New study challenges mantle oxidation theory

Smithsonian scientists have conducted new research on ancient ‘time capsule’ rocks that are at least 2.5 billion years old.

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 that sheds light on the chemical history of Earth’s mantle, the layer beneath the planet’s crust. Their findings advance our understanding of Earth’s earliest geologic processes and contribute to a long-standing scientific debate about the planet’s geologic history. Remarkably, 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 changes.

“This study tells us more about how the special place we live in came to be the way it is, how its unique surface and interior allowed life and liquid water to exist,” said Elizabeth Cottrell, chair of the museum’s Department of Mineralogy, curator of the National Rock Collection, and co-author of the study. “This is part of our story as humans because our origins are all connected to how the Earth formed and evolved.”

Study published in the journal Naturefocuses on a group of rocks collected from the sea floor that have unusual geochemical properties. That is, the rocks show evidence of extreme melting with very low levels of oxidation; oxidation occurs when a Atom In a chemical reaction, a molecule loses one or more electrons. With the help of additional analysis and modeling, the researchers used the unique properties of these rocks to show that they probably date back at least 2.5 billion years during the Archean Eon. Furthermore, the findings suggest that the Earth’s mantle as a whole has maintained a stable oxidation state since these rocks formed, which is contrary to theories previously suggested by other geologists.

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

“The ancient rocks we studied are up to 10,000 times less oxidized than typical modern mantle rocks, and we present evidence that this is because they were melted deep in the Earth during the Archean, when the mantle was much hotter than it is today,” Cottrell said. “Other researchers have tried to explain the high oxidation levels seen in rocks from today’s mantle by suggesting that an oxidation event or change occurred between the Archean and today. Our evidence suggests that the difference in oxidation levels can only be explained by the fact that the 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 study methodology

The research team — which included lead study author Suzanne Birner, who completed a pre-doctoral 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 connection between Earth’s solid mantle and modern seafloor volcanic rocks. The researchers began by studying a group of rocks that were dredged up from the sea floor at two oceanic ridges, where tectonic plates are spreading and the mantle is moving to the surface and creating 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, whereas faster spreading ridges full of volcanoes, such as the East Pacific Rise, are relatively quiet. This means that rocks collected from these slower spreading ridges are more likely to be samples of the mantle itself.

The stern of the research vessel, the R/V Knorr, while at sea in 2004. The A-frame structure holds a giant metal and chain bucket that is lowered more than 10,000 feet below the ocean surface and dragged along the sea floor to collect geological samples. Credit: Emily Van Ark

When the team analyzed mantle rocks collected from these two ridges, they discovered they had strange chemical properties in common. First, the rocks were much more molten than Earth’s mantle today. Second, the rocks were much less oxidized than most other samples from Earth’s mantle.

The researchers reasoned that to achieve such a high degree of melting, the rocks must have melted at very high temperatures deep inside the Earth. The only period in Earth’s geological history that includes such high temperatures was during the Archean Eon, 2.5 to 4 billion years ago. As a result, the researchers speculated that these mantle rocks must have melted during the Archean, when the temperature inside the planet was 360-540 degrees Fahrenheit (200-300 degrees Celsius) is warmer than today.

Being highly molten would have protected these rocks from further melting, which would have changed their chemical makeup, allowing them to circulate in the Earth’s mantle for billions of years without any significant changes to their chemistry.

“This fact alone doesn’t prove anything,” Cottrell said. “But it does suggest that these samples are true geologic time capsules from the Archean.”

Scientific explanation and insights

To explore geochemical scenarios that could explain the low oxidation levels of rocks collected at the Gakkel Ridge and Southwest Indian Ridge, the team applied several models to their measurements. The models showed that the low oxidation levels measured in their samples could be due to melting under extremely hot conditions deep in the Earth.

Both lines of evidence supported the interpretation that the rocks’ unusual properties reflect chemicals produced by melting deep in the Earth during the Archean epoch, when the mantle could generate extremely high temperatures.

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

Instead, the new findings support the view that the oxidation level of Earth’s mantle has remained largely constant for billions of years, and that the low oxidation seen in some samples of the mantle was formed under geological conditions that Earth can no longer produce as its mantle cools. Therefore, rather than any mechanism that created Earth’s mantle, the new findings support the view that the oxidation level of Earth’s mantle has remained largely constant for billions of years, and that the low oxidation seen in some samples of the mantle was formed under geological conditions that Earth can no longer produce as its mantle cools. More Because of oxidation over billions of years, the new study argues that the high temperatures of the Archean oxidized parts of the mantle Less Oxidized. Because Earth’s mantle has cooled since the Archean, it can no longer form rocks with very low oxidation levels. The process of the planet’s mantle cooling down offers a very simple explanation: Earth no longer forms rocks as it did before, Cottrell said.

Cottrell and his colleagues are now attempting to better understand the geochemical processes that shaped Archean mantle rocks from the Gakkel Ridge and the Southwest Indian Ridge by simulating the extremely high pressures and temperatures found in the Archean in the laboratory.

Reference: “Deep, hot, ancient melting recorded by ultralow oxygen fugas in peridotites” by Suzanne K. Birner, Elizabeth Cottrell, Fred A. Davis and Jessica M. Warren, July 24, 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 also co-authored the study.

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