Impact microcrater morphology on Australasian microtektites

Prasad, M.M.; Khedekar, V. D.

doi:10.1111/j.1945-5100.2003.tb00319.x

Cited by 14 publications

(13 citation statements)

References 29 publications

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“…Such features are only known to occur during high-energy atomic detonations and cosmic impacts, and, because differential velocities are too low † † , have never been reported to have been caused by volcanism, lightning, or anthropogenic processes. High-speed collisions can be either constructive, whereby partially molten, plastic spherules grow by the accretion of smaller melt droplets (35), or destructive, whereby collisions result in either annihilation of spherules or surface scarring, leaving small craters (36). In destructive collisions, small objects commonly display three types of collisions (36): (i) microcraters that display brittle fracturing; (ii) lower-velocity craters that are often elongated, along with very low-impact "furrows" resulting from oblique impacts (Fig.…”

Section: Sem or Eds Analyses To Determine Whether Their Spherules Arementioning

confidence: 99%

Very high-temperature impact melt products as evidence for cosmic airbursts and impacts 12,900 years ago

Bunch

Hermes

Moore

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

117

177

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It has been proposed that fragments of an asteroid or comet impacted Earth, deposited silica-and iron-rich microspherules and other proxies across several continents, and triggered the Younger Dryas cooling episode 12,900 years ago. Although many independent groups have confirmed the impact evidence, the hypothesis remains controversial because some groups have failed to do so. We examined sediment sequences from 18 dated Younger Dryas boundary (YDB) sites across three continents (North America, Europe, and Asia), spanning 12,000 km around nearly one-third of the planet. All sites display abundant microspherules in the YDB with none or few above and below. In addition, three sites (Abu Hureyra, Syria; Melrose, Pennsylvania; and Blackville, South Carolina) display vesicular, high-temperature, siliceous scoria-like objects, or SLOs, that match the spherules geochemically. We compared YDB objects with melt products from a known cosmic impact (Meteor Crater, Arizona) and from the 1945 Trinity nuclear airburst in Socorro, New Mexico, and found that all of these high-energy events produced material that is geochemically and morphologically comparable, including: (i) high-temperature, rapidly quenched microspherules and SLOs; (ii) corundum, mullite, and suessite (Fe 3 Si), a rare meteoritic mineral that forms under high temperatures; (iii) melted SiO 2 glass, or lechatelierite, with flow textures (or schlieren) that form at >2,200°C; and (iv) particles with features indicative of high-energy interparticle collisions. These results are inconsistent with anthropogenic, volcanic, authigenic, and cosmic materials, yet consistent with cosmic ejecta, supporting the hypothesis of extraterrestrial airbursts/impacts 12,900 years ago. The wide geographic distribution of SLOs is consistent with multiple impactors.tektite | microcraters | oxygen fugacity | trinitite Manuscript TextThe discovery of anomalous materials in a thin sedimentary layer up to a few cm thick and broadly distributed across several continents led Firestone et al. (1) to propose that a cosmic impact (note that "impact" denotes a collision by a cosmic object either with Earth's surface, producing a crater, or with its atmosphere, producing an airburst) occurred at 12.9 kiloannum (ka; all dates are in calendar or calibrated ka, unless otherwise indicated) near the onset of the Younger Dryas (YD) cooling episode. This stratum, called the YD boundary layer, or YDB, often occurs directly beneath an organic-rich layer, referred to as a black mat (2), that is distributed widely over North America and parts of South America, Europe, and Syria. Black mats also occur less frequently in quaternary deposits that are younger and older than 12.9 ka (2). The YDB layer contains elevated abundances of iron-and silica-rich microspherules (collectively called "spherules") that are interpreted to have originated by cosmic impact because of their unique properties, as discussed below. Other markers include sediment and magnetic grains with elevated iridium concentrations and exot...

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Section: Sem or Eds Analyses To Determine Whether Their Spherules Arementioning

confidence: 99%

Very high-temperature impact melt products as evidence for cosmic airbursts and impacts 12,900 years ago

Bunch

Hermes

Moore

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

117

177

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“…Subsequently, however, Australian tektites have been found in older horizons (Fudali 1993;Shoemaker and Uhlherr 1999), which is at variance with the concept of the age paradox. Further, microtektites belonging to the strewn field have been found mostly only in the oceans of over 50 locations (Glass and Pizzuto 1994;Prasad 1994;Lee and Wei 2000;Prasad and Khedekar 2003). In addition, microtektites have also been reported on land from Chinese loess (Wenzhu et al 1995).…”

Section: Introductionmentioning

confidence: 97%

Two layers of Australasian impact ejecta in the Indian Ocean?

