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.     

Potassium isotopic composition of Australasian tektites

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.     

Triple oxygen isotope composition of Australasian tektites

Major and trace element analyses and triple oxygen isotope measurements were performed on 11 individual specimens of Australasian tektites (AAT) with exactly known field positions from Laos. The sample set was dominated by Muong Nong‐type tektites (MNAAT), including separated layers of glass of different appearance and chemistry from four samples. This first larger set of oxygen isotope data of MNAAT revealed the δ¹⁸O range 8.7 ≤ δ¹⁸O ≤ 11.6‰ on VSMOW2 scale (12 analyses), only slightly wider than the previously reported range for splash‐form AAT. The Δ’¹⁷O values of MNAAT (?0.098 ≤ Δ’¹⁷O ≤ ?0.069‰; 12 analyses) and splash‐form AAT (?0.080 ≤ Δ’¹⁷O ≤ ?0.068‰; three analyses) are all in the range of data typical for terrestrial crustal rocks, with no mass‐independent oxygen isotope fractionation (from impactor or from exchange with atmospheric O₂) being observed.     

Anomalous Ne enrichments in tektites

Abstract— We measured noble gases and Ne isotopic compositions of five tektites collected from three different strewn fields. The elemental abundance patterns of noble gases in all samples show anomalous Ne enrichments relative to air. Ne isotopic compositions in tektites are in good agreement with that of atmospheric Ne, suggesting that Ne has diffused in from the atmosphere. It is conceivable that the high relative Ne abundance is essentially an equilibrium effect, i.e., storage of Ne in vesicles rather than the glass itself, facilitated by the relatively high diffusion coefficient of Ne.     

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.     

Fragmentation and hydration of tektites and microtektites

Abstract— An examination of data collected over the last 30 years indicates that the percent of glass fragments vs. whole splash forms in the Cenozoic microtektite strewn fields increases towards the source crater (or source region). We propose that this is due to thermal stress produced when tektites and larger microtektites fall into water near the source crater while still relatively hot (>1150 °C). We also find evidence (low major oxide totals, frothing when melted) for hydration of most of the North American tektite fragments and microtektites found in marine sediments. High-temperature mass spectrometry indicates that these tektite fragments and microtektites contain up to 3.8 wt% H₂O. The H₂O-release behavior during the high-temperature mass-spectrometric analysis, plus high CI abundances (0.05 wt%), indicate that the North American tektite fragments and microtektites were hydrated in the marine environment (i.e., the H₂O was not trapped solely on quenching from a melt). The younger Ivory Coast and Australasian microtektites do not exhibit much evidence of hydration (at least not in excess of 0.5 wt% H₂O); this suggests that the degree of hydration increases with age. In addition, we find that some glass spherules (with <65 wt% SiO₂) from the upper Eocene clinopyroxene-bearing spherule layer in the Indian Ocean have palagonitized rims. These spherules appear to have been altered in a similar fashion to the splash form K/T boundary spherules. Thus, our data indicate that tektites and microtektites that generally contain >65 wt% SiO₂ can undergo simple hydration in the marine environment, while impact glasses (with <65 wt% SiO₂) can also undergo palagonitization.     

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.     

Chemical variation within fragments of Australasian tektites

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.     

Krypton and xenon fractionation in North American tektites

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.     

Potassium isotope abundances in Australasian tektites and microtektites

Abstract— We report electron microprobe determinations of the elemental compositions of 11 Australasian layered tektites and 28 Australasian microtektites; and ion microprobe determinations of the ⁴¹K/³⁹K ratios of all 11 tektites and 13 of the microtektites. The elemental compositions agree well with literature values, although the average potassium concentrations measured here for microtektites, 1.1‐1.6 wt%, are lower than published average values, 1.9‐2.9 wt%. The potassium isotope abundances of the Australasian layered tektites vary little. The average value of ^δ41K, 0.02 ± 0.12%₀ (1 s? mean), is indistinguishable from the terrestrial value (= 0 by definition) as represented by our standard, thereby confirming four earlier tektite analyses of Humayun and Koeberl (2004). In agreement with those authors, we conclude that evaporation has significantly altered neither the isotopic nor the elemental composition of Australasian layered tektites for elements less volatile than potassium. Although the average ⁴¹K/³⁹K ratio of the microtektites, 1.1 ± 1.7%₀ (1 s? mean), is also statistically indistinguishable from the value for the standard, the individual ratios vary over a very large range, from ?10.6 ± 1.4%₀ to +13.8 ± 1.5%₀ and at least three of them are significantly different from zero. We interpret these larger variations in terms of the evaporation of isotopically light potassium; condensation of potassium in the vapor plume; partial or complete stirring and quenching of the melts; and the possible uptake of potassium from seawater. That the average ⁴¹K/³⁹K ratio of the microtektites equals the terrestrial value suggests that the microtektite‐forming system was compositionally closed with respect to potassium and less volatile elements. The possibility remains open that ⁴¹K/³⁹K ratios of microtektites vary systematically with location in the strewn field.     

