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Argon dating is a powerful tool for determining when volcanoes erupted in the past. This technique works on a multitude of volcanic materials—from Yellowstone ash to lava flows in New Mexico—and involves both natural radioactive decay and irradiation by nuclear reactor.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory.  This week's contribution is from Kellie Wall, geologist with the U.S. Geological Survey.

Knowing the eruptive history of volcanoes is critical to anticipating where and what kinds of eruptions are likely to occur in the future.

Scientists have several tools in their kit to determine the eruption ages of volcanic rocks. One such tool is 40Ar/39Ar dating, informally called argon dating. This technique takes advantage of the principle of radioactive decay, where an unstable "parent" nuclide (an atom with a specific number of protons and neutrons) decays to a stable "daughter" nuclide at a known rate over time. The more daughter nuclide is present relative to the parent, the more time has passed since the decay "clock" started.

A blue glow is emitted by radiation during operation of the USGS TRIGA Reactor, Denver, CO
A blue glow is emitted by radiation during operation of the USGS TRIGA® Reactor, a low-power nuclear research reactor in Denver, Colorado. Photo by Christopher Farwell, October 27, 2017.

Argon dating is an advancement of the long-used potassium-argon (K-Ar) dating method. Both techniques use the decay of unstable 40K to stable 40Ar. Potassium, including the 40K nuclide, is present in nearly every composition of magma and is stored readily in volcanic rocks as they cool and crystallize. Argon (including the 40Ar nuclide), on the other hand, is a gaseous element that escapes from magma and minerals at high temperatures. Only at low temperature—such as when erupted lava or ash cool at the surface—can atoms of argon be trapped in minerals. The advantage of this difference is that the radiometric "clock" starts at the time of eruption, when 40Ar starts building up in the rock. The disadvantage is that potassium and argon must be measured using different samples, which can create some uncertainty in the age calculation. 

The newer 40Ar/39Ar dating method was developed as an alternative approach. In this technique, samples are irradiated in a special type of nuclear reactor that is designed for scientific research. This type of reactor produces much lower energy than a power reactor, but it is enough to chemically change a rock sample. During irradiation,[PMP1]  some of the potassium isotopes in the rock are converted into argon isotopes, including 39Ar. The proportion of 40Ar to the reactor-made 39Ar can then be converted into a ratio of daughter 40Ar to parent 40K, yielding an age.

At some volcanoes, like Yellowstone or Valles Caldera, the mineral sanidine is abundant in rhyolite ash and lava flows, and it is ideal for argon dating. Sanidine is rich in potassium, including the radioactive parent 40K, and thus a large signal of radioactive daughter 40Ar can typically be measured, leading to a very precise age. Argon dating of Yellowstone lava flows, for example, has shown that they erupt in episodes of several lava flows over short time periods.

SP Crater in the San Francisco volcanic field of northern Arizona
SP Crater (right foreground), like many volcanoes in Arizona’s San Francisco volcanic field, erupted mafic lava that lacks sanidine crystals. Scientists dated this eruption at approximately 70,000 years old by using groundmass for argon dating, as well as another technique called cosmogenic nuclide surface exposure dating (Fenton et al., 2013; Fenton and Niedermann, 2014). Photo by Kellie Wall, 2022.

But not all volcanic rocks contain sanidine. This is especially true for basaltic lava flows like those found across the southwestern USA, for example, at Sunset Crater in Arizona and Bandera Crater in New Mexico. In these scenarios, scientists must turn to alternative materials for dating.

Microscopic view of different groundmass textures in rocks
Microscopic view of different groundmass textures in rocks. On the left, this groundmass is a good choice for argon dating, as it consists of abundant interconnected crystals. On the right, the groundmass consists predominantly of glass (black because it does not transmit cross-polarized light) and is a poor choice for argon dating. Photos by Andrew Calvert, 2024.

Perhaps the best alternative to sanidine is the groundmass of a volcanic rock—that is, the material between larger crystals that was once the liquid part of the magma. This is where the bulk of potassium becomes concentrated when no sanidine crystals are present to absorb it. But not just any groundmass will do—geochronologists (scientists who specialize in determining geologic ages) are very particular when it comes to this material. When erupted lava cools rapidly, the groundmass freezes into glass. Glassy groundmass can pose problems for argon dating because it can either release argon over time or trap excess argon from sources unrelated to the volcanic system, leading to less reliable age calculations. To avoid this problem, geochronologists seek groundmass that cooled slowly enough to form a network of tiny crystals, which better retains daughter 40Ar as it is produced and does not take up excess argon. The best groundmass with this "holocrystalline" texture is often found in the interior of a lava flow, where it was insulated and took longer to cool relative to the outer edges of the flow.

After the best material is sampled, a geochronologist will crush it into small pieces (less than a millimeter in diameter), wash it with water to remove dust, and inspect it under a microscope to remove foreign material, glassy pieces, and large crystals that do not contain any potassium. Then, a small amount of the cleanest material is packaged into a tiny foil packet, about the size of a green pea. Many of these packets are loaded together into a vial and then inserted into the nuclear research reactor for irradiation.

After a few hours of irradiation, the samples are returned to the geochronology laboratory where they are loaded into a mass spectrometer [PMP1] for analysis. Inside this instrument, a laser heats each sample to release the argon gas, and then the different forms argon, including 40Ar and 39Ar, are separated by the pull of an electromagnet so that each can be measured. When the ratio of 40Ar to 39Ar is determined, the rock’s age is finally revealed!

Mass spectrometer, used to measure the ration of atoms with different masses, in the USGS laboratory at Moffett Field, California
A mass spectrometer is used to measure the ratio of atoms with different masses—in this case, the different isotopes of argon gas, which can be used to determine the age of a volcanic rock. Left: a side view of a mass spectrometer at the USGS Argon Geochronology Laboratory in Moffett Field, CA. Right: a close-up view of the sample chamber in this mass spectrometer. Numbers 1 through 5 (right to left) and color-coded text boxes show the different parts and processes of the mass spectrometer. Photos by Genna Chiaro, 2024.

Recently, Dr. Matt Zimmerer at the New Mexico Bureau of Geology determined more than 100 new argon ages for eruptions from volcanic fields across New Mexico. These high-precision ages from mafic groundmass material show that volcanic eruptions occur in New Mexico approximately every few thousand years, including four eruptions during the past ~12,000 years. Similar work is ongoing by USGS scientists to determine the ages of eruptions in other areas of the southwestern USA (which is part of the Yellowstone Volcano Observatory’s area of responsibility), including the San Francisco Volcanic Field, near Flagstaff, Arizona. Each sample dated brings us one step closer to understanding the history—and anticipating the future—of volcanic eruptions in the United States.

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