egusphere-egu22-8653, 2022.

17:21–17:28
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EGU22-584
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ECS
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Virtual presentation
A Geochemical perspective on A-type granites of Nilgiri area, Southeastern Singhbhum Craton, Odisha India: Implication for petrogenesis
(withdrawn)
Manoj Kumar Sahoo and Sukanta Dey
17:28–17:35
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EGU22-5226
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ECS
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On-site presentation
Jonathan Lewis et al.

The origins of Eoarchean peridotites found in the Itsaq Gneiss Complex (IGG) of southern West Greenland represent a crucial record of igneous and geodynamic processes on the early Earth. The igneous and geodynamic origins of these rocks have, however, been the subject of controversy, with some researchers arguing that they represent the first known slivers of mantle emplaced by tectonic processes in the crust and others contending that they represent cumulates associated with the local basalt units. The geodynamic context for the formation of these rocks has also been disputed, with some researchers arguing that they formed in a horizontal tectonic setting analogous to a modern subduction zone, while others propose a vertical tectonic origin for all Eoarchean rocks. Here, we provide new insights into the history of these peridotites using multiple sulfur isotope signatures combined with Hf isotope compositions. Anomalously high εΗf values in some IGC peridotites identified in previous studies [1], as well as in metabasalts with boninite-like compositions [2] found in the Isua Supracrustal Belt (ISB) within the IGC, point to contributions from a mantle source already depleted in the Hadean [2]. The multiple sulfur isotope data of the IGC peridotites found south of the ISB reveal small but significant Δ33S anomalies, consistent with incorporation of surface-derived material of Archean age or older. Furthermore, correlations between sulfur isotope data and major and trace element abundances as well as initial Hf isotope values of IGC peridotites support the hypothesis that high-degree melt depletion occurred under hydrous conditions, followed by variable degrees of melt metasomatism. The involved fluid and melt components precipitated sulfides that incorporated surface-derived sulfur with different depositional origins. We propose that these findings are best explained by a horizontal tectonic regime similar to modern arc settings.

 

1. van de Löcht, J., et al., Preservation of Eoarchean mantle processes in ∼3.8 Ga peridotite enclaves in the Itsaq Gneiss Complex, southern West Greenland. Geochimica et Cosmochimica Acta, 2020. 280: p. 1-25.

2. Hoffmann, J.E., et al., Highly depleted Hadean mantle reservoirs in the sources of early Archean arc-like rocks, Isua supracrustal belt, southern West Greenland. Geochimica et Cosmochimica Acta, 2010. 74(24): p. 7236-7260.

How to cite: Lewis, J., Hoffmann, J. E., Schwarzenbach, E. M., Strauss, H., Li, C., Münker, C., and Rosing, M. T.: Sulfur and Hafnium Isotope evidence for Early Horizontal Tectonics in Eoarchean Peridotites, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5226, https://doi.org/10.5194/egusphere-egu22-5226, 2022.

17:35–17:42
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EGU22-7947
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ECS
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Virtual presentation
Antonia Thijssen et al.

The nature of Paleoarchean (>3.2 Ga) crustal accretion continues to be debated, in particular the onset and timing of subduction-like processes. Crust of this age is typically characterised by dome-and-keel geometry that is widely interpreted to be related to “sagduction” or the episodic dripping of denser, mafic volcanics into the mantle around buoyant silicic cratonic nuclei. This occurs during regional scale crust-mantle overturn events.

The exceptional preservation of the East Pilbara Terrane (EPT) has been instrumental in the development of this model and its role in Paleoarchean continental crust formation. The Emu Pool Supersuite (~3324-3290 Ma) represents a phase of voluminous silicic magmatism that has been attributed to overturn and sagduction within the EPT (e.g. Wiemer et al., 2018). However, the widespread occurrence of magmatic-hydrothermal Cu and Mo mineralisation, reported to be linked to this magmatic episode, have received little attention. Comparisons to Phanerozoic porphyry Cu-Mo deposits have been drawn (e.g. Barley & Pickard, 1999), which is intriguing as such porphyry-type deposits have a clear genetic link to arc magmatism and subduction processes as they require hydrous, Cl-rich magmatism (e.g. Tattich et al., 2021).

