Meteoritics

The 1908 Tunguska event was not related to the Earth's collision with a comet/asteroid and could be most probably a tectonic event caused by solar-lunar-terrestrial perturbances. The evidence: 1. The polarization effect 'spread' from the lower to the upper atmosphere, and not vice versa, as would be expected in the case of the penetration of comet matter. 2. Before the Tunguska explosion on June 30, the appearance of luminous clouds over Europa. For asteroids and inactive comets, preceding airglows are impossible. In the case of an active comet with tails, astronomers would notice her in advance. Since the airglows of the Tunguska event were also observed in the cone of the Earth's shadow, they could not be caused, as is generally believed, by NLCs after the explosion of a comet/asteroid. 3. The Tunguska event was dependent on physical processes associated with the solar eclipse on June 28, 1908. This was indicated by geomagnetic pulsations in Kiel on June 27-30, 1908. 4. The seismogram recorded in Irkutsk on June 30, 1908, confirms the tectonic-volcanic earthquake. 5. No traces of the chondrite fraction of an alleged comet were found. 6. The Kulikovskiy paleovolcano had five stages of activity from the times of Siberian Flood Basalts (~252 Myr ago) to the 1908 explosion. All events were connected with the 'Perm LLSVPs Anomaly' located under Eurasia on the core-mantle boundary. 7. Moissanite formations, in layers of both the mud-volcanic phase (7320 years) and 1908, were found. Their genesis is the decisive factor for the 1908 Tunguska event's nature.

Field work in and around the Lake St. Martin's meteorite crater has found evidence of shatter cones from this meteorite crater as well as two other structures not described previously from other meteorite craters that must be characteristic of a violent explosion going through the rocks. Evidence of "expulsion cones" or narrow, cone-shaped channels cut through the rock from small holes were found in dolomite just outside the crater. Loose, weathered rocks of impact breccias and melt rocks within the crater show a consistent inverted bowl structure with the proposed name of umbrella structure. These impact rocks are dominated by carbonate and chert and the structure could be the result of gas generated by partial gasification of the rock with the gas moving explosively through the rock and bending the rock around it.

In the book, questions of Earth's craters exploration and origin of tektites on our planet are brought up. It is shown that our knowledge in this field remains limited so far and demand revision of the data interpretation.
In the context of the Tunguska 1908 phenomenon, we suppose correlations between the Perm LLSVPs Anomaly and the explosion of paleovolcano in Siberia on 30 June 1908. We discuss magnetic pulsations in Kiel, Germany observed from 27 till 30 June 1908, a variation of gravitational constant during the solar eclipse on 28 June
1908, and the discrepancy of Moon longitude at the beginning of the 20th century. The non-typical registrations in Tasmania in the period of the Tunguska 1908 phenomenon and his connection with the Perm LLSVPs anomalies provide a highly probable solution to one of the greatest mysteries of the previous century.

For astro- and geophysicists and a wide range of readers who are interested in meteorites and planetology.

Descriptions of natural events, such as fireballs, and meteorite impacts, are found within Indigenous Australian oral traditions. Studies of oral traditions demonstrate that they extend beyond the realm of myth and legend; they contain structured knowledge about the natural world (science) as well as historic accounts of natural events and geo-hazards. These traditions could lead to the discovery of meteorites and impact sites previously unknown to Western science. In addition to benefiting the scientific study of meteoritics, this study can help social scientists better understand the nature and longevity of oral traditions and further support the growing body of evidence that oral traditions contain historical accounts of natural events. In a previous study led by the author in 2009, no meteorite-related oral traditions were identified that led to the discovery of meteorites and/or impact craters. This article challenges those initial findings.

