February 20, 2014
Two Sources Of Helium Produced By Ancient Yellowstone Activity
[ Watch the Video: Yellowstone Releasing Ancient Helium ]
Lawrence LeBlond for redOrbit.com - Your Universe Online
As one might expect, Yellowstone National Park’s hot springs, geysers and other hydrothermal vents release gases stored deep below the Earth, including carbon dioxide and methane. Another gas has also been discovered to be squeaking out of Yellowstone’s vents, and while it is a common gas released from volcanic rocks, in this region, most of it comes from a much different source.
Most of the helium that scientists have found rising from Yellowstone’s depths has originated deep in rocks where it has been stored for hundreds of millions of years, according to a new paper published in the journal Nature.
Helium, the second-most abundant element in the universe, is formed by nuclear fusion of hydrogen atoms, a process that powers stars. While helium is abundant elsewhere in space, it is pretty rare here on Earth. Still, it can be found in two main forms. Almost all of the helium found in Earth occurs as helium-4 – two protons and two neutrons – which can be produced during the radioactive decay of heavy elements such as uranium. A very small fraction occurs as helium-3 – two protons and one neutron – which has been present on Earth since the planet was formed billions of years ago.
Scientists from USGS had previously determined that most of the helium coming from Yellowstone should be that of the -3 variety. This has been theorized based on the knowledge that uranium and other heavy elements are incompatible with minerals typically found in the Earth’s mantle. Since these heavy elements are needed for the production of helium-4 and since Yellowstone is a hotbed of deep mantle activity, it is most assuredly helium-3 being released from within the park’s deep rocks.
Yellowstone’s hydrothermal system has been found to carry a high amount of helium-3 and scientists consider this to be evidence that the Yellowstone hotspot activity originates deep within the mantle. Furthermore, they believe the helium released from within has a relatively straight path to the surface.
Surprisingly, USGS scientists have found a large amount of helium-4 being released from the park as well. After a decade-long collection of gas samples from Yellowstone, the scientists have found that the amount of helium-4 coming from within Yellowstone “exceeds by several orders of magnitude the average amount they’d expect to find elsewhere,” Smithsonian Magazine’s Sarah Zeilinski reports.
Most of the helium-4 from Yellowstone is likely billions of years old, according to the scientists. Given the amount present, they conclude that the gas most likely comes from ancient rocks in very ancient crust from nearby sources. This old crust, which stems from the Archaean eon, was formed 2.5 billion years ago and contains uranium and other heavy elements that have been steadily decaying over time. This process would have allowed high concentrations of helium-4 to build up underground. Then, around two million years ago, Yellowstone’s mantle hotspot penetrated the helium-4 stores, releasing it along with the helium-3 brought up by the mantle.
Bill Evans, a researcher with the USGS in Menlo Park, California, explained the helium release through volcanism.
Volcanoes most always form on the edges of tectonic plates that make up the Earth’s crust. But Yellowstone sits directly over the middle of a plate, he said.
"It's a part of the crust that formed a very long time ago, billions of years ago, and it's basically been stable since that time," said Evans, noting that it is now moving over the Yellowstone hotspot and molten rock from deep within has been pushing up into these ancient rocks.
"They've had this boring, peaceful existence and now suddenly they're put on the front burner," Evans said in a statement to NPR’s Nell Greenfieldboyce. "They're really getting cooked."
It is this cooking that is driving out the helium that has been trapped inside the rock for hundreds of millions of years, if not longer.
Jake Lowenstern, a USGS researcher who collected gas samples from Yellowstone, noted that the amount of helium-4 being released is “far more than we would have predicted.”
"It's kind of an interesting thought to us, how these rocks behave," Evans said, "because it's very rare on the face of the Earth to have [volcanism] come into rocks that have been that stable for that long."
The USGS team said that the discovery may be “kind of interesting” for fans of Yellowstone, but there may be more important implications for the discovery. Helium, as well as other noble gases, is used to estimate groundwater residence times and the scientists assume that the more helium-4 present in water, the longer that water has been sitting in the rocks around it.
“But the study of helium at Yellowstone shows that some of these assumptions—specifically helium-4 produced by the steady decay of elements found only within the rocks and sediments of the local aquifer—aren’t quite right,” wrote Smithsonian’s Sarah Zeilinski.
“Helium can suddenly come into a system from unexpected places—a pocket of ancient rock, for instance, or an magma source—so the dates in past calculations, particularly those from aquifers in volcanic regions or near earthquake faults, might be way off because of that extra helium,” she continued.
While this discovery is significant for scientists, it really has no practical value. In other words, don’t expect to be hearing about any extra party balloons being filled with helium anytime soon.
And while scientists are busy studying Yellowstone’s helium sources, others, namely nearby residents, just want to know whether or not the supervolcano that sits below the park is going to erupt again. Yellowstone last erupted 640,000 years ago and based on the timeline of previous eruptions, it is nearly due for another.