The new study challenges the mantle oxidation theory

Smithsonian scientists conduct new research on 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-running scientific debate about the planet’s geological history. Notably, the study provides evidence that the oxidation state of much of Earth’s mantle has remained stable over geologic time, challenging previous assertions by other researchers about major transitions.

“This study tells us more about how this particular place we live in came to be the way it is, with its unique surface and interior that have allowed life and liquid water to exist,” said Elizabeth Cottrell, chair of the department of the museum. mineral sciences, curator of the National Rock Collection and co-author of the study. “It’s part of our history as humans, because our origins have all the traces of how the Earth was formed and how it has evolved.”

The study, published in the journal Nature, focused on a group of rocks collected from the sea floor that possessed unusual geochemical properties. Namely, the rocks show evidence of being melted to an extreme degree with very low levels of oxidation; oxidation is when one atom or 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 date back at least 2.5 billion years during the Archean Eon. Further, the findings indicate that the Earth’s mantle has generally maintained a stable oxidation state since the formation of these rocks, in contrast to what other geologists have previously theorized.

“The ancient rocks we studied are 10,000 times less oxidized than typical rocks of the modern mantle, 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 higher 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 change in oxidation levels can be explained simply by with 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 levels of oxidation.

Geological Evidence and Study Methodology

The research team — including 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 relationship between the rigid mantle of Modern land and seabed. volcanic rocks. The researchers began by studying a group of rocks that were excavated from the sea floor at two oceanic ridges where the tectonic plates are drifting apart and the mantle is spreading to the surface and producing 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 world’s slowest spreading tectonic plate boundaries. The slow rate of spreading at these oceanic ridges means they are relatively quiet, volcanically speaking, compared to faster-spreading ridges that are filled with volcanoes such as the East Pacific Ridge. This means that rocks collected from these slow-spreading ridges are more likely to be samples of the mantle itself.

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 the Earth’s mantle today. Second, the rocks were much less oxidized than most other samples of the Earth’s mantle.

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

Being so extremely molten would have protected these rocks from further melting that could have altered their chemical signature, allowing them to circulate in the Earth’s mantle for billions of years without significantly changing their chemical composition.

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

Interpretation and scientific knowledge

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 multiple models to their measurements. The models found that the low levels of oxidation they measured in their samples could have been caused by melting under extremely hot conditions deep in the Earth.

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

Previously, some geologists have interpreted mantle rocks with low oxidation levels as evidence that the Archean Earth’s mantle was less oxidized and that through 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 by subduction, and continued participation of the Earth’s core in mantle geochemistry. But, to date, the proponents of this view have not united around any 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 cannot produce. anymore because her cloak has since cooled. So instead of some mechanism that creates the Earth’s mantle more oxidized over billions of years, the new study argues that high Archean temperatures made it part of the mantle less oxidized. Because the Earth’s mantle has cooled since the Archean, it can no longer produce rocks with super-low oxidation levels. Cottrell said the cooling process of the planet’s mantle offers a much simpler explanation: Earth just isn’t making rock like it used to.

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

Reference: “Deep, hot, ancient melting recorded by ultra-low oxygen fugacity in peridotites” by Suzanne K. Birner, Elizabeth Cottrell, Fred A. Davis, and Jessica M. Warren, 24 July 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 were co-authors of the study.

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