THERMOCHRONOLOGICAL INSIGHTS ON THE TIMING OF THE SLATE ISLANDS IMPACT STRUCTURE, LAKE SUPERIOR, CANADA

The Slate Islands Archipelago is the remnants of a highly eroded impact structure in Lake Superior. Although it’s origin as a meteorite crater is well established, the timing of this impact has not been identified. Numerous potential dates have been suggested for the impact, ranging from 1.1 Ga to 282 Ma. More importantly, it has been suggested that the Slate Islands impact occurred concurrently with the Ordovician Meteorite Event, a period of increased meteorite activity from 470 Ma to 450 Ma. The OME has been well established in Sweden, and the findings from that country suggest the OME should have a worldwide impact. Thus far, no impact structures from outside of Eurasia have been tied to the OME by thermochronological means.

This project attempted to determine the age of the Slate Islands impact by analyzing zircon and K-feldspar, which were affected by crater formation. Those minerals would have originally formed during the initial emplacement of the local bedrock. During the crater formation, those minerals would have been both shock metamorphosed and been heated by the energy of the impact. In theory, this could have reset the internal thermochronometers (U-Pb for zircon and K-Ar for K-feldspar).

Samples of syenite and suevite were collected from the islands in Summer of 2017. The samples were processed to remove zircon at the University of Texas at Austin and to remove K-feldspar at Oregon State University. The minerals were dated using a variety of complementary techniques, including LA-ICP-MS at UT Austin (zircon), SIMS at the Heidelberg University (zircon) and the ⁴⁰Ar/³⁹Ar method at Oregon State University (K-Feldspar). The data suggest events raised the temperature of the area above the closure temperature for Pb in zircon. Of those events, the two which stand out the most include an event at ~2.7Ga, which corresponds with the initially emplacement of the syenite, and an event at 469±45 Ma, which falls within the range of the OME. This lends strong evidence to the hypothesis that the Slate Islands Impact did occur during the OME.