DETECTING METEORITIC COMPONENTS IN TERRESTRIAL IMPACT CRATERS AND THEIR EJECTA

The recognition of geological structures and ejecta layers on Earth as being of impact origin requires the detection of either shock metamorphic effects in minerals and rocks, and/or the presence of a meteoritic component in these rocks. Apart from studying meteorite impact craters, information can also be gained from the study of impact ejecta. In some cases, impact events have been identified solely from the discovery and study of regionally extensive or globally distributed impact ejecta. Geochemical methods have been used to determine the presence of the traces of such an extraterrestrial component. In the absence of actual meteorite fragments, it is necessary to chemically search for traces of meteoritic material that is mixed in with the target rocks in breccias and melt rocks. Meteoritic components have been identified for just about 45 impact structures, out of the more than 170 impact structures that have so far been identified on Earth. The presence of a meteoritic component can be verified by measuring abundances and interelement ratios of siderophile elements, especially the platinum group elements (PGE), which are much more abundant in meteorites than in terrestrial upper crustal rocks. Often the content of the element iridium is measured as a proxy for all PGEs, because it can be measured with the best detection limit of all PGEs by neutron activation analysis, but taken out of context, small Ir anomalies alone have little diagnostic power. More reliable results can be achieved by measuring whole suites of elements, for example, the PGEs, which also avoids some of the ambiguities that result if only moderately siderophile elements (e.g., Cr, Co, Ni) are used. It is difficult to distinguish among different chondrite types based on siderophile element (or even PGE) abundances, which has led to conflicting conclusions regarding the nature of the impactor at a number of structures. In such cases, the Os and Cr isotopic systems can be used to establish the presence of a meteoritic component in a number of impact melt rocks and breccias. Both of these methods are based on the observation that the isotopic compositions of the elements Os and Cr, respectively, are different in most meteorites compared to terrestrial rocks; the Cr isotopic method allows, in addition, the identification of the type of meteoritic material involved.