Variable microstructural response of baddeleyite to shock metamorphism in young basaltic shergottite NWA 5298 and improved U–Pb dating of Solar System events
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Baddeleyite
Shock metamorphism
Impact structure
Seismic profiles, drill core samples, and gravity data suggest that a complex impact crater ∼35.5 million years old and 90 kilometers in diameter is buried beneath the lower Chesapeake Bay. The breccia that fills the structure contains evidence of shock metamorphism, including impact melt breccias and multiple sets of planar deformation features (shock lamellae) in quartz and feldspar. The age of the crater and the composition of some breccia clasts are consistent with the Chesapeake Bay impact structure being the source of the North American tektites.
Breccia
Impact structure
Shock metamorphism
Chesapeake bay
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MAJOR objectives of this study have been to define the character of the igneous rocks present; to determine the number of separate igneous complexes, their relative ages, their variations, the relationships of these variations to structure, and the interpretation of their origin; to elucidate the mechanics of intrusion; and to discriminate primary magmatic structures from postconsolidation metamorphic features and to describe and interpret the mineralogic facies as a result of metamorphism.
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Gow Lake, in the Precambrian Shield of Saskatchewan, is circular, 4 km in diameter, and has a large central island. Granites and quartzofeldspathic gneisses are exposed around the perimeter of the lake, whereas the island is formed largely of brecciated equivalents. Most of the breccias are composed entirely of clastic material, but at one locality fine-grained felted matrices form a significant component of the breccias, and coronas of clear glass surround quartz grains. The latter breccias also contain microscopic features characteristic of shock metamorphism, among which multiple sets of planar deformation structures in quartz are particularly diagnostic. Similar shock metamorphic features have been widely reported from terrestrial meteorite craters; accordingly, Gow Lake is interpreted as a deeply eroded impact crater and the felted matrices as impact melts.A local negative gravity anomaly with an amplitude of 3 mGal centered on the lake is attributed mainly to highly fractured basement rocks underlying the lake, which model studies indicate may extend to a depth of 900 m. A provisional minimum age of 100 Ma is proposed for the crater.
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Shock metamorphism
Impact structure
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Chemical and isotopic signatures recorded by the accessory phase baddeleyite (ZrO2) yield important insights into the formation and evolution of mafic planetary crusts. However, little work has been conducted regarding the effects of structures on the mobilization and diffusion of substitutional and interstitial ions. Coupled nanometer-scale analyses of chemistry and structure in mineral phases are possible using the emerging technique of atom probe tomography (APT). Here we use this technique to describe a range of complex chemical nanostructures within shocked, annealed, and metamorphosed baddeleyite grains sampled in crater floor rocks ~550 m away from the contact with the Sudbury impact melt sheet. This has revealed a wide range of nanostructural phenomena, including domains of clustered incompatible cations (Fe), separated by subgrain boundaries or planar features exhibiting wave-like features decorated with trace amounts of Al, Si, and Fe likely generated by shock metamorphism. In some cases, these nanostructures have facilitated much later, and highly localized, postimpact Pb loss and Si gain ascribed to regional greenschist metamorphism. Characterizing nanoscale heterogeneities within complex, shocked baddeleyite grains using APT for resolution of different deformation pathways and a more confident interpretation of the geologic significance of micron-scale trace element and isotopic analyses.
Baddeleyite
Shock metamorphism
Atom probe
Trace element
Greenschist
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Baddeleyite
Large igneous province
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Abstract The Målingen structure in Sweden has for a long time been suspected to be the result of an impact; however, no hard evidence, i.e., shock metamorphic features or traces of the impactor, has so far been presented. Here we show that quartz grains displaying planar deformation features ( PDF s) oriented along crystallographic planes typical for shock metamorphism are present in drill core samples from the structure. The shocked material was recovered from basement breccias, below the sediment infill, and the distribution of the orientation of the shock‐produced PDF s indicates that the studied material experienced low shock pressures. Based on our findings, we can exclude that the material is transported from the nearby Lockne impact structure, which means that the Målingen structure is a separate impact structure, the seventh confirmed impact structure in Sweden. Furthermore, sedimentological and biostratigraphic aspects of the deposits that fill the depression at Målingen are very similar to features at the Lockne impact structure. This implies a coeval formation age and thus also the confirmation of the first known marine target doublet impact craters on Earth (i.e., the Lockne–Målingen pair).
