149 research outputs found
Shocked quartz grains in the polymict breccia of the Granby structure, Sweden-Verification of an impact
No full textThe Middle Ordovician Granby structure in Sweden is generally considered the result of an asteroidal or cometary collision with Earth, although no hard evidence, i.e., shock metamorphic features or traces of the impactor, have been presented to date. In this Study, drill core samples of a sedimentary breccia from the Granby structure have been investigated for microscopic shock metamorphic evidence in an attempt to verify the impact genesis of the Structure. The finding of multiple sets of decorated planar deformation features (PDFs) in quartz grains in these samples provides unambiguous evidence that the structure is impact derived. Furthermore, the orientation of the PDFs, e.g., omega{101 (3) over bar}, pi{101 (2) over bar} and r, z {101 (1) over bar}, is characteristic for impact deformation. The fact that a majority of the PDFs are decorated implies a water-bearing target. The shocked quartz grains can be divided into two groups; rounded grains found in the breccia matrix likely originated from mature sandstone,, and angular grains in fragments from crystalline target rocks. The absence of melt particles provides ail estimated maximum shock pressure for the sedimentary derived quartz of 15-20 GPa and the frequency distribution of PDF orientations in the bedrock quartz implies pressures of the order of 10 GPa
Shocked quartz grains in the polymict breccia of the Granby structure, SwedenVerification of an impact .
No full textThe Middle Ordovician Granby structure in Sweden is generally considered the result of an asteroidal or cometary collision with Earth, although no hard evidence, i.e., shock metamorphic features or traces of the impactor, have been presented to date. In this study, drill core samples of a sedimentary breccia from the Granby structure have been investigated for microscopic shock metamorphic evidence in an attempt to verify the impact genesis of the structure. The finding of multiple sets of decorated planar deformation features (PDFs) in quartz grains in these samples provides unambiguous evidence that the structure is impact derived. Furthermore, the orientation of the PDFs, e.g., {1013}, {1012} and r, z {1011}, is characteristic for impact deformation. The fact that a majority of the PDFs are decorated implies a water-bearing target. The shocked quartz grains can be divided into two groups; rounded grains found in the breccia matrix likely originated from mature sandstone, and angular grains in fragments from crystalline target rocks. The absence of melt particles provides an estimated maximum shock pressure for the sedimentary derived quartz of 15-20 GPa and the frequency distribution of PDF orientations in the bedrock quartz implies pressures of the order of 10 GPa
The Wabar impact craters, Saudi Arabia, revisited
No full textThe very young Wabar craters formed by impact of an iron meteorite and are known to the scientific community since 1933. We describe field observations made during a visit to the Wabar impact site, provide analytical data on the material collected, and combine these data with poorly known information discovered during the recovery of the largest meteorites. During our visit in March 2008, only two craters (Philby-B and 11 m) were visible; Philby-A was completely covered by sand. Mapping of the ejecta field showed that the outcrops are strongly changing over time. Combining information from different visitors with our own and satellite images, we estimate that the large seif dunes over the impact site migrate by approximately 1.0–2.0 m yr␣1 southward. Shock lithification took place even at the smallest, 11 m crater, but planar fractures (PFs) and undecorated planar deformation features (PDFs), as well as coesite and stishovite, have only been found in shock-lithified material from the two larger craters. Shock-lithified dune sand material shows perfectly preserved sedimentary structures including cross-bedding and animal burrows as well as postimpact structures such as open fractures perpendicular to the bedding, slickensides, and radiating striation resembling shatter cones. The composition of all impact melt glasses can be explained as mixtures of aeolian sand and iron meteorite. We observed a partial decoupling of Fe and Ni in the black impact glass, probably due to partitioning of Ni into unoxidized metal droplets. The absence of a Ca-enriched component demonstrates that the craters did not penetrate the bedrock below the sand sheet, which has an estimated thickness of 20–30 m
The origin of the Brunflo fossil meteorite and extraterrestrial chromite in mid-Ordovician limestone from the Gärde quarry (Jämtland, central Sweden)
No full textThe Brunflo fossil meteorite was found in the 1950s in mid-Ordovician marine limestone in the Grde quarry in Jmtland. It originates from strata that are about 5 million years younger than similar limestone that more recently has yielded 50 fossil meteorites in the Thorsberg quarry at Kinnekulle, 600 km to the south. Based primarily on the low TiO2 content (about 1.8 wt%) of its relict chromite the Brunflo meteorite had been tentatively classified as an H chondrite. The meteorite hence appears to be an anomaly in relation to the Kinnekulle meteorites, in which chromite composition, chondrule mean diameter and oxygen isotopic composition all indicate an L-chondritic origin, reflecting an enhanced flux of meteorites to Earth following the disruption of the L chondrite parent body 470 Ma. New chondrule-size measurements for the Brunflo meteorite indicate that it too is an L chondrite, related to the same parent-body breakup. Chromite maximum diameters and well-defined chondrule structures further show that Brunflo belongs to the L4 or L5 type. Chromites in recently fallen L4 chondrites commonly have low TiO2 contents similar to the Brunflo chromites, adding support for Brunflo being an L4 chondrite. The limestone in the Grde quarry is relatively rich (about 0.45 grain kg^(-1)) in sediment-dispersed extraterrestrial chromite grains (63 m) with chemical composition similar to those in L chondrites and the limestone (1-3 grains kg^(-1)) at Kinnekulle, suggesting that the enhanced flux of L chondrites prevailed, although somewhat diminished, at the time when the Brunflo meteorite fell
Microstructural Characterisation of Ilmenite (FeTiO3) from Melt-Bearing Impactites and Impact Ejecta.
