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Fingerprinting the K/T impact site and determining the time of impact by U-Pb dating of single shocked zircons from distal ejectaU-Pb isotopic dating of single 1 - 3 micrograms zircons from K/T distal ejecta from a site in the Raton Basin, Colorado provides a powerful new tool with which to determine both the time of the impact event and the age of the basement at the impact site. Data for the least shocked zircons are slightly displaced from the 544 +/- 5 Ma primary age for a component of the target site, while those for highly shocked and granular grains are strongly displaced towards the time of impact at 65.5 +/- 3.0 Ma. Such shocked and granular zircons have never been reported from any source, including explosive volcanic rocks. Zircon is refractory and has one of the highest thermal blocking temperatures; hence, it can record both shock features and primary and secondary ages without modification by post-crystallization processes. Unlike shocked quartz, which can come from almost anywhere on the Earth's crust, shocked zircons can be shown to come from a specific site because basement ages vary on the scale of meters to kilometers. With U-Pb zircon dating, it is now possible to correlate ejecta layers derived from the same target site, test the single versus multiple impact hypothesis, and identify the target source of impact ejecta. The ages obtained in this study indicate that the Manson impact site, Iowa, which has basement rocks that are mid-Proterozoic in age, cannot be the source of K/T distal ejecta. The K/T distal ejecta probably originated from a single impact site because most grains have the same primary age.
Document ID
19950038163
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
Authors
Krogh, T. E.
(Royal Ontario Museum Toronto, Canada)
Kamo, S. L.
(Royal Ontario Museum Toronto, Canada)
Bohor, B. F.
(U.S. Geological Survey, Denver, CO United States)
Date Acquired
August 16, 2013
Publication Date
September 1, 1993
Publication Information
Publication: Earth and Planetary Science Letters
Volume: 119
Issue: 3
ISSN: 0012-821X
Subject Category
Geophysics
Accession Number
95A69762
Funding Number(s)
CONTRACT_GRANT: NSERC-655
CONTRACT_GRANT: NSERC-4261
CONTRACT_GRANT: NASA ORDER T-5715
Distribution Limits
Public
Copyright
Other

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