New technologies to measure temporal variations of magmatic helium and CO2 emissions at Mammoth Mountain California

The injection of magma into subvolcanic magma reservoirs is one of the most significant triggers of volcanic eruptions. It is often manifested by an increase in CO₂ emissions and in the helium isotopic ratio (³He/⁴He) measured in discharged gases. Temporal variations of helium isotope ratios in volcanic gases preceded eruptions at Ontake Volcano in Japan (September 2014) and at Etna Volcano in Italy. However, temporal correlations in these examples were demonstrated only post-eruption, because real-time continuous gas data was lacking. The USGS National Innovation Center (https://geography.wr.usgs.gov/InnovationCenter/index.html) and Volcano Hazards Program co-funded a collaboration with the University of Hawaii to develop a new field-portable instrument to measure in-situ helium isotopic ratios in volcanic gases. This new instrument, with two compact mass spectrometers, an ion trap and a frequency-modified quadrupole mass spectrometer measures six helium isotopic values per day. In the summer of 2018, the USGS and partners from academia and industry deployed the new helium isotope instrument and a cavity ring-down spectrometer (CRDS) for CO₂ isotope analysis 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. The study site also has easy access, a building enclosure with electrical power, and established eddy covariance and gas analyzer systems that continuously measure surface CO₂ fluxes and soil CO₂ concentrations, respectively. We envision that successful development and demonstration of the various instruments will result in further enhancements (e.g., more compact, lower power consumption) that will enable them to be incorporated into monitoring networks at volcanoes worldwide.