La raccolta di micrometeoriti non e' cosa complessa o difficoltosa. Non occorre un'attrezzatura costosa, ma tanta voglia di scoprire la natura (anche a livello microscopico). A partire dal suggerimento di alcuni membri del Gruppo Astrofili Centesi, ho cominciato questa piccola ricerca a novembre 2017, raccogliendo campioni di polvere magnetica in due punti del mio giardino (secchio posto sotto la grondaia del magazzino e cisterna del pozzo). Questa piccola guida vuole illustrare la procedura di raccolta, partnedo dal materiale usato, fino a semplici elaborazioni statistiche basate sui dati raccolti.

The Carswell structure in the western Athabasca Basin, northern Saskatchewan (Canada), has previously been interpreted as an eroded impact structure with a minimum diameter of ~36 km, the outer margin of which is broadly defined by an outer ring of sediments composed of the only algal reefs observed in the Athabasca Group, the Carswell Formation. This ring surrounds an 18-km-wide uplifted basement core composed of gneiss units of Archean to Paleoproterozoic age that display shatter cones, planar deformation features (PDFs), pseudotachylyte veins, and impact melts and breccias (Cluff melt sheet, Cluff breccias) indicating that pressures and temperatures locally exceeded 60 GPa and 1500 °C. Detailed analysis of the basement–Athabasca Group contact from field and drill core samples indicates that shock features are not present in the Athabasca Group sediments in direct contact with the highly shocked basement gneisses. This pattern is inconsistent with a post–Athabasca Group age for the impact. Moreover, our study has revealed PDF-bearing quartz grains in basal units of the Athabasca Group 14 km south of the southern edge of the basement core (outside the estimated outer ring). The new proposed model suggests that the impact event is of pre-Athabasca (Proterozoic) age and that it produced a multiring structure that controlled the paleogeography of the Athabasca Group units in the western part of the basin. The model is well supported by basin analysis and gravity data. The Carswell Formation is the result of algal reefs building on peak-ring–related seamounts at the end of Athabasca Group deposition. The overturned bedding observed locally adjacent to the basement core is interpreted as the result of gravity-driven readjustment of the central uplift.

    Accepted 29 September 2009.

In this paper we explore Aboriginal oral traditions that relate to Australian meteorite craters. Using the literature, first-hand ethnographic records and fieldtrip data, we identify oral traditions and artworks associated with four impact sites: Gosses Bluff, Henbury, Liverpool and Wolfe Creek. Oral traditions describe impact origins for Gosses Bluff, Henbury and Wolfe Creek Craters, and non-impact origins for Liverpool Crater, with Henbury and Wolfe Creek stories having both impact and non-impact origins. Three impact sites that are believed to have been formed during human habitation of Australia. Dalgaranga, Veevers, and Boxhole do not have associated oral traditions that are reported in the literature.

A more exact dating of the Chiemgau meteorite impact in Bavaria, southeast Germany, that produced a large strewn field of more than 80 craters sized between a few meters and several hundred meters, may provide the indispensable fundament for evaluating its cultural implications and thus enable an extraordinary case study. A straightforward answer has not yet been provided due to e.g. scarce existence of diagnostic material, lack of specialised micromorphologists, absence of absolute
dating data etc. Here we report on a first OSL dating applied to a catastrophic impact layer that features both impact ejecta and tsunami characteristics attributed to proposed falls of projectiles into Lake Chiemsee in the impact event. The OSL dating was conducted on a quartzite cobble and four sediment samples collected from an excavated archaeological stratigraphy at Lake Chiemsee that comprised also the impact layer. In a first approach the analyses were based on the assumption of zero luminescence resetting clock from the induced impact shock for the quartzite cobble, and a solar bleaching of tsunamigenerated sediments. Optically Stimulated Luminescence (OSL) was applied using the Single Aliquot
Regeneration (SAR) protocol and relevant reliability criteria. For sediments the beta‐TL
method was also applied. Reported ages fall around the beginning of 2nd millennium BC. Special attention is given to the peculiar situation of OSL dating of material that may have been exposed to impact shock of strongly varying intensity, to excavation, ejection and ejecta emplacement, the latter overprinted by and mixed with tsunami transport processes resulting in possibly very complex bleaching scenarios largely differing from the original assumptions.

