10:00am - 10:15amTopics: 2.03 Setting the stage for a habitable planet: Solid Earth processes through time
Experimental and isotopic constraints on the formation of Archean continental crust
1Ruhr-Universität Bochum, Germany; 2Freie Universität Berlin, Germany
Remnants of Earth’s juvenile continental crust are preserved in the form of Archean Tonalite-Trondhjemite-Granodiorites (TTGs). However, much controversy surrounds the composition of TTG protoliths and whether the geodynamic setting involved convergent-style plate tectonics. Thus, a combination of high-pressure and high-temperature experiments combined with robust geochemical proxies are required to gain insight into TTG petrogenesis. Numerous experimental studies have demonstrated that partial melting of hydrated basalt at 0.8 to 2 GPa is sufficient to produce TTG-like melts. The compositions of starting materials used in these experiments vary significantly between studies and has profound implications for the solidus, melt composition, and the type, composition and modal abundance of solid phases. This is problematic especially given most studies have utilised MORB-like starting compositions, which differ significantly in composition to the least altered Archean metabasalts which posses higher MgO and lower Al2O3. We present the results of partial melting experiments conducted at 1-1.5 GPa and 940 to 1100 °C in a piston cylinder apparatus using synthetic starting materials with varying H2O (4-6 wt.%), based on the compositions of Eoarchean metabasalts from the Isua supracrustal belt in southern West Greenland. The run products of these experiments will be analysed to assess their phase assemblages and melt composition, and to constrain the magnitude of mineral-melt trace element and isotopic (Ti) fractionation during partial melting. Ultimately this experimental campaign will ascertain if partial melting of high-Mg, low-Al metabasalts is a viable mechanism to produce melt compositions resembling those of Archean TTGs.
10:15am - 10:30amTopics: 2.03 Setting the stage for a habitable planet: Solid Earth processes through time
Tectono-magmatic evolution of the Lewisian Gneiss Complex, NW Scotland: constraints from in situ U-Pb, Lu-Hf and trace element analysis of TTG-derived zircon.
1Ruhr-Universität Bochum, Germany; 2ETH Zürich, Siwtzerland; 3Curtin University, Australia; 4Paul Scherrer Institut
The mainland Lewisian Gneiss Complex (LGC) in NW Scotland is dominated by Archean tonalite-trondhjemite-granodiorite (TTG) gneisses. It consists of at least three distinct crustal blocks that have different magmatic and metamorphic histories. The LGC is regarded to either represent i) a once-contiguous fragment of Archean crust that was later disaggregated and reassembled along major shear zones into the northern, central and southern regions, or ii) a collage of discrete terranes. To better assess the tectono-magmatic evolution of the LGC, we use in situ U-Pb, Lu-Hf and trace element analysis of zircon grains derived from a representative set of TTG gneiss samples. The crystallization ages of TTGs range between 2778 and 2609 Ma in the northern region (Rhiconich terrane), 3003 and 2731 Ma in the central region (Assynt/Gruinard terranes), and 3110 and 2675 Ma in the southern region (Rona terrane). Zircon εHf(t) values of the oldest samples from the southern region are broadly chondritic. Samples from the southern part of the central region (Gruinard terrane) are more radiogenic, indicating a major period of juvenile magmatism at c. 2900-2800 Ma. Zircon derived from TTGs < 2800 Ma is characterised by chondritic to sub-chondritic Hf isotope signatures, pointing to increased crustal reworking during the late stages of TTG magmatism. Collectively, our data show that TTGs in the different parts of the LGC formed at different conditions and from different sources. We propose that the LGC is composed of at least three terranes that were assembled during the Neoarchean to Paleoproterozoic.
10:30am - 10:45amTopics: 2.03 Setting the stage for a habitable planet: Solid Earth processes through time
Metamorphism and the tectonic evolution of the Archean
1Freie Universität Berlin, Berlin; 2China University of Geosciences, Wuhan; 3University of Louisiana at Lafayette; 4Columbia University
The tectonics on the Archean Earth is intricate and contentious, with ongoing debate concerning the dominant surface processes controlled by either a plate tectonics regime or alternative forms of tectonics (stagnant lid, heat pipe, drip tectonics, sluggish plates and other planetary modes of heat loss). In this study, we assess the viability of interpretations of tectonics during the Archean using a newly compiled metamorphic database. We relate Archean cratons and continental history, crustal growth and reworking, and horizontal motion of ancient cratons to infer which tectonic styles and processes operated. Our analysis is synthetized by the highlighting of three distinct Archean periods with different tectonic activity, starting at 3.8 billion years (Ga), from when the first metamorphic data are available. We find that in the interval 3.8-3.5 Ga, tectonics was dominated by short-lived subduction tectonics and non-subduction tectonics, possibly in cohabitation. Between 3.4 and 3.0 Ga, subduction was present and contributed to the lateral growth of the continents. In the 2.8-2.5 Ga period, the assembly of supercontinent/supercratons signals the action of modern-style plate tectonics. In summary Archean metamorphic data allow timing the Earth progression from pre-modern tectonics to modern plate tectonics including the supercontinent cycle.
