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Smithsonian scientists are conducting a new study of ancient “time capsule” rocks dating back at least 2.5 billion years.

Researchers at the Smithsonian’s National Museum of Natural History have conducted a new analysis of rocks believed to be at least 2.5 billion years old, shedding light on the chemical history of Earth’s mantle, the layer beneath the planet’s crust. Their findings improve our understanding of Earth’s earliest geological processes and contribute to a long-standing scientific debate about the planet’s geological history. In particular, the study provides evidence that the oxidation state of most of Earth’s mantle has remained stable over geologic time, challenging previous claims by other researchers about major transitions.

“This study tells us more about how this special place we live came to be the way it is, with its unique surface and interior that allowed life and liquid water to exist,” said Elizabeth Cottrell, chair of the Department of the Museum of Mineral Sciences, curator of the National Rock Collection and co-author of the study. “It’s part of our history as humans, because our entire origins trace back to how the Earth formed and how it evolved.”

The study, published in Nature, centered on a group of rocks collected from the sea floor that possess unusual geochemical properties. Namely, the rocks show evidence of having been melted to an extreme degree with very low levels of oxidation; oxidation is when the an atom or the molecule loses one or more electrons in a chemical reaction. Using additional analysis and modeling, the researchers used the unique properties of these rocks to show that they likely date back at least 2.5 billion years during the Archean Eon. The findings also show that Earth’s mantle has generally maintained a stable oxidation state since these rocks formed, unlike what other geologists had previously theorized.

An ancient rock excavated from the sea floor and studied by the research team. Credit: Tom Kleindinst

“The ancient rocks we studied are 10,000 times less oxidized than typical modern mantle rocks, and we present evidence that this is because they melted deep in the Earth during the Archean, when the mantle was much cooler. -hotter than it is today,” Cottrell said. “Other researchers have attempted to explain the higher oxidation levels seen in present-day mantle rocks by suggesting that an oxidation event or change occurred between the Archaean and today.” Our evidence suggests that the difference in oxidation levels can simply be explained by the fact that Earth’s mantle has cooled over billions of years and is no longer hot enough to produce rocks with such low oxidation levels.

Geological Evidence and Research Methodology

The research team—including lead study author Suzanne Birner, who completed a predoctoral fellowship at the National Museum of Natural History and is now an assistant professor at Berea College in Kentucky—began their investigation to understand the relationship between Earth’s solid mantle and modern seafloor volcanic rocks. The researchers began by studying a group of rocks that were excavated from the sea floor at two ocean ridges, where tectonic plates are moving apart and the mantle rises to the surface and produces new crust.

The two locations from which the studied rocks were collected, the Gakkel Ridge near the North Pole and the southwest Indian Ridge between Africa and Antarctica, are two of the slowest spreading tectonic plate boundaries in the world. The slow rate of spreading at these oceanic ridges means they are relatively quiet, volcanically speaking, compared to faster-spreading ridges that are dotted with volcanoes like the East Pacific Rise. This means that rocks collected from these slowly spreading ridges are more likely to be samples of the mantle itself.

The stern of the research vessel, R/V Knorr, while at sea in 2004. The A-frame structure holds the giant metal and chain bucket, which descends more than 10,000 feet below the ocean’s surface and drags along the seabed. to collect geological samples. Credit: Emily Van Ark

When the team analyzed the mantle rocks they collected from these two ridges, they found they shared strange chemical properties. First, the rocks were melted to a much greater extent than is typical of Earth’s mantle today. Second, the rocks are much less oxidized than most other samples of Earth’s mantle.

To achieve such a high degree of melting, researchers believe that the rocks must have melted deep within the Earth at very high temperatures. The only period in Earth’s geologic history known to include such high temperatures was between 2.5 and 4 billion years ago during the Archean Eon. The researchers therefore concluded that these mantle rocks may have melted during the Archean, when the planet’s interior was 360–540 degrees Fahrenheit (200-300 degrees Celsius) hotter than today.

Being so highly molten would protect these rocks from further melting, which could change their chemical signature, allowing them to circulate in the Earth’s mantle for billions of years without significantly changing their chemistry.

“That fact alone doesn’t prove anything,” Cottrell said. “But it opens the door to these samples being real geological time capsules from the Archean period.”

Scientific interpretation and insights

To explore the geochemical scenarios that could explain the low oxidation levels of rocks collected at Gakkel Ridge and the Southwest Indian Ridge, the team applied multiple models to their measurements. The models revealed that the low levels of oxidation they measured in their samples may have been caused by melting under extremely hot conditions deep within the Earth.

Both lines of evidence supported the interpretation that the atypical properties of the rocks represented a chemical signature of melting deep within the Earth during the Archean period, when the mantle could produce extremely high temperatures.

Previously, some geologists interpreted mantle rocks with low levels of oxidation as evidence that the Archean Earth’s mantle was less oxidized and that by some mechanism it became more oxidized over time. Proposed oxidation mechanisms include a gradual increase in oxidation levels due to loss of gases to space, recycling of old seafloor through subduction, and continued involvement of the Earth’s core in mantle geochemistry. But to date, proponents of this view have not converged on a single explanation.

Instead, the new findings support the view that the oxidation level of Earth’s mantle has been largely stable for billions of years, and that the low oxidation seen in some mantle samples was created under geological conditions that Earth can no longer produce. as her mantle has since cooled. So, instead of some mechanism that makes the Earth’s mantle More ▼ oxidized over billions of years, new research claims Archaean high temperatures made parts of the mantle less oxidized. As Earth’s mantle cooled after the Archean, it could no longer produce rocks with super-low oxidation levels. Cottrell said the process of cooling the planet’s mantle provides a much simpler explanation: Earth just isn’t making rocks like it used to.

Cottrell and her collaborators are now seeking to better understand the geochemical processes that shaped the Archean mantle rocks of the Gakkel Ridge and Southwest Indian Ridge by simulating in the laboratory the extremely high pressures and temperatures found in the Archean period.

Reference: “Deep, hot, ancient melting recorded by ultralow oxygen fugacity in peridotites” by Suzanne K. Birner, Elizabeth Cottrell, Fred A. Davis, and Jessica M. Warren, 24 Jul 2024. Nature.
DOI: 10.1038/s41586-024-07603-w

In addition to Birner and Cottrell, Fred Davis of the University of Minnesota Duluth and Jessica Warren of the University of Delaware co-authored the study.

The research was supported by the Smithsonian and the National Science Foundation.