Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.513g&link_type=abstract
Meteoritics, vol. 30, no. 5, page 513
Mathematics
Logic
1
Ages, Ar-Ar, Craters, Impact, Gardnos, Terrestrial
Scientific paper
Introduction: The Gardnos structure (Norway) [1], has recently been identified as an impact crater [2,3], despite being deformed by a series of tectonic processes [2]. The age of the structure is uncertain, although attempts to date the impact event have been made on the basis of stratigraphic relationships [2,3]. Unfortunately, these estimates differ by more than 100 million years. The first study suggested that the impact corresponded roughly with the Cambro-Ordovician boundary, ~500 Ma [2], while a second stratigraphic interpretation suggested a Precambrian age of 650 Ma [3]. To try and resolve this discrepancy, we attempted to determine the age of the structure with radiometric techniques, which typically have an error of < 10 Ma. Method: We examined three samples from the crater, in an attempt to determine the age using the 40Ar/39Ar dating method. The samples (plagioclase feldspar, alkali feldspar, and shards of gray, semi-transparent, glassy-looking material) were obtained from the suevite breccia unit of [2]. This unit should be the most promising for 40Ar/39Ar dating, since it is melt bearing, and no other unit is likely to be less affected by subsequent tectonism and sedimentation. Results: Our experiments yielded complicated age spectra interpreted to reflect a thermal event at 385+/-5 Ma and another possible event at 312+/-5 Ma (Fig. 1). All three samples yielded similar results. If the stratigraphic interpretations are correct, it thus seems clear that the impact event itself was not dated. The 385 Ma age corresponds to the end of the Caledonian Orogeny, which significantly altered this section of Norway [4]. It is suspected that late Caledonian folding occurred during this time [4], and we therefore interpret the 385 Ma age as the thermal metamorphic overprint of the Caledonian Orogeny. Another event clearly occurred afterwards, which is responsible for the 312 Ma signature. The cause of this thermal overprint has not yet been identified, since geologic events during this range of time are less well known [4]. Implications: The high mobility of Ar during thermal events makes use of the 40Ar/39Ar age technique difficult in attempting to date this structure. The greatest chance of dating the impact with 40Ar/39Ar would be examination of a mineral such as hornblende, which is more Ar retentive. Yet it appears that even hornblende has been partially or completely reset by the subsequent orogenic events in Norway [5]. Of course examination of more retentive minerals from the unit in question (if such can be found and identified) should be conducted before ruling out this possibility. It may be that another radiometric technique (U-Pb, if suitable samples can be found) or continued detailed stratigraphic analyses will be more successful in determining the age of the impact event. References: [I] Broch O. A. (1945) Norsk. Geol. Tidsskr., 25, 16-25. [2] Dons J. A. and Naterstad J. (1992) Meteoritics, 27, 215. [3] French B. M. et al. (1995) LPS XXVI, 423-424. [4] Oftedahl C. (1980) The Geology of the European Countries, BORDAS and 2th Intl. Geo. Congress, 349-351. [5] Dallmeyer R. D. (1988) GSA Spec. Pap. 38, 365-379. Figure 1: 40Ar/39Ar release spectra for Gardnos plagioclase feldspar
Grier Jennifer Ann
Kring David A.
Melosh Henry Jay
Swindle Timothy D.
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