The fate of banded iron formations in the deep mantle

带状铁矿是地球早期历史上常见的沉积矿床，也是世界上最重要的铁矿石。它们被认为已经覆盖了数亿年的深海海底，但今天只有一小部分留在地球表面附近。这些矿床中的大多数都在俯冲带（subduction zone）与携带它们的大洋板块一起沿着到地幔中。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，矿床中的铁仍然以氧化形式存在，并沉入地幔底部;在这种情况下，它们可以解释最深地幔的异常地震特性，就在地核之上。另一种可能性是，沉积物中的氧化铁在类似于铁冶炼的过程中被还原为金属。如果是这样的话，大量的金属可能已经在地幔中形成，落入地核，并可能在地球中心形成地球内核结晶的模板。一个尚未得到解决的关键问题是，深部地幔中氧化铁的还原速度是否足够快，足以从俯冲带状铁地层中产生大量的金属。本项目将通过实验研究在地球深部地幔的压力和温度下氧化铁的还原速率，目前对氧化物还原动力学的了解几乎完全来自金属提取工业的低压实验。在低压下，还原通常非常迅速，快速的动力学通过快速的气相传输实现。这里提出的实验的目的是阐明在高压下的速率控制机制，气相运输被抑制，并描述的还原速率对压力，温度和氧逸度的依赖性。根据这些实验的结果，研究人员将能够评估BIF在俯冲到地幔后氧化铁的还原程度。这些结果将解决是否有可能保存在俯冲的BIF，这已被假设为占超低速区（ULVZ）在地幔的底部，或相反，是否有可能产生大型机构的金属，可能会影响地幔和核心的结构和动力学的丰富的FeO材料。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价值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.