В книге рассмотрены вопросы исследования земных
кратеров и происхождения тектитов на нашей планете.
Новые данные, связанные с необычными регистрациями на Тасмании в период Тунгусского феномена в 1908 г., с большой вероятностью позволяют решить его проблему.

В различных регионах Земли известны т.н. криптовзрывные структуры — линейные цепочки кратеров и полей диатрем, включая кимберлитовые. Долгое время их относили к эндогенным, но появилась и до сих пор доминирует метеоритно-ударная версия. Борьба доктрин длилась десятилетия, касаясь, в основном, известных структур: 1) кратеров Рис и Штайнхайм и соседнего с ними поля диатрем Урах, Германия; 2) кратеров и кимберлитовых полей структур Бушвельд—Вредефорт—Тромсбург, ЮАР; 3) кратеров Уэллс Крик и Хикс и диатрем поля Эйвон, США.
Поскольку кратер Рис связывают с удаленными от него молдавитами, решение проблемы криптовзрывных структур касается и тектитов.

Для астро- и геофизиков, а также широкого круга читателей, интересующихся метеоритами и планетологией.

In the wake of the discovery of clay minerals in Gale crater (an ancient martian crater) by NASA’s Curiosity rover, and numerous remote and in situ detections of hydrous minerals on the early surfaces of Mars, the absence of clays in the oldest martian meteorite, ALH 84001, remains conspicuous. ALH 84001 is an orthopyroxenite that contains carbonate minerals which were formed on Mars – probably as a result of low temperature, near-surface aqueous alteration of the rock – but it does not contain clays.
This body of work comprises a number of studies and different analytical techniques used to understand and constrain the composition of early martian fluids that lead to the alteration of the meteorite when it was emplaced near the surface of Mars ~ 3.9 Ga ago. Petrological and mineralogical studies; carbon, nitrogen and noble gas stepped-combustion analyses; geochemical modelling under different conditions; and aqueous alteration experiments were all carried out here for this purpose. A young martian meteorite, shergottite NWA 6234, was also investigated in the context of this work to understand whether aqueous alteration persisted in time.
Compositional clues described here point to varying degrees of aqueous alteration in ALH 84001, and possibly even in a shock melt vein in NWA 6234. The results of the extensive geochemical modelling strongly suggest that the fluid that altered ALH 84001 was near-neutral (pH ≈ 7), low temperature (~ 20 °C), and that the atmospheric CO2 pressure required to produce the carbonates was under 1 bar. The fluid was probably transient, and evaporated after precipitating the ubiquitous carbonates. A suite of other secondary products are predicted to have formed neighbouring ALH 84001, a few of which have been detected on Mars (such as clays), yet others are more surprising (such as talc and zeolites, and a possible pathway to the abiotic production of methane).

Indigenous Australian practices, developed and honed over thousands of years, weave science with storytelling. In this Indigenous science series, we’ll look at different aspects of First Australians' traditional life and uncover the knowledge behind them – starting today with astronomy.

Myths and legends are taught as an important component of cultural history and heritage, but are viewed by many as little more than fairy tales. Rife with supernatural deities, unphysical acts of strength and prowess, and fictional creatures, the very concept of examining them for scientific content is frowned upon, if not rejected outright. And with anti-science creationists pushing for teaching mythology-as-science, this is not unexpected. But researchers are finding out something oral cultures have always known - myths and legends (or to use the preferred term, 'oral traditions') contain information about nature, survival, cultural practices, and social structure for the peoples that developed them. This information actually does include a scientific component - explanations of the natural world that were used for navigation, calendars, food economics, and agricultural practices. The rising of the Pleiades at dawn may signal the arrival of winter. The behavior of some animals may predict the onset of bad weather. Certain plants may be used to treat ailments and disease. But these oral traditions can also contain information about past geological and astronomical events.

