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1.
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2.
Ten splash‐form tektites from the Australasian strewn field, with masses ranging from 21.20 to 175.00 g and exhibiting a variety of shapes (teardrop, ellipsoid, dumbbell, disk), have been imaged using a high‐resolution laser digitizer. Despite challenges due to the samples’ rounded shapes and pitted surfaces, the images were combined to create 3‐D tektite models, which captured surface features with a high fidelity (≈30 voxel mm?2) and from which volume could be measured noninvasively. The laser‐derived density for the tektites averaged 2.41 ± 0.11 g cm?3. Corresponding densities obtained via the Archimedean bead method averaged 2.36 ± 0.05 g cm?3. In addition to their curational value, the 3‐D models can be used to calculate the tektites’ moments of inertia and rotation periods while in flight, as a probe of their formation environment. Typical tektite rotation periods are estimated to be on the order of 1 s. Numerical simulations of air flow around the models at Reynolds numbers ranging from 1 to 106 suggest that the relative velocity of the tektites with respect to the air must have been <10 m s?1 during viscous deformation. This low relative velocity is consistent with tektite material being carried along by expanding gases in the early time following the impact.  相似文献   

3.
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. The compositions obtained agree well with those found in previous studies, supporting argument that tektites have been derived from terrestrial upper crustal sediments. Chemical variations within the tektite fragments are present, but do not show any systematic trends, probably reflecting incomplete mixing of parent rocks. The intra‐sample heterogeneity of the Muong Nong‐type tektite is more pronounced than that in the philippinite. For the Muong Nong‐type tektite, the intra‐sample variation in the trace element contents is higher than that for the major elements, again reflecting target rock properties. For the philippinite the intra‐sample variations mostly do not exceed the limits imposed by the precision of the analytical data, confirming that the splash form tektites are indeed well homogenized.  相似文献   

4.
Abstract— The size, shape, composition, and vesicle content of 6 kg of layered tektite fragments, excavated near the town of Huai Sai, Thailand, place some constraints on the formation of layered tektites. The mass, shape, and distribution of the fragments are not consistent with an origin as a “puddle” of impact melt but suggest that they were derived from a single equant block. The presence of vesicles up to 7 mm in mean diameter within the tektite fragments suggests that the material was too viscous to allow for significant gravity-driven flow. These results suggest that layered tektites may be analogous to lava bombs, which may have been stretched and deformed in flight but underwent little flow after landing. Rather than being a product of “unusual circumstances,” such as multiple impacts, layered tektites may differ from splash-form tektites only in initial temperature of formation, speed of ejection, and small differences in initial composition.  相似文献   

5.
Elgygytgyn crater (lat. 67–30 N, long. 172–00 E) in remote northeastern Siberia is proposed as the meteorite impact site from which the Australasian tektite strewnfield was splashed. The following points support this interpretation: 1, Elgygytgyn very likely is an impact crater and is of adequate size, 18 km across, to generate tektites; 2, the apex of the strewnfield points towards this crater; 3, the terrane is Mesozoic which fits the age of the tektite parental material from Sr/Rb data; 4, compositional and specific gravity lineations within the strewnfield are directed, in part, toward this crater; 5, the high velocity tektites, australites, are distal with respect to this crater while the low velocity tektites, splash forms and Muong Nong tektites, are proximal; 6, the loess deposits and mixed acid/basic rocks of the impact site provide a suitable subgraywacke-type source material; 7, the erosional state of Elgygytgyn suggests that its age may well be in accordance with that of the Australasian tektite event, i.e., 700,000 years.  相似文献   

6.
Moldavites represent tektites derived from the Ries impact structure (~24 km diameter, ~15 Myr old) in southern Germany. Two new localities with parautochthonous moldavites in southwestern Poland were found. In these localities, fluvial sediments of the so‐called Gozdnicka formation host the moldavites. Characteristic tektite features, especially bubbles and inclusions of lechatelierite, are reported. The moldavites' size distribution and their abraded shapes indicate that they were redeposited from the nearby Lusatia substrewn field.  相似文献   

7.
Abstract— We have analyzed the potassium isotopic composition of four tektites from the Australasian strewn field, spanning a wide diversity of thermal histories, inferred from textures and volatile element contents. Our results indicate no isotopic differences between tektites and terrestrial crustal rocks, placing stringent limits of ≤2% loss of potassium during the brief duration of high temperature heating experienced by these samples. This confirms that the chemical composition of tektites is entirely a reflection of source rock composition and has not been modified by the tektiteforming process for elements less volatile than potassium. Losses of more volatile components, e.g., the halogens and water, are not precluded by the present data. Coupling a radiative cooling temperature‐time path with potassium vapor pressure data indicates that tektite melt drops are not likely to develop bulk elemental fractionation during the brief heating episodes of tektites for peak temperatures <2273 K. The extent of K isotopic fractionation is independent of droplet size but dependent on peak heating temperature. The exact peak temperature depends on the choice of vapor pressure data used for K, which need to be better constrained.  相似文献   

