Geochemical studies of tektites from East Asia (original) (raw)
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Progress in Earth and Planetary Science
There are several reports of Australasian tektites found within a stratum called “laterite” layer widely distributed in Indochina. However, it has been debated whether these tektites are in situ or reworked. This uncertainty is because a detailed description of their field occurrence is lacking. Here, we describe the detailed occurrence of a cluster of tektite fragments recovered from the “laterite” layer near Huai Om, northeastern Thailand, and demonstrate the evidence of in situ occurrence of the tektites. At least 331 tektite fragments with a total weight of 713 g were found from a 40 × 30 cm area with 10 cm thickness in the uppermost part of the “laterite” layer. The very angular shapes and very poorly sorted nature of the fragments, restoration of larger tektite fragments into one ellipsoidal Muong Nong-type (MN) tektite mass, and the similar chemical composition of the fragments suggest that these MN tektite fragments represent a tektite mass that fragmented in situ. The fact ...
Chemical variation within fragments of Australasian tektites
Meteoritics & Planetary Science, 2005
available online at http://meteoritics.org 805 Abstract-In previous studies, intersample variation between compositions of different tektites from one particular group were studied and, in a few cases, major element variations within single tektites. No data for intra-sample trace element variations existed. Thus, we sectioned a Muong Nong-type tektite fragment from Vietnam and a splash-form tektite fragment from the Philippines into eleven and six pieces, respectively, and analyzed the individual fragments for major and trace element contents.
A sedimentary profile exposed in soil quarries a few kilometers north of Noen Sa-nga district, Chaiyaphum province, NE Thailand, reveals a Quaternary geological history. The lower part of the soil profile is a gravel deposit characterized by sub-angular to well-rounded pebbles representing an abandoned river sedimentary deposit. The gravel bed in some places is coated by iron oxide as a thin layer of ferricrete at the uppermost part. The upper part of the soil profile is a bright reddish brown structureless sand deposit with fining upward sedimentary structure at the basal portion, the Yasothon soil series. A piece of tektite was discovered at the contact boundary between the two sedimentary units. It is characterized by an irregular shape with a smooth concaved surface regarding as an external mold of a piece of well-rounded pebble. This piece of tektite indicates that a solidified tektite had fallen from high sky then was remelted into a plastic form prior to reach and partly cover a piece of well-rounded pebble. These evidences suggest that there was a meteoritic or cometary impact on our earth surface somewhere in the region then catapulted numbers of melted silica ejecta with vast volume of dust into the sky. The melted silica ejecta were solidified into splash-form tektites with various shapes while they were in the high sky. After that the tektites had fallen down and remelted into a plastic form prior to reach the ground surface and then solidified as a tektite deposit followed by larger-sized sediments and angular quartz fragments forming a fining upward sedimentary structure. The finer sediments were gradually settled down forming a bright reddish brown structureless sand deposit, the Yasothon soil series. This meteoritic impact event occurred at about 0.77 Ma ago as the evidence of the previous tektite radiometric dating.
Field recovery of layered tektites in northeast Thailand
Journal of Geophysical Research, 1995
We recovered Australasian tektites in place throughout a 40×130 km region in northeast Thailand extending from the Laotian border westward to a line connecting Na Pho Klang in the northeast through Det Udom to Nam Yun in the south. With two exceptions, in sites near the western edge of this region, all fragments are layered (muong-Nong-type) tektites. It appears that large layered tektites are mainly found by rice farmers in fields that were forested until the recent past. The presence of layered tektites in this 40×130 km area implies that impact melt that fell in these areas was not enough to flow if it was deposited on a sloping surface. The absence of splash-form tektites from the region indicates that the layer was still molten when masses having shapes (teardrops, dumbbells, etc.) produced by spinning reached the ground. To account for this and to allow time for the melt to flow a few tens of centimeters requires that the atmosphere remained hot (>2300 K) for a few minutes. Tektites that are in place are almost always associated with a wide-spread 10-cm to meter-thick layer of laterite. In two cases (one involving layered, one involving splash-form tektites), where accurate stratigraphic control demonstrated that the tektites were still in section, they were sited on top of the laterite layer just below a layer of aeolian sand.
Tektites, minitektites and microtektites from the Kalgoorlie region, Western Australia
2012
Introduction About 790 ka (Schneider et al. 1992), an asteroid or comet impacted in southeast Asia, melting crustal rocks (and regolith) and producing glassy impact debris, known as tektites or australites, which are found over more than 10% of the Earth’s surface (Schnetzler & McHone 1996), including much of Australia and surrounding oceans (Fig. 1). The tektites formed as molten “splash” material cooled during high-velocity movement through the air and range in size from spheres less than 1 mm (microtektites, found mainly in deep sea cores) to irregular blocks weighing up to more than 20 kg (Muong Nong tektites in SE Asia). The distribution, size and concentration of tektites in the Australasian strewn field indicate a likely impact site somewhere in southern Laos, northern Cambodia or eastern Thailand, but the location of the impact site has not yet been discovered.