40 Ar ⁄ 39 Ar dating of potassium feldspar (primary spherulitic-blocky and secondary idiomorphic K-feldspar) separated from impact-metamorphosed gneiss found near Videix in the western central part of the Rochechouart impact structure (NW Massif Central, France) yielded a Rhaetian combined age of 201 ± 2 Ma (2r), indistinguishable within uncertainty from the age of the Triassic ⁄ Jurassic boundary. Ballen quartz intergrown with the primary K-feldspar indicates post-shock temperatures exceeding approximately 1000°C that affected the precursor gneiss. Geochemically, both feldspar types represent essentially pure potassium end-members. Apart from the approximately 15 km diameter impact deposit area, the youngest crystallization age known for basement rocks in this part of the Massif Central is approximately 300 Ma. No endogenic magmatic-thermal events are known to have occurred later in this region. The K-feldspar recrystallized from local feldspar melts and superimposed post-shock hydrothermal crystallization, probably within some thousands of years after the impact. It is, therefore, suggested that the 40 Ar ⁄ 39 Ar age for the Videix gneiss (as a potassic ''impact metasomatite'') dates the Rochechouart impact, in consistence with evidence for K-metasomatism in the Rochechouart impactites. The new age value is distinctly younger than the previously obtained Karnian-Norian age for Rochechouart and, thus, contradicts the Late Triassic multiple impact theory postulated some years ago. In agreement with the paleogeographic conditions in the western Tethys domain around the Triassic ⁄ Jurassic boundary, the near-coastal to shallow marine Rochechouart impact is compatible with the formation of seismites and tsunami deposits in the latest Triassic of the British Isles and possible related deposits in other parts of Europe.

Eye-witness accounts of the Chelyabinsk airburst have provided unique information that is not recorded in the many security and dash-cam videos of the event. Accounts were collected from in-person interviews during a field study from March 9 to 25, 2013, and from phone and internet surveys. In total, about 2000 accounts were collected, which provide information about sensations of heat, smells, sounds, the occurrence of sunburn, and the nature of injuries.

Keywords: meteoritic ablation debris micrometeorites Antarctic ice cores meteoritic impacts Tunguska airburst Aggregates of microscopic spherules broadly similar in texture and composition to cosmic spherules or meteorite ablation spheres were discovered within the ∼ 1 Ma-old Transantarctic Mountain micrometeorite traps at Miller Butte, Victoria Land, Antarctica. Mineralogical and geochemical data obtained by means of field emission-scanning electron microscopy, electron microprobe analyses, synchrotron X-ray diffraction, and magnetization measurements show that they consist of a porous aggregate of quench-textured spherules, with individual spherules ranging from less than 1 to 65 µm in diameter. Spherule types include porphyritic olivine plus magnesioferrite spherules, dendritic magnesioferrite spherules, barred and feathered olivine spherules, and cryptocrystalline spherules. In contrast to the textural variations, the bulk composition of the individual spherules is fairly homogeneous and broadly chondritic. Likewise olivine has a nearly homogeneous composition Fa 16.3 ± 2.7. Olivine and magnesioferrite are characterized by high NiO contents (2.72 ± 1.6 and 4.68 ± 0.68 wt.%, respectively), as typically observed in ablation debris and meteorite fusion crusts. The bulk composition of the aggregates is similar to the fusion crust of ordinary and carbonaceous chondrites. We interpret the spherulitic aggregates as meteorite ablation debris formed during the atmospheric entry of a large meteorite of ordinary or carbonaceous chondritic composition. Comparison with the available literature data shows that the ablation debris found at Miller Butte is most likely paired with the extraterrestrial dust found in a ∼ 480 ka-old ice layer in the EPICA-Dome C and Dome Fuji ice cores (East Antarctic ice sheet), thereby documenting a continental-scale distribution of ablation debris associated with a major meteoritic impact event which occurred ∼ 480 ka ago. Based on estimates of the projectile mass (N 10 8 kg) and numerical simulation of small-scale impacts from literature, we propose that the continental-scale distribution of the ablation debris was generated by the deceleration of a giant impact plume associated with a Tunguska-like impact over Antarctica. Tunguska-like impacts have global frequencies on the order of 10 3 to 10 5 yr, consistent with several being recorded in the Antarctic ice sheet. Finally, our detailed petrographic description of the spherulitic aggregates could serve as guide to the search for Tunguska-like events in the geological record.

