Dating terrestrial impact events
Authors: Deutsch Alexander , Schärer Urs 
Published: June 2012

Abstract— Systematic examination of dating results from various craters indicates that about 90% of the rocks affected by an impact preserve their pre‐shock ages because shock and post‐shock conditions are not sufficient to disturb isotopic dating systems. In the other 10% of target lithologies, various geochronometers show significant shock‐induced effects. Major problems in dating impactites are caused by their non‐equlibrated character. They often display complex textures, where differently shocked and unshocked phases interfinger on the sub‐mm scale. Due to this, dating on whole rock samples or insufficiently pure mineral fractions often yielded ambiguous results that set broad age limits but are not sufficient to answer reliably questions such as a possible periodicity in cratering on Earth, or correlation of impact events with mass extinctions. Dating results from shock recovery experiments indicate that post‐shock annealing plays the most important role in resetting isotopic clocks. Therefore, the major criterion for sample selection in and around craters is the post‐shock thermal regime. Based on their different thermal evolution, the following geological impact formations can be distinguished: (1) the coherent impact melt layer, (2) allochthonous breccia deposits, (3) the crater basement, and (4) distant ejecta deposits.

Samples of the coherent impact melt layer are the most suitable candidates for dating. Excellent ages of high precision can be obtained by internal Rb‐Sr, and Sm‐Nd isochrons, U‐Pb analyses on newly crystallized accessory minerals, and K‐Ar (39Ar‐40Ar) dating of clast‐free melt rocks. Fission track counting on glassy material has yielded correct ages, and paleomagnetic measurements have been successfully applied to post‐Triassic craters. In the ideal case of a fast‐cooling impact melt layer, all these different techniques should give identical ages.

Allochthonous breccias contain shocked, unshocked, and/or glassy components in various proportions; and, hence, each of these ejecta deposits has its own individual thermal history, making sample evaluation difficult Glassy melt particles in suevitic breccias are well suited for fission track and Ar‐Ar dating. Weakly shocked material may yield reliable Ar‐Ar and fission track ages, if formation temperatures were high, and cooling rates moderate. In contrast, highly shocked but rapidly cooled lithologies show only disturbed and not reset isotopic systems. For ejecta deposits and the crater wall of young craters, dating with cosmogenic nuclides is a new and powerful technique.

Crater basement lithologies have a high potential in impact dating, although it has not been exploited so far. A prerequisite for resetting of isotopic clocks in these lithologies is the presence of an overlaying impact melt layer, which causes thermal metamorphism. Fission track and K‐Ar techniques are most promising, because both systems are easily reset at low temperatures. Good candidates for impact dating are long‐term annealed rocks, even if shock metamorphic overprint is very weak. In addition, Ar‐Ar dating dating of pseudotachylites appears promising. In large impact structures, where high temperatures persist for long times, polymict “footwall” breccias beneath the melt sheet are also appropriate for dating, using the isochron approach and U‐Pb on accessory minerals.

Distant ejecta material have undergone very fast cooling, and the ejecta deposits have ambient formation temperatures. Among this material, tektites and impact melt glass are ideal objects for Ar‐Ar and fission track impact dating. Dating on other material from distant ejecta deposits, such as U‐Pb analyses on zircons, offers new possibilities. Efforts to correlate distant ejecta with distinct craters critically depend on proper error assignment to a specific age. This aspect is illustrated on the K/T boundary example.