Formation conditions of Impact-induced Natural Glasses

Tektites and impact glasses are known as products by hypervelocity impacts onto the Earth surfaces. Their formation conditions, however, have been long debated, although the field geological works, mineralogical and geochemical data, and numerical simulations for hypervelocity impact and cratering have been accumulated so far. The difficulty to recover the post-shock samples in such extreme shock experiments suffers from direct evidences to unravel. The silicate glass structures and properties as the quenched melts may have been subjected to post-shock annealing during the release process and entry to the atmosphere and may not help to reveal the formation conditions due to such residual effects. The redox states of Fe were compared in the previous reports to understand the formation conditions of natural impact-induced glasses. We pay attention that x-ray absorption spectroscopy results on Ti K edge feature and found the presence of Ti³⁺ in glasses as a characteristic feature clearly to suggest a difference in the redox state at the time of hypervelocity impact, depended on the impact scale. The feature is closely related to high temperatures and found to be useful to characterize the formation conditions.