Abstract

The verification of an extraterrestrial component in impact-derived melt rocks or breccias can be of diagnostic value to provide confirming evidence for an impact origin of a geological structure. Geochemical methods are used to determine the presence of the minor traces of such a component. This is because even under ideal conditions, meteoritic fragments are preserved only at very young and small craters, but are destroyed in larger and older structures as a result of erosion and/or vaporization. The absence of meteorite fragments can, therefore, not be used as evidence against an impact origin of a crater structure. The determination of such an extraterrestrial component can be done by studying the concentrations and interlement ratios of siderophile elements, especially the platinum group elements, which are several orders of magnitude more abundant in meteorites than in terrestrial upper-crustal rocks. More recently, the Re-Os isotopic system has been used for identifying the presence of a meteoritic component in a number of impact melt rocks and breccias. This method avoids the ambiguities that may affect the usage of elemental abundances and ratios; for example, if the target rocks have high abundances of siderophile elements, or if the siderophile element concentrations in the impactites are very low. The presence of a meteoritic component has been established for about 41 impact structures (out of about 150 known on Earth), which reflects also the detail in which these structures were studied. In some of the smaller of these craters, fragments of the meteorite (the ‘projectile’) are preserved, but in most others, only a minor extraterrestrial component (mostly <1 wt %) is mixed in with impact melt rocks or breccias. The identification of the projectile type (i.e. meteorite type) is not straightforward, as vapour fractionation during the impact, as well as post-impact fractionation in the melt or hydrothermal mobilization, may lead to changes in the interlement ratios.