Lechatelierite inclusions in indochinites and the origin of tektites

Abstract

Abstract-The grain size distribution and shapes oflechatelierite 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. grain mounts, or chips, were prepared for microscopic study. The threedimensional shape of the inclusions was established with a universal stage. The lechatelierite inclusions were identified with a single refractive index, 1.46, using the immersion method. A Chaix Mexa microscope crushing stage was used to liberate the glass inclusions from the tektite chips into the immersion liquid. The crushing stage was used for the estimation of the degree of vacuum in the bubbles inside lechatelierite inclusions (see Roedder, 1970, for description of the method). The size of the inclusions was measured with an ocular micrometer on linear traverses of six indochinites. The modal content of the lechatelierite inclusions was determined from linear traverses with the modified Rosiwal method (Kinnunen, 1989). SHAPE AND SIZE DATAIn shape, the lechatelierite inclusions ranged from vesicular spongy fragments and fibre-like bodies to roundish blebs (Fig. 1). Their number percentages were as follows: vesicular spongy fragments 6%, vesicular prismatic bodies 48%, fibre-like bodies 7%, blocky fragments 2%, roundish blebs 28%, and undetermined 9%. These shape types evidently result from vesiculation of lechatelierite while it was confined in the melt.The modal content ofthe lechatelierite inclusions ranged from 0.15% to 0.17%. The number percentages of the lechatelierite inclusions for various size intervals were as follows: 500-250 m 0.7%, 250-125~m 14.0%, 125-62~m 36.1%, 62-32~m 27.9%, 32-16~m 11.8%, 16-8~m 5.1%, 8-4~m 3.7%, 4-2~m 0.7%. According to sedimentological classification, this distribution represents moderately sorted fine sand and coarse silt.

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

“…This size distribution appears to be consistent with the size distribution of lechatelierite particles, melted quartz, observed by Kinnunen (1990) in indochinites. Thus, it is possible that the rock fragments found in the microtektite layer represent the target deposits that were melted to produce the Australasian tektites and microtektites.…”

Discovery of the most distal Ries tektites found in Lower Silesia, southwestern Poland

“…The very high content of SiO 2 has also been noted in the moldavite literature (von Engelhardt et al. ; Kinnunen ; Knobloch ). The lechatelierite formation is an indicator of rapid and irregular flow during an early formation stage (Koeberl et al.…”

A review of volatile compounds in tektites, and carbon content and isotopic composition of moldavite glass

“…(2009) in a statistical analysis of geochemical data from a different set of moldavite samples from the Cheb Basin. The existence of a biogenic component in source materials for tektites was assumed earlier by Kinnunen (1990) who suggested that siliceous phytoliths—biogenic opal formed in plant tissues—was the precursor of lechatelierite grains in tektites.…”