New Study Challenges Mantle Oxidation Theory

Scientists at the Smithsonian Institution are conducting a new study of ancient “time capsule” rocks that date back at least 2.5 billion years.

The researchers Smithsonian National Museum of Natural History They conducted new analyses 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 Earth’s crust. Their findings advance our understanding of Earth’s earliest geological processes and contribute to a long-standing scientific debate about the Earth’s geological history. In particular, the study provides evidence that the oxidation state of most of Earth’s mantle has been stable over geological time, challenging previous claims by other researchers about major changes.

“This study tells us more about how this special place we live in came to be what it is today, with a unique surface and interior that allows life and liquid water to exist,” said Elizabeth Cottrell, head of the museum’s mineral sciences department, curator of the National Rock Collection, and co-author of the study. “It’s part of our human story, because our origins all go back to how the Earth formed and how it evolved.”

The study was published in the journal NatureThe study centers on a group of rocks from the ocean floor that have unusual geochemical properties: they show evidence of extreme melting with extremely low levels of oxidation. atom or the molecule loses one or more electrons in a chemical reaction. With the help of additional analysis and modeling, the researchers used the unique properties of these rocks to show that they likely date back to the Archean Era, at least 2.5 billion years ago. Furthermore, this discovery indicates that Earth’s mantle has maintained a stable oxidation state overall since these rocks formed, contrary to what other geologists have previously theorized.

Ancient rocks dredged from the ocean floor and studied by the research team. Photo by Tom Kleindinst

“We present evidence that the ancient rocks we studied are 10,000 times less oxidized than typical modern mantle rocks, and that this is due to melting deep in the Earth during the Archean, when the mantle was much hotter than it is today,” Cotterell said. “Other researchers have attempted to explain the high oxidation levels seen in mantle rocks today by suggesting that an oxidation event or change occurred between the Archean and the present. Our evidence suggests that the difference in oxidation levels can be simply 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 methods

The research team, which included lead study author Suzanne Berner, who completed a predoctoral fellowship at the National Museum of Natural History and is now an assistant professor at Berea College in Kentucky, set out to understand the relationship between Earth’s solid mantle and modern-day undersea volcanic rocks. The researchers began by studying rocks dredged from the ocean floor at two ocean ridges, where plates are spreading and mantle is churning up at the surface to form new crust.

The two locations where the rocks studied were collected — the Gakkel Ridge near the North Pole and the Southwest Indian Ridge between Africa and Antarctica — are two of the world’s most slowly spreading plate boundaries. The slow pace of spreading of these ridges means that volcanic activity is relatively quiet compared to faster-spreading ridges dotted with volcanoes, such as the East Pacific Ridge. This means that rocks from these slow-spreading ridges are more likely to be samples of the mantle itself.

The stern of the research vessel R/V Knorr at sea in 2004. The A-frame structure holds the giant metal-and-chain buckets that will be lowered more than 10,000 feet below the ocean’s surface and dragged along the ocean floor to collect geological samples. Photo by Emily Van Ark

When the team analyzed mantle rocks from the two ridges, they found they shared strange chemical properties. First, the rocks were much more highly molten than is typical of Earth’s mantle today. Second, the rocks were much less oxidized than most other samples of Earth’s mantle.

The researchers reasoned that to achieve such high degrees of melting, the rocks must have melted at extremely high temperatures deep within the Earth. The only other time in Earth’s geological history where such high temperatures are recorded is during the Archean Era, between 2.5 and 4 billion years ago. Therefore, the researchers speculated that these mantle rocks may have melted during the Archean Era, when temperatures inside the Earth ranged from 360 to 540 degrees. Fahrenheit (200 to 300 degrees Celsius) It’s hotter than today.

Being highly molten may have protected these rocks from further dissolution that would have altered their chemical signature, allowing them to circulate within Earth’s mantle for billions of years without significantly changing their chemistry.

“This fact alone doesn’t prove anything,” Cottrell says, “but it does open the door to the possibility that this sample could be a genuine geological time capsule from the Archean Era.”

Scientific interpretations and insights

To explore a geochemical scenario that could explain the low oxidation levels in rocks from the Gakkel Ridge and Southwest Indian Ridge, the team applied several models to their measurements. The models revealed that the low oxidation levels measured in the samples could be due to melting under extremely hot conditions deep inside the Earth.

Both lines of evidence support the interpretation that the rocks’ anomalous properties represent the chemical signature of deep Earth melting during the Archean, a time when the mantle could have produced extremely high temperatures.

Some geologists have interpreted mantle rocks with low levels of oxidation as evidence that the Archean Earth’s mantle was not very oxidized and that some mechanism caused it to become more oxidized over time. Proposed oxidation mechanisms include a gradual increase in oxidation levels due to the escape of gases into space, recycling of old ocean floors through subduction, and the Earth’s core continuing to contribute to the geochemistry of the mantle. However, to date, proponents of this view have not been able to agree on a single explanation.

Instead, the new findings support the view that the oxidation level of Earth’s mantle has remained fairly constant for billions of years, and that the low oxidation seen in some mantle samples was produced under geological conditions that Earth can no longer produce because the mantle has cooled. Thus, they suggest that there is no mechanism for oxidizing Earth’s mantle. more Parts of the mantle that have been oxidized for billions of years were formed by high temperatures in the Archean, a new study argues. few Oxidized. Earth’s mantle has cooled since the Archean, so rocks with extremely low oxidation levels can no longer form. Cottrell said the process of Earth’s mantle cooling offers a much simpler explanation: Earth simply isn’t producing the rocks it once did.

By simulating the extreme pressures and temperatures found during the Archean in the laboratory, Cotterell and his collaborators now aim to better understand the geochemical processes that formed the Archean mantle rocks of the Gakkel Ridge and Southwest Indian Ridge.

Reference: Suzanne K. Berner, Elizabeth Cottrell, Fred A. Davis, and Jessica M. Warren, “Deep high-temperature ancient melting recorded in extremely low-oxygen fugacity in peridotites,” 24 July 2024; Nature.
DOI: 10.1038/s41586-024-07603-w

In addition to Vilner and Cottrell, Fred Davis of the University of Minnesota Duluth and Jessica Warren of the University of Delaware are co-authors on the study.

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