Oxygen isotope exchange between suspected airburst glasses and air

Airbursts of extraterrestrial objects were likely common in the geological past but left few traces in the rock record. Three occurrences of texturally similar, vesicular natural glasses – Pica glass (Chile), Edeowie glass (Australia), and Dakhleh glass (Egypt), here termed PED glasses – may have formed during airburst events [e.g., 1] or, alternatively, through combustion metamorphism [e.g., 2]. Silicate melts can exchange oxygen isotopes with air if exposed to high temperatures for sufficient time, e.g., 50% exchange at T > 1500 °C within 15 s [1], as observed in irghizites – a type of impact glass formed in an impact-generated vapour plume. To investigate the formation conditions of PED glasses, we therefore analysed their triple oxygen isotope compositions, and compared these with a broad suite of natural glasses. The data show that PED glasses contain several tens of atom percent atmosphere-derived oxygen. Similarly, paralavas from the Canadian Arctic – melts formed via the combustion of bituminous shales and coal seams – also incorporate significant amounts of atmospheric oxygen. In contrast, tektites, basaltic glasses from subaerial eruptions, impact melts, and desert fulgurites show no detectable atmosphere-derived oxygen. Additionally, trinitite, the glass formed by the low-altitude (30 m) nuclear airburst in 1945, does not contain measurable atmospheric oxygen either. These results confirm that PED glasses formed under exceptionally high-temperature conditions, exhibiting a common formation mechanism – either similar to irghizites but not to trinitite or, alternatively, to paralavas.

[1] Schultz et al. (2022) Geology [2] Roperch et al. (2022) EPSL [3] Pack (2021) RiMG 86, 217–240