Lechatelierite inclusions in indochinites and the origin of tektites

Abstract— The grain size distribution and shapes of lechatelierite inclusions (silica glass inclusions) have been determined from 20 splash-form tektites from the Khorat Plateau, northeastern Thailand. The chemical and petrographic properties are reviewed, and the absence of any type of inclusion other than bubbles is confirmed. These data suggest that the parent material for the lechatelierite inclusions is not the conventional detrital quartz. One possible precursor is silica of plant origin in the form of biogenic opal-CT. According to this model, the lechatelierite inclusions are formed by shock melting of opal phytoliths in plants. These opal phytolith melts were included in the shock-melted soil and bedrock, jetted from the impact site. The expansion of the vapor plume ejected the melt droplets in ballistic trajectories. This model is extended to all tektite groups, because of the similarity between lechatelierite inclusions in them.     

A new discovery of parautochthonous moldavites in southwestern Poland,Central Europe

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.     

Tektites: Size estimates of their source craters and implications for their origin

From estimates of the total masses of tektites in three strewnfields, calculations by Orphal et al. (1980) of the amount of melt that could be ejected from impact craters, and equations relating kinetic energy of impact to crater diameter, it is possible to calculate minimum diameters of lunar craters capable of ejecting the liquid masses that could have formed the various tektite strewnfields. No lunar craters of the requisite sizes have been found that are young enough to correlate with the dates of formations of the strewnfields and it seems clear that the Moon must be eliminated as a source of tektites on the Earth. It is concluded that the associations of the Ivory Coast tektites with the Bosumtwi crater and the moldavites with the Rieskessel are real and the tektites are of terrestrial origin. It follows that if the Ivory Coast tektites came from the 10.5-km-wide Bosumtwi crater, the larger masses in the Australasian and North American strewnfields came from craters 17 km in diameter and between 33 and 65 km in diameter, respectively. No crater has yet been proven to be the parent of the Australisian tektites. The large crater that formed the North American tektites may not yet have been found, although the Mistastin Lake Crater may eventually be proven to be the source.     

Nd and Sr isotopic evidence for the origin of tektite material from DSDP site 612 off the New Jersey coast

Abstract— Late Eocene tektite material from DSDP site 612 is composed of angular to spherical tektites and microtektites containing abundant vesicles and a few unmelted to partially melted mineral inclusions. The major element compositions of the 612-tektites are generally comparable to those of North American tektites, but the physical features suggest that the DSDP-612 tektites were formed by less severe shock melting. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd compositions of 612-tektites: a) show much wider ranges than the tightly constrained group of North American tektites and microtektites, and b) are significantly different from those of other groups of tektites. The existence of large isotopic variations in tektites from DSDP site 612 requires that they were formed from a chemically and isotopically heterogeneous material in a regime that is distinctive from that of other groups of tektites. T^ND_CHUR and T^Sr_UR model ages of the 612-tektites indicate that they were formed from a crustal source of late Precambrian mean age (800–1000 Ma) which in middle Palaeozoic time (?400 Ma) was further enriched in Rb/Sr during sedimentary processes. These source characteristics suggest that the impact which produced the 612-tektites occurred in rocks of the Appalachian orogeny or sediments derived from this orogenic belt. Potential source materials for both 612-tektites and North American tektites are present on the eastern and southeastern part of the North American continent and its adjacent shelf. The distinct isotopic differences between 612-tektites and North American tektites indicate that the two groups of tektites were either formed by the impact of more than one bolide in the same general area, or by a single impact event that sampled different layers.     

Fluorine and boron geochemistry of tektites,impact glasses,and target rocks

Abstract— We have analyzed fluorine and boron in nine tektites from all four strewn fields, and in a suite of impact glasses and target rocks from the Zhamanshin and Darwin impact craters, as well as Libyan Desert Glass and Aouelloul impact glass samples. Fluorine and boron are useful indicators for the volatilization and temperature history of tektites and impact glasses. Tektites from different strewn fields show a limited range of F and B contents and have F/B ratios near unity. Most splash-form tektites have lower average F and B contents than Muong Nong type tektites, which is similar to the relation between irghizites and zhamanshinites. The F and B contents in target rocks from the Zhamanshin and Darwin impact craters are similar to normal terrestrial sediments. Fluorine in impact glasses and tektites is more depleted compared to their (known or inferred) target rocks than is boron, which is caused by the higher volatility of F. The F/B ratios therefore decrease with increasing temperature of formation (suggesting that irghizites were formed at a higher temperature than zhamanshinites, and Muong Nong type tektites at a lower temperature than splash-form tektites). Mixing of local country rocks together with partial loss of the volatiles F and B can reproduce the F and B contents of impact glasses.     

Variation of chemical composition in Australasian tektites from different localities in Vietnam

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.     

