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

小行星和彗星已经轰击太阳系的行星数十亿年了。 在地球上，活跃的地质过程已经摧毁或隐藏了大约90%的撞击坑，到目前为止，只有176个撞击结构被确定。 近年来，科学界已经认识到地球上的影响事件的重要性？的地质和生物历史，包括6500万年前的恐龙大灭绝事件，这是与大型希克苏鲁布撞击坑。 因此，确定这些已知撞击事件的准确和精确年龄对于我们了解地球历史至关重要。 不幸的是，大约80%的已确定的陆地撞击结构的年龄是未知的或只有大约已知的，因为传统的测年技术的应用并不是一致成功的。 该项目代表了一种新的测年技术的应用，初步研究表明，这种技术很有希望提高我们对地球上外星撞击历史的认识。 具体而言，该项目将涉及使用（U-Th）/He地质年代学技术来确定单个含U和Th晶体的年代（例如，锆石和磷灰石）的20个小型、中型和大型撞击结构的撞击材料。 放射性子产物（4 He）的独特性质，即，它的低闭合温度和高扩散率，使（U-Th）/He技术成为测定与撞击构造形成相关的短暂但非常强烈的热和冲击变质事件的理想方法。 该项目涉及一个由来自8个国家和13个科学机构的科学家组成的国家和国际小组，他们将提供撞击结构样本和专门知识，以检验这一假设。

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.