Australasian microtektites: early target-projectile interaction in large impacts on Earth

Microtektites are microscopic impact glass spherules produced by the melting and vapourisation of the Earth's crust upon hypervelocity impact of large asteroidal/cometary bodies. They are distal ejecta distributed in strewn fields extending for thousands of kilometres. We studied the geographic distribution of the impactor signature in microtektites from the Australasian strewn field using Ni contents as a proxy. Although still unidentified, geological evidence suggests an impact location in southeast Asia. Based on this assumption, the impactor signature (Ni concentrations of up to 678 mu g/g; one order of magnitude higher than continental crust values) decreases with ejection distance and is not detected in the most distal microtektites from Antarctica. This evidence, coupled with trends versus launch distance in the concentrations of cosmogenic nuclides, volatile elements, Fe isotopes, and compositional homogeneity documented in the literature, suggests the following constraints for tektite formation models: the parent melts of the microtektites launched further away formed first, experienced the highest thermal regimes and record no impactor-target materials interaction, whereas those microtektites ejected closer formed later, experienced lower thermal regimes and record variable impactor-target materials interaction. The lack of impactor contamination in the most distal microtektites suggests that the early formed tektite/microtektite melts originated shortly before touchdown, possibly through thermal radiation in a compressed air front preceding the incoming fireball.