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

解开地球撞击历史的下一步：撞击结构的 (U-Th)/He 年代测定

• 批准号: 0948143
• 负责人: Kip Hodges
• 金额: $ 30.66万
• 依托单位: Arizona State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-07-01 至 2015-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0948143&HistoricalAwards=false
• 关键词: Next; Step; Unravelling; Earth; Impact

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.

数十亿年来，小行星和彗星不断撞击太阳系的行星。在地球上，活跃的地质过程已经破坏或隐藏了大约90%的撞击坑，到目前为止，只有176个撞击结构被确定。近年来，科学界已经意识到撞击事件对地球的重要性。美国的地质和生物历史，包括6500万年前与希克苏鲁伯陨石坑有关的恐龙大灭绝事件。因此，确定这些已知撞击事件的准确年龄对我们了解地球历史至关重要。不幸的是，大约80%已确定的陆地撞击结构是未知的或只有大致已知的年龄，因为传统测年技术的应用并不总是成功的。这个项目代表了一种新的测年技术的应用，在初步研究中，它显示出极大的希望，可以提高我们对地球上的地外撞击历史的理解。具体来说，该项目将涉及使用(U- th)/He地质年表技术来确定来自20个小型、中型和大型撞击结构的撞击材料中单个含U和含th晶体（例如锆石和磷灰石）的年代。放射性成因子产物（4He）的独特性质，即其低闭合温度和高扩散率，使(U-Th)/He技术成为确定与撞击构造形成相关的短暂但非常强烈的热变质和冲击变质事件的理想方法。该项目涉及来自8个国家和13个科学机构的国家和国际科学家团队，他们将提供撞击结构样本和专业知识来验证这一假设。