8.
Abstract— One hundred and thirteen Australasian tektites from Vietnam (Hanoi, Vinh, Dalat, and Saigon areas) were analyzed for their major and trace element contents. The tektites are either of splash form or Muong Nong‐type. The splash‐form tektites have SiO2 contents ranging from 69.7 to 76.8 wt%, whereas Muong Nong‐type tektites, which are considerably larger than splash‐form tektites and have a blocky and chunky appearance, have slightly higher silica contents in the range of 74–81 wt%. Major‐element relationships, such as FeO versus major oxides, Na2O versus K2O, and oxide ratio plots, were used to distinguish the different groups of the tektites. In addition, correlation coefficients have been calculated for each tektite group of this study. Many chemical similarities are noted between Hanoi and Vinh tektites from the north of Vietnam, except that the Hanoi tektites contain higher contents of CaO than Vinh; the higher content of CaO might be due to some carbonate parent material. Both Dalat and Saigon tektites have nearly similar composition, whereas the bulk chemistries of the tektites from Hanoi and Vinh appear different from those of Saigon and Dalat. There are differences, especially in the lower CaO and Na2O and higher MgO, FeO, for the tektites of Dalat and Saigon in comparison to that of Hanoi tektites. Furthermore, the Dalat and Saigon tektites show enrichments by factors of 3 and 2 for the Ni and Cr contents, respectively, compared to those of Hanoi and Vinh. The difference in chemistry between the North Vietnam tektites (Hanoi, Vinh) to that of South Vietnam tektites (Saigon, Dalat) of this study indicate that the parent material was heterogeneous and possibly mixing between different source rocks took place. Muong Nong‐type tektites are enriched in the volatile elements such as Br, Zn, As, and Sb compared to the average splash‐form tektites of this study. The chemical compositions of the average splash‐form and Muong Nong‐type tektites of this study closely resemble published data for average splash‐form and Muong Nong‐type indochinites, indicating that they have the same source. The trace element ratios Ba/Rb (2.7), Th/U (5.2), Th/Sc (1.3), Th/Sm (2.2), and the rare earth element (REE) abundances of this study show close similarities to those of average upper continental crust.  相似文献   

9.
Abstract— Tektites are natural glasses formed from terrestrial material that was melted and displaced by the impact of an extraterrestrial body. The surface and near-surface compositions of tektite glass results from fractionation during impact and ejection, and/or postsolidification weathering. The first goal of this study was to characterise the surface and near-surface (in the order of tens of angstroms) chemical composition of two tektites by x-ray photoelectron spectroscopy (XPS), and to estimate the importance of weathering vs. fractionation during flying. In order to separate the chemical modification due to weathering from that due to fractionation during ballistic flight, we studied two samples from the Australasian tektite strewn field. One of them was collected in a hot desert area (Nullarbor Plain, Australia) and the other, in a humid climate (Thailand). Our study reveals the presence of well-developed leached layers in both tektites. In the Australian tektite, Si is depleted in the topmost layers (a few tens of angstroms). A more complex chemical zoning is defined in the tektite from Thailand. These leached layers are comparable to those observed in weathered glasses, and therefore we conclude that weathering is responsible for the chemical composition of the surface and near-surface compositions. The second goal was to investigate the chemical environment of O, N and C in the glass. The O peak was resolved into two bridging O components (Si-O-Si and Al-O-Si) that are comparable to O environments in artificial glasses. The binding energy of the C1s electron is typical for C-C and C-H bonds in hydrocarbons; minor organic acid components are also present. Nitrogen is only observed on the surface of the Thailand tektite. The binding energy of N1s is comparable to that of ammonia, and the surface enrichment in N is interpreted as due to sorption related to interactions between glass and fluid buffered by the organic material in the soil.  相似文献   

10.
Abstract– Tektites, natural silica‐rich glasses produced during impact events, commonly contain bubbles. The paper reviews published data on pressure and composition of a gas phase contained in the tektite bubbles and data on other volatile compounds which can be released from tektites by either high‐temperature melting or by crushing or milling under vacuum. Gas extraction from tektites using high‐temperature melting generally produced higher gas yield and different gas composition than the low‐temperature extraction using crushing or milling under vacuum. The high‐temperature extraction obviously releases volatiles not only from the bubbles, but also volatile compounds contained directly in the glass. Moreover, the gas composition can be modified by reactions between the released gases and the glass melt. Published data indicate that besides CO2 and/or CO in the bubbles, another carbon reservoir is present directly in the tektite glass. To clarify the problem of carbon content and carbon isotopic composition of the tektite glass, three samples from the Central European tektite strewn field—moldavites—were analyzed. The samples contained only 35–41 ppm C with δ13C values in the range from ?28.5 to ?29.9‰ VPDB. This indicates that terrestrial organic matter was a dominant carbon source during moldavite formation.  相似文献   

