Computer Science
Scientific paper
Mar 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990metic..25....1m&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 25, March 1990, p. 1-10.
Computer Science
18
Argon Isotopes, Earth Surface, Hypervelocity Impact, Meteorite Craters, Planetary Evolution, Planetary Surfaces, Rocks, Earth, Impacts, Impact Craters, Craters, Age, Age Dating, Isotopes, Dating Methods, Isotopic Ratios, Argon, Samples, Terrestrial, Isochrons, Dellen Crater, Sweden, Janisjarvi Crater, Ussr, Spectra, Argon 40, Data, Saaksjarvi Crater, Finland, Laboratory Studies, Devitrification, Melts, Production Rate, Petrography, Photomicrographs, Analysis
Scientific paper
Ar-40 - Ar-39 age measurements were made for three whole rock melt samples produced during impact events which formed the Dellen, Janisjarvi, and Saaksjarvi craters on the Baltic Shield. An age of 109.6 + or - 1.0 Ma was obtained for the Dellen sample based on an age spectrum plateau. The age spectrum shows a small (7 percent) loss of radiogenic Ar-40 from low temperature fractions. Ages of 698 + or - 22 Ma and 56 + or - 12 Ma were obtained from isochrons for the Janisjarvi and Saaksjarvi samples, respectively. Data obtained by laser degassing support the Saaksjarvi results. The presence of excess Ar-40 is indicated in lower temperature fractions for both samples and is correlated with K concentrations in the Saaksjarvi sample. Models explaining these results may require a change in the local 'atmospheric' Ar isotopic composition as cooling of melt rocks proceeded. However, it cannot be excluded that devitrification and/or alteration changed the Ar budget. A crater production rate on the Baltic Shield based on measured ages of 6 craters is (0.3 + or - 0.2) x 10 to the -14th 20-km-and-larger craters per sq km per year, in satisfactory agreement with previous estimates.
Hartung Jack B.
Jessberger Elmar K.
Muller Nicolas
Reimold Wolf Uwe
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