Coesite in a Muong Nong‐type tektite from Muong Phin, Laos: Description, formation, and survival

Abstract

We examined 16 white opaque inclusions exposed on two polished slices of a Muong Nong‐type Australasian tektite from Muong Phin, Laos. The inclusions usually consist of a core, surrounded by a froth layer, and a quartz neoblast layer. The cores are composed primarily of a mixture of silica glass, coesite, and quartz in varying proportions. A thin (up to ~4 μm) layer of SiO2‐poor glass enriched in FeO, MgO, CaO, Al2O3, and TiO2 is observed as a bright halo in backscattered electron images around the quartz neoblasts and in places contains μm‐sized crystals, which may be Fe,Mg‐rich spinel. The distribution and textural relationships between the coesite‐bearing inclusions and the tektite matrix point to an in situ formation of the coesite due to an impact, rather than to infall, from a nearby impact, into tektite melt produced by the aerial burst of a bolide. The quartz neoblasts probably formed by crystallization of silica melt squeezed out of the inclusion core during the development of the froth layer. The bright halo may be the result of silica diffusing from the adjacent tektite melt into the growing quartz neoblasts. We propose that the survival of coesite was possible due to the froth layer that acted as a heat sink during bubble expansion and then as a thermal insulator.

In situ occurrence of Muong Nong-type Australasian tektite fragments from the Quaternary deposits near Huai Om, northeastern Thailand

“…Mineral inclusions which are several micrometers in diameter are sparsely contained, but they are too small to identify mineral species under an optical microscope. These characteristics are typical for MN Australasian tektites found in Indochina (e.g., Wasson 1991;Glass et al 2020). Three-hundred and thirty-one MN tektite fragments were found in the small (~40 cm × 30 cm) area with 10 cm in thickness in the upper part of Unit 2 (Fig.…”

“…Analyzed elements were Si, Al, Fe, K, Mg, Ca, Na, Ti, and Mn. Sodium and potassium were analyzed before other elements to reduce possible volatilization effects followed by the method of Glass et al (2020). The precision of the EPMA was evaluated by the analysis of the composition-known magnetite.…”

3D X-ray tomographic analysis reveals how coesite is preserved in Muong Nong-type tektites

“…9 Zeiss Research Microscopy Solutions, Carl Zeiss SAS, Marly-le-Roi, France. 10 Dipartimento di Scienze, Università degli Studi Roma Tre, Roma, Italy. 11 Department of Physics, Faculty of Natural Sciences, National University of Laos, Vientiane, Laos.…”

“…12 Instituto di Geologia y Geofisica CIGEO, Universidad Nacional Autónoma de Nicaragua, Managua, Nicaragua. * email: matteo.masotta@ unipi.it higher water content (up to 300 ppm) as compared to splashform tektites (up to 209 ppm) 13,14 and microtektites collected in Antarctica (< 20-206 ppm) 15 and a lower concentration of 10 Be as compared to ablated tektites from Australia 16 and microtektites from Antarctica 17 . These geochemical and isotopic data indicate a deeper origin in the target rock of MN tektites and a consequently lower ejection velocity, which suggests that they are closer to the source crater than tektites from the rest of the strewn field outside of Indochina 18 .…”

“…In the Australasian tektite strewn field, coesite was found as microcrystalline grains in partly melted mineral inclusions within impact melt glasses (namely tektites 9,10 ) and in shocked ejecta found in the Australasian microtektite layer 7,11 . Muong Nong (MN) tektites constitute one of the three tektite types associated with the Australasian strewn field and the only one where coesite was occasionally found 9,10 . Compared to splashform and ablated types, MN tektites typically occur as small fragments (with the exception of a few very large samples, weighting some tens of kg) with no signs of ablation.…”