Nature http://dx.doi.org/10.1038/nature12992 (2014)
The fluids seeping from the Yellowstone caldera are rich in helium. The isotopic signature of this gas suggests that it is escaping from the underlying crust, where it had been accumulating for hundreds of millions of years.
Jacob Lowenstern and colleagues at the US Geological Survey measured the chemical and isotopic composition of gases escaping from hot springs and fumaroles in the Yellowstone Park in the northwest US over the past decade. Although much of the helium in these fluids comes from the mantle, the amount of 4He and the composition of the coincident gases suggest an additional source. The authors argue that the helium is coming from a crustal reservoir that has been isolated for at least 172 million years — and possibly even billions of years. The most likely source is therefore the Archaean-aged rocks that underlie the caldera, which have been largely undisturbed for the past 2 billion years.
The onset of volcanism around the Yellowstone caldera about 2 million years ago probably mobilized this reservoir through metamorphism, fracturing and the migration of magmatic and hydrothermal fluids.
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Newton, A. Helium escape. Nature Geosci 7, 163 (2014). https://doi.org/10.1038/ngeo2111