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.     

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.     

Impact microcrater morphology on Australasian microtektites

Abstract— Scanning electron microscopy of 137 Australasian microtektites and fragments from 4 sediment cores in the Central Indian Ocean reveals more than 2000 impact‐generated features in the size range of 0.3 to 600 μm. Three distinct impact types are recognized: destructive, erosive, and accretionery. A large variation in impact energy is seen in terms of catastrophic destruction demonstrated by fragmented microtektites through erosive impacts comprising glass‐lined pit craters, stylus pit craters, pitless craters, and a small number of accretionery features as well. The size range of observed microtektites is from 180 to 2320 μm, and not only are the smaller microtektites seen to have the largest number of impacts, but most of these impacts are also of the erosive category, indicating that target temperature is an important factor for retaining impact‐generated features. Further, microcratering due to collisions in impact‐generated plumes seems to exist on a larger and more violent scale than previously known. Although the microcraters are produced in a terrestrially generated impact plume, they resemble lunar microcraters in many ways: 1) the size range of impacts and crater morphology variation with increasing size; 2) dominant crater number densities in μm and sub‐μm sizes. Therefore, tektite‐producing impacts can lead to the generation of microcraters that mimic those found on lunar surface materials, and for the lunar rocks to qualify as reliable cosmic dust flux detectors, their tumbling histories and lunar surface orientations have to be known precisely.     

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.     

New occurrences of Australasian microtektites in the Central Indian Basin

Abstract— Thirty-three microtektites have been recovered from four different sites in the Central Indian Basin. Based on their physical properties, geographical occurrence and chemical composition, they are identified as belonging to the Australasian tektite strewn field. Microtektites from three of the sites under study are from surficial samples implying bioturbation and/or erosion or non-deposition.     

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.     

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.     

Australasian microtektites from Antarctica: XAS determination of the Fe oxidation state

The Fe oxidation state and coordination number of 29 impact glass spherules recently recovered from the Transantarctic Mountains (Antarctica) have been determined by X‐ray absorption near edge structure (XANES) spectroscopy. Based on geochemical, isotopic, and fission track data, these spherules are considered as microtektites from the Australasian tektite/microtektite strewn field. Their find location is the farthest so far discovered from the possible source crater region, and their alkali content is the lowest compared with other published data on Australasian microtektite glasses. The Fe³⁺/(Fe²⁺+Fe³⁺) ratio, determined from the analysis of the pre‐edge peak energy position and integrated intensity, is below 0.1 (±0.04) for all the samples, and is comparable to that of most tektites and microtektites from the Australasian strewn field. Also, the pre‐edge peak integrated intensity, which is sensitive to the average Fe coordination geometry, is comparable to that of other Australasian microtektites reported in the literature. The agreement of the Fe oxidation state and coordination number, between the Transantarctic Mountain microtektites (TAM) and the Australasian tektites and microtektites, further confirms the impact origin of these glass spherules and provides an independent suggestion that they represent a major extension southeastward of the Australasian strewn field. The fact that similar redox conditions are observed in tektites and microtektites within the Australasian strewn field regardless of the distance from the source crater area (up to approximately 11000 km) could be an important constraint for better understanding the different processes affecting microtektite formation and transport. The fact that the Fe oxidation state of microtektites does not increase with distance, as in the case of North American microtektites, means that thermal and redox histories of Australasian and TAM microtektites could differ significantly from those of North American microtektites.     

Microimpact phenomena on Australasian microtektites: Implications for ejecta plume characteristics and lunar surface processes

Abstract— Detailed investigations of the microimpact phenomena on Australasian microtektites from four samples from the Central Indian Basin reveal an array of features, such as very low-velocity captured droplets, welded projectiles, angular fragments and dust, craters generated by projectiles defining an oblique trajectory, high-velocity “pitless” craters, and the conventional hypervelocity craters with well-defined central pits and radial and concentric cracks—found commonly on lunar surface materials. The microimpacts are a consequence of interparticle collisions within the ejecta plume (as suggested by their chemistry) subsequent to a major impact and, therefore, reveal processes inherent in an impact-generated plume. All the impact phenomena observed here have taken place while the targets and projectiles were in flight and are therefore secondary impacts in lunar terms. However, some of the resultant features are analogous to lunar micro-craters attributed to primary impacts by cosmic dust. Therefore, ballistic sedimentation on the Moon is likely to contain plume collisional debris as well.     

