Iron-rich rocks are opening up new insights into Earth’s planetary history

A metamorphic wrought iron formation from southern Wyoming shows fine lamination. The rock is approximately 2.7 billion years old. Dark bands are iron oxides (magnetite, hematite) and red-orange bands are chert with iron oxide inclusions (jasper). Credit (Photo by Linda Welsenbach-Fries/Rice University)

Visually striking layers of burnt orange, yellow, silver, brown and blue-black colors characteristic of bound iron systems, sedimentary rocks may have fueled the largest volcanic eruptions in Earth’s history, according to new research from Rice University.

The rocks contain iron oxides that long ago sank to the bottom of the oceans, forming thick layers that eventually turned into stone. The study, published this week in Nature Geoscience, suggests that iron-rich layers could link ancient changes to Earth’s surface — such as the emergence of photosynthetic life — to planetary processes like volcanism and plate tectonics.

In addition to linking planetary processes that are generally thought to be unconnected, this study could reframe scientists’ understanding of Earth’s early history and provide insight into the processes that could create habitable exoplanets far from our solar system.

„These rocks tell — literally — the story of a changing planetary environment,” said Duncan Keller, the study’s lead author and a postdoctoral researcher at Rice in the Department of Earth, Environmental and Planetary Sciences. „They create a change in atmospheric and ocean chemistry.”

Bound iron formations are chemical deposits deposited directly from ancient seawater rich in dissolved iron. The metabolic activities of microbes, including photosynthesis, are thought to have facilitated the precipitation of minerals, which along with chert (microcrystalline silicon dioxide) formed as layers over time. 2.5 billion years ago, the largest deposit was formed as oxygen accumulated in Earth’s atmosphere.

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„These rocks formed in ancient oceans, and we know that those oceans were later covered laterally by plate tectonic processes,” Keller explained.

The mantle, though solid, flows like a fluid as fingernails grow. Tectonic plates—the continental-sized portions of the crust and uppermost mantle—are constantly moving, often as a result of thermal convection currents in the crust. Earth’s tectonic processes control the life cycles of the oceans.

A metamorphic wrought iron formation from the Hammersley Group of Western Australia. This rock is about 2.5 billion years old. Dark bands are iron oxides (hematite, magnetite), red bands are iron oxide inclusions (jasper) and gold bands are amphibole and quartz. Specimen collected by Chin-Tye Lee. Credit (Photo by Linda Welsenbach-Fries/Rice University)

Low-Res_BIF6-0567_775.JP „As the Pacific Ocean closes today — it’s subducting under Japan and under South America — ancient ocean basins were tectonically destroyed,” he said. „These rocks either had to be pushed onto the continents and preserved — and we see some preserved, which is where the rocks we see today come from — or buried within the mantle.”

Due to their high iron content, the bound iron formations are denser than the mantle, which made Keller wonder if the lower parts of the formations all sank down and settled in the lowest part of the mantle near the top of the Earth’s core. There, under immense temperature and pressure, their minerals would have undergone profound changes as they acquired different structures.

„There is some interesting work on the properties of iron oxides under those conditions,” Keller said. „They can become highly thermal and electrically conductive. Some of them transfer heat as easily as metals. So, once in the lower mantle, these rocks become highly conductive lumps like hot plates.

Keller and his colleagues suggest that regions enriched in subducted iron systems may help create mantle plumes, where thermal anomalies rise above thermal anomalies in the lower mantle that can create massive volcanoes like the ones that formed the Hawaiian Islands. „Beneath Hawaii, the seismic data show a hot passage of the upper mantle,” Keller said. “Imagine a hot spot on your stove burner. As the water in your pot boils, you will see more bubbles in a column of water rising in that area. Mantle plumes are a giant version of that.”

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„We looked at the depositional ages of banded iron formations and the ages of large basaltic eruption events, called large igneous provinces, and found a correlation,” Keller said. „Many igniting events—10 or 15 at most could have been enough to regenerate the entire planet—give or take 15 million over a period of about 241 million years. That’s a strong connection to a mechanism that makes sense.

The study shows that trapped iron forms can first be drawn deep into the lower mantle and then affect heat flow to drive a plume thousands of kilometers above the Earth’s surface.

In an effort to trace the journey of bound iron forms, Keller crossed disciplinary boundaries and ran into unexpected insights.

„What happens in the early oceans, after microbes chemically change the surface environment, eventually creates lava eruptions somewhere else on Earth 250 million years later, which means these processes are interconnected and 'talk’,” Keller said. It is possible to have quantities. To infer this, we had to draw on data from various disciplines such as mineralogy, geochemistry, geophysics and sedimentology.

Keller hopes the study will spur further research. „I hope it inspires people in the various fields it touches,” he said. „I think it’s really cool to get people talking to each other in renewed ways about how different parts of the Earth system are connected.”

Keller is part of the Intelligent Planets: Cycles of Life-Essential Volatile Elements Project on Rocky Planets, a team of scientists led by Rajdeep Dasgupta. Environmental and Planetary Sciences.

It is a very diverse collaboration that investigates how the volatile elements important to biology – carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur – behave on planets, how planets acquire these elements and the role they play in planet formation. livable,” Keller said.

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„We’re using Earth as the best example, but we’re trying to figure out what the presence or absence of one or some of these elements means for planets in general,” he added.

Chin-Tye Lee, Rice’s Harry Carothers Vice Professor of Geography, Earth, Environmental and Planetary Sciences, and Dasgupta are co-authors of the study. Other co-authors are Santiago Tassara, an assistant professor at the University of Bernardo O’Higgins in Chile, and Leslie Robbins, an assistant professor at the University of Regina in Canada, both of whom did postdoctoral work at Yale University, and Yale Professor of Earth and Planetary Sciences Jay Ogg, Keller’s doctoral advisor.

The research was supported by NASA (80NSSC18K0828) and the Natural Sciences and Engineering Research Council of Canada (RGPIN-2021-02523).

Links between large igneous province volcanism and subducted iron systemsNature

Astronomy

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