A thin slice of the ancient rocks collected at the Gakkel Ridge near the North Pole was photographed under a microscope and viewed under cross-polarized light. Field width ~14 mm. Thin section rock analysis helps geologists identify and characterize minerals in a rock. The analyses provide information about the mineral composition, texture, and history of the rock, such as how it was formed and what changes it later underwent. Researchers use the identification and chemical composition of the minerals in these ancient rocks from the Earth’s mantle to determine the conditions under which these rocks melted. Image credit: E. Cottrell, Smithsonian
Scientists at the Smithsonian Institute are conducting 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, shedding light on the chemical history of the Earth’s mantle, the layer beneath the Earth’s crust. Their findings expand our understanding of Earth’s earliest geologic processes and contribute to a long-standing scientific debate about the planet’s geologic history. In particular, the study provides evidence that the oxidation state of most of the Earth’s mantle has remained stable over geologic time, challenging previous claims by other researchers of major transitions.
“This study tells us more about how this special place we live in became what it is, with its unique surface and interior that have supported life and liquid water,” 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 history as humans, because our origins all go back to the formation and evolution of the Earth.”
The study was published in the journal Naturewhich focused on a group of rocks collected from the sea floor that possessed unusual geochemical properties. Namely, the rocks show signs of having been melted to an extreme degree with very low levels of oxidation; oxidation is when a atom or a molecule loses one or more electrons in a chemical reaction. Using additional analysis and modeling, the researchers took advantage of the unique properties of these rocks to show that they likely date from at least 2.5 billion years ago, to the Archean period. In addition, the results show that the Earth’s mantle has overall maintained a stable oxidation state since these rocks formed, contrary to previous theories by other geologists.
An ancient stone recovered from the sea floor and examined by the research team. Photo credit: Tom Kleindinst
“The ancient rocks we studied are 10,000 times less oxidized than typical modern mantle rocks, and we present evidence that this is because they melted 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 in rocks from the present-day mantle by suspecting 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 study author Suzanne Birner, who completed a 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 Earth’s solid mantle and the modern volcanic rocks of the seafloor. The researchers began by examining a group of rocks dredged from the seafloor at two ocean ridges, where tectonic plates drift apart and the mantle flows to the surface, forming 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 slowest spreading tectonic plate boundaries in the world. The slow rate of spreading at these ocean ridges means they are relatively quiet volcanically speaking, compared to faster spreading ridges that are dotted 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 Earth’s mantle itself.
The stern of the research vessel R/V Knorr at sea in 2004. The A-shaped structure holds the huge metal and chain vessel that will be lowered more than 3,000 meters below the ocean surface and dragged along the sea floor to collect geological samples. Photo credit: Emily Van Ark
When the team analyzed the mantle rocks they collected from these two ridges, they discovered that they shared strange chemical properties. First, the rocks were much more molten than is typical of Earth’s mantle today. Second, the rocks were much less oxidized than most other samples of Earth’s mantle.
To achieve such a high degree of melting, the researchers concluded, the rocks deep within the Earth must have melted at very high temperatures. The only known period in Earth’s history that had such high temperatures was 2.5 to 4 billion years ago during the Archean. From this, the researchers concluded that these mantle rocks could have melted during the Archean, when the planet’s interior was 360–540 degrees Celsius. Fahrenheit (200–300 degrees Celsius) hotter than today.
The extreme melting would have protected these rocks from further melting, which could have altered their chemical signature, and would have allowed them to circulate in the Earth’s mantle for billions of years without significantly changing their chemical composition.
“This fact alone does not prove anything,” said Cottrell. “But it does suggest that these samples are genuine geological time capsules from the Archean period.”
Scientific interpretation and findings
To investigate the geochemical scenarios that could explain the low oxidation levels of the rocks collected from the Gakkel Ridge and the Southwest Indian Ridge, the team applied several models to their measurements. The models showed that the low oxidation levels they measured in their samples could have been caused by melting under extremely hot conditions deep within the Earth.
Both lines of evidence supported the interpretation that the rock’s atypical properties represented a chemical signature resulting from melting deep within the Earth during the Archean, when the Earth’s mantle was capable of generating extremely high temperatures.
To date, some geologists have interpreted low-oxidation mantle rocks as evidence that the Earth’s mantle was less oxidized in the Archean and became more oxidized over time by some mechanism. Proposed oxidation mechanisms include a gradual increase in oxidation due to the loss of gases to space, recycling of ancient seafloor by subduction, and the continued involvement of the Earth’s core in mantle geochemistry. To date, however, proponents of this view have not agreed on a unified explanation.
Instead, the new findings support the view that the oxidation level of the Earth’s mantle has been largely constant for billions of years and that the low oxidation observed in some mantle samples arose under geological conditions that can no longer occur on Earth, since its mantle has since cooled. So, instead of having a mechanism that more The new study argues that the high temperatures of the Archean oxidized parts of the mantle over billions of years. fewer oxidized. Because the Earth’s mantle has cooled since the Archean, it can no longer produce rocks with extremely low levels of oxidation. Cottrell said the process of mantle cooling offers a much simpler explanation: Earth simply no longer produces rocks like it used to.
Cottrell and her collaborators are now trying to better understand the geochemical processes that shaped the Archean mantle rocks of the Gakkel Ridge and the Southwest Indian Ridge by simulating the extremely high pressures and temperatures of the Archean in the laboratory.
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.