Gas content in bubbles of tektites and other natural glasses
Abstract
The gas content of individual bubbles in tektites and other natural glasses was investigated using a highly sensitive gas chromatograph. In indochinites, moldavites and dense No¨rdlinger Ries glass, the nitrogen/oxygen ratios were found to be roughly similar to the ratio in terrestrial atmosphere. Bubbles in Muong Nong and Muong Nong-type glass of East Thailand contain air as well as nitrogen and carbon dioxide. In bubbles of Libyan desert glass nitrogen and carbon dioxide were found but no oxygen could be detected. In none of the analyzed bubbles carbon monoxide or other gases were present in measurable quantities. Rare gases were not determined.
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Cited by (15)
<sup>40</sup>Ar-<sup>39</sup>Ar laser dating of tektites from the Cheb Basin (Czech Republic): Evidence for coevality with moldavites and influence of the dating standard on the age of the Ries impact
2009, Geochimica et Cosmochimica ActaMoldavites (Central European tektites) are genetically related to the impact event that produced the ∼24-km diameter Ries crater in Germany, representing one of the youngest large impact structures on Earth. Although several geochronological studies have been completed, there is still no agreement among 40Ar–39Ar ages on both moldavites and glasses from Ries suevites. Even recently published data yielded within-sample mean ages with a nominal spread of more than 0.6 Ma (14.24–14.88 Ma). This age spread, which significantly exceeds current internal errors, must be in part ascribed to geological and/or analytical causes.
This study reports the results of a detailed geochronological investigation of moldavites from the Cheb area (Czech Republic), which have never been dated before, and, for comparison, of two samples from type localities, one in southern Bohemia and the other in western Moravia. We used 40Ar–39Ar laser step-heating and total fusion techniques in conjunction with microscale petrographic and chemical characterization. In addition, with the purpose of ascertaining the influence of the dating standards on the age of the Ries impact and making data from this study and literature consistent with the now widely used Fish Canyon sanidine (FCs) standard, we performed a direct calibration of multi-grain splits of the Fish Canyon biotite (FCT-3) with FCs. The intercalibration factors (), determined for eight stack positions in one of the three performed irradiations, were indistinguishable within errors and gave an arithmetic mean and a standard deviation of 1.0086 ± 0.0031 (±2σ), in agreement with previous works suggesting that biotite from the Fish Canyon Tuff is somewhat older (∼0.8%) than the coexisting sanidine.
Laser total fusion analysis of milligram to sub-milligram splits of five tektite samples from the Cheb area yielded mostly concordant intrasample 40Ar–39Ar ages, and within-sample weighted mean ages of 14.66 ± 0.08–14.75 ± 0.12 Ma (±2σ internal errors, ages relative to FCs) that overlap within errors. These ages match those obtained for samples from western Moravia (14.66 ± 0.08 Ma) and southern Bohemia (14.68 ± 0.11 Ma), supporting the genetic link between Cheb Basin tektites and moldavites, and, consequently, between Cheb Basin tektites and the Ries impact. In contrast to samples from the Cheb area and Moravia, 40Ar–39Ar ages from total fusion experiments on the Bohemian specimen ranged widely from ∼14.6 to ∼17.0 Ma. Older apparent ages, however, were systematically obtained from fragments characterized by visible surface alteration. Laser step-heating experiments, although displaying slightly disturbed age profiles, were in line with total fusion analyses and yielded well-defined plateau ages of 14.64 ± 0.11–14.71 ± 0.11 Ma (±2σ internal errors, ages relative to FCs).
A thorough comparison of our and previous 40Ar–39Ar ages on both moldavites and Ries suevite glasses, recalculated relative to the 40Ar∗/40K ratio recently determined for FCs using intercalibration factors available in or derivable from the literature, reveals some inconsistencies which may be ascribed to either geological or analytical causes. Based on our data, decay constants in current use in geochronology, and ages calculated relative to FCs, we infer that the age of moldavites is 14.68 ± 0.11 Ma (±2σ, neglecting uncertainties in the 40K decay constants).
