| 1. |
- Kenny, Gavin G., et al.
(författare)
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A Late Paleocene age for Greenland’s Hiawatha impact structure
- 2022
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Ingår i: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 8:10
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Tidskriftsartikel (refereegranskat)abstract
- The ~31-km-wide Hiawatha structure, located beneath Hiawatha Glacier in northwestern Greenland, has been proposed as an impact structure that may have formed after the Pleistocene inception of the Greenland Ice Sheet. To date the structure, we conducted 40Ar/39Ar analyses on glaciofluvial sand and U-Pb analyses on zircon separated from glaciofluvial pebbles of impact melt rock, all sampled immediately downstream of Hiawatha Glacier. Unshocked zircon in the impact melt rocks dates to ~1915 million years (Ma), consistent with felsic intrusions found in local bedrock. The 40Ar/39Ar data indicate Late Paleocene resetting and shocked zircon dates to 57.99 ± 0.54 Ma, which we interpret as the impact age. Consequently, the Hiawatha impact structure far predates Pleistocene glaciation and is unrelated to either the Paleocene-Eocene Thermal Maximum or flood basalt volcanism in east Greenland. However, it was contemporaneous with the Paleocene Carbon Isotope Maximum, although the impact’s exact paleoenvironmental and climatic significance awaits further investigation.
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| 2. |
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| 3. |
- Garde, Adam A., et al.
(författare)
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The Finnefjeld domain, Maniitsoq structure, West Greenland: Differential rheological features and mechanical homogenisation in response to impacting?
- 2014
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268. ; 255, s. 791-808
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Tidskriftsartikel (refereegranskat)abstract
- The 35 by 50 km large, Mesoarchaean Finnefjeld domain near Maniitsoq in the North Atlantic craton of southern West Greenland constitutes the central part of the previously proposed, deeply eroded Maniitsoq impact structure with an age of 3.0 Ga. The Finnefjeld domain is an exceedingly homogeneous, quartzo-feldspathic rock mass which superficially appears to be a late-orogenic, deep-crustal, intrusive granitoid pluton, and which was described as such for decades. However, new observations confirm and qualify the first observations from 1962 of these rocks as 'cataclastic'. The Finnefjeld domain is characterised by a highly unusual, mixed rheological behaviour. Plagioclase displays brittle behaviour with cataclasis, quartz was ductilely deformed, and K-feldspar was melted. The deformation and homogenisation of the Finnefjeld domain was caused by an intense event of heating and deformation, which was coseismic in nature and comprised numerous increments of pure shear strain. This type of intense, brittle, regional deformation and concomitant direct mineral melting in the deep crust is unknown from endogenic orogenic events and is ascribed to deep-crustal effects of impacting. (C) 2014 Elsevier B.V. All rights reserved.
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| 4. |
- Hyde, William R., et al.
(författare)
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Microstructural and isotopic analysis of shocked monazite from the Hiawatha impact structure : development of porosity and its utility in dating impact craters
- 2024
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Ingår i: Contributions to Mineralogy and Petrology. - 0010-7999. ; 179:3
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Tidskriftsartikel (refereegranskat)abstract
- U–Pb geochronology of shocked monazite can be used to date hypervelocity impact events. Impact-induced recrystallisation and formation of mechanical twins in monazite have been shown to result in radiogenic Pb loss and thus constrain impact ages. However, little is known about the effect of porosity on the U–Pb system in shocked monazite. Here we investigate monazite in two impact melt rocks from the Hiawatha impact structure, Greenland by means of nano- and micrometre-scale techniques. Microstructural characterisation by scanning electron and transmission electron microscopy imaging and electron backscatter diffraction reveals shock recrystallisation, microtwins and the development of widespread micrometre- to nanometre-scale porosity. For the first time in shocked monazite, nanophases identified as cubic Pb, Pb3O4, and cerussite (PbCO3) were observed. We also find evidence for interaction with impact melt and fluids, with the formation of micrometre-scale melt-bearing channels, and the precipitation of the Pb-rich nanophases by dissolution–precipitation reactions involving pre-existing Pb-rich high-density clusters. To shed light on the response of monazite to shock metamorphism, high-spatial-resolution U–Pb dating by secondary ion mass spectrometry was completed. Recrystallised grains show the most advanced Pb loss, and together with porous grains yield concordia intercept ages within uncertainty of the previously established zircon U–Pb impact age attributed to the Hiawatha impact structure. Although porous grains alone yielded a less precise age, they are demonstrably useful in constraining impact ages. Observed relatively old apparent ages can be explained by significant retention of radiogenic lead in the form of widespread Pb nanophases. Lastly, we demonstrate that porous monazite is a valuable microtexture to search for when attempting to date poorly constrained impact structures, especially when shocked zircon or recrystallised monazite grains are not present.
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| 5. |
- Scherstén, Anders, et al.
(författare)
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Complete hydrothermal re-equilibration of zircon in the Maniitsoq structure, West Greenland: A 3001Ma minimum age of impact?
- 2013
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Ingår i: Meteoritics and Planetary Science. - : Wiley. - 1086-9379. ; 48:8, s. 1472-1498
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Tidskriftsartikel (refereegranskat)abstract
- Zircon in five samples of variably comminuted, melted, and hydrothermally altered orthogneiss from the Maniitsoq structure of southern West Greenland yield a weighted mean 207Pb/206Pb age of 3000.9 +/- 1.9Ma (ion probe data, n=37). The age data constitute a rare example of pervasive and nearly complete isotopic resetting of zircon during a regional hydrothermal event. Many zircon grains are homogeneous or display weak flame-like patterns in backscattered electron images. Other grains show complex internal textures, where homogeneous, high-U fronts commonly cut across relict igneous-type oscillatory zonation. Inclusions of quartz, plagioclase, mica, and other Al +/- Na +/- Ca +/- Fe-bearing silicates are very common. In two samples, selective replacement of zircon with baddeleyite occurs along concentric zones with relict igneous zonation, and as specks a few microns large within recrystallized, high-U areas. We interpret the 3000.9 +/- 1.9Ma date as the minimum age of the recently proposed impact structure at Maniitsoq. The great geographical extent and intensity of the hydrothermal event suggest massive invasion of water into the currently exposed crust, implying that the age of the hydrothermal alteration would closely approximate the age of the proposed impact at Maniitsoq. At the western margin of the Taserssuaq tonalite complex, which postdates the Maniitsoq event, a 207Pb/206Pb mean age of 2994.6 +/- 3.4Ma obtained from zircon has mostly retained igneous-type oscillatory zonation. A subsequent thermal event at approximately 2975Ma is recorded in several samples by zircon with baddeleyite replacement textures.
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