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| 2. |
- Kenny, Gavin, et al.
(författare)
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Age of the Sääksjärvi impact structure, Finland: reconciling the timing of small impacts in crystalline basement with regional basin development
- 2020
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Ingår i: Journal of the Geological Society. - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 177, s. 1231-1243
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Tidskriftsartikel (refereegranskat)abstract
- We report a new age for the Sääksjärvi impact structure, Finland, a 6 km diameter feature that formed in crystalline rocks of the Precambrian Baltic Shield. Two previous studies reported 40Ar/39Ar data for Sääksjärvi and suggested conflicting formation ages of ≤330 Ma or c. 560 Ma. The former age represents a possible complication for models which indicate that the region was covered by sediments of the Caledonian foreland basin throughout much of the Phanerozoic. We conducted a study combining imaging, microstructural analysis and U–Pb dating of shocked zircon from Sääksjärvi. The U–Pb dataset indicates a c. 600 Ma impact into predominantly c. 1850 Ma target rocks. A concordia age of 608 ± 8 Ma (2σ) confirms Sääksjärvi as the first known Ediacaran impact structure in the Baltic Shield and only the second worldwide. Our data indicate that the Sääksjärvi impact structure formed in exposed crystalline basement rocks of the Baltic Shield prior to the development of the Caledonian foreland basin. Given that most impact structures on Earth are relatively small features, radiometric dating of small impact structures in crystalline basement may place boundaries on the timing and spatial extent of palaeobasins that might otherwise be difficult to constrain.
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| 3. |
- Kenny, Gavin, et al.
(författare)
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Recrystallization and chemical changes in apatite in response to hypervelocity impact
- 2020
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 48:1, s. 19-23
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Tidskriftsartikel (refereegranskat)abstract
- Despite the wide utility of apatite, Ca5(PO4)3(F,Cl,OH), in the geosciences, including tracing volatile abundances on the Moon and Mars, little is known about how the mineral responds to the extreme temperatures and pressures associated with hypervelocity impacts. To address this deficiency, we here present the first microstructural analysis and chemical mapping of shocked apatite from a terrestrial impact crater. Apatite grains from the Paasselkä impact structure, Finland, display intragrain crystal-plastic deformation as well as pervasive recrystallization—the first such report in terrestrial apatite. A partially recrystallized grain offers the opportunity to investigate the effect of shock recrystallization on the chemical composition of apatite. The recrystallized portion of the fluorapatite grain is depleted in Mg and Fe relative to the remnant non-recrystallized domain. Strikingly, the recrystallized region alone hosts inclusions of (Mg,Fe)2(PO4)F, wagnerite or a polymorph thereof. These are interpreted to be a product of phase separation during recrystallization and to be related to the reduced abundances of certain elements in the recrystallized domain. The shock-induced recrystallization of apatite, which we show to be related to changes in the mineral’s chemical composition, is not always readily visible in traditional imaging techniques (such as backscattered electron imaging of polished interior surfaces), thus highlighting the need for correlated microstructural, chemical, and isotopic studies of phosphates. This is particularly relevant for extraterrestrial phosphates that may have been exposed to impacts, and we urge the consideration of microstructural data in the interpretation of the primary or secondary nature of elemental abundances and isotopic compositions.
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| 5. |
- Gnos, Edwin, et al.
(författare)
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Ash Shutbah: A possible impact structure in Saudi Arabia
- 2014
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Ingår i: Meteoritics and Planetary Science. - : Wiley. - 1086-9379. ; 49:10, s. 1902-1914
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the Ash Shutbah circular structure in central Saudi Arabia (21 degrees 37N 45 degrees 39E) using satellite imagery, field mapping, thin-section petrography, and X-ray diffraction of collected samples. The approximately 2.1km sized structure located in flat-lying Jurassic Tuwaiq Mountain Limestone has been nearly peneplained by erosional processes. Satellite and structural data show a central area consisting of Dhruma Formation sandstones with steep bedding and tight folds plunging radially outward. Open folding occurs in displaced, younger Tuwaiq Mountain Limestone Formation blocks surrounding the central area, but is absent outside the circular structure. An approximately 60cm thick, unique folded and disrupted orthoquartzitic sandstone marker bed occurring in the central area of the structure is found 140m deeper in undisturbed escarpment outcrops located a few hundred meters west of the structure. With exception of a possible concave shatter cone found in the orthoquartzite of the central area, other diagnostic shock features are lacking. Some quartz-rich sandstones from the central area show pervasive fracturing of quartz grains with common concussion fractures. This deformation was followed by an event of quartz dissolution and calcite precipitation consistent with local sea- or groundwater heating. The combination of central stratigraphic uplift of 140m, concussion features in discolored sandstone, outward-dipping concentric folds in the central area, deformation restricted to the rocks of the ring structure, a complex circular structure of 2.1km diameter that appears broadly consistent with what one would expect from an impact structure in sedimentary targets, and a possible shatter cone all point to an impact origin of the Ash Shutbah structure. In fact, the Ash Shutbah structure appears to be a textbook example of an eroded, complex impact crater located in flat-lying sedimentary rocks, where the undisturbed stratigraphic section can be studied in escarpment outcrops in the vicinity of the structure.
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| 6. |
- Goderis, Steven, et al.
(författare)
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Globally distributed iridium layer preserved within the Chicxulub impact structure
- 2021
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Ingår i: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 7:9
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Tidskriftsartikel (refereegranskat)abstract
- The Cretaceous-Paleogene (K-Pg) mass extinction is marked globally by elevated concentrations of iridium, emplaced by a hypervelocity impact event 66 million years ago. Here, we report new data from four independent laboratories that reveal a positive iridium anomaly within the peak-ring sequence of the Chicxulub impact structure, in drill core recovered by IODP-ICDP Expedition 364. The highest concentration of ultrafine meteoritic matter occurs in the post-impact sediments that cover the crater peak ring, just below the lowermost Danian pelagic limestone. Within years to decades after the impact event, this part of the Chicxulub impact basin returned to a relatively low-energy depositional environment, recording in unprecedented detail the recovery of life during the succeeding millennia. The iridium layer provides a key temporal horizon precisely linking Chicxulub to K-Pg boundary sections worldwide.
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