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Abstract:
Accurate and precise dating of meteorite impacts is a prime intent of stratigraphic, paleoclimatic, and paleonto-logical research. Robust age estimates, however, exist only for 11 of 174 confirmed impacts structures (ref. 1) including the Ries crater in Germany, one of the best studied impact structures on Earth. This is probably also the impact site with the highest number of radiometric age determinations. So far, published ages are exclusively based on the analysis of impact-generated melts, i.e. suevite glass and/or moldavites (tektites) (e.g., ref. 2-7) and feldspar melt (ref. 8). However, despite more than 60 individual isotopic age data obtained during 50 years of research there is no consensus as to the accurate age of the impact. For example, 40Ar-39Ar ages range from 15.2 Ma (ref. 2) to 14.3 Ma (ref. 6,7), with the young ages representing most of the recent age determinations. Accordingly, the long-standing estimate of about 15 +- 0.2 Ma (ref. 2,3) has been criticized recently as being too old by more than 0.5 Ma (e.g., ref. 1, 5-8) and only confirmed once (ref. 9). Ages < 14.6 Ma, however, pose severe problems on geophysical and biostratigraphic interpretations. Most significantly and central to the discussion, they are in conflict with a reversed magnetic field during impact (ref. 10,11), because a normal magnetic field persisted from about 14.6 to 14.15 Ma (ref. 12), i.e. impact ages < 14.6 Ma are impossible from a paleomagnetic point of
view.