Major and trace element compositions of georgiaites: Clues to the source of North American tektites

Abstract— Electron microprobe and laser ablation, inductively‐coupled plasma mass spectrometer analyses of 24 georgiaites show that these tektites are all Si‐rich (79–83 wt% SiO₂) glasses with variable major and trace element abundances (e.g., FeO varies from 2.1 to 3.7 wt%). Glass compositions are similar to but not identical with average upper continental crust. For example, georgiaites are light rare earth element enriched with small negative Eu anomalies (Eu/Eu*=0.73‐0.86) and La‐Th‐Sc systematics are intermediate between that of Archean and post‐Archean continental crust. When the georgiaite data are placed in the context of data for all North American tektites, triangular arrays appear on some oxide‐oxide plots (e.g., FeO‐MgO). Large variations in refractory element abundances and ratios compared to the variation in SiO₂ favors mixing over volatilization as a cause of the compositional variation. If all the tektites formed as a result of a single impact, then triangular arrays in oxide‐oxide variation diagrams require at least three source components. These components include a Si‐rich material, probably a quartz‐rich sand that was predominant in the formation of georgiaites. Two relatively silica‐poor and Fe‐rich components have compositional characteristics similar to shales and greywackes. The La‐Th‐Sc systematics of the georgiaites and most other North American tektites are distinctive and could potentially be used to link the tektites to Eocene sediments at the Chesapeake Bay impact structure.     

Constraints on the formation of layered tektites from the excavation and analysis of layered tektites from northeast Thailand

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.     

A review of volatile compounds in tektites,and carbon content and isotopic composition of moldavite glass

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.     

COMPARING INVESTIGATIONS ON THE SURFACE STRUCTURES OF IRGHIZITES AND PYROCLASTICS BY SEM

The surface structures of irghizites from the Zhamanshin crater in Kazachstan, USSR, play an important role in the discussion of their genesis. These surface structures were compared with those of typical tektites (australites) and pyroclastics (obsidians, lapilli) based on investigation by electron microscope. The results of these investigations indicate that there are no unambiguous genetical relationships between the morphology of irghizites and the surface features of tektites and pyroclastics. The surface shapes of irghizites result from several simultaneous or successive processes, in the course of which globules of different size melted, stuck together and were eaten into by corrosive gases after their solidification. The assumption that the verrucose swellings were caused by expanding gas bubbles immediately below the surface of the glass bulk can be excluded. The verrucose glass globules are identical in chemical composition to the glass bulk of the irghizites.     

ZHAMANSHIN AND AOUELLOUL: CRATERS PRODUCED BY IMPACT OF TEKTITE-LIKE GLASSES?

It is shown that the enhanced abundance of siderophile elements and chromium in tektite-like glasses from the two impact craters of Zhamanshin and Aouelloul cannot be explained as a result of contamination of the country rock by meteorites nor, probably, comets. The pattern is, however, like that found in certain Australasian tektites, and in Ivory Coast tektites. It is concluded, in agreement with earlier suggestions by Campbell-Smith and Hey, that these craters were formed by the impact of large masses of tektite-like glass, of which the glasses which were studied are fragments. It follows that it is necessary, in considering an impact crater, to bear in mind that the projectile may have been a glass.     

IRGHIZITES AND ZHAMANSHINITES: ZHAMANSHIN CRATER,USSR

Two irghizites, three zhamanshinites and one sample each of lechatelierite, vein-quartz, Palaeogene silty clay and Palaeogene quartzite were analyzed using neutron activation analysis. A silicate analysis of the Palaeogene silty clay has also been performed, as well as an incomplete analysis of the Palaeogene quartzite from the Zhamanshin impact crater. The REE abundances of irghizites resemble those of sedimentary rocks. On the Köhler and Raaz diagram all projection points of irghizites lie inside the field of tektites, and indicate that they were derived from terrestrial sedimentary rocks. The Zhamanshin impact glasses may be divided into three types: (a) silica-rich zhamanshinites (x?SiO₂ = 73.89%), (b) zhamanshinites (x?SiO₂ = 54.34%), and (c) silica-poor zhamanshinites (x?SiO₂ = 39.64%). These are also characterized by varying proportions of alkalis and Al. Mn and Ca contents. Irghizites and silica-rich zhamanshinites display a depletion of Eu. Zhamanshinites do not show this Eu depletion. Partial melting is assumed to be an important process in the origin of zhamanshinites.     