To date the chronological relationships of the magmatic-hydrothermal deposits to the major dome forming silicic magmatism is poorly constrained. In one deposit, hydrothermal activity is constrained by 187Re-187Os geochronology (Stein et al., 2007) to late to post Emu Pool Supersuite magmatism, yet this interpretation is hampered by issues relating to the λ187Re uncertainty. Furthermore, interpretation of Paleoarchean geodynamics and magmatic evolution generally relies on micro-beam zircon U-Pb geochronological analyses, typically reported at single 207Pb/206Pb date precision at >±10 Myrs (2s), and presents challenges for accurately resolving geological processes and events.

We demonstrate that high-precision CA-ID-TIMS (Chemical Abrasion-Thermal-Ionisation Mass Spectrometry) zircon U-Pb geochronology, utilising ATONA low-noise detectors, can now routinely obtain precision of  ~<±200 kyrs (2s) on 207Pb/206Pb dates of single zircon or fragments at ~3.3 Ga. By combining detailed field relationships, with unprecedented temporal precision, we show that the Mo-Cu hydrothermal mineralisation can be demonstrably linked to their host plutons and formation timescales can even be constrained to ~1 Myrs, comparable to Phanerozoic porphyry deposits. This study identifies that magmatic-hydrothermal systems were not synchronous across the EPT. Instead they occurred over >7 Myrs during the early phase of Emu Pool Supersuite and silicic magmatism within domes.

Whilst the geodynamic trigger for Mo and Cu magmatic-hydrothermal mineralisation at ~3.3 Ga remains enigmatic, we highlight their timing and occurrence should be accommodated within Paleoarchean geodynamic models. Furthermore, the results illustrate the potential of modern high-precision U-Pb geochronology to routinely examine Paleoarchean magmatic records at timescales that closely approximate known plutonic accretion rates within the Phanerozoic.

 

References

Barley, & Pickard, (1999) Precambrian Research, 96, 41-62

Stein et al., (2007) Geochimica et Cosmochimica Acta, 71

Tattitch et al., (2021) Nature communications, 12, 1-11.

Wiemer et al., (2018) Nature Geoscience, 11, 357-361.

How to cite: Thijssen, A., Tapster, S., and Parkinson, I.: Pinpointing Paleoarchean magmatic-hydrothermal events during the geodynamic and crustal evolution of the East Pilbara Terrane , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7947, https://doi.org/10.5194/egusphere-egu22-7947, 2022.

17:42–17:49
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EGU22-594
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ECS
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Aniruddha Mitra and Sukanta Dey

Singhbhum Craton, eastern India, exposes an array of Paleoarchean granitoids (e.g., TTGs and diorites, transitional TTG, and K-rich granite) ranging in age from ~3.53─3.25 Ga, thus making it a suitable archive for understanding crustal architecture and dynamics during that era. Granitoids cover the core of the craton as a composite dome and are fenced by keels of contemporaneous iron ore bearing greenstone belts from east, west, and south giving rise to a dome-and-keel architecture.  Change in granitoid chemistry and isotope signature over time and space can provide a window into the change of crustal evolution mechanism as well as geodynamics of the crust formation if put into a robust tectonic framework. Most of such earlier studies addressed the secular evolution of granitoid chemistry and isotopic changes as an expression of a shift in tectonic paradigms. This tectonic shift is explained broadly as a response to a progressively cooling earth. However, the timing of the transition (advent of a new tectonic setting) varies globally; hence, each craton needs to be studied separately and without any prior bias.

Spatial variation represented by contour diagrams from the cratonic core show two distinct areas exposing dominantly 3.35–3.25 Ga high-silica, low-magnesiam, high K2O/Na2O (K/Na>0.60) granitoids of shallow crustal origin, indicated by their low pressure-sensitive ratios (eg. Eu/Eu*, Sr/Y, Gd/Er, La/Yb). These two areas are surrounded by older intermediate granitoids (>3.35 Ga TTGs). Based on the spatial distribution, it is being suggested that these spatial arrangement of granitoids are related to “partial convective overturn (PCO)” process where the >3.35 Ga TTG basements were subjected to greenstone load while they were soft. As a result some part of the newly formed softer >3.35 Ga TTG crust melted as these overburdens helped in bringing the TTGs to a potential melting depth. The greenstones then sank into the partially molten TTGs along steep-dipping sinistral shear zones by forming synformal keels. The moderate- to- low-pressure TTG-derived partial melts then rose to the shallower level and formed the 3.35–3.25 Ga high-silica, low-Mg# potassic granitoids.

