The composition of the Earth’s mantle varies due to the planet’s formation and evolution. Earth gathered minerals and gasses from space during its formation. Different processes separated the chemicals during Earth’s evolution, resulting in a diversified mantle.
Ocean island basalts are derived from volcanic islands and provide information about these various mantle chemical mixtures. They aid in our comprehension of how these mixtures developed and endured across time.
To learn more about Earth’s past, graduate student Judy Zhang and assistant professor Rita Parai examined minute levels of noble gases in volcanic rocks. Prof. Parai studied these gases in rocks using precise equipment, which can provide valuable insights into the planet’s historical evolution.
Parai and graduate student Judy Zhang analyzed noble gases in volcanic rocks from the Cook and Austral islands. According to their research, plate tectonics has been moving gases from the Earth’s surface deep inside the planet for more than 2.5 billion years.
Researchers can use elemental systematics and noble gas isotope systematics as effective techniques for examining the formation and evolution of mantle geochemical reservoirs to learn more about accretion, early and long-term mantle degassing, and the injection of air volatiles into the mantle.
Noble gases are chemically inert. When the Earth first formed 4.5 billion years ago, some of its isotopes were trapped in the interior, leaving a lasting imprint on the rocks.
The team measured several noble gas elements and their isotopes, including the rare element xenon. Noble gas abundance patterns and their characteristic isotopic signatures presented a significant story.
Zhang explained, “The Cook-Austral Island rocks were delivered from deep in the planet’s mantle to the Earth’s surface through volcanic eruptions. The signature noble gases in the rocks showed that deep in the past, tectonic plates were relatively cool when they underwent subduction — the process where surface rock sinks to the mantle — some 2.5 billion years ago.”
According to some experts, Earth’s history may have been markedly more intense and hotter than it is now. The finding of xenon in the rocks suggests that conditions similar to those of today’s subduction—the process by which one tectonic plate descends beneath another—occurred.
Zhang said, “If the rock had been subducted at much higher temperatures, it would have lost all of its gases.”
“This is a fascinating finding.”
Finding xenon atoms in rocks is incredibly challenging. It’s like trying to find a tiny needle in a vast field of hay. Noble gases are already rare in stones, and detecting xenon, which can reveal ancient plate tectonic activity, is even more difficult.
“There are only about 100,000 xenon atoms for every gram of material,” Parai said. “That’s about 1 out of every ten quadrillion atoms.”
The team extracted gas from large amounts of the mineral olivine in these rocks to get the faintest signal. It was like finding a way to squeeze blood from a stone.
Journal Reference:
- Xinmu J. Zhang, Rita Parai, John C. Lassiter et al. Primordial and recycled noble gases in the Cook-Austral HIMU mantle: Insights into the onset of volatile subduction. Earth and Planetary Science Letters. DOI: 10.1016/j.epsl.2024.118591