Geochemistry and origin of Muong Nong-type tektites
1992, Geochimica et Cosmochimica ActaMuong Nong-type tektites are one of three tektite groups occurring on land (the others being splash-form and aerodynamically shaped tektites). They differ in appearance from splash-form tektites by having irregular, blocky shapes and a layered structure. In thin sections, dark and light colored layers alternate, with dark layers being less abundant and embedded in a lighter glass matrix. The dark layers contain fewer bubbles than the lighter zones, in which bubbles are much more abundant than in splash-form tektites. Lechatelierite is often frothy, indicating that homogenization with the surrounding glass was not as efficient as in splash-form tektites. All nineteen samples studied here belong to the high silica group. The major element contents show an inverse correlation with the SiO2 content. Additionally, forty-four trace elements have been determined in all samples, using various methods. Muong Nong-type tektites are enriched in volatile elements compared to splash-form tektites. The halogens F, Cl, Br, and I, and several other volatile elements (e.g., B, Cu, Zn, Ga, As, Se, Sb, and Pb), show enrichment factors that vary between about 1.5 and 25, with the highest enrichments being shown by Cl, Br, and Zn. Compared to volatile element contents of possible target rocks, Muong Nong-type tektites are only slightly depleted compared to the target rocks, while splash-form tektites show considerable depletions. Some volatilization and selective element loss affected the tektites during their production, but only the volatile elements were affected, in contrast to the suggestion that volatilization of silica took place. The water contents are also slightly higher in Muong Nong-type tektites than in splash-form tektites (0.014 wt% H2O vs. 0.008 wt% H2O). Trace element ratios such as , , , or of Muong Nong-type tektites are very similar to those of the average upper continental crust. The chondrite-normalized REE patterns of the Muong Nong-type tektites are very similar to those of post-Archean upper crustal sediments. Local soil samples have different REE patterns, La/Yb slopes, and Ce and Eu anomalies. Mixing of local soils, or with some related loess samples, cannot reproduce the tektite REE patterns, and any basaltic, oceanic, or extraterrestrial rocks can be excluded as source rocks as well. The ratio of Muong Nong-type tektites is additional evidence for an origin from post-Archean sediments.
Major and trace elements have been analyzed in chips of dark and light layers, showing that a distinct chemical difference exists between the layers. Light layers have higher contents of Al2O3, FeO, TiO2, and MgO, and lower contents of SiO2, but the enrichment is not in linear correlation with the SiO2 content, thus simple dilution with silica cannot account for these differences. Trace element abundances, element ratios (e.g., , , and ), and REE patterns show marked differences between layers. This indicates incomplete mixing of different (but not completely dissimilar) parent rocks. Ferric/ferrous iron ratios were determined in all samples, yielding an average of 0.133, which is slightly higher than the ratio determined for two thailandite samples (0.07), but not different from the average ratio of 0.14 that was determined by previous analyses for australites.
Muong Nong-type tektites differ in the following criteria from splash-form tektites: (1) higher concentrations of volatile elements (e.g., Cl, Br, Zn, Cu, Pb); (2) chemically inhomogeneous on a millimeter scale; (3) dark and light layers with different chemical compositions; (4) may contain relict mineral inclusions (e.g., zircon, chromite, rutile, quartz, monazite); (5) large and more abundant bubbles that may be elliptical, showing glass flow; (6) large and irregular sample size with no sign of ablation. Muong Nong-type tektites have most probably originated during impact melting from a mixture of post-Archean sediments with compositions close to that of the upper crust (e.g., greywacke, sandstone, shale, etc.). Local loess and soil mixtures may reproduce the major element chemistry of average Muong Nong-type tektites, but the trace element ratios and REE patterns differ, and isotopic studies of Muong Nong-type tektites exclude recent young sediments such as soil or loess as tektite source materials. The data are in agreement with older sediments (with a sedimentation age of about 167 Ma) such as shales or greywacke. The chemical and isotopic data also do not support an origin of Muong Nong-type tektites from a multitude of very small impact craters. A single large impact, maybe occurring at an oblique angle, was probably responsible for all tektites in the Australasian strewn field. The crater is likely to be situated on or near Indochina, e.g., underwater, on the continental slope east of Vietnam, or on land (i.e., the Cambodian lake of Tonle Sap). The production of tektites seems to require special impact conditions because otherwise there should be more than four tektite strewn fields. Muong Nong-type tektites have not travelled far from the site of the impact, which most probably occurred somewhere in Indochina into post-Archean upper crustal sediments.
