Laboratory experiments help explain what happens when hot rocks, water and gas interact in Yellowstone’s hydrothermal system

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Shaul Hurwitz, Research Hydrologist with the U.S. Geological Survey and Jeffrey Cullen, Research Scientist, and Jaime Barnes, Associate Professor, both at the Jackson School of Geosciences, University of Texas at Austin.

In Yellowstone, large volumes of dilute rain and snowfall (meteoric water) seep into the subsurface and are heated, mainly by the underlying magma. The hot groundwater, after flowing through the subsurface rhyolite rocks, discharges at the ground surface through Yellowstone’s numerous springs. These discharged hot waters have diverse chemical compositions which significantly differ from the composition of the cold meteoric water that seeped into the subsurface. Thus, the chemical composition of groundwater is significantly modified during flow from recharge (seepage) to discharge. The alteration minerals in core samples obtained during scientific drilling in Yellowstone also establishes that the rhyolite is modified by interaction with the hot groundwater.

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To simulate the chemical reactions that modify groundwater and rhyolite compositions between water recharge and discharge, laboratory experiments were conducted under pressures and temperatures similar to those in Yellowstone’s hydrothermal system. Some of the questions that the experiments tried to address include: At what temperatures and pressures do groundwater and rhyolite react to generate the chemical compositions of Yellowstone’s hot waters? What are the alteration minerals that form when groundwater and rhyolite react at different temperatures? What are the causes for water chemistry variations between different thermal areas? What is the effect of CO₂ gas derived from magma on the chemical reactions?

To conduct the experiments, small fragments of rhyolite glass (obsidian) and water were inserted into gold bags, which are chemically inert and therefore will not react with the water or rock, and then sealed. Tests were then carried out at temperatures ranging from 150 to 350 degrees Celsius (about 300 to 660 degrees Fahrenheit) for approximately 90 days. In one experiment carbon dioxide (CO₂) gas was added to the water. At the end of each experiment the gold bag was quickly cooled (quenched) to room temperature, and the reacted waters and rhyolite were recovered. Different types of chemical analyses were performed on the experimental products.

Results suggest: 1) The rhyolite glass progressively absorbs water (hydrates) with increasing temperature between 150 and 275 degrees Celsius (302 to 527 degrees F). 2) At temperatures higher than 275 degrees Celsius, the rhyolite glass loses its structure and alteration minerals form. 3) The pH of the water decreases (becomes more acidic) with increasing temperature. 4) At temperatures of up to 250 degrees Celsius nearly all the chlorine is retained in the rhyolite, whereas at higher temperatures, nearly all the chlorine is leached out of the rhyolite. 5) Fluorine concentrations in the reacted water increase between 150 and 250 degrees Celsius, but then gradually decrease at higher temperatures. 6) At temperatures higher than 250 degrees Celsius, fluorine is incorporated into the alteration minerals zeolite and possibly also biotite. 7) At 250 degrees Celsius, reacted waters with CO₂ are significantly more concentrated in dissolved ions compared with reacted water at a similar temperature without CO₂.

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In summary, it seems that major changes to Yellowstone’s rhyolite rocks take place when exposed to water that is hotter than 250-275 degrees Celsius (482 to 527 degrees F). This temperature represents something of a threshold for many chemical reactions to take place.  By characterizing the chemistry of hot water at the surface, scientists can get a sense of groundwater temperature without having to directly measure that temperature deep in the ground.

Results from this study are published in the journal Geochimica et Cosmochimica Acta and provide guidance for interpreting changes in the chemical composition of Yellowstone’s thermal waters. Variations in water chemistry could signal changes in the subsurface temperature and hydrothermal activity. For example, YVO monitors the activity of the hydrothermal system by measuring chloride flux through rivers as a proxy for heat discharge. The lower temperature in which chlorine is leached from rhyolite in the laboratory experiments compared with previous estimates, probably suggests that less heat is discharged from Yellowstone than was previously postulated.

Experimental results help explain many other observations. For example, the absorption of water by rhyolite glass has implications for applying the Obsidian hydration dating method which is used for dating young volcanic events and is also commonly used by archeologists to date artifacts. Results also help to explain differences in chemical compositions of hot waters in the Yellowstone Caldera and compositions in Norris Geyser Basin, to the north of the caldera. In the experiments, the ratio of chlorine to fluorine concentration in the reacted waters decreases between 150 and 250 degrees Celsius but substantially increases at higher temperatures. The higher ratio in Norris Geyser Basin is consistent with observations of higher subsurface temperatures there compared with temperatures in Yellowstone Caldera.

For centuries, results from laboratory experiments have provided significant insights on many fundamental physical and chemical processes on Earth. Laboratory experiments have also been used for testing contrasting concepts and quantifying myriad processes. Within this context, the experimental study on the high-temperature interaction between rhyolite, water, and gas adds new insights into the Yellowstone hydrothermal system that could not have been obtained otherwise.