TEXTURAL EVOLUTION IN HIGH TEMPERATURE EXPERIMENTS ON TEKTITES

Tektites are natural glasses formed as a result of hypervelocity impacts on Earth. Tektites are roughly rhyolitic in composition, and most contain inclusions of lechatelierite, nearly pure SiO₂ glass, formed by the melting and subsequent quenching of quartz grains.

This study analyzed backscattered electron (BSE) and secondary electron (SE) images from high temperature experiments of Macris et al. (2018) to learn about the formation and evolution of lechatelierite and bubbles in tektites over a range of temperatures and times in the impact plume. The experiments from Macris et al. (2018) were done in a High Temperature Conical Nozzle Levitation (HT-CNL) system on natural powdered tektite mixed with 60-100 mm quartz grains as starting material. Experimental temperatures ranged from 1800-2400 °C, with heating times between 1-120 s. The experimental spheres are glassy dark-grey to black with diameters ranging from ~1.5-2.3 mm. These spheres were cut in half and polished for imaging.

Here we use ImageJ to quantitatively assess the area and diameters/lengths of vesicles and lechatelierite in BSE and SE images of the experimental glasses at a single temperature (2000 °C) and a range of times (1-120 s). In the shortest experiment at this temperature, vesicles cover 9.1% of the exposed area of the sphere. This value decreases to a minimum of 1.3% at 50 s, before climbing to the highest value of 22.1% of the total exposed area of the sphere in the longest experiment (120 s). The area covered by lechatelierite in the same spheres starts out as 7.1% at 1 s, and decreases in roughly linear fashion to 0.5% in the 120 s experiment. The average vesicle diameter generally increased with experiment duration. The increase is gradual between 1 and 80 seconds, rising from 19.7 to 37.5 µm, followed by a sharp increase to 138.2 µm in the 120 s experiment. The average lechatelierite length (measured along the longest dimension) is 65.7 µm in the 1 s experiment, decreasing gradually to 40.3 µm at 120 s. Future work will incorporate image analysis data from the full suite of experiments and compare the trends to data from natural tektites.

Macris et al., Geochim Cosmochim Acta 241 (2018)