The fate of banded iron formations in the deep mantle

Case Western Reserve University, Cleveland OH

Investigators

Abstract

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.