Geochronological constraints on two proposed Ordovician meteorite event impact structures in North America
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Following the breakup of the L-Chondrite Parent Body in the Mid-Ordovician, the Earth experienced a drastic increase in the number of meteorite impacts. This event, the Ordovician Meteorite Event (OME), has been confirmed from impact structures or meteoritic debris in Sweden, Russia and China. Yet despite the long lasting and global nature of this event, only two OME impact structures have been identified outside of Sweden, with no OME evidence having been found in the Western Hemisphere. In North America, there is a series of impact structures which are speculated to have formed during the OME. All of these structures have been tentatively dated to the mid-Ordovician, but absolute formation dates have not yet been obtained. The ages of these structures have been constrained based on stratigraphic and biostratigraphic evidence, but radiometric ages have proven difficult to determine. This project attempts to determine the formation age and likely origin of two of those Impact Structures. The Ames Astrobleme (OK, USA) and Slate Islands Archipelago (ON, Canada) were chosen due to previous work and the ease of accessing impact generated material. For this project, shock metamorphosed minerals from impactites was dated via thermochronological techniques. Samples of impactite and shocked target rock were collected from each of the impact structures. K-feldspar (⁴⁰Ar/³⁹Ar) and zircon (U/Pb) were dated from the impacted rocks. The hypothesis was that these minerals would be ‘reset’ from the heat and/or pressure generated by the impact, and could have the potential to provide the age of formation of the crater. This is similar to the way traditional metamorphic events are dated using these minerals, although the events in question occur over vastly different time scales. The analysis determined significant events in the thermal histories of the two impact structures. At the Ames Astrobleme, initial cooling of the basement granodiorite was confirmed to be 1428±31 Ma with a significant thermal episode at 372±42 Ma. At the Slate Islands, initial cooling of the basement syenite was confirmed to be 2706±20 Ma with significant thermal episodes at 614±27 Ma, 481±13 Ma, and 331±9 Ma. In both cases, the data does not conclusively confirm an OME related origin for the impact structures. Of all the zircons analyzed, only three from the Slate Islands have concordant dates which match the time frame of the OME. Even more puzzling, most of the young dates cannot be explained by nearby tectonic events. It is likely that these discrepancies are due to a combination of factors, including a lack of understanding of the regional geologic history, an incorrect determination of stratigraphic constraints on the timing of impact structure formation, and the difficulties of using shock metamorphosed minerals when dating impact structures.