The Next Step in Unravelling the Earth's Impact History: (U-Th)/He Dating of Impact Structures

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Asteroids and comets have bombarded the planets of the Solar System for billions of years. Here on Earth, active geological processes have either destroyed or hidden about 90% of these impact craters, and thus far, only 176 impact structures have been identified. In recent years the scientific community has realized the importance of impact events on Earth?s geological and biological history, including the 65 million years old dinosaur mass extinction event that is linked to the large Chicxulub impact crater. Therefore, determining accurate and precise ages of these known impact events is vital in our understanding of the Earth's history. Unfortunately, about 80% of identified terrestrial impact structures are of unknown or only approximately known age because the application of traditional dating techniques is not uniformly successful. This project represents the application of a new dating technique, which in preliminary studies has shown great promise for improving our understanding of the extraterrestrial impact history on Earth. Specifically, this project will involve the use of the (U-Th)/He geochronological technique to date individual U- and Th-bearing crystals (e.g., zircon and apatite) from impact materials from 20 small, medium and large impact structures. The unique properties of the radiogenic daughter product (4He), i.e., its low closure temperature and high diffusivity, make the (U-Th)/He technique ideal for dating the brief, but very intense, thermal and shock metamorphism episode associated with the formation of impact structures. This project involves a national and international team of scientists from 8 countries and 13 scientific institutions, who will provide impact structure samples and expertise to test this hypothesis.