11.
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12.
Abstract— During Leg 150 of the Ocean Drilling Project (ODP), two sites (903C and 904A) were cored that have sediments of the same biostratigraphic age as the upper Eocene tektite-bearing ejecta layer at Deep Sea Drilling Project (DSDP) Site 612. Core 45X from ODP Site 904A (～4 km north of Site 612) contains a 5 cm thick tektite-bearing ejecta layer, and Core 56 from Site 903C (～8 km north-northwest of Site 904) contains a 2 cm thick layer of impact ejecta without any tektite or impact glass. Shocked quartz and feldspar grains, with multiple sets of planar deformation features (PDFs), and abundant coesite-bearing grains are present at both sites. The major oxide contents, trace element compositions, and rare earth element (REE) patterns of the Site 904 tektites are similar to those of the Site 612 tektites and to North American tektites (especially bediasites). The ?Sr and ?Nd values for one composite tektite sample from Site 904 fall within the range previously obtained for the Site 612 tektites, which defines a linear trend that, if extrapolated, would intersect the values obtained for North American tektites. The water contents of eight tektite fragments from Site 904 range from 0.017 to 0.098 wt%, and, thus, are somewhat higher than is typical for tektites. The heavy mineral assemblages of the 63–125 μm size fractions from the ejecta layers at Sites 612, 903, and 904 are all similar. Therefore, we conclude that the ejecta layer at all three sites is from the same impact event and that the tektites at Sites 904 and 612 belong to the North American tektite strewn field. Clinopyroxene-bearing (cpx) spherules occur below, or in the lower part of, the main ejecta layer at all three sites. At all three sites, the cpx spherules have been partly or completely replaced with pyrite that preserved the original crystalline textures. Site 612, 903, and 904 cpx spherules are similar to those found in the Caribbean Sea, Gulf of Mexico, central equatorial Pacific, western equatorial Pacific, and eastern Indian Ocean. The cpx event appears to have preceded the North American tektite event by 10–15 ka or less. The fining-upward sequence at all three sites and concentration of the denser, unmelted impact ejecta at the top of the tektite layer at Sites 612 and 904 suggest that the tektite-bearing ejecta layers are not the result of downslope redeposition and that the unmelted ejecta landed after the glass. Geographic variations in thickness of the tektite-bearing ejecta layer, the lack of carbonate clasts in the ejecta layer, and the low CaO content of the tektite glass suggest that the ejecta (including the tektite glass) were derived from the Chesapeake Bay structure rather than from the Toms Canyon structure. A sharp decline in microfossil abundances suggests that local environmental changes caused by the impact may have had adverse effects on benthic foraminifera, radiolaria, sponges, and fish as well as the planktic foraminifera.  相似文献   

13.
Abstract— Five indochinites from Hainan Island and the Leizhou Peninsula, China were analyzed for noble gas abundances and isotopic ratios. These splash‐form tektites show vesiculation ranging from 0.4 vol% to 8 vol%, as determined by digital image analysis (software SXM®) on thin section photographs. To study the distribution of noble gases in vesicles and in glass, the gases were extracted by heating and by crushing, respectively, on 2 aliquots of the same sample. The results show that 5 to 53% of the total measured 20Ne resides in vesicles. The calculated concentration of neon dissolved in the glass is higher (0.7‐1 times 10?7 cm3STP/g) than that expected from solubility equilibrium (1 times 10?8 cm3STP/g), assuming solubility data from MORB glasses. The neon concentration of splash‐form tektites, those analyzed in this work and those from other strewn‐fields worldwide, is correlated with the SiO2 content of glass and with the nonbridging oxygen per tetrahedral cation in the melt (hereafter NBO/T ratio), the latter being an index of the free‐volume in the silica network where neon could be dissolved. These correlations suggest that the glass structure of tektite has a larger free‐volume available for dissolving noble gases than MORB glasses.  相似文献   

14.
Abstract— We have recovered 18 kg of layered tektites from 10 tektite-bearing localities in Laos and central Vietnam, including 5 localities around the town of Muong Nong (Laos). Several of these deposits originally contained several hundred kilograms of layered tektite fragments, and one fragmented mass may have been as large as 1000 kg. This is the largest single deposit of tektites yet reported. In this region, layered tektite fragments are found in isolated clusters usually associated with a pebbly laterite horizon that is 0–1 m below the surface. Near Khe Sanh, Vietnam, we estimate the abundance of layered tektite fragments to be ～100 g/m2. This is greater than five times the abundance estimated for northeast Thailand (Fiske et al., 1996). In a region that extends from northeast Thailand, through central Laos, and into central Vietnam, we found only layered tektites, which confirmed the existence of a large (>50 000 km2) subfield of the Australasian strewn field with only layered tektites. The east-west extent of the “layered-only” subfield is well constrained, but little field data exist to constrain its north-south extent.  相似文献   