Preserved rock record in the Singhbhum Craton indicates that the granitoid magmatism started at ~3.47 Ga with emplacement of high-silica, low alumina tonalite, characterized by low Sr/Y, (Gd/Er)N, (La/Yb)N, Eu/Eu* and Sr. The 3.47 to 3.32 Ga TTG record from the Singhbhum Craton show a progressive increase in Al2O3, Sr/Y, (Gd/Er)N, (La/Yb)N, Eu/Eu* and Sr and decrease in Na2O. The increase in the pressure-sensitive ratios reached peak during 3.32 Ga and then started decreasing until ~3.28 Ga followed by another increase during ~3.28 to ~3.25 Ga before ceasing of Paleoarcehan magmatism in the Singhbhum Craton. Such variation in geochemical tracers is explained in terms of episodic crustal thickening by periodic mantle upwelling and associated delamination along with time-progressive changes in bulk chemical composition of the continental crust from mafic to felsic.

How to cite: Mitra, A. and Dey, S.: Time-space evolution of an ancient continent, a window to crustal evolution: Insight from granitoids of Singhbhum Craton, eastern India, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-594, https://doi.org/10.5194/egusphere-egu22-594, 2022.

17:49–17:56
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EGU22-1666
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ECS
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On-site presentation
Martin Hugo Senger et al.

The Precambrian comprises the vast majority of Earth’s history. Preserved archives contain essential information about the first few billion years for planetary evolution of our planet. Despite covering a large part of the history of our planet, these outcrops are not so abundant due to erosion and frequently occur in disparate areas. In order to relate them and to establish a timeline of geological events in a world lacking biochronology, we rely on accurate radio-isotopic age determinations. These are, however, rather scarce and still leave several hundreds of million years long time intervals undated. In this study, we present U-Pb age determinations from volcanic and sedimentary units of the Paleoproterozoic Transvaal Supergroup, South Africa. The Transvaal Supergroup is an exceptionally well preserved sequence and therefore accounts for a very large amount of geochemical data. Due to its capacity to produce large data sets the preferred technique in U-Pb zircon geochronology for ancient sediments is LA-ICP-MS. It allows the aqcuisition of maximum depositional ages (MDA) in a fast way and at a relatively low cost. However, the large analytical uncertainty preclude the temporal resolution to distinguish between different processes in such old rocks. Moreover, the standard dating procedure rarely includes zircon treatment via chemical abrasion to mitigate common problems such as open system behavior due to radioactive decay damage related Pb loss. In consequence, interpreted ages might be severely disturbed and may yield MDA’s that are tens to hundreds of million years too young. As an alternative, the much more work-intensive CA-ID-TIMS technique allows the obtention of more accurate and more precise ages, preferably using zircon grains that have previously been screened for their LA-ICP-MS U-Pb age.

 Our new combined LA-ICP-MS and CA-ID-TIMS data indicates that the glaciogenic Makganyene Formation has a MDA of ~2.42 Ga. Younger age clusters at around ~2.2 Ga from LA-ICP-MS dating disappear with chemical abrasion and have to be interpreted as artifacts of radiation-damage related Pb loss. These new results have important implications for both environmental evolution during the Neoarchean/Paleoproterozoic, as well as for the regional geology. The Makganyene diamictites are thought to represent the oldest Paleoproterozoic glaciation in South Africa. The data also corroborate the hypothesis that the directly overlying-to-locally-interfingered mafic volcanic Ongeluk Formation is ~200 Ma older than the volcanic rocks ~2250 Ma Hekpoort Formation in the East Transvaal basin. We therefore reject the long-standing correlation between both units, as previously published.