Comparison between clasts of the Tatahouine diogenite, collected the day of the fall in 193 1 and 63 years later in 1994, allows the evaluation of mineralogical and chemical effects of terrestrial residence on meteorites. Secondary minerals are found in the 1994 samples: iron stains and carbonate rosettes. Major and trace element abundances have been determined on fallen and found clasts. No significant differences have been observed for most elements with the exception of Rb, Sr, and (in a single case) the light rare earth elements ( L E E ) . In this case, the REE pattern of a 1994 clast displays a weak positive Ce anomaly, probably linked to the presence o f iron hydroxide. The contents o f R b and Sr are significantly higher in the 1994 samples than in the 193 1 clasts and reflect the formation of calcite inside some o f the clast fractures. These results demonstrate that weathering processes may change the chemistry o f meteorites in a very short time.

... The fall of the St-Robert meteorite PETER BROWN1*, ALAN R. HILDEBRAND2, DANIEL WE GREEN3, DENIS PAGE~, CLIFF JACOBS~, DOUG ... The associated strewn field is well delineated with more recovered fragments than any other Cana-in the diameter range of 0.5 to 5 ...

The stories about meteors, fireballs
and meteorite falls are in the legends
of many civilizations around the globe.
South American indigenous peoples have
many stories in which they describe some
phenomena produced by the interaction
of a meteoroid in the Earth’s atmosphere
and in certain instances it is related to
causes of fire. A search of manuscripts
and books in the Nueva Granada in the
eighteenth and nineteenth century
produced eight stories that adequately
describe the passage of fireballs and sonic
waves in the following locations and
dates: Píritu (Venezuela), 1680; Santafé
de Bogotá (Colombia), 1687; Maracaibo
(Venezuela), 1692; Popayán (Colombia),
1816; central region of Colombia, 1827;
Antioquia (Colombia), 1854 and Cundinamarca
(Colombia), 1883.

available online at Abstract-Impact cratering is a ubiquitous geological process on the terrestrial planets. Meteorite impact craters are the most visible product of impact events, but there is a growing recognition that large aerial bursts or airbursts should occur relatively frequently throughout geological time. In this contribution, we report on an unusual impact glass-the Dakhleh Glass (DG)-which is distributed over an area of ~400 km 2 of the Dakhleh Oasis, Egypt. This region preserves a rich history of habitation stretching back to over 400,000 years before the emergence of Homo sapiens. We report on observations made during recent fieldwork and subsequent analytical analyses that strengthen previous suggestions that the DG formed during an impact event. The wide distribution and large size of DG specimens (up to ~50 cm across), the chemistry (e.g., CaO and Al 2 O 3 contents up to ~25 and 18 wt%, respectively), the presence of lechatelierite and burnt sediments, and the inclusion of clasts and spherules in the DG is inconsistent with known terrestrial processes of glass formation. The age and other textural characteristics rule out a human origin. Instead, we draw upon recent numerical modeling of airbursts to suggest that the properties of DG, coupled with the absence of a confirmed crater, can best be explained by melting of surficial sediments as a result of a large airburst event. We suggest that glass produced by such events should, therefore, be more common in the rock record than impact craters, assuming that the glass formed in a suitable preserving environment.

