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

基岩中次生氧化铁矿物普遍存在于浅层地壳的断层、裂缝和局部高孔隙带中。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一段时间内与流体运移有关的反应的结果。二级铁氧化物中保存的放射性同位素日期和成分信息有可能揭示区域流体流动历史的时间和动力学，以及在基岩中形成高渗透带（如脆性变形）的过程。尽管它们很有潜力，但事实证明，用放射性同位素的方法测定基岩中次生氧化物的年代是很困难的。然而，新的结果表明，在许多情况下，(U-Th)/He定年提供了明显可靠且地质上一致的日期，这表明该方法具有很大的潜力。该项目将开发基岩次生氧化铁矿物的放射性同位素定年和地球化学表征方法，并主要关注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表征。目标是利用这些次级相的日期和组成来限制脆性变形的时间和条件，这些脆性变形产生了矿物质形成的空隙，并了解导致它们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，在那里可以将结果与其他地质和地质年代学限制进行比较。次生氧化铁的日期和成分有可能提高对一系列重要过程的时间和条件的理解，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时期。该项目还将为社区继续使用亚利桑那放射性氦测年实验室提供支持，并为低温热年代学暑期学生研讨会提供支持，使访问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。

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.