A thin slice of ancient rock taken from Gakkel Ridge near the North Pole, photographed under a microscope and viewed under cross-polarized light. Width of field ~14 mm. Thin-section rock analysis helps geologists identify and characterize the minerals in a rock. The analyses reveal information about the mineral composition, texture, and history of the rock, such as how it formed and what changes it underwent later. Researchers use the identification and chemical composition of minerals in these ancient rocks from the Earth’s mantle to determine the conditions under which the rocks were melted. Credit: E. Cottrell, Smithsonian
Smithsonian scientists are conducting new research on ancient rocks known as “time capsules,” 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 that are 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 improve our understanding of Earth’s early 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 much of Earth’s mantle has remained stable over geological time, challenging previous claims by other researchers about major transitions.
“This study tells us more about how this particular place we live in became what it is today, with its unique surface and interior that allowed life and liquid water to exist,” said Elizabeth Cottrell, chair of the museum’s Department of Mineral Sciences, curator of the National Rock Collection and co-author of the study. “It’s part of our story as human beings, because our origins all go back to the formation of the Earth and its evolution.”
The study, published in the journal Naturefocused on a group of rocks taken from the seafloor and possessing unusual geochemical properties. Indeed, the rocks show signs of extreme fusion with very low levels of oxidation; oxidation occurs when a
” data-gt-translate-attributes=”({“attribute”:”data-cmtooltip”, “format”:”html”})” tabindex=”0″ role=”link”>atom or a molecule loses one or more electrons during 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, to the Archean eon. In addition, the results show that the Earth’s mantle has generally maintained a stable oxidation state since these rocks formed, contrary to what other geologists had previously theorized.
An ancient rock extracted from the seabed and studied by the research team. Credit: Tom Kleindinst
“The ancient rocks we studied are 10,000 times less oxidized than modern mantle rocks, and we show that this is because they melted deep within the Earth during the Archean, when the mantle was much warmer than it is today,” Cottrell said. “Other researchers have tried to explain the higher oxidation levels seen in modern mantle rocks by suggesting that an oxidation event or shift occurred between the Archean and today. Our evidence suggests that the difference in oxidation levels can simply 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, including lead author Suzanne Birner, who did her undergraduate training at the National Museum of Natural History and is now an assistant professor at Berea College in Kentucky, began their research to understand the relationship between Earth’s solid mantle and modern-day seafloor volcanic rocks. The researchers began by studying a group of rocks that had been dredged from the seafloor at two mid-ocean ridges where tectonic plates are moving apart and the mantle is rising to the surface and producing new crust.
The two sites 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 slowest spreading tectonic plate boundaries in the world. The slow spreading of these mid-ocean ridges means that they are relatively quiet, volcanically speaking, compared to faster-spreading ridges that are dotted with volcanoes like the East Pacific Ridge. This means that rocks collected 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 bucket that is lowered more than 10,000 feet below the ocean surface and dragged along the seafloor to collect geological samples. Credit: Emily Van Ark
When the team analyzed the mantle rocks they had collected from these two ridges, they found that they had strange chemical properties in common. First, the rocks had melted to a much greater extent than Earth’s mantle today. Second, the rocks were much less oxidized than most other samples of Earth’s mantle.
To achieve such a degree of melting, the researchers estimated that the rocks must have melted deep inside the Earth at very 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. Therefore, the researchers deduced that these mantle rocks could have melted during the Archean, when the planet’s interior was at 360-540 degrees
” data-gt-translate-attributes=”({“attribute”:”data-cmtooltip”, “format”:”html”})” tabindex=”0″ role=”link”>Fahrenheit (200–300 degrees
” data-gt-translate-attributes=”({“attribute”:”data-cmtooltip”, “format”:”html”})” tabindex=”0″ role=”link”>Celsius) it’s warmer 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 chemistry.
“This fact alone doesn’t prove anything,” Cottrell said. “But it does open the door to the possibility that these samples are true geological time capsules from the Archean.”
Interpretation and scientific knowledge
To explore geochemical scenarios that could explain the low oxidation levels in rocks collected from Gakkel Ridge and Southwest Indian Ridge, the team applied several models to their measurements. The models revealed that the low oxidation levels measured in their samples could have been caused by melting under extreme heat conditions deep within the Earth.
Both lines of evidence support the interpretation that the rocks’ atypical properties represent a chemical signature from their melting deep within the Earth during the Archean, when the mantle could produce extremely high temperatures.
Some geologists have 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 gas to space, recycling of ancient seafloor by subduction, and continued participation of the Earth’s core in mantle geochemistry. But to date, proponents of this theory have not agreed on a single explanation.
Instead, the new findings support the idea that the level of oxidation in Earth’s mantle has remained largely stable for billions of years, and that the low oxidation observed in some mantle samples was created under geological conditions that Earth can no longer produce because its mantle has since cooled. So, instead of a mechanism that creates Earth’s mantle more oxidized for billions of years, the new study argues that high temperatures during the Archean transformed parts of the mantle less oxidized. Since the Earth’s mantle has cooled since the Archean, it can no longer produce rocks with extremely low levels of oxidation. According to Cottrell, the process of cooling the Earth’s mantle provides a much simpler explanation: the Earth simply does not produce rocks as it used to.
Cottrell and his collaborators are now seeking to better understand the geochemical processes that shaped the Archean mantle rocks of the Gakkel Ridge and the Southwest Indian Ridge by simulating in the laboratory the extremely high pressures and temperatures observed in the Archean.
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, 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 were co-authors of the study.
The research was supported by the Smithsonian and the National Science Foundation.