New Study Challenges Mantle Oxidation Theory

A thin slice of ancient rock collected from the Gakkel Ridge near the North Pole, imaged under a microscope and viewed under cross-polarized light. Field width ~14 mm. Analysis of rocks in thin sections helps geologists identify and describe the minerals within the rock. The analyses reveal information about the rock’s mineral composition, texture, and history, such as how it formed and any subsequent changes it underwent. Researchers use the determination of the chemical composition of minerals in these ancient rocks from the Earth’s mantle to determine the conditions under which these rocks melted. Copyright: E. Cottrell, Smithsonian

Smithsonian scientists are conducting new research on ancient “time capsule” rocks, dating back at least 2.5 billion years.

Researchers at the Smithsonian Institution’s National Museum of Natural History have conducted a new analysis of rocks thought 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 advance our understanding of Earth’s earliest geological processes and contribute to a long-standing scientific debate about the planet’s geological history. Notably, the study provides evidence that the oxidation state of most of Earth’s mantle has remained stable over geologic time, challenging previous assertions by other researchers about major shifts.

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

The study, published in the journal naturefocuses on a group of rocks collected from the seafloor that have unusual geochemical properties. Specifically, the rocks show evidence of being highly dissolved with very low levels of oxidation; oxidation is when corn 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 at least 2.5 billion years during the Archean. What’s more, the results show that Earth’s mantle has generally maintained a stable oxidation state since these rocks formed, contrary to what other geologists had previously assumed.

An ancient rock was extracted from the seabed and studied by the research team. Image credit: Tom Kleindienst

“The ancient rocks we studied are 10,000 times less oxidized than typical modern mantle rocks, and we provide evidence that this is due to their melting deep within 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 the mantle today by suggesting that an oxidation event or change occurred between the Archean and today. Our evidence suggests that the difference in oxidation levels can be explained simply 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, including lead author Susan Berner, 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 volcanic rocks on the seafloor. The researchers began by studying a set of rocks extracted from the seafloor at two ocean ridges where tectonic plates are moving apart and the mantle is moving to the surface, producing new crust.

The two places where the rocks were collected, the Jackal Ridge near the North Pole and the Southwest Indian Ridge between Africa and Antarctica, are among the slowest-spreading tectonic plate boundaries in the world. The slow rate of spreading at these oceanic ridges means that they are relatively quiet, volcanically speaking, compared to faster-spreading ridges that are punctuated by volcanoes such as the East Pacific Ridge. This means that rocks collected from these slow-spreading ridges are likely to be samples of the mantle itself.

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

When the team analyzed the mantle rocks they collected from these two hills, they discovered that they shared strange chemical properties. First, the rocks were much more melted than is typical for mantle rocks today. Second, the rocks were much less oxidized than most other mantle rock samples.

To reach this high degree of melting, the researchers conclude that the rocks must have melted deep in the Earth at extremely high temperatures. The only period in Earth’s geological history known to have included such high temperatures was between 2.5 and 4 billion years ago during the Archean Eon. The researchers therefore conclude that these mantle rocks probably melted during the Archean Eon, when the planet’s interior was between 360 and 540 degrees. F (200-300 degrees Celsius) hotter than it is today.

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

“This fact alone doesn’t prove anything, but it opens the door to the possibility that these specimens are true geological time capsules dating back to the Archean,” Cottrell said.

Scientific interpretation and visions

To explore geochemical scenarios that might explain the low oxidation levels of the rocks collected at Jackel Ridge and southwest Indian Ridge, the team ran several models on their measurements. The models revealed that the low oxidation levels they measured in their samples were likely caused by melting under extremely hot conditions deep in the Earth.

Both lines of evidence support the interpretation that the rocks’ atypical properties represent a chemical signature resulting from their melting deep within the Earth during the Archean, when the mantle was capable of producing extremely high temperatures.

Previously, some geologists interpreted mantle rocks with low oxidation levels as evidence that the Archean 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 the loss of gases to space, the recycling of ancient seafloor material by subduction, and the continued involvement of the Earth’s core in mantle chemistry. But until now, proponents of this view have not agreed on any single explanation.

Instead, the new findings support the view that the oxidation level of Earth’s mantle has been largely constant for billions of years, and that the low oxidation seen in some mantle samples arose under geological conditions that Earth can no longer produce because its mantle has since cooled. So, instead of some mechanism that makes Earth’s mantle more Oxidized over billions of years, the new study claims that high temperatures in the Archean period made parts of the mantle less Because the Earth’s mantle has cooled since the Archean, it can no longer produce rocks with very low oxidation levels. Cottrell says the cooling process offers a much simpler explanation: The Earth simply doesn’t make rocks as it did in the past.

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

Reference: “Deep, Hot, Ancient Melting Recorded by Extremely Low Oxygen Levels in Peridotite” by Susan K. Berner, Elizabeth Cottrell, Fred A. Davis, and Jessica M. Warren, July 24, 2024, nature.
doi: 10.1038/s41586-024-07603-s

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

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