Arguing from a critical reading of the text, and scientific evidence on the ground, the authors show that the myth of Phaethon – the delinquent celestial charioteer – remembers the impact of a massive meteorite that hit the Chiemgau region in Bavaria between 2000 and 428 BC.

available online at http://meteoritics.org 627 Abstract-A multiple fall of a stony meteorite occurred near the town of Dergaon in Assam, India, on March 2, 2001. Several fragments weighing 2 kg and a single large fragment weighing a10 kg were recovered from the strewn field, which extended over several tens of square kilometers. Chemical, petrographic, and oxygen isotopic studies indicate it to be, in most aspects, a typical H5 chondrite, except the unusually low K content of a340 ppm. A cosmic ray exposure of 9.7 Ma is inferred from the cosmogenic noble gas records. Activities of eleven cosmogenic radionuclides were measured. 26 Al and 22 Na activities as well as the 22 Na/ 26 Al activity ratio are close to the values expected on the basis of solar modulation of galactic cosmic rays. The low 60 Co activity (1 dpm/kg) is indicative of a small preatmospheric size of the meteorite. Cosmic ray heavy nuclei track densities in olivine grains range from a10 6 cm 2 in samples from the largest fragment toapproximately (4-9) u10 5 cm 2 in one of the smaller fragments. The combined track, radionuclide, and noble gas data suggest a preatmospheric radius of a20 cm for the Dergaon meteorite.

An impressive daylight fireball was observed from Spain, Portugal, and the south of France at 16h46m45s UTC on January 4, 2004. The meteoroid penetrated into the atmosphere, generating shock waves that reached the ground and produced audible booms. The associated airwave was recorded at a seismic station located 90 km north of the fireball trajectory in Spain, and at an infrasound station in France located 750 km northeast of the fireball. The absolute magnitude of the bolide has been determined to be −18 ± 1 from a casual video record. The energy released in the atmosphere determined from photometric, seismic, and infrasound data was about 0.02 kilotons (kt). A massive fragmentation occurred at a height of 28 ± 0.2 km, resulting in a meteorite strewn field of 20 × 6 km. The first meteorite specimen was found on January 11, 2004, near the village of Villalbeto de la Peña, in northern Palencia (Spain). To date, about 4.6 kg of meteorite mass have been recovered during several recovery campaigns. The meteorite is a moderately shocked (S4) L6 ordinary chondrite with a cosmic-ray-exposure age of 48 ± 5 Ma. Radioisotope analysis shows that the original body had a mass of 760 ± 150 kg, which is in agreement with the estimated mass obtained from photometric and seismic measurements.

Abstract— An impressive daylight fireball was observed from Spain, Portugal, and the south of France at 16h46m45s UTC on January 4, 2004. The meteoroid penetrated into the atmosphere, generating shock waves that reached the ground and produced audible booms. The associated airwave was recorded at a seismic station located 90 km north of the fireball trajectory in Spain, and at an infrasound station in France located 750 km north-east of the fireball. The absolute magnitude of the bolide has been determined to be −18 ± 1 from a casual video record. The energy released in the atmosphere determined from photometric, seismic, and infrasound data was about 0.02 kilotons (kt). A massive fragmentation occurred at a height of 28 ± 0.2 km, resulting in a meteorite strewn field of 20 × 6 km. The first meteorite specimen was found on January 11, 2004, near the village of Villalbeto de la Peña, in northern Palencia (Spain). To date, about 4.6 kg of meteorite mass have been recovered during several recovery campaigns. The meteorite is a moderately shocked (S4) L6 ordinary chondrite with a cosmic-ray-exposure age of 48 ± 5 Ma. Radioisotope analysis shows that the original body had a mass of 760 ± 150 kg, which is in agreement with the estimated mass obtained from photometric and seismic measurements.

