Muong-Nong-Tektite ( 368,1 g )

Muong 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 KU, ThU, LaTh, or ZrHf 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 LaTh 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., KU, ThU, and LaYb), 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 SmNd-RbSr 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.