HR: 15:15h
AN: V33E-06    [Abstracts]
TI: Fake Statistically Valid Isotopic Ages in Impact Crater Geochronology
AU: * Jourdan, F
AF: Western Australian Argon Isotope Facility, Dep. of Applied Geol. & JdL-CMS, Curtin Univ. of Tech., GPO Box U1987, Perth, WA 6845, Australia
AU: Schmieder, M
AF: 2Institut für Planetologie, Univ. Stuttgart, Herdweg 51, Stuttgart, D-70174, Germany
AU: McWilliams, M M
AF: CSIRO Exploration & Mining, Queensland Centre for Advanced Technologies, PO Box 883, Kenmore, QLD 4069, Australia
AU: Buchner, E
AF: 2Institut für Planetologie, Univ. Stuttgart, Herdweg 51, Stuttgart, D-70174, Germany
AB: Precise dating of impact structures is crucial in several fundamental aspects, such as correlating effects on the bio- and geosphere caused by these catastrophic processes. Among the 176 listed impact structures [1], only 25 have a stated age precision better than ± 2%. Statistical investigation of these 25 ages showed that 11 ages are accurate, 12 are at best ambiguous, and 2 are not well characterized [2]. In this study, we show that even with statistically valid isotope ages, the age of an impact can be "missed" by several hundred millions of years. We present a new 40Ar/39Ar plateau age of 444 ± 4 Ma for the Acraman structure (real age ∼590 Ma [3]) and four plateau ages ranging from 81.07 ± 0.76 Ma to 74.6 ± 1.5 Ma for the Brent structure (estimated real age ∼453 Ma [4]). In addition, we discuss a 40Ar/39Ar plateau age of 994 ± 11, recently obtained by [5] on the Dhala structure (real age ∼2.0 Ga [5]). Despite careful sample preparations (single grain handpicking and HF leaching, in order to remove alteration phases), these results are much younger than the impact ages. Petrographic observations show that Acraman and Dhala grain separates all have an orange color and show evidence of alteration. This suggests that these ages are the results of hydrothermal events that triggered intensive 40Ar* loss and crystallization of secondary phases. More intriguing are the Brent samples (glassy melt rocks obtained from a drill core) that appeared very fresh under the microscope. The Brent glass might be a Cretaceous pseudotachylite generated by a late adjustment of the structure and/or by a local earthquake. Because we know the approximate age of the craters with stratigraphic evidences, these outliers are easy to identify. However, this is a red flag for any uncritical interpretation of isotopic ages (including e.g., 40Ar/39Ar, U/Pb, or U-Th/He [6]). In this paper, we encourage a multi-technique approach (i.e., isotopic, stratigraphic, paleogeographic [7,8]) and cross- calibrations in order to obtain both accurate and precise impact ages. [1] Earth Impact Database, Univ. New Brunswick, Canada (accessed Feb 28, 2009), [2] Jourdan et al., submitted to EPSL, [3] Baldwin et al., AJES 1991, [4] Grieve, Impact structures in Canada, GEOText 5, Geol. Assoc. Canada, 2006, [5] Jourdan et al., LPSC 39, 2008., [6] van Soest et al., LPSC 40, 2009, [7] Schmieder et al., Geol. Mag. 145, 2008, [8] Buchner et al., LPSC 40, 2009.
DE: 1115 Radioisotope geochronology
DE: 1194 Instruments and techniques
DE: 5420 Impact phenomena, cratering (6022, 8136)
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2009 Joint Assembly