Mathematics – Logic
Scientific paper
May 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29..301d&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 3, p. 301-322
Mathematics
Logic
44
Earth Surface, Ejecta, Geochronology, Impact Melts, Lithology, Metamorphism (Geology), Meteorite Craters, Meteorites, Meteoritic Composition, Radioactive Age Determination, Rocks, Accuracy, Biogeochemistry, Cooling, Glass, High Temperature, Isotopes, Melting, Mineralogy, Shock Waves, Stratigraphy
Scientific paper
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 (Ar-39-Ar-40) 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.
Deutsch Alexander
Schaerer Urs
No associations
LandOfFree
Dating terrestrial impact events does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Dating terrestrial impact events, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Dating terrestrial impact events will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-887759