"This study tells us more about how this special place in which we live 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 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 humans because our origins all trace back to how Earth formed and how it has evolved."
Published in Nature, the study focused on seafloor rocks exhibiting unusual geochemical characteristics, indicating they melted to an extreme degree with low oxidation levels. Oxidation occurs when an atom or molecule loses electrons in a chemical reaction.
Through additional analyses and modeling, the researchers determined these rocks likely date back to at least 2.5 billion years ago during the Archean Eon. Their findings suggest that the Earth's mantle has maintained a stable oxidation state since then, opposing earlier theories that proposed significant changes.
"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 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 has taken place between the Archean and today. Our evidence suggests that the difference in oxidation levels can simply be 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."
The research team, led by 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, aimed to understand the relationship between Earth's mantle and modern seafloor volcanic rocks. They examined rocks dredged from the Gakkel Ridge near the North Pole and the Southwest Indian Ridge between Africa and Antarctica, two of the slowest-spreading tectonic plate boundaries. These regions are relatively quiet volcanically, making them ideal for sampling mantle rocks.
Analyzing the mantle rocks from these ridges, the team found they had been melted to a much greater extent and were less oxidized than most modern mantle samples. The high degree of melting likely occurred deep in the Earth at very high temperatures, characteristic of the Archean Eon.
Being extremely melted would have preserved these rocks' chemical signatures, allowing them to remain unchanged for billions of years.
"This fact alone doesn't prove anything," Cottrell said. "But it opens the door to these samples being genuine geologic time capsules from the Archean."
The researchers applied multiple models to their measurements to explore possible geochemical scenarios. The models supported the idea that the low oxidation levels were due to melting under very hot conditions deep within the Earth.
Previous theories suggested that Earth's mantle became more oxidized over time due to various mechanisms. However, this study supports the view that the mantle's oxidation state has remained stable, with low oxidation levels in some mantle samples resulting from high temperatures during the Archean.
Cottrell emphasized that the cooling of Earth's mantle over billions of years means it can no longer produce rocks with extremely low oxidation levels, simplifying the explanation of oxidation differences.
Cottrell and her team are now working to simulate the high pressures and temperatures of the Archean in the lab to further understand the geochemical processes that shaped these ancient mantle rocks.
This research is part of the Smithsonian's Our Unique Planet initiative, a public-private partnership exploring Earth's origins, oceans, continents, and the role of minerals in life's development.
In addition to Birner and Cottrell, Fred Davis of the University of Minnesota Duluth and Jessica Warren of the University of Delaware co-authored the study. The research was supported by the Smithsonian and the National Science Foundation.
Research Report:Deep, hot, ancient melting recorded by ultra-low oxygen fugacity in peridotites
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