10:45am - 11:00amTopics: 2.03 Setting the stage for a habitable planet: Solid Earth processes through time
Growth of non-typical garnet textures during amphibolite facies metamorphism: Dwalile Supracrustal Suite, Ancient Gneiss Complex, Eswatini
1Helmholtz Centre Potsdam Deutsches GeoForschungsZentrum GFZ, Germany; 2Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland; 3Centre for Crustal Petrology, Department of Earth Sciences, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa; 4Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstrasse 74-100, 12249 Berlin, Germany; 5Department of Earth Sciences, Utrecht University, Budapestlaan 4, 3584CD Utrecht, The Netherlands
The Dwalile Supracrustal Suite of the Ancient Gneiss Complex (Eswatini) represents one of the oldest greenstone belts in the world with a crustal evolution history from Palaeo- to Mesoarchaean times. The main metapelitic rock type is dominated by garnet and staurolite porphyroblasts in a layered matrix of biotite, muscovite, quartz and retrograde sericite. Minor components are andalusite, chlorite and chloritoid with accessory ilmenite and monazite. The age for the amphibolite facies metamorphism is recorded by monazite at ca. 3.15 Ga.
The garnet-staurolite bearing metapelites have thus similar mineralogy and bulk rock compositions, but differ due to their unusual garnet microstructures. In some samples the garnet grains are distributed as thin layers consisting of elongated ribbons, with local resorption textures and peninsular features together with coarse recrystallised quartz. The euhedral garnet cores are only visible in compositional maps, which show a typical bell-shaped growth zoning.
EBSD maps of the crystallographic orientations of the garnet are created for four samples and two samples were imaged using high-resolution X-ray tomography for the visualization of the garnet morphology in order to test the relationship of the garnet porphyroblasts and consider the implications for the formation mechanism. For example, do the garnets share the same crystallographic orientation or is there evidence for deformation and/or rotation processes during growth. How are the garnets orientated in 3D space and are the ribbon textures build up by subgrains caused by deformation or are they separate grains formed by multiple nucleation events, possibly caused by fluid-rock interaction processes.
11:00am - 11:15amTopics: 2.03 Setting the stage for a habitable planet: Solid Earth processes through time
In-situ analysis of lead and multiple sulfur isotopes in southern West Greenland peridotite sulfide grains reveal evidence for Eoarchean crustal recycling
1Freie Universität Berlin, Germany; 2University of Fribourg, Switzerland; 3Universität zu Köln, Germany; 4Westfälische Wilhelms-Universität Münster, Germany; 5University of Copenhagen, Denmark; 6Naturhistoriska riksmuseet, Stockholm, Sweden
Eoarchean peridotites from the area south of the Isua Supracrustal Belt (SOISB) in southern West Greenland have been found by previous studies to contain sulfur subject to mass independent fractionation (MIF-S), with positive Δ33S values indicating that these rocks have incorporated sedimentary sulfur recycled from Earth’s surface . New in-situ secondary ion mass spectrometry measurements of sulfide grains found within these peridotites reveal that this MIF-S is hosted within the sulfide grains. Electron microprobe analyses of the sulfide grains reveal that they are predominantly composed of pentlandite and pyrrhotite, consistent with the typical sulfide mineralogy of mantle rocks. The peridotites displaying the least petrographic evidence for melt overprint (Group 1) were found to contain the highest average in-situ Δ33S values, +0.20±0.02‰, while those bearing more evidence for melt overprint (Group 2) had lower average in-situ Δ33S values, +0.09±0.03‰. These findings are in good agreement with previous bulk rock S isotope results . In-situ Pb isotope measurements of the sulfide grains reveal unradiogenic compositions consistent with an Archean origin. Furthermore, petrographic observations reveal that the sulfide grains are crosscut by amphiboles, indicating that the sulfide grains predate Neoarchean amphibolite-facies metamorphism. Our data support previous interpretations that these rocks are the oldest known mantle peridotites. These findings reinforce previous interpretations that SOISB peridotites preserve evidence of crustal recycling in the Eoarchean.
 Lewis et al. (2022) EGU 22-5226