Geochronology of Bedrock-hosted Secondary Iron Oxides and Shallow Crustal Fluid-flow and Brittle Deformation Histories

基岩中次生铁氧化物的地质年代学和浅地壳流体流动和脆性变形历史

• 批准号: 1219653
• 负责人: Peter Reiners
• 金额: $ 35.05万
• 依托单位: University of Arizona
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-10-01 至 2016-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1219653&HistoricalAwards=false
• 关键词: Geochronology; Bedrock; hosted; Secondary; Iron

Bedrock-hosted secondary Fe-oxide minerals are practically ubiquitous in faults, fractures, and localized high porosity zones in shallow crustal rocks. Unlike primary minerals, their formation is usually the result of reactions associated with fluid migration long after formation of the host rock. Radioisotopic dates and compositional information preserved in secondary Fe-oxides have the potential to reveal much about the timing and dynamics of regional fluid flow histories, as well as processes that create high permeability zones, like brittle deformation, in bedrock. Despite their potential, bedrock-hosted secondary oxides have proven difficult to date by radioisotopic means. However, new results show that, in many cases, (U-Th)/He dating provides apparently reliable and geologically consistent dates suggesting significant potential for this approach. This project will develop methods for radioisotopic dating and geochemical characterization of bedrock-hosted secondary iron-oxide minerals and focuses primarily on (U-Th)/He geo- and thermochronology of hematite and associated iron-oxide minerals in these environments, combined with characterization of trace element and oxygen isotopic compositions of the minerals. The goal is to use dates and compositions of these secondary phases to constrain the timing and conditions of brittle deformation that created the voids in which the minerals formed, and understand the timing and conditions of fluid flow that resulted in their precipitation. This project develops these approaches in a range of geologic settings, primarily in the western U.S., where results can be compared with other geologic and geochronologic constraints.Dates and compositions of secondary iron-oxides have the potential to improve the understanding of the timing and conditions of a wide range of important processes ranging from brittle fault movement, to groundwater migration, to atmospheric conditions, to ancient episodes of bedrock exposure. This project will also provide support for continued community use of the Arizona Radiogenic Helium Dating Lab, and support for a summer student workshop on low-temperature thermochronology, allowing visiting students to perform analyses and learn how to use and interpret thermochronology in geologic research.

基岩中的次生氧化铁矿物普遍存在于地壳浅部岩石的断层、裂隙和局部高孔隙带中。与原生矿物不同，它们的形成通常是在寄主岩石形成后很长时间内与流体迁移有关的反应的结果。放射性同位素测年和成分信息保存在次生铁氧化物有可能揭示区域流体流动历史的时间和动力学，以及在基岩中创建高渗透区（如脆性变形）的过程。尽管有潜力，但已证明很难通过放射性同位素方法确定以托克为主体的次生氧化物的年代。然而，新的结果表明，在许多情况下，（U-Th）/He测年提供了明显可靠和地质上一致的日期，这表明这种方法的巨大潜力。该项目将制定放射性同位素定年和地球化学鉴定法，鉴定以图尔库为主体的次生氧化铁矿物，主要侧重于这些环境中赤铁矿和伴生氧化铁矿物的（U-Th）/He地质和热年代学，并结合矿物的微量元素和氧同位素组成的鉴定。我们的目标是使用这些次生相的日期和成分来限制脆性变形的时间和条件，这些脆性变形产生了矿物形成的空隙，并了解导致其沉淀的流体流动的时间和条件。该项目在一系列地质环境中开发了这些方法，主要是在美国西部，其中的结果可以与其他地质和地质年代学的约束条件进行比较。二次氧化铁的年代和成分有可能提高对各种重要过程的时间和条件的理解，这些过程从脆性断层运动到地下水迁移，到大气条件，再到基岩暴露的古代事件。该项目还将为社区继续使用亚利桑那州放射性氦测年实验室提供支持，并支持关于低温热年代学的暑期学生讲习班，使来访的学生能够进行分析，并学习如何在地质研究中使用和解释热年代学。