Helium isotopes carry messages from the mantle

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

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Rock outcrops and other landforms around Yellowstone National Park provide records of three cycles of explosive volcanism that occurred over the past 2.1 million years. Although no magmatic eruptions have occurred for 70,000 years, visitors today can still see evidence of Yellowstone's volcanic nature in the geysers, hot springs, and areas of steaming ground that are part of Yellowstone's modern hydrothermal systems. These features are visible evidence for the large amount of heat stored deep in the subsurface.

Scientists who work at Yellowstone are interested in finding physical and chemical signals from the deep magmatic system, both to better understand the nature of the system and also to monitor for possible changes. Some of that research involves collection and analyses of gas and water from thermal areas to look for chemical tracers that can be directly linked to the magma.

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Helium is an inert gas that is an excellent tracer of magmatic processes. There are two important isotopes of helium, helium-3 (³He), which is stored deep in the earth's mantle where it was trapped during formation of the earth, and helium-4 (⁴He), which is produced during radioactive decay of uranium and thorium, two elements commonly found in earth's crust. ³He is very rare at the earth's surface; for example, the ³He/⁴He ratio in air is 1.4 x 10^-6, or about one ³He for every million ⁴He atoms. However, magma rising from the mantle transports some of the trapped ³He to the surface.

We use the air ratio (R_A) as a basis for comparison when we discuss the ratios in samples of gas. If gas has been stored for a long time in the earth's crust, it will likely have very high concentrations of ⁴He relative to ³He, and the ³He/⁴He ratio will be low, giving it a smaller R_A value. In contrast, a gas sample collected from an erupting volcano will likely have more ³He relative to ⁴He, and the ratio will be high, meaning a larger R_A value. To simplify things, we express the R_A value of air as 1. Values much greater than 1 indicate that there is a greater proportion of ³He (more mantle component) than what is found in air.

To put it simply, a high R_A means that the gas has a magmatic origin. A low R_A means there is not a magmatic source.

Years of study of Yellowstone's thermal areas have shown that gas at several areas is closely linked to the magmatic system. Gas from a fumarole at Mud Geyser in the Mud Volcano thermal area has the highest ³He/⁴He ratio of any feature at Yellowstone, up to 17 R_A, and nearby thermal areas have degassing features with R_A values >15. Likewise, the ³He/⁴He ratios in gas from several features in the Gibbon Geyser Basin, on the margin of Yellowstone's caldera, are also very high, but here the maximum R_A value is ~12.6. The waters in both of these thermal areas tend to be acidic with high concentrations of sulfur, and both areas have super-heated fumaroles, where the temperature of the gas exiting the ground is higher than the temperature of boiling water.

Relatively high R_A values are also found in gas outside of the caldera boundary. The thermal areas at Mammoth Hot Springs in the north part of the park and Boundary Creek in the southwest corner of the park differ greatly from the Mud Volcano area. Temperatures of the thermal features do not exceed boiling and the pH of the hot springs are close to neutral. In spite of the cooler nature of these areas and the distance from the main upflow of magmatic gas there is a strong signal from the magmatic system.

The R_A values of thermal areas around Yellowstone Lake and other thermal features in the southeast part of the park are generally low (less than 5). These low ratios indicate more ⁴He, which is the isotope of the gas that is stored in the rocks of the crust. Heating caused by the Yellowstone magma system has caused this form of helium to be released from the crust in abundant quantities, but the source of this gas is not the magma itself.

Helium is an exceptional indicator of magmatic activity beneath the surface, but isotopes matter! Knowing whether the gas is ³He or ⁴He tells the story of its origin: from the mantle and transported to the surface by magma, or from billion-year-old crustal rocks where the gas has formed by the process of radioactive decay. Yellowstone displays both forms of helium, so measuring the ratio of the two isotopes is critical to understanding its source!