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Here we report on findings for four rock samples with melt texture found in a gravel pit within a glaciofluvial deposit near the small town of Kitkiöjärvi in northernmost Sweden. The samples are comprised of granitic clasts embedded in a brown fine‐grained melt matrix. The samples all contain quartz grains and/or clasts exhibiting multiple sets of planar deformation features oriented parallel to crystallographic planes characteristic of shock metamorphism. The samples also contain Former Reidite In Granular Neoblastic (FRIGN) zircon. We therefore conclude that the investigated samples represent impact melt rock. We interpret a U‐Pb concordia age of 658.9 ± 6.9 Ma (Cryogenian) derived using secondary‐ion mass spectrometry analysis of shocked zircon, as the best estimate for the age of the impact event that formed the melt rocks. Zircon grains from two of the samples yield younger lower intercept ages, raising the possibility that the samples came from multiple impact events of different ages. Although we cannot exclude this possibility, we interpret the younger ages from the clast‐rich melt rocks to reflect non‐impact‐related Pb loss events and suggest that all samples likely came from the same structure. Analysis of the glaciofluvial history of the region, along with the relatively high frequency of finds (five in total, as one similar melt rock was found in the pit in 2018), points to a short‐distance glacial transportation of the samples from the southwest. Since there are no known impact structures in Sweden within that area and/or of similar age, we conclude that an old (the oldest known yet) impact structure in Sweden potentially is yet to be discovered somewhere in the vicinity of the gravel pit. [ABSTRACT FROM AUTHOR]

Introduction: Hypervelocity impacts induce extreme pressures and temperatures that are not normally produced by endogenic geological processes. Features created in response to these conditions can thus be used to infer the occurrence of a bolide impact and confirm a suspected impact structure (planar deformation features, PDFs, in quartz are perhaps the best known of these [1]). The mineral zircon (ZrSiO4) responds to extreme pressure-temperature (P-T) conditions in a variety of crystallographically controlled ways [2]. Given the use of zircon in studies of shock metamorphism, it is critical to understand which features in zircon are unique to hypervelocity impacts and which may form as a result of other extreme P-T excursions, such as lightning strikes. Additionally, analysis of deformation and phase transitions in zircon may shed light on the P-T conditions of such events. Previous studies of fulgurites (tubular bodies of glass and fused clasts that form in response to lightning) have noted zircon with rims of ZrO2 [3,4] and one study documented a Zr-rich patch of glass interpreted to represent a melted zircon grain [4]. With the aim to recognize features that form in response to extreme P-T excursions but are not unique to hypervelocity impacts, and to understand better the P-T conditions of these events, we undertook imaging and microstructural analysis of zircon and baddeleyite (monoclinic ZrO2) in a recently formed fulgurite. As an experimental control, we also imaged zircon from nearby soil and rock that was not exposed to the lightning strike. Samples: The fulgurite sample studied here formed as lightning struck vegetated soil in York County, Pennsylvania, USA, in 2004. The rock control sample collected near the fulgurite is a phyllite, consistent with local bedrock being mapped as metasedimentary rocks of the late Proterozoic or early Cambrian Harpers Formation [5]. The soil control sample contains mm-scale clasts of phyllite, supporting a local provenance. Methods: A portion of the fulgurite was mounted in epoxy and cut perpendicular to it exterior wall. This exposed an approximately circular cross-section 20 mm in diameter, which was polished with diamond suspension and colloidal silica. Imaging and microstructural characterization of zircon and baddeleyite by electron backscatter diffraction analysis were performed on an FEI Quanta FEG 650 scanning electron microscope. Microstructural data were processed in Oxford Instruments Channel 5 software and further data analysis was done in ARPGE software [6]. Results: Zircon grains from the control samples displayed a range of features, including xenotime rims and porosity. However, none displayed rims or inclusions of baddeleyite. Of the 91 Zr-rich grains identified in the fulgurite, five are composed entirely of vermicular baddeleyite. The other 86 are zircon grains, with 19 of these displaying granular baddeleyite and 29 having rims of vermicular baddeleyite. The abundance of baddeleyite in the zircon grains decreases with increasing distance from the fulgurite's central void, indicating that the baddeleyite formed in response to the lightning strike. Microstructural analysis of a rim of vermicular ZrO2 on a zircon grain revealed that it is composed of systematically oriented clusters of baddeleyite, consistent with cubic to monoclinic transformation and the former presence of cubic ZrO2 in the sample [7]. Cubic ZrO2 is a high-temperature phase (forming at T>2370°C at low pressure) and previously documented natural occurrences were associated with hypervelocity impacts [8-10]. Implications: Evidence for precursor cubic ZrO2 in baddeleyite rims on zircon grains in a fulgurite indicate that cubic ZrO2 - despite indicating extreme P-T conditions - is not unique to, or diagnostic of, hypervelocity impacts. Furthermore, the observation adds new constraints to the P-T conditions of lightning strikes. Comprehensive results and discussion are presented in [11]. Acknowledgments: This project received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Individual Fellowship Grant Agreement No. 792030, supporting Kenny. We thank Cyril Cayron for helpful discussions and support with ARPGE software. [ABSTRACT FROM AUTHOR]

