Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v51h..06w&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #V51H-06
Mathematics
Logic
1100 Geochronology, 1115 Radioisotope Geochronology
Scientific paper
Dating impact events is important for our understanding of the frequency of bolide impacts on Earth, which might provide clues to the influence of large impacts on the development and/or extinction of life on Earth. It can also provide information on the flux of large bodies to the inner solar system, and in the future when material from other planetary bodies becomes available, help us gain a better understanding of the bombardment history of those planets. However, determining the age of an impact event is not straightforward, as most geochronometers are not completely reset unless the energy released upon impact produces a significant amount of melt material. Aside from the question as to whether or not the impact was large enough to produce a significant impact melt, there is the issue of conservation of the melt throughout geologic time. Many of these impact melts are poorly preserved. There are, however, other deposits that form during impacts, which are predominantly unmelted, which might record the age of the impact event if a suitable geochronometer can be found to date them. (U-Th)/He geochronology, a dating method not commonly applied to address this problem, might be a suitable method. It utilizes minerals (e.g., apatite and zircon) that are often present in crystalline impact melts as well as in unmelted but shocked material in impact breccias that can be reset even if an impact does not produce enough heat to create a significant amount of crystalline impact melt. So far our focus has been on a crystalline impact melt from the Manicouagan impact structure for which there is high precision U/Pb data. Single crystal (U-Th)/He zircon ages from the deposit give results (216.6 +/- 4.7 Ma 2SD) indistinguishable from the U/Pb age of 215.5 Ma (Ramezani et al. 2005), thus demonstrating the ability of the (U-Th)/He geochronometer to accurately date crystalline impact melts. We are investigating the application of (U-Th)/He geochronology on other dateable minerals in impact breccias: such as unmelted shocked grains, hydrothermally altered grains, and neoblastic minerals crystallized during shock metamorphism or post-shock hydrothermal metamorphism. Ramezani et al. (2005) Goldschmidt Conf. 2005, Moscow, ID, USA.
Hodges Kip V.
Monteleone Brian D.
van Soest Matthijs C.
Wartho J. C.
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