United States Meteorite Impact Craters

[note - this text may seem disjointed because this page is significantly incomplete]

Only in a few, very small impact craters, have fragments of the crater-forming impactor been found in the form of meteorites.  The are typically less than 1 km in diameter.  The energy of impact associated with the formation of larger craters is generally sufficient to 'destroy' the impactor, meaning it is reduced to dust and vapor, and is dispersed at low concentrations amidst shattered or melted rock remnants.  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.  These are, again, less than 1 km in diameter.  Such glass bomblets are typically a composite of melted or partially melted target material, unmelted rock clasts and occasional small bits of melted impactor.  The FeNi blebs found in this context are typically substantially enriched in Ni compared to the impactor from which they formed.

Large craters also tend to be older, meaning remnants of the impactor have a greater opportunity to be reduced through terrestrialization processes (meaning interactions with water and atmospheric gasses) to stable clays and oxides, minerals that derive from chemical and physical weathering.    In only one case of a multi-km complex impact crater, Morokweng, has a cm-scale fragment of an impactor been found been found.

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.  Because the remnants are re-incorporated in melt rocks, however, they can be detected through isotope systematics in these target rocks, in the form of unusual ratios of elements.  There is a catch even in this, however, since only impacts in silicate (typically meaning igneous in this context) target rocks produce volumes of melt material.  A substantial amount of very good work has been done on answering the question of whether sedimentary target rocks can produce impact melts, but it is still not 100% clear.  [note to self - or has this been resolved? Ask G. O.]

When impactor traces are present in target rocks, they can be recognized through a variety of processes of instrumental analysis, as changes in isotopic or bulk elemental ratios, relative to unaltered target rocks from the same region.  The results can be pretty  amazing, translating parts per thousand or less to not only confirmation of an impact origin for a structure, but the specific or general type of meteorite that produced the impact.

Work on identifying impactor components distributed within target rocks is a a fairly young facet of impact crater research.  Some of the earliest work was done by Palme et al., in 1978.  The techniques have been substantially refined in subsequent decades, but have still not been applied at the majority of large impact craters.

No trace of the impactor has been reported for any of the 3 Ozark Plateau impact sites.  Numerical models have shown that the dynamics of a large impact should result in the complete destruction of the impactor. In such an impact, the meteorite should be reduced to gas, plasma, and very small particles.  An exception to this (Maier, 2006) was discovered at the 70 kilometer Morokweng impact crater in South Africa, where a 25 cm chondrite fragment was recovered within the impact melt sheet.  Other impacts over 4km, in keeping with models, have not produced any remants of impactors above microscopic sizes (Martel, 2006), though relatively successful attempts to characterize the impactors associated with certain craters have been undertaken using trace element abundances and very small fragments.

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).

References:

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

Palme H., Janssens M. J., Takahashi H., Anders E., Hertogen J. (1978) Meteoritic material at five large impact craters. Geochimica et Cosmochimica Acta, Volume 42, pages 313-332.

Impactors