Nitrogen and trace elements in tektites and impact glasses
1989, Earth and Planetary Science LettersNitrogen and several trace elements have been studied in samples of one Muong Nong tektite, irghizites (tektite-like glass)/zhamanshinites (impactites) and several obsidian glasses. A detailed investigation of inter- and intra-layer samples from the Muong Nong tektite shows a chemical heterogeneity both within and between the layers. In general, the elemental variations seem to be due to a varying FeO content. Irghizites, both SiO2-rich and SiO2-poor, are compositionally more homogeneous than the zhamanshinites. The heterogeneous distribution of nitrogen, observed in the Muong Nong tektite and the zhamanshinites, is unlike that in any other tektite studied so far. This shows clearly that zhamanshinites have suffered lower peak temperatures than irghizites and Muong Nong tektites lower ones than normal “splash-form” tektites. Available chemical and petrological data on Muong Nong tektites suggest that they were formed by in situ melting of soil.
Libyan Desert glass: A review
1984, Journal of Non-Crystalline SolidsLibyan Desert Glass occurs as a concentration of glass fragments of a broad range of sizes strewn across the desert surface between the NNW-SSE-trending linear sand dunes along the southwestern margin of the Great Sand Sea in western Egypt; it was discovered by the international scientific community in 1932.
Despite the difficulty of access to the region where the glass occurs, at least 20 geologists have examined its field context in the course of at least 10 expeditions since 1932. Many more individuals have undertaken laboratory analyses of materials brought back by the various field parties.
These analyses have established that the gross chemical composition of Libyan Desert Glass approximates that of both the sandstone of the Nubia Formation that underlies the Great Sand Sea and the quartz aggregate of the dunes. A fissiontrack age of 28.5×106 y has been assigned to the glass. The quality of internal equilibration among various components distributed through the glass supports the interference that the fusion process that presumably formed the glass occurred at a high temperature: that temperature was sustained for a long time; and that the glass cooled to ambient temperature slowly.
The 1981 expedition, described herein, established that the present mass of glass exceeds 1.4×109 g; the original mass of glass may have been 10 000 times greater. The glass has been dispersed across a limited area of the desert surface by Earth-surface processes during a period of erosional lowering that is still in progress. No glass fragment representing an edge of the original mass of glass or a transition zone with the precursor material from which it presumably formed has yet been found.
The origin of Libyan Desert Glass remains as much a physical and geologic enigma today as it was in 1932.
Nitrogen in tektites and natural glasses
1979, Earth and Planetary Science LettersNitrogen concentrations in various tektites and natural glasses have been measured using RNAA and are found to be almost constant at about 15 ppm. A terrestrial origin for tektite formation is favoured.
Mass spectrometric analysis of gas inclusions in Muong Nong glass and Libyan Desert glass
1972, Earth and Planetary Science LettersNoble and non-noble gases in bubbles of Muong Nong- and Libyan Desert glass were released by vacuum crushing at room temperature and measured by high sensitivity mass spectrometry. The N2:Ar:Kr:Xe ratio as well as the rare gas isotope ratios were found to be atmospheric, indicating the terrestrial origin of these glasses. The concentrations of the active gases O2, CO2, CO and SO2 vary highly between adjacent bubbles. Total gas pressure in the bubbles of the glasses is in the 100 mm range, much higher than that found for other types of tektites.