The fate of banded iron formations in the deep mantle

条带状铁建造是地球早期历史中常见的沉积矿床，也是世界上最重要的铁矿石。它们被认为覆盖了几亿年来的大部分深海海底，但今天只有一小部分留在了地球表面附近。这些沉积物大多在俯冲带沉入地幔，与携带它们的大洋板块一起下沉。然而，它们在地幔中的最终命运是高度不确定的。一种可能性是，沉积物中的铁仍以氧化的形式存在，并沉入地幔底部；在这种情况下，它们可能解释了地核上方最深地幔的异常地震性质。另一种可能性是，矿床中的铁氧化物在类似于铁冶炼的过程中被还原为金属。如果是这样的话，巨大的金属体可能已经在地幔中形成，落入地核，并可能形成了地球核心在地球中心结晶的模板。尚未解决的一个关键问题是，地幔深处的氧化铁还原速度是否足够快，足以从俯冲的带状铁建造中产生大量金属。这个项目将在地球深部地幔的压力和温度下对氧化铁的还原速率进行实验研究。目前已知的氧化物还原动力学几乎完全来自金属提取行业的低压实验。在低压下，还原通常是非常迅速的，快速的气相传输使快速的动力学成为可能。本文提出的实验旨在阐明高压下气相传输受到抑制的速率控制机制，并描述还原速率与压力、温度和氧逸度的关系。根据这些实验的结果，研究人员将能够评估BIF在俯冲到地幔后的氧化铁还原程度。这些结果将讨论是否有可能在俯冲的BIF中保存富含FeO的物质，这些物质被假设为解释了地幔底部的超低速带(ULVZ)，或者相反，是否有可能产生可能影响地幔和核的结构和动力学的大型金属体。该项目还将支持一名研究生、资深科学家和东克利夫兰高中生的教育和职业发展，他们都来自美国STEM领域代表性不足的群体。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.