Widespread reworking of Hadean-to-Eoarchean continents during Earth’s thermal peak
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Abstract The nature and evolution of Earth’s crust during the Hadean and Eoarchean is largely unknown owing to a paucity of material preserved from this period. However, clues may be found in the chemical composition of refractory minerals that initially grew in primordial material but were subsequently incorporated into younger rocks and sediment during lithospheric reworking. Here we report Hf isotopic data in 3.9 to 1.8 billion year old detrital zircon from modern stream sediment samples from West Greenland, which document successive reworking of felsic Hadean-to-Eoarchean crust during subsequent periods of magmatism. Combined with global zircon Hf data, we show a planetary shift towards, on average, more juvenile Hf values 3.2 to 3.0 billion years ago. This crustal rejuvenation was coincident with peak mantle potential temperatures that imply greater degrees of mantle melting and injection of hot mafic-ultramafic magmas into older Hadean-to-Eoarchean felsic crust at this time. Given the repeated recognition of felsic Hadean-to-Eoarchean diluted signatures, ancient crust appears to have acted as buoyant life-rafts with enhanced preservation-potential that facilitated later rapid crustal growth during the Meso-and-Neoarchean.Keywords:
Hadean
Early Earth
Abiogenesis
The mantle was probably oxidized early, during and shortly after accretion, and so the early atmosphere of Earth was likely dominated by CO2 and N2, not by CH4 and NH3. CO2 declined from multibar levels during the early Hadean to perhaps a few tenths of a bar by the mid- to late Archean. Published geochemical constraints on Archean CO2 concentrations from paleosols are highly uncertain, and those from banded iron formations are probably invalid. Thus, CO2 could have been sufficiently abundant during the Archean to have provided most of the greenhouse...
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Early Earth
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Conditions at the surface of the young (Hadean and early Archean) Earth were suitable for the emergence and evolution of life. After an initial hot period, surface temperatures in the late Hadean may have been clement beneath an atmosphere containing greenhouse gases over an ocean-dominated planetary surface. The first crust was mafic and it internally melted repeatedly to produce the felsic rocks that crystallized the Jack Hills zircons. This crust was destabilized during late heavy bombardment. Plate tectonics probably started soon after and had produced voluminous continental crust by the mid Archean, but ocean volumes were sufficient to submerge much of this crust. In the Hadean and early Archean, hydrothermal systems around abundant komatiitic volcanism may have provided suitable sites to host the earliest living communities and for the evolution of key enzymes. Evidence from the Isua Belt, Greenland, suggests life was present by 3.8 Gya, and by the mid-Archean, the geological record both in the Pilbara in Western Australia and the Barberton Greenstone Belt in South Africa shows that microbial life was abundant, probably using anoxygenic photosynthesis. By the late Archean, oxygenic photosynthesis had evolved, transforming the atmosphere and permitting the evolution of eukaryotes.
Hadean
Early Earth
Abiogenesis
Greenstone belt
Felsic
Banded iron formation
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The magmato-tectonic environment(s) of origin for Earth’s earliest crust are enigmatic and fiercely debated. Revealing the composition of the melts from which Hadean (>4.02 Ga) zircons crystallized might clarify conditions of initial crust construction. We calculate model melts using Ti-calibrated zircon/melt partition coefficients (KdZrc(Ti)) and published trace element data for Hadean and Archean zircons. The same treatment is applied to zircons from possible analogue environments (MORB, Iceland, arcs, lunar), to constrain potential petrogenetic similarities and distinctions between the early and modern world. Model melts from oceanic environments (MORB, oceanic arc, Iceland) have higher heavy rare earth element (HREE) contents and shallower middle REE (MREE) to HREE/chondrite (ch) slopes than those from continental arcs and tonalite-trondhjemite-granodiorite suites (TTGs). Hadean and Archean model melts are nearly indistinguishable from one another, both resembling TTGs and continental arcs, with pronounced depletion of HREE and slope reversal in heaviest REE. A limited number of samples > 4.25 Ga yield model melts with broadly similar characteristics to those from younger Hadean and Archean zircons, but with relatively elevated REE (~half order of magnitude) and higher LREE and MREE relative to HREE. Rare earth element patterns of early Earth model melts suggest a common petrogenetic history in the Hadean and Archean, involving garnet +/-amphibole in relatively low-temperature, high-pressure, environments.
Hadean
Early Earth
Rare-earth element
Felsic
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The close of the Hadean and opening of the so-called Archean eon is defined and characterized by the oldest whole rock samples found on Earth, 4.0 billion years old. At the opening of the Archean, Earth had an atmosphere rich in carbon dioxide, with perhaps some nitrogen and methane but little molecular oxygen, and liquid water was stable on its surface. Mantle convection had begun producing oceanic basalts and continental-type granitic rocks. The rate of impacts of asteroidal and cometary fragments had decreased significantly. The Moon, formed from Earth at the end of accretion some half billion years before, could be seen in the terrestrial sky.
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Early Earth
Abiogenesis
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Abstract Elucidating the compositions of melts from which Hadean zircons crystallized can provide insight into early crust construction. We calculated model melts using Ti-calibrated zircon/melt partition coefficients and trace element data for zircons from the Hadean, Archean, and possible analogue environments (e.g., rifts, hotspots, arcs) to constrain petrogenetic relationships. Model melts from oceanic settings (mid-ocean ridges, arcs, Iceland) showed higher heavy rare earth element (HREE) contents and shallower middle REE (MREE) to HREE/chondrite (ch) slopes than those from continental arcs and tonalite-trondhjemite-granodiorite suites (TTGs). However, Hadean and Archean model melts were consistently similar to each other and to those from continental arcs, hotspots, and TTGs (and dissimilar to oceanic settings), with depleted HREE contents and slope reversal in heaviest REEch. Despite close similarities that suggest comparable petrogenesis of Hadean and early Archean magmas from which Jack Hills detrital zircons crystallized, subtle variabilities in REEch and Zr/Hf suggest thickening crust and evolving igneous systems through time.
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Early Earth
Petrogenesis
Rare-earth element
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