Using Radioactive Radium To Better Understand Yellowstone’s Hydrothermal System

Editor’s noteYellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week’s contribution is from Andrew Miller, Ph.D. student, and Ken Sims, professor of geology and geophysics, both at the University of Wyoming.

In 1898, Marie and Pierre Curie acquired a large amount of rock called pitchblende that was known to have a high concentration of the radioactive element uranium. Through arduous mechanical and chemical separations, Marie Curie isolated a new element that was 300 times more radioactive than uranium. The substance she identified was the element radium, and the discovery resulted in Marie and Pierre Curie, along with Henry Becquerel, being awarded the Nobel Prize in Physics in 1903. Marie Curie later, in 1911, was awarded the Nobel Prize in Chemistry for isolating radium in its metallic state.

Perhaps unexpectedly, radium was one of the earliest elements studied in the Yellowstone hydrothermal system. Before the dangers of radioactivity were understood, radium was extracted for use in industrial goods, like self-luminescent paints (you may have heard of the “Radium Girls,” which refers to a lawsuit in the 1930s that won compensation for women who contracted radiation poisoning while painting watch faces with paint containing radium), and it continues to this day to be used in treatment for illnesses like cancer. In the early 1900s chemist Hermon Schlundt, from the University of Missouri, was recruited by the U.S. Geological Survey to “[determine] the radioactive properties of the thermal waters of the Yellowstone National Park.” During trips from 1906 to 1908, Dr. Schlundt and his colleagues measured hot spring waters and gases across many of Yellowstone’s hydrothermal basins and did find measurable radium in many of the features.

Photo of Dr. Hermon Schlundt and a park ranger sampling a pool in Norris Geyser Basin for radium. The description of the photo states that this was the site where the radioactive element thorium was first discovered in the United States./Plate IIIB from Schlundt and Moore, 1909 (USGS Bulletin 395).

While Dr. Schlundt was correct in his claim that he had found radium in Yellowstone, the measurements he made were not actually radium. In 1993, Dr. Neil Sturchio, a University of Delaware scientist studying radium geochemistry in the Yellowstone, remarked that while Dr. Schlundt had intended to measure radium, he was actually measuring the noble gas radon, which is the radioactive decay product of radium, and the measured radium concentrations he claimed were five to more than 50 times too large. Despite not measuring the correct element and even though he was not a geologist, Dr. Schlundt noted:

We have indicated a possible application of some of the data to a geologic question – the age of some of the spring deposits; but no systematic attempt has been made to relate the facts of radioactivity to the hydrography of the region or to the geology of the park.

Although he was not specifically studying the ages of the hot springs, Dr. Schlundt estimated the age of the travertine terraces in Mammoth to be about 20,000 years—the first estimate of the age of a feature in Yellowstone. Dr. Schlundt came to this conclusion through his understanding of the rate at which radium radioactively decays. He was working under the understanding that the half-life (the rate at which it takes half of the radioactive element to decay away) of radium was 2,600 years, although this number was ultimately found to be about 1,000 years off (the currently accepted value is 1,599 years). Dr. Schlundt identified the radium concentration of travertines at Mammoth to be 1 percent of the surrounding ‘recent’ deposits. For only 1 percent of radium to remain, around 6–7 half-lives have passed. This would suggest the travertine is 15,600–18,200 years old. Dr. Schlundt seems to have rounded up or presumed around 8 half-lives had passed to reach his conclusion of 20,000 years old.

Angel Terrace, Mammoth Hot Springs, Yellowstone National Park

Angel Terrace, Mammoth Hot Springs, Yellowstone National Park. Travertine deposits are abundant in the area/JoAnn Holloway, 2003.

It appears that Dr. Schlundt returned to Yellowstone sometime in the late 1920s or early 1930s to continue working on the age of Mammoth Hot Springs, but all that remains of this work is a reference to a reading of unpublished results at the Annual Meeting of the American Association for the Advancement of Science in Chicago in 1933.

Dr. Schlundt returned to Yellowstone one last time in 1935 and repeated the results from his first study using more advanced instruments. One major observation he emphasized, consistent with those made by visitors, researchers, and employees of the park, was the ephemeral nature of some hydrothermal features. Dr. Schlundt died in 1937, and his work Radioactivity of the Thermal Waters, Gases, and Deposits of Yellowstone National Park, published posthumously in 1938, was likely one of his final contributions to the scientific community.

Building on Dr. Schlundt’s work, University of Wyoming scientists have now developed a technique that utilizes all four natural occurring radium isotopes—forms of an element with different atomic masses—which have very different half-lives: 226Ra (t1/2 = 1600 years), 228Ra (t1/2 = 5.75 years), 224Ra (t1/2 = 3.63 days) and 223Ra (t1/2 = 11.43 days). This new method can constrain the timescales of hydrothermal fluid circulation across different feature types in Yellowstone. Combining these very hard-to-measure isotopes with other radiogenic isotopes, like lead and strontium, has provided, for the first-time, an understanding of the timescales of water-rock interaction beneath the surface of Yellowstone.

The University of Wyoming work indicates that it takes hundreds to thousands of years for hydrothermal fluids in neutral-chloride features (like Old Faithful and Grand Prismatic Spring) to ascend to the surface from the deep hydrothermal reservoir in Norris Geyser Basin. Hydrothermal fluids in acidic mud pots and frying pans (like Mud Volcano) ascend over tens of years.

The results of this study emphasize the importance of radium as a tool for further understanding the complex interplay of geology and biology on the formation of hydrothermal features around Yellowstone. What started as the search for a single element by Dr. Schlundt over one hundred years ago has led to new insights into Yellowstone’s hydrothermal system!

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