We demonstrate that LA-ICP-MS is not capable to provide a robust and reliable MDA’s in ancient clastic sediments. CA-ID-TIMS analysis provides dates of significantly higher accuracy, because the chemical abrasion is minimizing Pb-loss in the crystal. Therefore, for studies relying on U-Pb zircon geochronology, we encourage the application of CA-ID-TIMS in the youngest populations previously identified with the LA-ICP-MS. This is particularly important for establishing reliable maximum depositional ages in sedimentary rocks.

How to cite: Senger, M. H., Davies, J., Ovtcharova, M., Beukes, N., Gumsley, A., Gaynor, S. P., Ulyanov, A., and Schaltegger, U.: U-Pb zircon geochronology combining both in-situ and bulk-grain techniques in the Transvaal Supergroup, South Africa., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1666, https://doi.org/10.5194/egusphere-egu22-1666, 2022.

17:56–18:03
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EGU22-3181
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ECS
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On-site presentation
Kira Musiyachenko et al.

Much of the continental lithosphere developed during the Archean, which was an Eon of change in terms of global geodynamics and geochemical cycles. Uncovering the causal links between crust forming processes and prevailing geodynamic mechanisms is crucial for understanding the origins and composition of the present-day continental lithosphere. Pristine Archean crust is scarce yet can be found in cratons worldwide. Many of these occurrences comprise rocks of the tonalite-trondhjemite-granodiorite (TTG) suite, which represent a prevalent component of the Archean continental crust. TTGs are generally considered to have formed by partial melting of amphibolite or eclogite source rocks that had basaltic precursors originally extracted from a depleted mantle (e.g., [1]). The age of the source rocks (i.e., the time between the basalt extraction from the mantle and TTG formation) can be determined from the initial radiogenic isotope compositions of TTGs, provided that the P/D ratio of the source can be reliably estimated and is significantly different from that of the depleted mantle.

Based on this principle, we estimated the age of basaltic sources of TTGs from cratons of different age and paleogeography from initial 87Sr/86Sr compositions determined by in-situ Sr isotope analysis of primary igneous apatite (LA-MC-ICPMS). The 87Sr/86Sr of these apatites show that prior to 3.4 Ga TTGs were derived from relatively old mafic sources and that the average time between formation of basaltic material from the mantle and subsequent remelting under amphibolite to eclogite facies conditions decreased drastically during the Paleoarchean. This secular change indicates a rapid global increase in the efficiency of TTG production or the emergence of a new TTG-forming process at c. 3.4 Ga [2].

In this contribution we explore this hypothesis by comparing the 87Sr/86Sr signature of the TTGs with their trace-element compositions, as well as with 176Hf/177Hf zircon data for these rocks and contemporary TTGs from other studies. This combined geochronological, isotope and geochemical analyses will provide new constraints on the age of TTG sources during the Archean and will allow investigation into the nature and probable causes of the apparent rejuvenation at 3.4 Ga, as indicated by Sr isotopes.

[1] Hoffmann, J.E. et al. (2011) Geochim. Cosmochim. Acta 75, 4157-4178.

[2] Caton, S., et al., (in review) Chem. Geol.

How to cite: Musiyachenko, K., Smit, M., Caton, S., B. Emo, R., Kielman-Schmitt, M., Kooijman, E., Scherstén, A., Halla, J., Bleeker, W., Hoffmann, J. E., Prakash Pandey, O., Ravindran, A., Maltese, A., and Mezger, K.: Secular change in the age of TTG sources during the Archean from in-situ Sr and Hf isotope analysis by LA-MC-ICPMS, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3181, https://doi.org/10.5194/egusphere-egu22-3181, 2022.

18:03–18:10
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EGU22-10873
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On-site presentation
Bradford Foley

The tectonic processes responsible for the formation of early Earth felsic crust (predominantly composed of tonalite-trondhjemite-granodiorite, or TTGs) inform the global regime of mantle convection that operated at this time. Many models have been proposed to explain the formation of Archean TTGs, including melting of downgoing crust in hot subduction zone settings, or melting of crust that is buried by lava flows and founders into the mantle. Formation in a subduction zone setting would imply at least some form of mobile-lid tectonics on the early Earth, while TTG formation via crustal burial and foundering does not require subduction or plate tectonics, and can thus occur in a stagnant-lid regime.  

