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Earth's first stable continents did not form by subduction
Tim E. Johnson,Michael Brown,Nicholas J. Gardiner,Christopher L. Kirkland,R. Hugh Smithies +4 more
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Johnson et al. as discussed by the authors performed phase equilibria modelling of the Coucal basalts from Western Australia and confirmed their suitability as parent rocks of the early continental crust of the Earth's first continents.Abstract:
Phase equilibria modelling of rocks from Western Australia confirms that the ancient continental crust could have formed by multistage melting of basaltic ‘parents’ along high geothermal gradients—a process incompatible with modern-style subduction Tim Johnson et al perform phase equilibria modelling of the Coucal basalts from Western Australia and confirm their suitability as parent rocks of the Archaean continental crust The authors suggest that these early crustal rocks were produced by 20–30 per cent melting along high geothermal gradients They conclude that the production and stabilization of the first continents required a protracted, multistage process When coupled with the high geothermal gradients, this suggests that the continents did not form by subduction Instead it favours a 'stagnant lid' regime in the early Archaean eon in which a single, rigid plate lay over the mantle The geodynamic environment in which Earth’s first continents formed and were stabilized remains controversial1 Most exposed continental crust that can be dated back to the Archaean eon (4 billion to 25 billion years ago) comprises tonalite–trondhjemite–granodiorite rocks (TTGs) that were formed through partial melting of hydrated low-magnesium basaltic rocks2; notably, these TTGs have ‘arc-like’ signatures of trace elements and thus resemble the continental crust produced in modern subduction settings3 In the East Pilbara Terrane, Western Australia, low-magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup have trace-element compositions that are consistent with these being source rocks for TTGs These basalts may be the remnants of a thick (more than 35 kilometres thick), ancient (more than 35 billion years old) basaltic crust4,5 that is predicted to have existed if Archaean mantle temperatures were much hotter than today’s6,7,8 Here, using phase equilibria modelling of the Coucal basalts, we confirm their suitability as TTG ‘parents’, and suggest that TTGs were produced by around 20 per cent to 30 per cent melting of the Coucal basalts along high geothermal gradients (of more than 700 degrees Celsius per gigapascal) We also analyse the trace-element composition of the Coucal basalts, and propose that these rocks were themselves derived from an earlier generation of high-magnesium basaltic rocks, suggesting that the arc-like signature in Archaean TTGs was inherited from an ancestral source lineage This protracted, multistage process for the production and stabilization of the first continents—coupled with the high geothermal gradients—is incompatible with modern-style plate tectonics, and favours instead the formation of TTGs near the base of thick, plateau-like basaltic crust9 Thus subduction was not required to produce TTGs in the early Archaean eonread more
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Secular change in metamorphism and the onset of global plate tectonics
Michael Brown,Tim E. Johnson +1 more
TL;DR: In this article, thermal gradients of metamorphic rocks were analyzed for 456 localities from the Eoarchean to Cenozoic Eras to test the null hypothesis that thermal gradient through time did not vary outside of the range expected for each of these distinct plate tectonic settings.
Journal ArticleDOI
Two distinct origins for Archean greenstone belts
R. Hugh Smithies,Timothy J. Ivanic,Jack R. Lowrey,Paul A. Morris,Stephen Barnes,Stephen Wyche,Yongjun Lu,Yongjun Lu +7 more
TL;DR: In this paper, the authors apply the Th/Yb-Nb-Yb plot of Pearce (2008) to the well-studied Archean greenstone sequences of Western Australia and show that individual volcanic sequences evolved through one of two distinct processes reflecting different modes of crust-mantle interaction.
Journal ArticleDOI
Onset of plate tectonics by the Eoarchean
TL;DR: In this paper, a review of the main Eoarchean supracrustal belts of the world, constrained by relevant geochemical/isotopic data, is presented evidence that suggests that from at least ca. 4.0 -4.2 -Ga to 2.7 -2.5 -Ga Earth produced considerable juvenile mafic crust and consequent island arcs by Accretionary Cycle Plate Tectonics.
Journal ArticleDOI
Crustal evolution and mantle dynamics through Earth history.
TL;DR: A tentative hypothesis is presented for the evolution of mantle dynamics and its relation to surface environment; the early onset of plate tectonics and gradual mantle hydration are responsible not only for the formation of continental crust but also for its preservation as well as its emergence above sea level.
References
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Macrobiont: Cradle for the Origin of Life and Creation of a Biosphere.
Benton C. Clark,Vera M. Kolb +1 more
TL;DR: Comparison of ponds with other macrobionts allows a conclusion that more than one possible macrobIONt locale could enable an OoL, and Pond size-sensitivities for several figures of merit are mathematically formulated, leading to the expectation that the optimum pond size for the OoL is intermediate, but biased toward smaller sizes.
Journal ArticleDOI
Palaeoarchaean TTGs of the Pilbara and Kaapvaal cratons compared; an early Vaalbara supercraton evaluated
Nicholas J. Gardiner,Nicholas J. Gardiner,Jacob A. Mulder,Christopher L. Kirkland,Tim E. Johnson,Tim E. Johnson,Oliver Nebel +6 more
TL;DR: In this article, the authors compare the geochemical and isotopic record of the Palaeoarchaean East Pilbara Terrane (EPT), Pilbara Craton, Western Australia, and the Barberton Granite-Greenstone Belt (BGGB), Kaapvaal Craton.
Journal ArticleDOI
Geochronology and geochemistry of the Xiaoqinling Taihua Complex in the southern Trans-North China Orogen: Implications for magmatism during the early Paleoproterozoic global tectono-magmatic shutdown
TL;DR: Based on zircon U−Pb geochronological and whole-rock geochemical data, the 2.30 Ga granitic gneisses in the Taihua Complex can be subdivided into three groups.
Journal ArticleDOI
The Mesoarchaean Dominion Group and the onset of intracontinental volcanism on the Kaapvaal craton – Geological, geochemical and temporal constraints
Andrea Agangi,Andrea Agangi,Axel Hofmann,Ernst Hegner,Hangqiang Xie,Claudia Teschner,A. I. Slabunov,A. I. Slabunov,Sergei A. Svetov +8 more
TL;DR: In this article, the authors report SHRIMP U Pb zircon ages, elemental and Nd-isotope bulk-rock analyses of drill core samples and interpret their petrogenesis in the context of a Mesoarchaean continental setting.
Journal ArticleDOI
Geochemistry and petrogenesis of the early Archean mafic crust from the Saglek-Hebron Complex (Northern Labrador)
TL;DR: In this paper, the authors presented the largest whole-rock major and trace element geochemical dataset (over 100 samples) on the mantle-derived rocks from the Saglek-Hebron Complex to constrain their petrogenesis.