Australasian microtektites and associated impact ejecta in the South China Sea and the Middle Pleistocene supereruption of Toba

Abstract— Australasian microtektites were discovered in Ocean Drilling Program (ODP) Hole 1143A in the central part of the South China Sea. Unmelted ejecta were found associated with the microtektites at this site and with Australasian microtektites in Core SO95–17957–2 and ODP Hole 1144A from the central and northern part of the South China Sea, respectively. A few opaque, irregular, rounded, partly melted particles containing highly fractured mineral inclusions (generally quartz and some K feldspar) and some partially melted mineral grains, in a glassy matrix were also found in the microtektite layer. The unmelted ejecta at all three sites include abundant white, opaque grains consisting of mixtures of quartz, coesite, and stishovite, and abundant rock fragments which also contain coesite and, rarely, stishovite. This is the first time that shock‐metamorphosed rock fragments have been found in the Australasian microtektite layer. The rock fragments have major and trace element contents similar to the Australasian microtektites and tektites, except for higher volatile element contents. Assuming that the Australasian tektites and microtektites were formed from the same target material as the rock fragments, the parent material for the Australasian tektites and microtektites appears to have been a fine‐grained sedimentary deposit. Hole 1144A has the highest abundance of microtektites (number/cm²) of any known Australasian microtektite‐bearing site and may be closer to the source crater than any previously identified Australasian microtektite‐bearing site. A source crater in the vicinity of 22° N and 104° E seems to explain geographic variations in abundance of both the microtektites and the unmelted ejecta the best; however, a region extending NW into southern China and SE into the Gulf of Tonkin explains the geographic variation in abundance of microtektites and unmelted ejecta almost as well. The size of the source crater is estimated to be 43 ± 9 km based on estimated thickness of the ejecta layer at each site and distance from the proposed source. A volcanic ash layer occurs just above the Australasian microtektite layer, which some authors suggest is from a supereruption of the Toba caldera complex. We estimate that deposition of the ash occurred ?800 ka ago and that it is spread over an area of at least 3.7 times 10⁷ km².     

Mechanism of Muong Nong-type tektite formation and speculation on the source of Australasian tektites

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.     

Ultraprecise age and formation temperature of the Australasian tektites constrained by 40Ar/39Ar analyses

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.     

Changes in abundance and nature of microimpact craters on the surfaces of Australasian microtektites with distance from the proposed source crater location

Abstract– Over 4600 Australasian microtektites from 11 sediment cores along an N–S transect in the Central Indian Ocean have been investigated optically for microimpact features on their surfaces. Detailed scanning electron microscope examination of 68 microtektites along this transect shows 4091 such features. These samples are located between approximate distances of 3900–5000 km from the suggested impact site in Indochina and therefore constitute distal ejecta. The morphology of the microimpacts seems to show distinct variations with distance from the source crater. The total number of microcraters on each microtektite decreases drastically from North to South indicating systematic decrease in the spatial density of the ejecta, and decrease in collisional activity between microtektites with distance from the proposed source crater location. Closer to the proposed source crater location, the microcraters are predominantly small (few μm), pit bearing with radial and concentric cracks, suggestive of violent interparticle collisions. The scenario is reverse farther from the source crater with smaller numbers of impacted microtektites due to increased dispersion of the ejecta and the microcraters are large and shallow, implying gentle collisions with larger particles. These observations provide systematic ground truth for the processes that take place as the ejecta of a large oblique impact which generated the Australasian tektite strewn field is emplaced. The microimpacts appear to take place during the descent of the ejecta and their intensity and number density decrease as a function of the spatial density of the ejecta at any given place and with distance from the source region. These features could help understand processes that take place during ejecta emplacement on planets with substantial atmosphere such as Mars and Venus.     

Australasian microtektites in the South China Sea and the West Philippine Sea: Implications for age,size, and location of the impact crater

Abstract— Microtektites from two deep‐sea cores in the South China Sea and the West Philippine Sea are identified as belonging to the Australasian tektite strewn field based on the morphology, chronostratigraphic occurrence, and geographical location of these microtektites. The higher concentrations of microtektites (>1000/cm²) in the marginal seas of the western Pacific, with the peak concentration in the South China Sea, support the hypothesis of a large impact crater in Indochina. These two new occurrences lead to a more precise dating of the impact event at 793 ka, whereas the size of the Australasian source crater on the Indochina Peninsula is estimated to be 90–116 km.     

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.     

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.     

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.     

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.     

Noble gas content and isotope abundances in phases of the Saint‐Aubin (UNGR) iron meteorite

Abstract— We analyzed the noble gas isotopes in the Fe‐Ni metal and inclusions of the Saint‐Aubin iron meteorite, utilizing the stepwise heating technique to separate the various components of noble gases. The light noble gases in all samples are mostly cosmogenic, with some admixture from the terrestrial atmosphere. Total abundances of noble gases in metal are one of the lowest found so far in iron meteorites and the ⁴He/²¹Ne ratio is as high as 503, suggesting that the Saint‐Aubin iron meteorite was derived from a very large meteoroid in space. The exposure ages obtained from cosmogenic ³He were 9–16 Ma. Saint‐Aubin is very peculiar because it contains very large chromite crystals, which—like the metal—contain only cosmogenic and atmospheric noble gases. The noble gases in all the samples do not reveal any primordial components. The only exception is the 1000 °C fraction of schreibersite which contained about 5% of the Xe‐HL component. The Xe‐Q and the El Taco Xe components were not found and only the Xe‐HL is present in this fraction. Some presolar diamond, the only carrier for the HL component known today, must have been available during growth of the schreibersite. However, it is also possible that this excess is due to the addition of cosmogenic and fission components. In this case, all the primordial components are masked (or lost) by the later events such as cosmic‐ray irradiation, heating, and radioactive decay.