A review of evidence for a Gulf of Tonkin location for the Australasian tektite source crater
Thai Geoscience Journal, 2021
Australasian tektites (AAT) occur across Southeast Asia, Australia, the Indian Ocean, and southwest Pacific Ocean. AAT form the youngest and most extensive major tektite strewn field. Unlike other tektite strewn fields, AAT have no known source crater. Review of the literature establishes that a single ~ 43 km post-impact diameter crater exists, possibly significantly enlarged by slumping. The obliquity of the impact that formed the AAT would result in a crater that is less pervasive in depth but with greater downrange shock effects and melt ejection. Multiple lines of evidence, historically viewed in isolation, were examined, concatenated, contextualized, and discussed. Tektite morphology and distribution; microtektite regressions; geochemical considerations, comparisons, and iso-concentration regressions; lithological characteristics; age of source rock; and regional geological considerations are reviewed. The source material is predicted to be an abnormally thick sequence of rapidly deposited, poorly compacted, deltaic to shallow marine, shales to clay-rich siltstones of early Pleistocene to Pliocene age. The impact likely occurred in a shallow marine environment. Fortytwo maps of positive and negative parameters are presented and overlain. These indicate the AAT source crater probably lies in the central to northwestern Yinggehai-Song Hong Basin / Gulf of Tonkin. This geochemically optimal setting is characterized by exceptionally high sedimentation rates that explain the 10 Be and Rb-Sr age discrepancy, the seawater signature, and apparent absence of a crater by rapid burial.
Australasian Tektites: Near and Far
Meteorite Magazine, 2009
Tektites are formed from silica-rich terrestrial deposits that are melted and ejected by cosmic impacts. The tektites re-enter the atmosphere minutes later. It takes special circumstances to form tektites. The target rock must be siliceous, which is the main reason tektites are not expected on most planetary bodies; the impact must be of sufficient size; an oblique impact of around 30 degrees (Artemieva and Pierazzo 2003) optimizes tektite production and a high velocity impact will likely aid tektite production and ejection. The Australasian strewnfield, being the youngest and freshest, affords us a unique insight into tektite formation and the aftermath of cosmic impacts. Tektites are usually reworked into porous and permeable gravel and then etched to non-existence. The remaining tektites will devitrify and turn to clay minerals, obliterating the geological record; hence the absence of true tektites older than the Tertiary period. When an asteroid or comet impacts the planetary surface, the kinetic energy melts the terrestrial rock. Sedimentary sands and shales, which are high in silica, are ideal for tektite production. Some of the momentum of the impactor is conserved and passed on to the melted rock. The lower the impact angle, the more effectively the momentum can be transferred to the melted rock. The melted rock must, however, traverse the dense atmosphere. An impact/ ejection angle that is too oblique prevents the melted rock effectively leaving the atmosphere (Artemieva and Pierazzo 2003). The melted rock is not ejected as individual tektites, which would be rapidly stopped by the dense atmosphere of the Earth. Instead, as with a splash of water, the melted rock is ejected as a sheet (Fig 1.). This sheet is eventually disrupted and breaks into isolated tektite droplets. By this stage, though, the melt sheet has passed through the densest layers of the atmosphere and there is little in the way of atmospheric impedance to stop these tektites. For roughly every 5.5 km ascended, the atmospheric pressure halves. So, roughly 50°/o of the atmosphere lies below 5.5 km; 7 5°/o below 11 km; 87 .5°/o below 16.5km; 93.8°/o below 22 km; 96.9°/o below 27.5 km; 98.4°/o below 33 km; and 99 .2°/o below 38.5 km. It is clear that by the time the melt sheet is disrupted and the small isolated tektite bodies start their journey, the dense atmosphere below is no longer an issue until they return to Earth. .:-· .,.
Basaltic rocks frorn Khorat Plateau, Thailand, dated at 0.9 Ma, coincide in time with the lithospheric extension of continental southeast Asia that began in the mid-Cenozoic. Dominated by alkali-olivine basalt and hawaiite compositions, they are generally alkalic and show specific petrologic and geochemical variations. Their trace-element and isotopic compositions are generally similar to those of ocean island basalts, and define two distinct groups. The group-I rocks have moderately depleted and relatively homogeneous isotopic ratios with '43Nd/ lUNd = 0.51287-0.51296, "Sr/ "Sr = 0.70354-0.70388 and Pb-isotopic ratios that are fairly nonradiogentic (*06Pb/ 204Pb = 18.23-18.32, *"Pb/ *04Pb = 15.47-15.53 and *'*Pb/ '04Pb = 38.16-38.27).