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After Yellowstone’s sizable earthquake on March 30, there was a lot of craziness. People were throwing around theories that animals were running from the park in fear, that the earthquake would trigger an eruption and that helium emissions were rising in the caldera, meaning an eruption was coming. Now, in my fervor to stifle such fearmongering, I said that no, none of these events are in any way related to a potential upcoming eruption at Yellowstone. Well, I received an email from Dr. Jacob Lowenstern, the scientist-in-charge of the Yellowstone Volcano Observatory, that brought me back from getting carried away because one of those three could actually tell us about activity in the caldera. No, it wasn’t the animals — they’re just migrating — and no, the earthquake isn’t going to trigger an eruption (I covered that last week). However, there is an interesting story in regards to helium and magmatic activity — and it turns out that helium might be able to help in volcano monitoring in the future.
Before we get too far, let’s make one thing abundantly clear: helium emissions in Yellowstone in no way suggest an eruption is in the works. Let that sink in.
However, helium, and especially the ratio between helium’s two naturally-occuring isotopes ( 3 He and 4 He), can tell us about the source of the helium. Within the Earth, helium can come from two main sources: (1) the mantle, where it is primordial helium from the formation of the planet and (2) the crust, where it comes from the radioactive decay of elements like uranium and thorium. These two sources of helium, however, have different flavors. Mantle-derived primordial helium is dominated by the lighter 3 He (2 protons, 1 neutron) while the decay of elements in the crust will produce the heavier 4 He (2 protons, 2 neutron, also known as an alpha particle, one of the ways elements decay radioactively).
This means that when you measure the isotopic ratio of helium being released in soils, hot springs, wells or fumaroles, you can determine how much of that helium is being derived from either degassing of magma coming from the mantle or from the radioactive decay of uranium and thorium in the crust. If the 3 He/ 4 He is high, then the mantle source is dominating. If the 3He/4He is low, then the crust source is dominating. When we talk about these ratios, normally we compare them to the 3 He/ 4 He ratio in the atmosphere (called R A, which is ~0.000001384), so if the helium ratios tend to be reported a multiple of R A. For mantle sources, that is something like 16R A or more and for more crustal sources, it is 1-3R A.
Two recent studies looked at helium emissions in active volcanic areas. The first is looking at the events that lead up to the October 12, 2011 eruption at El Hierro in the Canary Islands. A study by Padrón and others (2013) in Geology examined the relationship between helium emissions measured in soils across the island and 3 He/ 4 He in water from a well on the island. What they found is that as the earthquakes increasing during late summer into early fall, so did the helium emissions across the island, from 9 kg/day to over 24 kg/day just prior to the eruption. Hand in hand with that, the 3 He/ 4 He also increased, from 2-3R A to over 8R A. Padrón and others (2013) suggest that the earthquakes helped create fractures for the helium to escape (like a natural “fracking” process) as the magma rose and degassing under the island. However, the crustal helium signature (low RA) was also overwhelmed by the mantle helium signature (high R A) from the magma. So, the closer the magma got to the surface, the more helium was released and the higher the 3 He/ 4 He became.
Seismicity (grey), helium emissions (blue line) and He isotopic composition (red line) for El Hierro during the months leading up to and past the eruption in October 2011. Image: Padron and others (2013)However, it wasn’t that simple. After the eruption in early October, the helium emissions decreased with the decreasing seismicity. When seismicity picked back up in late October/early November, the helium emissions went back up again … but the 3 He/ 4 He ratio didn’t. Instead, it stayed below 5R A, suggesting that the helium was coming from the crust that makes up the island, not mantle-derived magma. The new earthquakes in the northern part of the island created cracks for the helium to escape, but that helium wasn’t entirely related to new magma. In fact, helium emissions overall peaked at 38 kg/day well after the initial eruption began, while 3 He/ 4 He peaked with the eruption itself. Overall, the helium emissions seem to track very closely with the amount of seismicity (see above), while the 3He/4He tracked with the rising of magma into the crust until eruption. Rising 3 He/ 4 He related to seismicity has also been noticed at Mammoth Mountain near Long Valley in California, which is interpreted as the evidence that moving magma deep in the crust is driving the earthquake swarms.
At Yellowstone, helium tells a different story. A study in Nature by Lowenstern and others (2014) examined the voluminous helium that is released within and around the Yellowstone caldera. What they found there is the most productive areas of helium emissions, like Heart Lake Geyser Basin in the southern margin of the caldera, are actually mainly releasing helium derived from the crust, not any magma underneath Yellowstone. If you look at Yellowstone, the highest 3 He/ 4 He ratios are in the heart of the caldera (see below), with ~10-17R A (relatively close to the assumed mantle hotspot composition of ~22R A). At Heart Lake Geyser Basin, the 3 He/ 4 He is <2.5R A, so very strongly controlled by crustal sources.
Helium isotopic composition of gases sampled in the Yellowstone caldera. The highest values (most mantle-like) are in the middle of the caldera (large red dots) while the lowest values (most crust-like) are on the edges, like the Heart Lake Geyser Basin (bottom, small green dots). Image: Lowenstern and others (2014)
Lowenstern and others (2014) calculated how much 4 He the crust underneath Yellowstone could produced based on the uranium and thorium content and found that the Yellowstone area releases almost 600 times more 4He than it should based on the decay of uranium and thorium. This means that it is likely released helium that has been trapped in the crust for millions to billions of years — and parts of Yellowstone sit on crust that is over 3 billion years old. This helium at Yellowstone is in no way related to the magma underneath the caldera, but has likely been freed from the crust by the earthquakes and heating of the crust done by the magma (much like what happened at El Hierro). You can begin to see the problem we currently have with using helium for volcano monitoring. The amount of helium being released doesn’t tell us much, as helium of any flavor might be liberated by earthquakes under a volcano. We need to know the ratio of 3He/4He of that helium to understand whether the changes in emissions are actually related to magma. So, why is that a problem? Well, there isn’t an easy way to measure 3 He/ 4 He ratios in the field. Instead samples have to be taken back to a lab to be analyzed, so getting fast (and cheap) 3 He/ 4 He ratios isn’t possible right now. If you only consider the amount of helium being released at the volcano, you’re only getting a piece of the full picture. Imagine you notice a flood going down your street. It could be coming from the downpour that going on, or it could be from a broken water main up the street. Merely measuring how much water is running into the gutter isn’t going to sufficient to tell the source.
These two studies clearly show that we can learn a lot from measuring helium emissions and their isotopic composition. At El Hierro, it is clear that there is a disconnect at times between the helium emissions and the composition of that helium (mantle versus crust source), while at Yellowstone, there is a significant volume of stored helium in the crust that can be released by processes unrelated to anything that could lead to an eruption. Step by step, we are moving towards better being able to anticipate the action at a volcano, but sometimes you need to be careful not to get carried away without understanding exactly what is happening (me included).
References
Lowenstern, J.B., Evans, W.C., Bergfeld, D., and Hunt, A.G., 2014, Prodigious degassing of a billion years of accumulated radiogenic helium at YellowstoneNature, v. 506, no. 7488, p. 355–358, doi: 10.1038/nature12992.
Padrón, E., Perez, N.M., Hernandez, P.A., Sumino, H., Melian, G.V., Barrancos, J., Nolasco, D., Padilla, G., Dionis, S., Rodriguez, F., Hernandez, I., Calvo, D., Peraza, M.D., and Nagao, K., 2013, Diffusive helium emissions as a precursory sign of volcanic unrest: Geology, v. 41, no. 5, p. 539–542, doi: 10.1130/G34027.1.
