by: Charles O’Dale
The name tektite comes from the Greek word ‘tektos’, meaning ‘molten’. Tektites do not contain any water. They can be mistaken for obsidian or pitchstone (black volcanic glasses), but these will emit some water on strong heating. Their density is similar to, or a little lighter than, quartz beach sand.
Natural, silica-rich, homogeneous glasses produced by complete melting and dispersed as aerodynamically shaped droplets during terrestrial impact events. The process of tektite formation is disputed, but many researchers believe that they are formed in the early contact and compression stage of impact cratering. They range in color from black or dark brown to gray or green. Tektites have been found in “strewn fields” on the Earth’s surface.
TEKTITE STREWN FIELDS
|Tektite Strewn Field||Age (Ma)||Crater Source||Notes|
|Aouellou||3.1 ± 0.3||Aouellou||Mauritania|
|Australasian||0.7881 ± .002.8||Bolaven volcanic field||Southern Laos|
|Darwin Glass||0.816 ± .007||Darwin Crater||Tasmania/td>|
|Libyan Desert Glass||~29||North Africa||Possible Air Burst|
|Moldavite||14.808 ± .038||Ries||Germany|
|K-Pg (KT) Boundary||66.043||Chicxulub||Yucatan Peninsula|
|Irghizite||0.9 ± 0.1||Zhamanshin||Kazakhstan|
Aouelloul tektites are associated with a nearby impact crater in Mauritania – tektite collected by Robert F. Fudali of the Smithsonian Institution. (Scientific American 1978)
Aouelloul impact crater, Mauritania, is located in the Akchar Desert, approximately 50 km southeast of Atar. The crater is 390 metres wide and roughly circular. The rim rises up to 53 metres above the bottom of the crater. Sediments in the crater are approximately 23 metres thick. Its age is estimated to be 3.1 ± 0.3 million years (Pliocene).
Tektite is found around the crater, although very few meteorites have been found. Zerga meteorite was found in 1973 at the bottom of the crater, but scientists are unsure if it is the same meteorite (or even a part of it) that formed the crater.
Kerry Sieha, Jason Herrina, Brian Jichab, Dayana Schonwalder Angela, James D. P. Moorea, Paramesh Banerjeea, Weerachat Wiwegwinc, Vanpheng Sihavongd, Brad Singerb, Tawachai Chualaowanichc, and Punya Charusirie
The crater and proximal effects of the largest known young meteorite impact on Earth have eluded discovery for nearly a century. We present 4 lines of evidence that the 0.79-Ma impact crater of the Australasian tektites lies buried beneath lavas of a long-lived, 910-km3 volcanic field in Southern Laos:
1) Tektite geochemistry implies the presence of young, weathered basalts at
the site at the time of the impact.
2) Geologic mapping and 40Ar-39Ar dates confirm that both pre- and postimpact basaltic lavas exist at the proposed impact site and that postimpact basalts wholly cover it.
3) A gravity anomaly there may also reflect the presence of a buried ∼17 × 13-km crater.
4) The nature of an outcrop of thick, crudely layered, bouldery sandstone and mudstone breccia 10–20 km from the center of the impact and fractured quartz grains within its boulder clasts support its being part of the proximal ejecta blanket.
52. P. S. Fiske et al., Layered tektites of southeast Asia: Field studies in central Laos and Vietnam. Meteorite. Planet. Sci. 34, 757–761 (1999).
53. S. M. Barr, A. S. Macdonald, Geochemistry and geochronology of late Cenozoic basalts of southeast Asia. Geol. Soc. Am. Bull. 92, 1069–1142 (1981)
54. J. L. Whitford-Stark, A Survey of Cenozoic Volcanism on Mainland Asia (Geological Society of America Special Papers, 1987), vol. 213, pp. 1–74, 10.1130/SPE1213-p1131.
55. J.-S. Ren et al., 1:5 million international geological map of Asia. Acta Geoscientica Sinica 34, 24–30 (2013).
56. L. Folco, M. D’Orazio, M. Gemelli, P. Rochette, Stretching out the Australasian microtektite strewn field in Victoria Land Transantarctic Mountains. Polar Sci. 2, 147–159 (2016).
Ultraprecise age and formation temperature of the Australasian tektites constrained by 40Ar/39Ar analyses
Fred Jourdan, Sebastien Nomade, Michael T. D. Wingate, Ela Eroglu, Al Deino
The Australasian tektites are quench melt glass ejecta particles distributed over the Asian, Australian, and Antarctic regions, the source crater of which is currently elusive. New 40Ar/39Ar age data from four tektites: one each from Thailand, China, Vietnam, and Australia measured using three different instruments from two different laboratories and combined with published 40Ar/39Ar data yield a weighted mean age of 788.1 ± 2.8 ka (±3.0 ka, including all sources of uncertainties) (P = 0.54). This age is five times more precise compared to previous results thanks, in part, to the multicollection capabilities of the ARGUS VI noble gas mass spectrometer, which allows an improvement of almost fourfold on a single plateau age measurement. Diffusion experiments on tektites combined with synthetic age spectra and Monte Carlo diffusion models suggest that the minimum temperature of formation of the Thai tektite is between 2350 °C and 3950 °C, hence a strict minimum value of 2350 °C
Dating K-T Tektites
One of the most exciting and important scientific findings in decades was the 1980 discovery that a large asteroid, about 10 kilometers diameter, struck the earth at the end of the Cretaceous Period. The collision threw many tons of debris into the atmosphere and possibly led to the extinction of the dinosaurs and many other life forms. The fallout from this enormous impact, including shocked quartz and high concentrations of the element iridium, has been found in sedimentary rocks at more than 100 locations worldwide at the precise stratigraphic location of the Cretaceous-Tertiary (K-T) boundary (Alvarez and Asaro 1990; Alvarez 1998). We now know that the impact site is located on the Yucatan Peninsula. Measuring the age of this impact event independently of the stratigraphic evidence is an obvious test for radiometric methods, and a number of scientists in laboratories around the world set to work.