£ state svil uppato un metoda elettrochimico per la detcrminarione del ferro, nichd e oobalto metallici nelle meteoriti. Lc fasi Jnetallichc: SOIlO It:lte sc:Iettivamcnu: disciolte e gli elementi metallici in esse presc:mi sono stati quindi determinati nella soluzione me:diante assorbimento atomico. Questo metodo e state utilizzato per analizza re miscde di una lega . di Fe·Ni con polveri del meteorite di Allende per simulare i1 contenutO metallico delle condriti tipo H, L eLL. £ stata altrcsl anaiizz8ta una condrite naturale LL.

The two Clearwater Lake impact structures (Québec, Canada) are generally interpreted as a crater doublet formed by the impact of a binary asteroid. Here, arguments are presented that raise important questions about the proposed double impact scenario. New 40 Ar/ 39 Ar dating of two virtually fresh impact melt rock samples from the P36 km West Clearwater Lake impact structure yielded two statistically robust Early Permian plateau ages with a weighted mean of 286.2 ± 2.2 (2.6) Ma (2r; MSWD = 0.33; P = 0.57). In contrast, 40 Ar/ 39 Ar results for two chloritized melt rocks from the $26 km East Clearwater Lake impact structure produced disturbed age spectra suggestive of a distinct extraneous argon component. Although individually weakly robust, age spectra corrected for the trapped argon component and inverse isochron plots for the East Clearwater melt rocks consistently yielded apparent ages around $460-470 Ma. No Permian signal was found in either of these melt aliquots. Our new 40 Ar/ 39 Ar results reproduce earlier 40 Ar/ 39 Ar plateau ages ($283 Ma and $465 Ma, respectively) for the two impact structures by and are in conflict with a previous, statistically non-robust Rb-Sr age of 287 [293] ± 26 Ma for East Clearwater. The combined cluster of apparent ages of $460-470 Ma, derived from four different samples across the impact melt sheet, is very unlikely to represent a 'false age effect' due to the incorporation of extraneous argon into the melt; instead, it strongly favors a Middle Ordovician age for the East Clearwater impact and impact-induced hydrothermal chloritization. Moreover, the Clearwater impact structures are characterized by different natural remanent magnetizations testifying to separate geologic histories, an effect unexpected in the case of a Permian double impact. Whereas the West Clearwater impact affected Ordovician carbonates incorporated into the impact breccia, drill core reports from the 1960s concluded that clasts of Ordovician sedimentary rocks are seemingly absent in the impact breccia lens of the East Clearwater Lake impact structure, which is overlain by >100 m of post-impact sandstones, shales and carbonates. No resolvable impactor contamination has so far been detected in the West Clearwater impact melt rocks, whereas East Clearwater carries a distinct ordinary (possibly L-) chondritic impactor signature in its melt rocks. East Clearwater Lake might thus represent one among a long list of Ordovician impact structures in North America and northern Europe that were presumably formed in response to the L-chondrite asteroid breakup event $470 Ma ago. Paleogeographic reconstructions show that the http://dx.Ordovician East Clearwater impact probably occurred in a near-coastal to shallow marine setting, while the Permian West Clearwater impact hit continental Pangaea. Along with the new 40 Ar/ 39 Ar data, the paleomagnetic, sedimentologic, and geochemical findings suggest that the close spatial arrangement of the two Clearwater Lakes is most likely pure coincidence. The two impact structures seem to represent a 'false doublet' struck by impacts separated by $180 million years in time. The new results for the Clearwater Lake impact structures have major implications for the reliable identification of doublet impact craters and the rate of binary asteroid impacts on Earth and on other planetary bodies in the inner Solar System.

