New geochemical tool reveals origin of Yellowstone's deep nitrogen

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Pete Barry, geochemist in the Department of Marine Chemistry and Geochemistry at Woods Hole Oceanographic Institution.

In the air we breathe, Oxygen plays an obvious and important role, but it is not the most abundant gas in the atmosphere. That honor belongs to nitrogen. But where did this nitrogen come from? And how much nitrogen is there deep within the Earth? It turns out that measurements at Yellowstone are helping to address these questions and others!

Researchers at the University of California Los Angeles (UCLA) and collaborators at Woods Hole Oceanographic Institution and several other universities around the globe developed a new geochemical tool to gain insight into the origin of nitrogen (N) and other gaseous elements in the Yellowstone volcanic province.

In September 2005, USGS/YVO Postdoctoral Fellow Brita Graham Wall used a radio-controlled camera, attached to a helium-filled balloon to take photos from the sky above the Norris Geyser Basin. Brita used these photos to gain insight into the distribution of cracks and fractures that feed the hot springs at Norris.

"We discovered that a lot of nitrogen molecules coming out of volcanoes at Yellowstone are actually composed of nitrogen molecules from air," said Jabrane Labidi, the paper's lead author. "Basically, air is contaminating the volcanic gases."

To reach this conclusion, researchers sampled gases from inside the Yellowstone Caldera and along its margins, and they analyzed them for gas compositions — namely nitrogen isotopes.

Isotopes are two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons, and hence differ in atomic mass but not in chemical properties. With two stable isotopes (¹⁴N and ¹⁵N), the molecule N₂ can occur as ¹⁴N¹⁴N, ¹⁴N¹⁵N and ¹⁵N¹⁵N. Because the vast majority (~99%) of nitrogen is ¹⁴N, the most common combination of isotopes (also called an isotopologue) is ¹⁴N¹⁴N. Substitution of a single ¹⁵N for ¹⁴N is rare; a doubly substituted isotopologue, ¹⁵N¹⁵N, is even less common.

An earlier study from UCLA showed that atmospheric N₂ displays a unique proportion of 15N15N, considerably different from the composition of N₂ that is found in Earth's mantle (the layer located beneath the crust that makes up the bulk silicate portion of Earth). Labidi and co-authors simply used this anomaly to track any atmospheric N₂ in hydrothermal fluids from Yellowstone. Their findings were published April 16, 2020, in the journal Nature.

The relationship between Δ30 and N2/3He ratios in hydrothermal gases from Iceland and Yellowstone. Δ30 and N2/3He ratios are shown for samples collected from gases in Iceland (yellow circles) and Yellowstone (red circles). The Yellowstone mantle-endmember is arguably indistinguishable in terms of N2/3He from the convecting upper mantle (grey diamonds). Modified from Labidi et al., 2020.

Nitrogen isotopologue measurements were made by using a custom-built instrument at UCLA to determine the amount of air- versus mantle-derived N₂ in the Yellowstone hydrothermal system. The team coupled these new-generation nitrogen data with state-of-the-art noble gas isotope measurements. Noble gases are chemically inert and are found in trace amounts in the Earth, but can reveal important information about Earth evolution due to their unique chemical properties. The authors showed that this new coupled noble gas and clumped nitrogen approach can help disentangle atmospheric gases from mantle gases.

The results revealed the presence of contaminating air in samples that were previously considered to be coming directly from magmatic sources. By accounting for the contaminating effect, the data revealed evidence that some mantle nitrogen, still contributing to present-day volcanism, could have been present in the deep Earth since our planet initially formed.

Woods Hole Oceanographic Institution Scientist Pete Barry, a co-author on the paper, said "Once air contamination is accounted for, we can gain new and valuable insights into the origin of nitrogen and the evolution of our planet."

The researchers also compared the compositions of gases from Yellowstone National Park with gas emissions from volcanoes in Iceland and around the globe (see accompanying YouTube video of what gas emissions look like from above). Nitrogen measurements in these volcanoes allowed Labidi and his coauthors to distinguish between nitrogen sources coming from air and those coming from inside the Earth's mantle. These results ultimately help these researchers understand how our planet and atmosphere formed.

This study was supported by the Deep Carbon Observatory and the Sloan Foundation.