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Abstract The Kamenetsk impact structure is a deeply eroded simple crater that formed in crystalline rocks of the Ukrainian Shield. This study presents structural, lithologic, and shock metamorphic evidence for an impact origin of the Kamenetsk structure, which was previously described as a paleovolcano. The Kamenetsk structure is an oval depression that is 1.0–1.2 km in diameter and 130 m deep. The structure is deeply eroded, and only the lower part of the sequence of lithic breccia has been preserved in the deepest part of the crater to recent time, while the predominant part of impact rocks and postimpact sediments was eroded. Manifestations of shock metamorphism of minerals, especially planar deformation features in quartz and feldspars, were determined by petrographic investigations of lithic breccia that allowed us to determine the impact origin of the Kamenetsk structure. The erosion of the crater and surrounding target to a minimal depth of 220 m preceded the deposition of the postimpact sediments. The time of the formation of the Kamenetsk structure is bracketed within a wide interval from 2.0 to 2.1 Ga, the age of the crystalline target rocks, to the Late Miocene age of the sediments overlaying the crater. The deep erosion of the structure suggests it is probably Paleozoic in age.
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Shock metamorphism
Breccia
Lithology
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Five Cretaceous alkaline-carbonatite igneous complexes are reported from the Assam-Meghalaya Plateau, These alkaline intrusions have been interpreted to be coeval and associated with the 117-105 Ma Rajmahal-Sylhet flood basalt province. With the existing age information it is possible that this alkaline magmatism may be a late magmatic stage of the Rajmahal-Sylhet large igneolls province. Therefore, it is essential to determine high.precision ages for these alkaline complexes in order to understand the detailed temporal evolution and genesis of this basaltic and alkaline magmatism. Out of five igneous complexes, Sung Valley, Swangkre and Samchampi have been dated, but tbe emplacement ages of the other two, i.e, Jasra and Barpung, are poorly constrained. The present communication reports a new, high precision U-Pb zircon/baddeleyite age for a djfferentiated portion of gabbro phase of the Jasra igneous complex.
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Carbonatite
Flood basalt
Large igneous province
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Abstract Meteorite impact processes are ubiquitous on the surfaces of rocky and icy bodies in the Solar System, including the Moon. One of the most common accessory minerals, zircon, when shocked, produces specific micro-structures that may become indicative of the age and shock conditions of these impact processes. To better understand the shock mechanisms in zircon from Apollo 15 and 16 impact breccias, we applied transmission electron microscopy (TEM) and studied nano-structures in eight lunar zircons displaying four different morphologies from breccias 15455, 67915, and 67955. Our observations revealed a range of shock-related features in zircon: (1) planar and non-planar fractures, (2) “columnar” zircon rims around baddeleyite cores, (3) granular textured zircon, in most cases with sub-µm-size inclusions of monoclinic ZrO 2 (baddeleyite) and cubic ZrO 2 (zirconia), (4) silica-rich glass and metal inclusions of FeS and FeNi present at triple junctions in granular zircon and in baddeleyite, (5) inclusions of rutile in shocked baddeleyite, (6) amorphous domains, (7) recrystallized domains. In many grain aggregates, shock-related micro-structures overprint each other, indicating either different stages of a single impact process or multiple impact events. During shock, some zircons were transformed to diaplectic glass (6), and others (7) were completely decomposed into SiO 2 and Zr-oxide, evident from the observed round shapes of cubic zirconia and silica-rich glass filling triple junctions of zircon granules. Despite the highly variable effect on textures and Zr phases, shock-related features show no correlation with relatively homogeneous U–Pb or 207 Pb/ 206 Pb ages of zircons. Either the shock events occurred very soon after the solidification or recrystallization of the different Zr phases, or the shock events were too brief to result in noticeable Pb loss during shock metamorphism.
Baddeleyite
Impact structure
Breccia
Shock metamorphism
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