No full textThe origin of the Brunflo fossil meteorite and extraterrestrial chromite in mid-Ordovician limestone from the Gärde quarry (Jämtland, central Sweden)
No full textThe Brunflo fossil meteorite was found in the 1950s in mid-Ordovician marine limestone in the Grde quarry in Jmtland. It originates from strata that are about 5 million years younger than similar limestone that more recently has yielded >50 fossil meteorites in the Thorsberg quarry at Kinnekulle, 600 km to the south. Based primarily on the low TiO2 content (about 1.8 wt%) of its relict chromite the Brunflo meteorite had been tentatively classified as an H chondrite. The meteorite hence appears to be an anomaly in relation to the Kinnekulle meteorites, in which chromite composition, chondrule mean diameter and oxygen isotopic composition all indicate an L-chondritic origin, reflecting an enhanced flux of meteorites to Earth following the disruption of the L chondrite parent body 470 Ma. New chondrule-size measurements for the Brunflo meteorite indicate that it too is an L chondrite, related to the same parent-body breakup. Chromite maximum diameters and well-defined chondrule structures further show that Brunflo belongs to the L4 or L5 type. Chromites in recently fallen L4 chondrites commonly have low TiO2 contents similar to the Brunflo chromites, adding support for Brunflo being an L4 chondrite. The limestone in the Grde quarry is relatively rich (about 0.45 grain kg^(-1)) in sediment-dispersed extraterrestrial chromite grains (63 m) with chemical composition similar to those in L chondrites and the limestone (1-3 grains kg^(-1)) at Kinnekulle, suggesting that the enhanced flux of L chondrites prevailed, although somewhat diminished, at the time when the Brunflo meteorite fell.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202
Estimating average shock pressures recorded by impactite samples based on universal stage investigations of planar deformation features in quartz-Sources of error and recommendations
No full textPlanar deformation features (PDFs) in quartz are the most widely used indicator of shock metamorphism in terrestrial rocks. They can also be used for estimating average shock pressures that quartz-bearing rocks have been subjected to. Here we report on a number of observations and problems that we have encountered when performing universal stage measurements and crystallographically indexing of PDF orientations in quartz. These include a comparison between manual and automated methods of indexing PDFs, an evaluation of the new stereographic projection template, and observations regarding the PDF statistics related to the c-axis position and rhombohedral plane symmetry. We further discuss the implications that our findings have for shock barometry studies. Our study shows that the currently used stereographic projection template for indexing PDFs in quartz might induce an overestimation of rhombohedral planes with low Miller-Bravais indices. We suggest, based on a comparison of different shock barometry methods, that a unified method of assigning shock pressures to samples based on PDFs in quartz is necessary to allow comparison of data sets. This method needs to take into account not only the average number of PDF sets/grain but also the number of high Miller-Bravais index planes, both of which are important factors according to our study. Finally, we present a suggestion for such a method (which is valid for nonporous quartz-bearing rock types), which consists of assigning quartz grains into types (A-E) based on the PDF orientation pattern, and then calculation of a mean shock pressure for each sample
Further evidence for an impact origin of the Tsenkher structure in the Gobi-Altai, Mongolia : Geology of a 3.7 km crater with a well-preserved ejecta blanket
No full textThe Tsenkher structure in the Gobi-Altai, Mongolia is a c. 3.7 km diameter crater with a well-preserved ejecta blanket. It has been hypothesized to be either of impact or volcanic origin in our previous work. Observations during our 2007 expedition and related sample analyses give further support for an impact origin. The evidence includes the presence of a structurally uplifted near-circular rim surrounded by an ejecta blanket, and abundant breccias, some of which are melt- A nd millimetre-scale spherule-bearing. Planar deformation features (PDFs) were found in one quartz grain in a breccia sample. Fe-rich grains are found in a vesicular melt sample that is also characterized by elevated platinum group element (PGE) abundances with respect to the sedimentary bedrock of the area (approximately an order of magnitude). Noble gas analysis of one breccia sample yielded an elevated 3He/4He value of (5.0±0.2) × 10-6. Although not conclusive alone, these geochemical results are consistent with a contribution of meteoritic components. A volcanic origin, in particular a maar formation, would require explanations for the unusual conditions associated with Tsenkher, including its large size occurring in isolation, the structurally uplifted rim and the lack of a bedded base surge deposit. A pronounced rampart structure observed at the eastern ejecta is also unusual for any volcanic origin. 40Ar-39Ar dating of a vesicular melt sample gives an age of the Tsenkher structure of 4.9±0.9 Ma. The rampart structure could provide insights into the formation of similar ejecta morphologies associated with numerous impact craters on Mars
Shocked quartz grains from the Malingen structure, Sweden-Evidence for a twin crater of the Lockne impact structure
No full textThe Malingen 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 (PDFs) 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 PDFs 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 Malingen 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 Malingen 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-Malingen pair)
Relict silicate inclusions in extraterrestrial chromite and their use in the classification of fossil chondritic material
No full textChromite is the only common meteoritic mineral surviving long-term exposure on Earth, however, the present study of relict chromite from numerous Ordovician (470 Ma) fossil meteorites and micrometeorites from Sweden, reveals that when encapsulated in chromite, other minerals can survive for hundreds of millions of years maintaining their primary composition. The most common minerals identified, in the form of small (1.5 mu m) in chromite from the Ordovician fossil chondritic material plot within the L-chondrite field, which is in accordance with previous classifications. The concordance in classification together with the fact that inclusions are relatively common makes them an accurate and useful tool in the classification of extraterrestrial material that lacks matrix silicates, such as fossil meteorites and sediment-dispersed chromite grains originating primarily from decomposed micrometeorites but also from larger impacts. (C) 2008 Elsevier Ltd. All rights reserved
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