Prasad

Gupta

Kodagali

2003

Meteorit & Planetary Scien

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Abstract-Only 2 Australasian tektites have been found in the Indian Ocean, and both are associated with surficial sediments. We collected cores from both locations where the tektites have been reported. The microtektites in these cores (and both the tektites, as reported earlier) have chemical compositions within the compositional range previously reported for Australasian tektites and microtektites. In both locations, while the tektites are occurring at the sediment/water interface, the microtektites are found buried in older horizons beneath the seafloor at stratigraphic levels, conforming to the radiometric age of the strewn field. Thus, at first glance, there appear to be 2 layers of Australasian impact ejecta in the Indian Ocean. However, the manganese nodules are associated with the tektites which, although millions of years old, are invariably resting on recent sediments. Therefore, the mechanism that retains nodules at the seafloor also seems to be operative on the tektites, thus leading to this apparent "age paradox" of tektite/microtektite distribution in the Indian Ocean, although they both belong to the same impact event.

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“…At best, microcraters having diameters of approximately 20 μm could be recognized under the binocular microscope without much ambiguity. This aspect has been described in detail earlier (Prasad and Khedekar 2003). However, the error would remain uniform for all the samples.…”

Section: Methodsmentioning

confidence: 79%

“…The microcraters along the transect are therefore described for each sample below. The description of microcraters follows the classification carried out earlier by Prasad and Khedekar (2003) into two major classes:…”

Section: Description Of Impacted Microtektitesmentioning

confidence: 99%

“…The Australasian microtektites contain microimpact features on their surfaces which have provided evidence for processes that take place as distal ejecta re‐enters the atmosphere (Prasad and Sudhakar 1996, 1998; Prasad and Khedekar 2003). In this study, we have examined surface features on 4604 Australasian microtektites ranging from 180 to 1530 μm diameter, in 11 sediment cores along a N–S transect stretching over 1300 km in the Central Indian Ocean.…”

Section: Introductionmentioning

confidence: 99%

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Changes in abundance and nature of microimpact craters on the surfaces of Australasian microtektites with distance from the proposed source crater location

Prasad¹,

Roy²,

Gupta³

2010

Meteoritics &Amp; Planetary Science

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Abstract-Over 4600 Australasian microtektites from 11 sediment cores along an N-S transect in the Central Indian Ocean have been investigated optically for microimpact features on their surfaces. Detailed scanning electron microscope examination of 68 microtektites along this transect shows 4091 such features. These samples are located between approximate distances of 3900-5000 km from the suggested impact site in Indochina and therefore constitute distal ejecta. The morphology of the microimpacts seems to show distinct variations with distance from the source crater. The total number of microcraters on each microtektite decreases drastically from North to South indicating systematic decrease in the spatial density of the ejecta, and decrease in collisional activity between microtektites with distance from the proposed source crater location. Closer to the proposed source crater location, the microcraters are predominantly small (few lm), pit bearing with radial and concentric cracks, suggestive of violent interparticle collisions. The scenario is reverse farther from the source crater with smaller numbers of impacted microtektites due to increased dispersion of the ejecta and the microcraters are large and shallow, implying gentle collisions with larger particles. These observations provide systematic ground truth for the processes that take place as the ejecta of a large oblique impact which generated the Australasian tektite strewn field is emplaced. The microimpacts appear to take place during the descent of the ejecta and their intensity and number density decrease as a function of the spatial density of the ejecta at any given place and with distance from the source region. These features could help understand processes that take place during ejecta emplacement on planets with substantial atmosphere such as Mars and Venus.

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Impact microcrater morphology on Australasian microtektites

Cited by 14 publications

References 29 publications

Very high-temperature impact melt products as evidence for cosmic airbursts and impacts 12,900 years ago

Very high-temperature impact melt products as evidence for cosmic airbursts and impacts 12,900 years ago

Two layers of Australasian impact ejecta in the Indian Ocean?

Changes in abundance and nature of microimpact craters on the surfaces of Australasian microtektites with distance from the proposed source crater location

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