Regardless of tectonic setting, TTGs can only form if hydrated basaltic protocrust melts before it experiences metamorphic dehydration. Previous work has argued that this constraint may preclude a subduction origin to TTGs. Regional scale numerical models have found that slabs sink quickly and steeply through the mantle at Archean mantle temperatures, such that they dehydrate before melting. However, these models do not consider evolution of grainsize in the mantle interior and in plate boundaries. Using numerical models of mantle convection with grain damage, a mechanism for generating mobile-lid convection via grain size reduction, I show that a sluggish, drip-like style of subduction emerges at early Earth conditions. This subduction style is a result of plate boundaries becoming effectively stronger with increasing mantle temperature, and leads to significant slab heating at shallow depths.

To test whether TTGs can form from this style of sluggish subduction, I use scaling laws developed from numerical models combined with a simple model of the evolution of the vertical temperature profile through a slab. Results show that the slower sinking speed of slabs caused by grain size evolution in plate boundaries allows for crustal melting for a much wider range of mantle temperatures and subducting plate thicknesses than if the effects of grain size evolution were ignored. Overriding plate thickness is also important, with thin overriding plates favored for TTG formation. These results have important implications for the settings where subduction could generate Archean TTGs, and for potential episodicity in TTG formation resulting from both short- and long-term episodicity in subduction.  

How to cite: Foley, B.: Generation of Archean TTGs by slab melting during sluggish, drip-like subduction, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10873, https://doi.org/10.5194/egusphere-egu22-10873, 2022.

18:10–18:17
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EGU22-4850
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ECS
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On-site presentation
Charitra Jain and Stephan Sobolev

The present-day Earth exhibits subduction-driven plate tectonics, which is a surface expression of processes happening in the deep interior. For the early Earth, following the magma ocean solidification stage, a variety of tectonic regimes have been proposed albeit without any consensus: heat-pipe tectonics, plutonic-squishy lid, stagnant lid. Furthermore, the rheological changes required to make the (supposedly gradual) transition to modern style plate tectonics on Earth remain hotly debated. Also, different estimates of mantle potential temperature (Herzberg et al., 2010; Aulbach and Arndt, 2019) for the Archean have been proposed.

Recently, it has been proposed that sediments accumulated at continental margins as a result of surface erosion processes could have acted as a lubricant to stabilise subduction and aid with the initiation of plate tectonics after the emergence of continents around 3 Ga (Sobolev and Brown, 2019). Before that time, the flux of sediments to the ocean was very limited. It was further suggested that subduction zones were already present at that time but were likely initiated only above hot mantle plumes. This tectonic regime of regional plume-induced retreating subduction zones was very different from the modern type of plate tectonics, but nevertheless might have been efficient in production of early continental crust and recycling of water and pre-existing crust into the deep mantle.

In this work, we test this hypothesis of surface-erosion controlled plate tectonics preceded by plume-induced retreating subduction tectonic regime in global convection models by introducing magmatic weakening of lithosphere above hot mantle plumes. We also adapt the effective friction coefficient in brittle deformation regime to mimic the lubricating effect of sediments. Furthermore, these models employ a more realistic upper mantle rheology and are capable of self-consistently generating oceanic and continental crust while considering both intrusive (plutonic) and eruptive (volcanic) magmatism (Jain et al., 2019). We also investigate the influence of lower mantle potential temperatures on crust production and compare our models with geological data.

When compared to models with just diffusion creep, the models with composite rheology (diffusion creep and dislocation creep proxy) result in more efficient mantle cooling, higher production of continental crust, and higher recycling of basaltic-eclogitic crust through delamination and dripping processes. These models also show higher mobilities (Tackley, 2000), which have been previously shown for diffusion creep models only with low surface yield stress values (Lourenço et al., 2020). Preliminary results from models initialised with lower mantle potential temperatures show an affect on the initial growth of TTG rocks over time. However, no considerable differences in terms of total crust production or mantle cooling are observed.

How to cite: Jain, C. and Sobolev, S.: Using composite rheology models to explore the interplay between continent formation, surface erosion, and the evolution of plate tectonics on Earth, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4850, https://doi.org/10.5194/egusphere-egu22-4850, 2022.