Computer Science
Scientific paper
Sep 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997m%26ps...32..647k&link_type=abstract
Meteoritics, vol. 32, pages 647-670
Computer Science
14
Scientific paper
To contribute to the understanding of the impact history of asteroids we performed a high resolution 40Ar-39Ar study of ten moderately to highly shocked chondrites, which we selected according to the shock classification given by Stoffler et al. (1991). Two recent shocked chondrite falls and two highly shocked eucrites completed our sample suite. When possible we separated impact melt from host rock for separate analysis. In total we studied 28 samples from 14 meteorites. In some cases atmospheric argon that we associate with terrestrial weathering was identified and corrected for. The ages we obtained range between ~100 EMa and ~4.1 EGa and are clearly distinct from primordial ages that correspond to solar system formation. We reproduced the previously reported cluster of L Echondrite ages ~500 EMa. The most prominent result of our study is that - in case of chondrites - melts generally are older than host rocks or melt-embedded unmolten rocks. To solve this apparent paradox we propose that the melt-forming event, which was the most severe shock episode in the history of these meteorites, has not been the only occasion affecting their K-Ar systems. At least one later impact metamorphism must have occured. The response of the K-Ar clock to this second event was more severe in the host rock than in the previously (in the first event) generated melt veins and pockets because of different Ar retentivities. Hence, impact metamorphism on meteorite parent bodies indeed was a multi-stage process extending in time over billions of years.
Falter Martina
Jessberger Elmar K.
Kunz Joachim
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