Collaborative Research: U and Th decay series dating of seafloor hydrothermal deposits

合作研究：海底热液矿床的 U 和 Th 衰变系列测年

• 批准号: 1259202
• 负责人: Margaret Tivey
• 金额: $ 12.62万
• 依托单位: Woods Hole Oceanographic Institution
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-08-01 至 2017-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1259202&HistoricalAwards=false
• 关键词: Collaborative; Research; Th; decay; series

Seafloor hydrothermal deposits form on time-scales of minutes to thousands of years, are products of significant heat and mass transfer from the oceanic crust to the oceans, and form in conjunction with activity that supports significant life in the deep-sea. Understanding time-scales over which these deposits form and whether they form continuously or episodically can provide important information about (1) energy and mass transfer between the ocean and crust; (2) environments, speciation rates, larvae dispersal, energy, and nutrient sources for hydrothermal biological communities; and (3) relationships between hydrothermal structure, crustal structure, and heat transfer within the crust. Unfortunately, there are currently no dependable methods to confidently date seafloor hydrothermal deposits to help us constrain some of these major processes. The most promising method involves measuring the radioactive decay of isotopes in the 238U- and 232Th- decay chains. The half-lives of target isotopes (238U-234U-230Th-226Ra-210Pb-210Po; 232Th-228Ra-228Th) range from 138 days to 75 kyrs, and allow investigation of samples ranging from ~1 to 105 years. These time-scales are appropriate for geologically young seafloor hydrothermal deposits. In addition, application of these isotopes is uniquely suited for dating minerals formed in seafloor hydrothermal deposits because geochemical processes isolate specific parent-daughter pairs in the U- and Th- series that can be exploited for age information. The motivation for this research is based on scientific need for a reliable method for dating seafloor hydrothermal deposits and to test the assumptions required for realization of the full potential of the 238U- and 232Th- method. This research combines a solid understanding of hydrothermal processes with high-precision isotopic measurements to rigorously test the assumptions that are central to full development of the analytical method and interpretation of the results and their application to seafloor hydrothermal vent chimney deposits. Goals will be to define the utility and limitations of the method and, hopefully, enable reliable dating of hydrothermal vent deposits of unknown age. The data produced on samples of known age, together with measurements of additional samples of unknown age, will be used to investigate the time-scales of hydrothermal processes at three distinct locations. Samples selected for analysis are in-hand and from (1) the Trans-Atlantic Geotraverse hydrothermal system (i.e., TAG); (2) the PACMANUS hydrothermal field in the Manus Basin in the Pacific Ocean near New Guinea; and (3) the Eastern Lau Spreading Center/Valu Fa Ridge in the Lau Basin of the western Pacific. Broader impacts of the study include the development of new analytical techniques and age pairs for geological applications, implications for ore deposit formation, public outreach, and broad dissemination of results to K-12 teachers, students, and the general public. Support of a graduate student and support of an institution in an EPSCoR stated (Wyoming) are additional impacts.

海底热液沉积物形成的时间尺度为几分钟到数千年，是从洋壳到海洋的大量热量和质量传递的产物，并与支持深海重要生命的活动一起形成。了解这些沉积物形成的时间尺度以及它们是连续形成还是间歇形成可以提供以下重要信息：（1）海洋与地壳之间的能量和质量传递； (2) 热液生物群落的环境、物种形成率、幼虫扩散、能量和营养来源； (3)热液结构、地壳结构和地壳内传热之间的关系。 不幸的是，目前还没有可靠的方法来确定海底热液沉积物的年代，以帮助我们限制其中一些主要过程。最有前途的方法是测量 238U 和 232Th 衰变链中同位素的放射性衰变。目标同位素（238U-234U-230Th-226Ra-210Pb-210Po；232Th-228Ra-228Th）的半衰期范围为 138 天至 75 kyrs，允许对样本进行约 1 至 105 年的研究。这些时间尺度适合地质上年轻的海底热液矿床。此外，这些同位素的应用特别适合对海底热液矿床中形成的矿物进行测年，因为地球化学过程隔离了 U 系列和 Th 系列中的特定母子对，可用于获取年龄信息。这项研究的动机是基于科学需要一种可靠的方法来测定海底热液沉积物的年代，并测试实现 238U- 和 232Th- 方法的全部潜力所需的假设。 这项研究将对热液过程的深入理解与高精度同位素测量相结合，严格测试对于分析方法的全面发展和结果解释及其在海底热液喷口烟囱沉积物中的应用至关重要的假设。目标是确定该方法的实用性和局限性，并希望能够对年龄未知的热液喷口沉积物进行可靠的测年。已知年龄样本产生的数据以及未知年龄其他样本的测量数据将用于研究三个不同地点的热液过程的时间尺度。选择用于分析的样品是现有的，来自 (1) 跨大西洋 Geotraverse 热液系统（即 TAG）； (2)新几内亚附近太平洋马努斯盆地的PACMANUS热液田； (3) 西太平洋劳盆地的东劳扩散中心/瓦鲁法海脊。该研究的更广泛影响包括开发新的分析技术和地质应用年龄对、对矿床形成的影响、公众宣传以及向 K-12 教师、学生和公众广泛传播结果。研究生的支持和 EPSCoR 指定机构（怀俄明州）的支持是额外的影响。