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.

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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.

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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.

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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!

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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.