The multi-coloured layers of banded iron formations could connect ancient changes on earth’s surface—like the emergence of photosynthetic life—to planetary processes like volcanism and plate tectonics, according to new research.
In a paper published in the journal Nature Geoscience, Rice University researchers say that in addition to linking planetary processes that were generally thought to be unconnected, their study could reframe the general understanding of earth’s early history and provide insight into processes that could produce habitable exoplanets far from our solar system.
Banded iron formations are chemical sediments precipitated directly from ancient seawater rich in dissolved iron. Metabolic actions of microorganisms, including photosynthesis, are thought to have facilitated the precipitation of the minerals, which formed layer upon layer over time along with chert (microcrystalline silicon dioxide). The largest deposits formed as oxygen accumulated on earth’s atmosphere about 2.5 billion years ago.
“These rocks formed in the ancient oceans, and we know that those oceans were later closed up laterally by plate tectonic processes,” Duncan Keller, the paper’s lead author, said in a media statement.
The mantle, though solid, flows like a fluid at about the rate that fingernails grow. Tectonic plates—continent-sized sections of the crust and uppermost mantle—are constantly on the move, largely as a result of thermal convection currents in the mantle. Earth’s tectonic processes control the life cycles of oceans.
“Just like the Pacific Ocean is being closed today—it’s subducting under Japan and under South America—ancient ocean basins were destroyed tectonically,” Keller said. “These rocks either had to get pushed up onto continents and be preserved—and we do see some preserved, that’s where the ones we’re looking at today come from—or subducted into the mantle.”
Sunken iron oxides
Because of their high iron content, banded iron formations are denser than the mantle, which made Keller wonder whether subducted chunks of the formations sank all the way down and settled in the lowest region of the mantle near the top of earth’s core. There, under immense temperature and pressure, they would have undergone profound changes as their minerals took on different structures.
“There’s some very interesting work on the properties of iron oxides at those conditions,” Keller said. “They can become highly thermally and electrically conductive. Some of them transfer heat as easily as metals do. So it’s possible that, once in the lower mantle, these rocks would turn into extremely conductive lumps like hot plates.”
Keller and his co-workers posit that regions enriched in subducted iron formations might aid the formation of mantle plumes, rising conduits of hot rock above thermal anomalies in the lower mantle that can produce enormous volcanoes like the ones that formed the Hawaiian Islands.
“Underneath Hawaii, seismological data show us a hot conduit of upwelling mantle,” Keller said. “Imagine a hot spot on your stove burner. As the water in your pot is boiling, you’ll see more bubbles over a column of rising water in that area. Mantle plumes are sort of a giant version of that.”
Large igneous provinces
The researcher pointed out that he and his team looked at the depositional ages of banded iron formations and the ages of large basaltic eruption events called large igneous provinces. They found that there’s a correlation.
“Many of the igneous events—which were so massive that the 10 or 15 largest may have been enough to resurface the entire planet—were preceded by banded iron formation deposition at intervals of roughly 241 million years, give or take 15 million. It’s a strong correlation with a mechanism that makes sense,” the scientists noted.
The study showed that there was a plausible length of time for banded iron formations to first be drawn deep into the lower mantle and to then influence heat flow to drive a plume toward earth’s surface thousands of kilometres above.
“If what’s happening in the early oceans, after microorganisms chemically change surface environments, ultimately creates an enormous outpouring of lava somewhere else on earth 250 million years later, that means these processes are related and ‘talking’ to each other,” Keller said. “It also means it’s possible for related processes to have length scales that are far greater than people expected. To be able to infer this, we’ve had to draw on data from many different fields across mineralogy, geochemistry, geophysics and sedimentology.”
