Projectile Identification in Terrestrial Impact Structures and Ejecta Material

Abstract

Next to the rare finds of meteorite fragments in or near impact structures, a handful of geochemical methods can be applied to confirm the impact origin of a structure and to constrain the nature of the impacted projectile. During crater formation on a solid planetary surface, a small amount of projectile material (generally <1 wt%) is incorporated into the produced impactites. This extraterrestrial contamination constitutes a measurable geochemical signal that differs from the crustal signature. Due to their geochemical behavior, the moderately and highly siderophile chromium (Cr), cobalt (Co), nickel (Ni), platinum group elements [PGE: ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), platinum (Pt)], and/or gold (Au) are enriched by several orders of magnitude in various types of meteorites (and their asteroid or comet parent body precursors) compared to terrestrial crustal rocks, inducing a unique “extraterrestrial or meteoritic” fingerprint in the impactites. In addition, meteoritic contributions can result in atypical Os and Cr isotope ratios. The determination of the 187Os/188Os ratio is the most sensitive method for detecting a minute projectile contribution in terrestrial lithologies (unless a significant ultramafic component occurs within the target). However, it does not distinguish between different types of meteorites. The 53Cr/52Cr and 54Cr/52Cr isotope ratios can discriminate between carbonaceous and other chondrites; and in favorable cases, between enstatite and ordinary chondrites (CHAPTER 2). Although, since the formulation of the K/Pg (Cretaceous-Paleogene) hypothesis, analysis for Ir has remained the tool of predilection for the recognition of extraterrestrial components in sedimentary sequences and impactites, an Ir anomaly alone is not always strong evidence for impact (e.g., for the Younger Dryas event; CHAPTER 8). More reliable results can be obtained by measuring the content and ratios of a larger suite of siderophile elements and demonstrating that these results are consistent with a meteoritic component. The apparent lack of extraterrestrial siderophile enrichments in samples from suspected or confirmed impact structures (e.g., Chesapeake Bay; CHAPTER 6), does not always establish the absence of a meteorite impact event. Some meteorites (e.g., achondrites) are not enriched in siderophile elements, and these projectiles do not produce siderophile anomalies in an impact crater. The presence of vaporized or melted projectile material in an impact structure is generally restricted to those materials that were originally close enough to the point of impact to have been penetrated by dispersed projectile melt or vapor: craterfilling allochthonous breccias, impact melt bodies, and glasses and other materials distributed in proximal and distal ejecta deposits. That the absence of a PGE meteoritic component does not necessarily imply the absence of a PGE-rich projectile is best illustrated by the Chicxulub impact structure. Its impactites yielded no detectable meteoritic contamination despite the fact that worldwide the K/Pg boundary ejecta layer is highly enriched in PGE. Additionally, the meteoritic signature can be modified by fractionation, alteration, and post-impact remobilization, as observed at the K/Pg boundary (CHAPTER 7). Although the ejecta, and possibly fallback material, of the Bosumtwi impact structure contain a meteoritic component, the presence of any meteoritic component in the impactites collected at the crater rim and in other parts of the Bosumtwi ICDP cores are obscured by the higher than average crustal siderophile element abundances in the target rocks and breccias (CHAPTER 4). The only impact structure in this study that shows a strong meteoritic signature (Ir up to 1.926 ng/g) is Gardnos in Norway (CHAPTER 5), likely produced by an IAB-IIICD nonmagmatic iron meteoritic (NMI) projectile. This interpretation is based on the Ni/Ir and Cr/Ir ratios, as well as non-chondritic PGE ratios similar to those observed in NMI. Projectile determination within a fraction of the impact structures recognized on Earth today indicates that (ordinary) chondrites represent the main type of terrestrial impactors, in agreement with their distribution in the present meteorite population. Precise age constraints on impact craters coupled with projectile identification suggest the existence of impact clusters in the geological record (e.g., during the late Eocene, mid-Ordovician, and Precambrian spherule layer windows; CHAPTER 9). The more precisely the nature of a projectile is constrained, the more likely its provenance and parent-asteroid can be determined, and a mechanism for its delivery towards Earth can be defined. Clearly, more work is required to confirm the pulsated nature of the terrestrial impact flux that could be the result of collisional disruptions in the Main Asteroid Belt. However, projectile identification in terrestrial impact craters is not always a straightforward and easy task. More research is needed to better understand the admixing of projectile during cratering events and to further characterize the largest fraction of the extraterrestrial flux to Earth.