Introduction: 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 [1]. However, available age constraints for the Hiawatha structure have only been tentative. The bedrock under the structure is likely a continuation of the highly metamorphosed 1.95-1.75 Ga [2] bedrock exposed immediately adjacent to Hiawatha Glacier, providing a maximum constraint on the structure's age [1]. Further age estimates have been based on indirect constraints, such as estimates of erosion rates and the structure's relationship to the Greenland Ice Sheet [1]. Here we dated the structure using detrital materials sampled immediately downstream thereof. We report 40Ar/39Ar analyses of impact-related glaciofluvial sand collected in 2016 and U-Pb analyses of shocked zircon from pebble-sized glaciofluvial clasts of impact melt rock collected in 2019. Results: Of the 50 grains that underwent 40Ar/39Ar step-heating analysis, 29 developed 'U'- or 'saddle'-shaped age spectra, with 23 giving minimum apparent ages in the Paleocene. Two mini-plateaus gave Late Paleocene ages of 58.5 ± 0.3 Ma and 60.2 ± 0.5 Ma, respectively. The 62 U-Pb analysis performed on 43 zircon grains using secondary ion mass spectrometry (SIMS) defined a discordant array in concordia space, spreading between an upper intercept of ~1915 Ma and a Late Paleocene lower intercept. The ~1915 Ma age for the majority of unshocked zircon grains from the impact melt rocks agrees with known geological events and zircon ages from the bedrock adjacent to Hiawatha Glacier [2], supporting a local origin for the samples. Of the 45 analyses performed on shocked zircon grains, the youngest nine data points are statistically indistinguishable, giving a robust concordia age and a weighted mean 206Pb/238U age of 57.99 ± 0.54 Ma (MSWD = 1.18; probability = 0.30; n = 9). We consider this to be the most robust estimate for the age of the impact structure [3]. Discussion: Impact melt rocks collected less than 10 km downstream from the edge of the Hiawatha structure contain unshocked zircon with ages matching that of the bedrock adjacent to Hiawatha Glacier. Shocked zircon in these samples gives a ~58 Ma U-Pb age, which agrees with the younger of two 40Ar/39Ar mini-plateau ages from the sand sample. The samples were thus derived from a Late Paleocene impact structure somewhere upstream but sufficiently proximal that the target geology is indistinguishable geochronologically from that exposed where the samples were recovered. Given existing geomorphic evidence for an eroded complex impact structure beneath Hiawatha Glacier, we suggest this is the source of the samples and thus that the Hiawatha structure is a relatively large impact structure that formed in the Late Paleocene. The new age indicates that the Hiawatha impact structure significantly predates Pleistocene glaciation and is unrelated to either the Paleocene-Eocene Thermal Maximum or flood basalt volcanism in east Greenland. The impact was contemporaneous with the Paleocene Carbon Isotope Maximum, although the impact's exact paleoenvironmental and climatic significance await further investigation. [ABSTRACT FROM AUTHOR]

Introduction: U-Pb dating of shocked zircon from impact melt rocks is a robust and proven method for determining the age of terrestrial impact structures. More recently, monazite alongside zircon has been used to date impact craters, such as the Araguainha impact structure, Brazil [1-2]. A critical methological advance has been documenting shock microstructures in monazite, for example microtwins and neoblasts, by electron backscatter diffraction (EBSD) analysis [3] and integrating this with U-Pb analysis by secondary ion mass spectrometry (SIMS) [2, 4-5]. Here we further explore the utility of monazite in dating impacts by analyzing shocked monazite from two impact melt rock samples from the Hiawatha impact crater, NW Greenland [6], that were both previously dated using in situ zircon U-Pb dating, revealing a Late Paleocene age for the crater at 57.99 ± 0.54 Ma [7]. Methods: Monazite grains were separated from two hemicrystalline clast-rich melt rocks collected proximal to the Hiawatha impact structure. Backscattered electron (BSE) imaging and microstructural characterization by EBSD were performed on an FEI Quanta FEG 650 scanning electron microscope (SEM) at the Swedish Museum of Natural History, Stockholm. The grains were then analyzed for U-Pb isotopic composition and age by SIMS on the CAMECA IMS1280 ion microprobe at the NordSIMS Laboratory, also at the Swedish Museum of Natural History. We applied a ~6 µm spot diameter, the smallest U-Pb analytical spot yet applied to shocked monazite, almost half the size of previous analyses performed on shocked monazite (~10 µm; [2, 4-5]), enabled by the recent installation of a Hyperion H201 RF plasma high-brightness oxygen source. Results: Microstructural mapping of shocked monazite by EBSD revealed various shock deformation features, including shock microtwins and neoblasts. We also report monazite grains rich in densely packed sub-micrometer pores. To the best of our knowledge, similar porous grains have only been reported at one other impact structure [5] and never in the absence of other features of shock deformation, i.e., neoblasts or microtwins. In situ U-Pb analyses of porous and neoblastic domains define an impact-related discordant array, consistent with a Late Paleocene impact into Paleoproterozoic bedrock. The upper intercept age is within error of known ages of local bedrock in Inglefield Land, immediately adjacent to the structure (~1915 Ma) [8], despite no unshocked grains having been analyzed. The full discordant array, including granular and porous monazite grains, gives a lower intercept age of 63 ± 25 Ma, within uncertainty of the 57.99 ± 0.54 Ma age of the structure [7]. Targeted analyses on neoblasts plot closer to the lower intercept, giving a more precise lower intercept age of 73 ± 15 Ma, whereas analyses on porous monazite grains give a less precise, but still accurate, lower intercept age of 59 ± 36 Ma. Discussion: In-situ U-Pb dating of shocked monazite grains here provides valuable insights into dating impact structures. Recrystallized domains of monazite grains show the greatest Pb loss, giving ages closest to concordant ages of previously dated shocked zircon [7]. However, while porous monazite shows less complete Pb loss, when analyzed alone they still give accurate impact ages. Therefore, we suggest that future studies should also consider porous monazite a promising target in dating impacts, especially when neoblastic monazite has not been found. [ABSTRACT FROM AUTHOR]