The Earth's Early Evolution
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The Archean crust contains direct geochemical information of the Earth's early planetary differentiation. A major outstanding question in the Earth sciences is whether the volume of continental crust today represents nearly all that formed over Earth's history or whether its rates of creation and destruction have been approximately balanced since the Archean. Analysis of neodymium isotopic data from the oldest remnants of Archean crust suggests that crustal recycling is important and that preserved continental crust comprises fragments of crust that escaped recycling. Furthermore, the data suggest that the isotopic evolution of Earth's mantle reflects progressive eradication of primordial heterogeneities related to early differentiation.Keywords:
Hadean
Early Earth
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...
Hadean
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 Archean crust contains direct geochemical information of the Earth's early planetary differentiation. A major outstanding question in the Earth sciences is whether the volume of continental crust today represents nearly all that formed over Earth's history or whether its rates of creation and destruction have been approximately balanced since the Archean. Analysis of neodymium isotopic data from the oldest remnants of Archean crust suggests that crustal recycling is important and that preserved continental crust comprises fragments of crust that escaped recycling. Furthermore, the data suggest that the isotopic evolution of Earth's mantle reflects progressive eradication of primordial heterogeneities related to early differentiation.
Hadean
Early Earth
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Hadean
Early Earth
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Significance The crust or its chemically weathered derivatives likely served as a substrate for the origin of life, which could have occurred by 4.1 Ga. Yet no known bona fide terrestrial rocks from this time remain. Studies have thus turned to geochemical signatures within detrital zircons from this time. While zircons do not directly record low-temperature weathering processes, they inherit isotopic information upon recycling and remelting of sediment. We developed a method to fingerprint the identity of material involved in water–rock interactions >4 Ga, bolstered by a large Si and O isotopic dataset of more modern zircon samples. The data presented here provide evidence for chemical sediments, such as cherts and banded iron formations on Earth >4 Ga.
Hadean
Early Earth
Geologic record
Abiogenesis
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Our planet is some 4540 million years old. There is little record of Earth's history for the first half billion years, but rocks have been found in Canada that date back some 4000 million years. The record of life on Earth goes back much further in time than the Cambrian Period, perhaps nearly as far as the record of the rocks. Earth's history is enormous from a human perspective, and has been divided up into more manageable packets of time, comprising four eons, the Hadean, the Archean, the Proterozoic, and the Phanerozoic; the Hadean, Archean, and Proterozoic are jointly termed the Precambrian. If we accept the combined evidence from microfossils, microbial mats, stromatolites and carbon isotopes, then it appears that life may have begun on Earth some 3500 million years ago, or possibly somewhat earlier, and that these life forms included microorganisms that could generate their own energy by chemoor photosynthetic processes.
Hadean
Early Earth
Geologic record
Deep time
Geologic time scale
Stromatolite
Earth history
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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.
Hadean
Early Earth
Petrogenesis
Rare-earth element
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Hydration of komatiite can be a source of significant amount of hydrogen in the Hadean and early Archean ocean floor. We report the first direct evidence for this process based on the results of our hydrothermal alteration experiments on the synthetic komatiitic glass at 300°C and 500 bars. Komatiitic glass was synthesized by dehydration and remelting of serpentinized Al-depleted komatiite collected from the early Archean Komati Formation, the Barberton Greenstone Belt, South Africa. Though the run is still continuing in our lab, the observed accumulation of hydrogen (2.4 mmol/kg) over 2,600 hours is truly significant and is comparable to those observed by hydration of peridotitic rocks. Our results suggest that hydrothermal alteration of komatiites may have provided the source of H2 in the vicinity of hydrothermal vents that fueled the early evolution of living ecosystems in the Hadean and early Archean.
Hadean
Early Earth
Greenstone belt
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