In addition to shocked quartz grains and high concentrations of iridium, the K-T impact produced tektites, which are small glass spherules that form from rock that is instantaneously melted by a large impact. The K-T tektites were ejected into the atmosphere and deposited some distance away. Tektites are easily recognizable and form in no other way, so the discovery of a sedimentary bed (the Beloc Formation) in Haiti that contained tektites and that, from fossil evidence, coincided with the K-T boundary provided an obvious candidate for dating. Scientists from the US Geological Survey were the first to obtain radiometric ages for the tektites and laboratories in Berkeley, Stanford, Canada, and France soon followed suit. The results from all of the laboratories were remarkably consistent with the measured ages ranging only from 64.4 to 65.1 Ma (Table 2). Similar tektites were also found in Mexico, and the Berkeley lab found that they were the same age as the Haiti tektites. But the story doesn’t end there.
The K-T boundary is recorded in numerous sedimentary beds around the world. The Z-coal, the Ferris coal, and the Nevis coal in Montana and Saskatchewan all occur immediately above the K-T boundary. Numerous thin beds of volcanic ash occur within these coals just centimeters above the K-T boundary, and some of these ash beds contain minerals that can be dated radiometrically. Ash beds from each of these coals have been dated by 40Ar/39Ar, K-Ar, Rb-Sr, and U-Pb methods in several laboratories in the US and Canada. Since both the ash beds and the tektites occur either at or very near the K-T boundary, as determined by diagnostic fossils, the tektites and the ash beds should be very nearly the same age, and they are (Table 2).
There are several important things to note about these results. First, the Cretaceous and Tertiary periods were defined by geologists in the early 1800s. The boundary between these periods (the K-T boundary) is marked by an abrupt change in fossils found in sedimentary rocks worldwide. Its exact location in the stratigraphic column at any locality has nothing to do with radiometric dating — it is located by careful study of the fossils and the rocks that contain them, and nothing more. Second, the radiometric age measurements, 187 of them, were made on 3 different minerals and on glass by 3 distinctly different dating methods (K-Ar and 40Ar/39Ar are technical variations that use the same parent-daughter decay scheme), each involving different elements with different half-lives. Furthermore, the dating was done in 6 different laboratories and the materials were collected from 5 different locations in the Western Hemisphere.
Darwin Crater (42°18.39′S, 145°39.41′E), is the assumed source crater for the glass.
IRGHIZITES, from Kazakhstan in the U.S.S.R., are the most recently discovered tektites. These specimens, which range from about .8 inch to 1.1 inches long (portions were removed from two of them for analysis), were made available to U.S. investigators by Institute of Geology in U.S.S.R. Academy of Sciences through P. V. Florensky, who first reported on them.
Zhamanshin 14 km crater N 48° 24′ E 60° 58′
LIBYAN DESERT GLASS
High-pressure evidence from zircon in Libyan Desert Glass
Aaron J. Cavosie, Christian Koeberl
Enigmatic natural glasses have been cited as geologic evidence of the threat posed by large airbursts. Libyan Desert Glass (LDG) is a natural glass found in western Egypt that formed ~29 m.y. ago, however its origin is disputed; the two main formation hypotheses include melting by meteorite impact or melting by a large, 100 Mt–class airburst. High-temperature fusion occurs during both processes, however airbursts do not produce shocked minerals; airbursts generate overpressures at the level of thousands of pascals in the atmosphere, whereas crater-forming impacts generate shockwaves at the level of billions of pascals on the ground. Here we report the presence in LDG of granular zircon grains that are comprised of neoblasts that preserve systematic crystallographic orientation relations that uniquely form during reversion from reidite, a 30 GPa high-pressure ZrSiO4 polymorph, back to zircon. Evidence of former reidite provides the first unequivocal substantiation that LDG was generated during an event that produced high-pressure shock waves; these results thus preclude an origin of LDG by airburst alone.
Nördlinger Ries’s status as an impact crater did not become apparent until the 1960s. Prior to that time, many geologists suspected the crater had been formed by volcanic activity. One line of evidence supporting the impact theory included shocked quartz grains, which are formed by meteorite impacts. Another was the building material used for the church of St. George.
[see – DATING – TEKTITES]
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