The University of Arizona

40Ar-39Ar ages of H-chondrite impact melt breccias

T. D. SWINDLE, C. E. ISACHSEN, J. R. WEIRICH, D. A. KRING

Abstract


40Ar-39Ar analyses of a total of 26 samples from eight shock-darkened impact melt breccias of H-chondrite affinity (Gao-Guenie, LAP 02240, LAP 03922, LAP 031125, LAP 031173, LAP 031308, NWA 2058, and Ourique) are reported. These appear to record impacts ranging in time from 303 ± 56 Ma (Gao-Guenie) to 4360 ± 120 Ma (Ourique) ago. Three record impacts 300-400 Ma ago, while two others record impacts 3900-4000 Ma ago. Combining these with other impact ages from H chondrites in the literature, it appears that H chondrites record impacts in the first 100 Ma of solar system history, during the era of the lunar cataclysm and shortly thereafter (3500-4000 Ma ago), one or more impacts ~300 Ma ago, and perhaps an impact ~500 Ma ago (near the time of the L chondrite parent body disruption). Records of impacts on the H chondrite parent body are rare or absent between the era of planetary accretion and the lunar cataclysm (4400-4050 Ma), during the long stretch between heavy bombardment and recent breakup events (3500-1000 Ma), or at the time of final breakup into meteorite-sized bodies (<50 Ma).

Keywords


H ordinary chondrite Meteorite(s);Ar-Ar Ages;Late heavy bombardment;Impact breccia

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