The fate of banded iron formations in the deep mantle

地幔深处带状铁地层的命运

• 批准号: 2114525
• 负责人: James Van Orman
• 金额: $ 36.51万
• 依托单位: Case Western Reserve University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-07-01 至 2024-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2114525&HistoricalAwards=false
• 关键词: fate; banded; iron; formations; deep

Banded iron formations were common sedimentary deposits during much of Earth’s early history and are the most important iron ores in the world. They are thought to have covered much of the deep seafloor for hundreds of millions of years, but today only a small fraction remains near Earth’s surface. Most of these deposits sank into Earth’s mantle at subduction zones, along with the oceanic plates that carried them. Their ultimate fate in the mantle, however, is highly uncertain. One possibility is that iron within the deposits remained in an oxidized form, and sank to the base of the mantle; in this case they may explain the anomalous seismic properties of the deepest mantle, just above the core. Another possibility is that the iron oxides in the deposits was reduced to metal in a process analogous to iron smelting. If so, large bodies of metal may have formed in the mantle, dropped into the core, and potentially formed a template for the crystallization of Earth’s inner core at the center of the Earth. A critical question that has not yet been addressed is whether the rates of iron oxide reduction in the deep mantle are fast enough to generate large volumes of metal from subducted banded iron formations. This project will investigate the rates of iron oxide reduction experimentally, at the pressures and temperatures of Earth’s deep mantle.What is presently known about oxide reduction kinetics comes almost exclusively from low-pressure experiments from the metal extraction industry. At low pressures, reduction is often extremely rapid, with the fast kinetics enabled by rapid gas-phase transport. The experiments proposed here are designed to elucidate the rate-controlling mechanism at high pressure, where gas-phase transport is suppressed, and to describe the dependence of the reduction rate on pressure, temperature and oxygen fugacity. Based on the results of these experiments, the researchers will be able to evaluate the extent of iron oxide reduction in BIFs after their subduction into the mantle. These results will address whether it is possible to preserve FeO-rich materials in subducted BIFs, which have been hypothesized to account for ultra-low velocity zones (ULVZs) at the base of the mantle, or conversely whether it is possible to produce large bodies of metal which may affect the structure and dynamics of the mantle and core. This project will also support the education and career development of a graduate student, senior scientist, and East Cleveland high school students, all of whom are from groups that are underrepresented in STEM fields in the United States.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

在地球早期的大部分历史上，带状铁层是常见的沉积沉积物，是世界上最重要的铁矿石。人们认为他们已经覆盖了大部分深海地板已有数亿年了，但如今，只有一小部分位于地球表面附近。这些沉积物中的大多数与携带它们的海洋板一起沉入了地球的地幔中。但是，他们在地幔中的最终命运是高度不确定的。一种可能性是，沉积物中的铁保持为氧化物形式，并沉入地幔的底部。在这种情况下，他们可以解释最深地幔的异常地震特性，即核心上方。另一种可能性是，在类似于铁冶炼的过程中，沉积物中的铁氧化物被还原为金属。如果是这样的话，大量金属可能已经在地幔中形成，掉入核心中，并有可能形成一个模板，以使地球内部的结晶一个尚未解决的关键问题是，深幔中氧化铁降低的速率是否足够快，可以产生大量的金属，从俯冲带有铁的铁形成。该项目将在地球深地幔的压力和温度下实验研究氧化铁降低的速度。关于氧化物还原动力学的介绍几乎完全来自金属提取行业的低压实验。在低压下，减少通常非常迅速，快速的气相传输可以实现快速动力学。此处提出的实验旨在阐明在高压下抑制气相传输的速率控制机制，并描述还原速率对压力，温度和氧气散热性的依赖性。根据这些实验的结果，研究人员将能够评估BIF俯冲到地幔后的氧化铁降低程度。这些结果将解决是否可以保留俯冲的BIF中的Feo富含材料，这些材料已被认为是在地幔底部的超低速度区域（ULVZ），或者相反，是否可以产生可能影响地幔和核心结构的大型金属物体。该项目还将支持研究生，高级科学家和东克利夫兰高中生的教育和职业发展，所有这些学生都是来自美国STEM领域中代表不足的团体。这项奖项反映了NSF的法定任务，并被认为是通过评估基金会的知识分子和更广泛影响的评估来审查Criteria的评估，并被认为是珍贵的支持。