Fluid inclusions studies in quartz and calcite in samples from the ICDP-Chicxulub drill core Yaxcopoil-1 (Yax-1) have revealed compelling evidence for impact-induced hydrothermal alteration. Fluid circulation through the melt breccia and the underlying sedimentary rocks was not homogeneous in time and space. The formation of euhedral quartz crystals in vugs hosted by Cretaceous limestones is related to the migration of hot (>200 °C), highly saline, metal-rich, hydrocarbon-bearing brines. Hydrocarbons present in some inclusions in quartz are assumed to derive from cracking of pre-impact organic matter. The center of the crater is assumed to be the source of the hot quartz-forming brines. Fluid inclusions in abundant newly-formed calcite indicate lower cyrstallization temperatures (75-100 °C). Calcite crystallization is likely related to a later stage of hydrothermal alteration. Calcite precipitated from saline fluids, most probably from formation water. Carbon and oxygen isotope compositions and REE distributions in calcites and carbonate host rocks suggest that the calcite-forming fluids have achieved close equilibrium conditions with the Cretaceous limestones. The precipitation of calcite may be related to the convection of local pore fluids, possibly triggered by impact-induced conductive heating of the sediments.

Scientists have observed that the Himalayan range, which includes some of the highest mountains in the world, is covered in fossilised seashells, sea lilies, fish and other marine fossils. There can therefore be no doubt that it once formed part of the sea floor. They theorise that the adjacent sub-continent of India slowly pushed northwards and crumpled the sea floor up into this very high ridge over tens or hundreds of millions of years, though they concede that there are many problems with this scenario. An alternative view is that the Himalayas appeared, overnight, as a result of a meteorite impact.

Aboriginal stories dating back many thousands of years talk of a fire from the sky in an area now home to the Henbury meteorite craters, in the Northern Territory. Imagine going about your normal day when a brilliant light races across the sky. It explodes, showering the ground with small stones and sending a shock wave across the land. The accompanying boom is deafening and leaves people running and screaming. This was the description of an incident that occurred over the skies of Chelyabinsk, Russia on February 15, 2013, one of the best recorded meteoritic events in history. This airburst was photographed and videoed by many people so we have a good record of what occurred, which helped explain the nature of the event.

–About half of the IIE nonmagmatic iron meteorites contain silicate inclusions with a primitive to differentiated nature. The presence of preserved chondrules has been reported for two IIE meteorites so far, Netscha€ evo and Mont Dieu, which represent the most primitive silicate material within this group. In this study, silicate inclusions from two samples of Netscha€ evo were examined. Both silicate inclusions are characterized by a porphyritic texture dominated by clusters of coarse-grained olivine and pyroxene, set in a fine-grained groundmass that consists of new crystals of olivine and a glassy appearing matrix. This texture does not correspond to the description of the previously examined pieces of Netscha€ evo, which consist of primitive chondrule-bearing angular clasts. Detailed petrographic observations and geochemical analyses suggest that the investigated samples of Netscha€ evo consist of quenched impact melt. This implies that Netscha€ evo is a breccia containing metamorphosed and impact-melt rock (IMR) clasts and that collisions played a major role in the formation of the IIE group.

available online at Abstract-Recently, samples of allochthonous melt rocks from the ∼10 km and ≤1.9 Ga Paasselkä impact structure, SE Finland, were obtained. In this study, we present a first detailed petrographic and geochemical description of clast-rich Paasselkä impact melt rocks. Shock metamorphic features comprise shocked feldspar grains, intensely shocked and toasted quartz, marginally molten and recrystallized clasts thought to have been diaplectic quartz glass, largely fresh and recrystallized feldspar glasses, decomposed biotite flakes, recrystallized fluidal silica glass (originally probably lechatelierite) in partially molten sandstone clasts, all set into a glassy to cryptocrystalline melt matrix. The degree of shock metamorphism of clasts suggests initial whole-rock melting at peak shock pressures of ≥35 GPa and post-shock temperatures of up to ∼1500 °C. Glass components vary in geochemical composition corresponding to the mixed character of the crystalline target rock (i.e., representing different monomineralic and mixed-mineral melts). Feldspar glasses and the fresh glassy to cryptocrystalline melt matrix indicate that the Paasselkä melt rocks are not intensely altered. The geochemical composition of the Paasselkä impact melt rocks is roughly consistent with the compositions of melt rocks from a number of impact structures located within the crystalline basement of the Baltic Shield.