Noble gases in Muong Nong‐type tektites and their implications

Abstract— We have the elemental abundances and isotopic compositions of noble gases in Muong Nong‐type tektites from the Australasian strewn field by crushing and by total fusion of the samples. We found that the abundances of the heavy noble gases are significantly enriched in Muong Nong‐type tektites compared to those in normal splash‐form tektites from the same strewn field. Neon enrichments were also observed in the Muong Nong‐type tektites, but the Ne/Ar ratios were lower than those in splash‐form tektites because of the higher Ar contents in the former. The absolute concentrations of the heavy noble gases in Muong Nong‐type tektites are similar to those in impact glasses. The isotopic ratios of the noble gases in Muong Nong‐type tektites are mostly identical to those in air, except for the presence of radiogenic ⁴⁰Ar. The obtained K‐Ar ages for Muong Nong‐type tektites were about 0.7 Myr, similar to ages of other Australasian tektites. The crushing experiments suggest that the noble gases in the Muong Nong‐type tektites reside mostly in vesicles, although Xe was largely affected by adsorbed atmosphere after crushing. We used the partial pressure of the heavy noble gases in vesicles to estimate the barometric pressure in the vesicles of the Muong Nong‐type tektites. Likely, Muong Nong‐type tektites solidified at the altitude (between the surface and a maximum height of 8–30 km) lower than that for splash‐form tektites.     

Two layers of Australasian impact ejecta in the Indian Ocean?

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.     

Origin of tektites: Constraints from heavy noble gas concentrations

Abstract— Heavy noble gas concentrations in tektites (splash-form type) are considerably lower than those in impact glasses. This can not be explained only by high formation temperatures for tektites, as might be expected from low concentrations of water and most volatile elements in tektites, and indicates that tektites solidified in an atmosphere with an ambient pressure of much less than 1 atm. The heavy noble gas concentrations may be an indicator of the height to which tektites were carried by the impact before they solidified.     

Stratigraphic relations of australites in the Port Campbell Embayment,Victoria

Abstract— In the Port Campbell Embayment of Victoria, australites have been found in situ in channel deposits of the Hanson Plain Sand of Pliocene and Pleistocene age. The large majority of the australites, however, occur as a lag deposit at the basal contact of the Sturgess Sand of late Pleistocene and Holocene age and are spatially correlated with ferruginous sandstone clasts that are derived from the Hanson Plain Sand. Some of the tektites are imbedded in or bonded to the ferruginous sandstone clasts, but most are found as individual tektite fragments. A few percent of the tektites have nearly perfectly preserved, complete aerodynamically shaped forms. The sandstone clasts and associated tektites have been reworked from the much older underlying Hanson Plain and have been locally concentrated in the lag deposit. Some tektites also occur at higher levels in the Sturgess Sand, almost invariably in association with stone flakes, exotic stones transported by the aborigines and, locally, with middens of mollusc shells. Circumstantial evidence indicates that the aborigines transported the tektites found in the upper part of the Sturgess, particularly at Stanhope Bay. As Port Campbell australites unequivocally occur in strata much older than the late Pleistocene and Holocene Sturgess, there is no longer any conflict between the apparent stratigraphic age of the tektites and the middle Pleistocene ages obtained by various chronometric methods.     

Halogens in tektites and impact glasses

Abstract— Impact glasses, tektites and some related basement rocks were analyzed for F, Cl, Br and I. The tektite and impact glasses show similar abundance patterns within the groups. Muong Nong-type tektites indicate that the halogens have been depleted in the order I > Br > Cl > F in their melt under oxidizing conditions. For Darwin Glass selective volatilization of F from the melt is a major depleting process. Cl, Br and I are lost to a lesser extent.     

Australites from Northern Australia

Abstract— Abundant tektites have been found in buried alluvial gravels at the south end of Lake Argyle in far northern Australia. The recovered tektites are unusual in that an astounding number are very large and 75% of those analyzed to date are HMg tektites, heretofore a very rare type in Australia. It is now clear that the paucity of tektites in northern Australia is due to the former high energy erosional environment rather than to crossing the strewnfield boundary, as has been previously suggested. The N-S extent of the australite strewnfield encompasses the entire continent.     

The chemical variation of moldavite tektites: Simple mixing of terrestrial sediments

Abstract— To test different hypotheses of moldavite formation, a major and trace-element study of 25 moldavite tektites and Sm-Nd isotope measurement of three moldavite tektites was completed. The samples were selected from the classical substrewnfields and the newly described locations in Lusatia (Saxony, Germany). Samples with unusual bulk composition were also included. The results confirm earlier studies that the variation in the chemical composition can be explained by single impact and through incomplete mixing of at least three lithographical components dominated by one of the three minerals or mineral groups: dolomite, clay minerals and quartz. An additional endmember, possibly a rare Earth's mantle component, containing high Co, Cr and Ni concentrations is also needed to explain the observed variations in compatible elements of some tektites. Volatile element abundances are low but not necessarily the result of selective volatilization.     

Upper Eocene tektite and impact ejecta layer on the continental slope off New Jersey

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.