Noble gases and nitrogen in Muong Nong tektites

Abstract— Three samples of Muong Nong tektites have been studied for N and noble gases. The isotopic composition of noble gases is airlike. The noble gas amounts are much higher in Muong Nong tektites than in splash-form tektites. As compared to air, He and Ne have been enriched, most likely due to inward diffuion from ambient air, subsequent to glass formation. Nitrogen contents range from 0.3 to 1.34 ppm, with a non-atmospheric δ¹⁵N ranging from 8 to 17%. The release pattern of δ¹⁵N clearly shows the presence of two N components. Higher N/³⁶Ar values than those of air, together with positive δ¹⁵N, show that a major portion of N in Muong Nong tektites is a remnant from the sedimentary source material.     

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 laboratory model of splash‐form tektites

Abstract— Splash‐form tektites are generally acknowledged to have the form of bodies of revolution. However, no detailed fluid dynamical investigation of their form and stability has yet been undertaken. Here, we review the dynamics and stability of spinning, translating fluid drops with a view to making inferences concerning the dynamic history of tektites. We conclude that, unless the differential speed between the molten tektite and ambient is substantially less than the terminal velocity, molten tektites can exist as equilibrium bodies of revolution only up to sizes of 3 mm. Larger tektites are necessarily non‐equilibrium forms and so indicate the importance of cooling and solidification during flight. An examination of the shapes of rotating, translating drops indicates that rotating silicate drops in air will assume the shapes of bodies of rotation if their rotational speed is 1% or more of their translational speed. This requirement of only a very small rotational component explains why most splash‐form tektites correspond to bodies of revolution. A laboratory model that consists of rolling or tumbling molten metallic drops reproduces all of the known forms of splashform tektites, including spheres, oblate ellipsoids, dumbbells, teardrops, and tori. The laboratory also highlights important differences between rolling drops and tumbling drops in flight. For example, toroidal drops are much more stable in the former than in the latter situation.     

Source of Australasian tektites: Investigating possible impact sites in Laos

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.     

Water in tektites and impact glasses by fourier-transformed infrared spectrometry

Abstract— To improve the scarce data base of H₂O content in tektites and impact glasses, we analyzed 26 tektites from all four strewn fields and 25 impact glass samples for their H₂O content. We used the fourier-transformed infrared (FTIR) spectrometry method, which permits measurement of areas of ～40 μm in diameter. Our results show that the tektites have H₂O contents ranging from 0.002 to 0.030 wt% (average 0.014 ± 0.008 wt%). Ivory Coast tektites have the lowest H₂O abundances (0.002–0.003 wt%), and Muong Nong-type indochinites and some North American tektites having the highest contents (up to ～0.03 wt%). Impact glass samples (from the Zhamanshin, Aouelloul, and Rio Cuarto craters) yielded H₂O contents of 0.008 to 0.13 wt% H₂O. Typical impact glasses from the Aouelloul and Zhamanshin craters have low H₂O contents (0.008 to 0.063 wt%). Libyan Desert Glasses and Rio Cuarto glasses have higher H₂O contents (～0.11 wt%). We also analyzed glasses of unknown origin (e.g., urengoites; glass fragments from Tikal), which showed very low H₂O contents, in agreement with an origin by impact. Our data confirm that all tektites found on land have very low H₂O contents (<0.03 wt% H₂O), while impact glasses have slightly higher H₂O contents. Both glass types are very dry compared to volcanic glasses. This study confirms that the low H₂O contents (<0.05 wt%) of such glasses can be considered good evidence for an origin by impact.     

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.     

Report of centimeter-sized tektites in Pima County,Arizona, cannot be verified

Abstract— Glassy objects reportedly found in Pima County, Arizona, have been identified as tektites. A field survey of the area where they were said to occur, however, did not produce any other tektites, nor did it reveal any other geologic features that might indicate a nearby impact crater. The major-, minor-, and trace-element composition of one specimen is similar to those measured in indochinites, which suggests the objects reportedly from Pima County were instead transported to southern Arizona from Indochina by people.     

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.     

Layered tektites of southeast Asia: Field studies in central Laos and Vietnam

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.     

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.