Only in a few, very small impact craters, have fragments of the crater-forming impactor been found in the form of meteorites. In a few additional cases, microscopic bits of impactor material have been found as iron-nickel blebs or spherules within cm-scale glass bomblets formed and ejected during impact. Henbury, Monturaqui and Aouelloul are good examples of this. Such glass bomblets are typically a composite of melted or partially melted target material, unmelted rock clasts and occasional small bits of metled impactor. The FeNi blebs found in this context are typically substantially enriched in Ni compared to the impactor from which they formed. In only one case, Morokweng, has a cm-scale fragment of an impactor been found been found at a large impact crater.
The physics associated with large scale impacts results in very high temperatures near the point of contact between the impactor and target rock, and the impactor is simply reduced to vapor and plasma in multi-kilometer impacts. It is re-incorporated in melt rocks, however, and can be detected through isotope systematics in these target rocks, in the form of unusual ratios of elements.
The results can actually provide not only confirmation of an impact origin for a structure, but the specific or general type of meteorite that produced the impact.
Re-Os isotope analysis has also been used to recognize an impactor component in glass bomblets produced at small craters such as Aouelloul (Koeberl et al., 1998) and in impact breccia at Kalkkop (Koeberl, 1994; Koeberl et al., 1994).
Koeberl, C., African meteorite impact craters: Characteristics and geological importance. Journal of African Sciences, v. 18, pp. 263-295. 1994.
Koeberl, C., Reimold, W.U., Shirey, S.B. and Le Roux, F.G. 1994a. Kalkkop crater, Cape Province, South Africa: Confirmation of impact origin using osmium isotope systematics. Geochimica Cosmochimica Acta 58, 1229-1234.
Koeberl, C., Reimold, W.U. and Shirey,S.B., The Aouelloul crater, Mauritania: On the problem of confirming the impact origin of a small crater, Meteoritics & Planetary Science 33, pp. 513-517. 1998