15.
Abstract— A 10.79-kg layered tektite from Hainan is the largest tektite from China, and the fifth largest reported to date. It together with a 1.9-kg Hainan tektite described by Yuan (1981) greatly extends the area of the Australasian field within which layered tektites having masses > 1 kg have been found.  相似文献   

16.
Abstract The source crater of the youngest and largest of the tektite strewnfields, the Australasian strewnfield, has not been located. A number of lines of evidence indicate that the Muong Nong-type tektites, primarily found in Indochina, are more primitive than the much more abundant and widespread splash-form tektites, and are proximal to the source. In this study the spatial distribution of Muong Nong-type tektite sites and chemical character have been used to indicate the approximate location of the source. The variation of Muong Nong-type tektite chemical composition appears to be caused by mixing of two silicate rock end-members and a small amount of limestone, and not by vapor fractionation. The variation in composition is not random, and does not support in-situ melting or multiple impact theories. The distribution of both Muong Nong and splash-form tektite sites suggest the source is in a limited area near the southern part of the Thailand-Laos border.  相似文献   

17.
Abstract— Elemental and isotopic compositions of the noble gases have been determined in six North American tektites (4 bediasites and 2 georgiaites) and one Ivory Coast tektite. Radiogenically produced 4He may explain the large 4He/36Ar ratios measured relative to air, despite significant diffusive losses. The Ne isotopic composition is enriched in 20Ne consistent with a single stage mass fractionation process. The enormous 20Ne/36Ar enrichments observed in all tektite samples, similar to those reported from other tektites and impact glasses, are attributed to atmospheric diffusion into the samples following solidification. The North American tektites show a systematic increase in 84Kr/36Ar and 132Xe/36Ar relative to air, with enrichments greater than those determined for any other tektite group or terrestrial samples other than shales. These enrichments are inconsistent with existing models of dissolving Kr and Xe in tektite glass without elemental fractionation at atmospheric pressures equivalent to ∼40 km altitude. The Kr and Xe isotopic compositions are indistinguishable from atmospheric within experimental uncertainty.  相似文献   

18.
Abstract— The origin of tektites has been obscure because of the following dilemma. The application of physical principles to the data available on tektites points strongly to origin from one or more lunar volcanoes; but few glasses of tektite composition have hitherto been reported from the lunar samples. Instead, the lunar silicic glasses consist chiefly of a material very rich in K2O and poor in MgO. The ratio of K2O/MgO is higher in these glasses than in any tektites reported. The solution of the dilemma seems to come from the study of some recently discovered terrestrial deposits of tektite glass with high values of K2O/MgO at the Cretaceous-Tertiary boundary. These glasses are found to be very vulnerable to crystallization into sandine or to alteration to smectite. These end products are known and are more abundant than any terrestrial deposits of tektite glass. It seems possible that, in fact, the moon produces tektite glass, mostly of the high K2O-low MgO type; but on Earth these deposits are destroyed. The much less abundant deposits with lower K and higher Mg are observed because they survive. Other objections to the lunar origin hypothesis appear to be answerable.  相似文献   

19.
Abstract— The site of an impact event that spread ejecta in the form of tektites and microtektites over ～5 × 107 km2 of the southern Pacific and Indian Ocean area has not yet been discovered. A number of lines of evidence point toward a source in eastern Indochina. From an examination of a digital topographic data set and Landsat imagery, we identified four candidate structures in southern Laos, and we visited these sites in 1995 February. No evidence of impact origin of these structures could be found; flat-lying, undisturbed Mesozoic sedimentary rocks similar to those on Thailand's Khorat Plateau were found over the region. Small layered tektite fragments are relatively common in a lateritic horizon that is characterized by the presence of quartz pebbles. This scene is identical to the situation found several hundred kilometers to the southeast in Thailand. New tektite sites identified on this trip support a previous suggestion that there is a large region in southern NE Thailand and Laos that is rich in Muong Nong-type (layered) tektites but seemingly devoid of the splash-form type tektites.  相似文献   

20.
A possible crater representing the source of Australasian tektites is identified in northeastern Cambodia at longitude, 106° 34′E., and latitude, 13° 55′N. The crater is an incomplete oval ring of hills 10 km long and 6 km wide located near the center of the Muong Nong-type tektite strewnfield. The morphology of the structure may have been significantly changed by relatively recent erosion and deposition processes. The types of rocks in the area of the crater are consistent with the compositional requirements for the tektite source rocks. Collection of impactite material from the vicinity of the structure will be required to confirm its association with Australasian tektites.  相似文献   

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