Development of a field-portable helium isotope detector for continuous, real-time monitoring of active volcanoes

In 2014, the Ontake Volcano in Japan erupted, resulting in more than 60 fatalities. This eruption was preceded by a decade-long increase in the ratio of helium-3 to helium-4 in gases emitted near the summit. Such changes in the discharge rate and the chemical composition of gases emitted from volcanoes are some of the best precursors of an impending eruption. Despite a strong motivation to implement real-time, continuous measurements of these gas emissions, the process for doing so has previously been expensive, laborious, and time-consuming. Scientists from the USGS Volcano Hazards Program have been developing various technologies that allow for real-time and continuous measurements of gas concentrations in active volcanoes with the goal of improving baseline monitoring and early detection of volcanic unrest.

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Helium is a noble gas that is typically enriched in gases emitted from volcanoes. The abundance of the isotopes helium-3 and helium-4 in the Earth's mantle and crust can vary by orders of magnitude. ³He to ⁴He (expressed as ³He/⁴He) in volcanic gas emissions changes based on the relative amount of helium from the earth’s mantle, which is enriched in ³He, and helium originating from different sources in Earth’s crust, which are enriched in 4He. When scientists compare the ³He/⁴He in a sample to the ratio in Earth's atmosphere, they find that ³He/⁴He in volcanic gases is much higher. The utility of measuring temporal changes in helium isotope ratios in volcanic gases was demonstrated in a pioneering study carried out at the Etna Volcano in Italy, which revealed that major eruptive episodes are preceded by an increase of ³He/⁴He.

In order to use the ³He/⁴He for early detection of volcanic unrest, however, scientists first have to travel to active volcanoes to collect gas samples, which are then taken to laboratories for helium isotope analysis. This rather laborious process often takes weeks to months, which is typically insufficient for providing an early warning.

A photo showing the new portable instrument that measures the helium isotope concentrations continuously in soil gas near Horseshoe Lake on the flanks of Mammoth Mountain volcano in eastern California. Photo by Gary McMurtry (U. Hawaii)

To overcome this challenge, the USGS National Innovation Center and Volcano Hazards Program partnered with scientists and engineers from the School of Ocean and Earth Science and Technology (SOEST) at the University of Hawaii and MKS Instruments, Inc. in Santa Clara, California, to develop a new field-portable instrument that can measure helium isotopic ratios in volcanic gases on its own. The portable instrument is about five feet long and only 20 inches tall, a fraction of the size of traditional instruments that are used in laboratories to measure helium isotope ratios. In addition, it runs unattended, measuring the helium isotope ratio four times per day. The data are transmitted to the volcano observatories via cellular modem.

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This new instrument is the first to make helium isotope measurements in-situ at an actively degassing volcano. In the summer of 2018, a prototype of the instrument was deployed near Horseshoe Lake on the flanks of Mammoth Mountain volcano in eastern California. This volcano is characterized by high CO2 emissions that have varied along with helium isotopic compositions during periods of volcanic unrest.

It is envisioned that successful development and demonstration of the prototype instrument will result in further enhancements that include improved software, smaller size, and reduced power consumption. The modified instrument could then be incorporated into volcano monitoring networks and allow scientists to correlate helium isotope time-series with seismic and deformation events. This development could improve the understanding of how volcanic systems work, which could lead to more accurate forecasts of unrest and earlier warning for people impacted by eruptions.

Authors Shaul Hurwitz, Sara E. Peek, Peter J. Kelly and Stuart K. Wilkinson are part of the USGS Volcano Science Center. Gary M. McMurtry is a Professor at the School of Ocean and Earth Science and Technology (SOEST) at the University of Hawaii, Manoa.