Study challenges conventional understanding of mantle gas migration

Scientists from the Woods Hole Oceanographic Institution and the University of Manchester discovered clear evidence showing that noble gases from the Earth’s mantle are reaching the Columbia Plateau aquifer system, even though there’s no recent volcanic activity in the region to explain it.

The release of gases like helium and argon from Earth’s interior has been tied to tectonic or volcanic activity, as per the previous scientific norms. But this study shows that even in geologically stable areas like the Palouse Basin in eastern Washington and Idaho, gases from the subcontinental lithospheric mantle (SCLM) are steadily moving into the shallow crust and can be clearly detected.

The researchers sampled 17 wells in the Palouse Basin Aquifer (PBA) within the Columbia Plateau Regional Aquifer (CPRA), which is made up of basalt layers and sediments. They measured helium and argon isotopes in groundwater that had been underground for 9 000 to 27 000 years.

While noble gases in groundwater usually come from nearby rocks or the atmosphere, the team found extra helium-3 (³He) and argon-40 (⁴⁰Ar) that couldn’t be traced to local sources alone.

“Think of it like having a small puncture in your car tire. We’ve discovered a steady trickle of gases coming from deep within Earth, even though there’s no obvious volcanic activity on the surface,” lead author Dr. Rebecca Tyne said.

Using high-precision mass spectrometry and a three-isotope method for argon, the researchers could isolate small contributions of radiogenic 40Ar from the background of atmospheric argon. They found a strong correlation between helium and argon isotope excesses. Because helium-3 almost always comes from the mantle, its presence alongside extra argon-40 suggests that both gases are rising from deep inside the Earth.

The study ruled out local sources as the main cause of the gases. Even under generous assumptions about how much argon-40 could be produced and released by the surrounding rocks, the models showed it made up less than 4 percent of what was actually found in the groundwater. This suggests the gases are coming from deeper underground, likely through passive degassing from the sub-continental lithospheric mantle.

Using a Monte Carlo mixing model based on helium and argon ratios, the researchers wanted to know how much of the gas came from the mantle. The results support the idea that these gases can slowly move up through the lithosphere, even without faults, uplift, or other tectonic activity. The isotopic signatures matched those observed in the subcontinental lithospheric mantle and not those from mid-ocean ridges or deep mantle plumes.

This kind of passive gas release changes how we think about the movement of gases from deep inside the Earth. It demonstrates that mantle gases don’t need volcanic activity to reach the surface and offers a new way to study Earth’s long-term volatile cycle.

Finding mantle gases in young aquifers shows that groundwater can carry a long-term record of what’s happening deep inside the Earth. It also means geochemists may need to rethink how they interpret noble gas data, especially when using helium for dating purposes.

By improving techniques like triple-argon-isotope analysis and using them in shallow aquifers, this research offers a new method for tracking the movement of gases from deep inside the Earth.

References:

¹ Passive degassing of lithospheric volatiles recorded in shallow young groundwater – R. L. Tyne, M. W. Broadley, D. V. Bekaert, P. H. Barry, O. Warr, J. B. Langman, I. Musan, W. J. Jenkins and A. M. Seltzer – Nature Geoscience – June 5, 2025 – DOI https://doi.org/10.1038/s41561-025-01702-7 – OPEN ACCESS

² Earth’s mantle gases found seeping into groundwater far from volcanic zones – The University of Manchester – June 5, 2025