Monitoring “foamy” magmatic gases for eruption signals

The composition of gases can warn of increased volcanic activity.

Volcanic eruptions are dangerous, but unfortunately they are also difficult to predict. Now a team of scientists from the University of Tokyo has discovered that the ratio of atoms in certain gases released from volcanic fumaroles (gaps in the Earth’s surface) can be an indicator of what is happening to the magma deep below. It works much like a blood test to check your health. This could indicate that things may be “heating up” volcanically.

In particular, changes in the ratio of argon-40 to helium-3 can indicate how foamy the magma is, signaling the risk of different types of eruptions. Understanding which ratios of which gases indicate a certain type of magma activity is a big step. Next, the researchers hope to build a 24-hour volcanic activity monitoring and early warning system by developing portable equipment that can provide real-time on-site measurements.

Does the thought of standing on a volcano give you shivers of excitement or fear? For many people, living in the shadow of a volcano is part of everyday life. Japan has 111 active volcanoes and an average of 15 volcanic “events”, including eruptions, each year. However, these potentially life-threatening events are notoriously difficult to predict.

In 2014, Mount Ontake, Japan’s second highest volcano and a popular tourist destination, unexpectedly erupted, leaving 58 people dead and five missing. Although earthquake activity is usually an early warning signal, some eruptions (including the Ontake eruption) can occur without clear earthquake signals. Thus, disaster mitigation professionals such as the Japan Meteorological Agency will benefit from other reliable ways to warn residents of the next potential disaster.

Fumaroles are holes and cracks in the Earth’s surface (crust) that release gas and steam and often occur around volcanoes. The emitted gas consists of a mixture of chemicals. Its composition can give us insight into what happens beneath the Earth’s crust in the mantle, where magma (molten rock) forms and pushes upward, eventually erupting as lava. Researchers already know that the ratio of isotopes (atoms of an element with the same chemical properties but different masses) of certain gases can indicate hidden magma activity.

“We knew that the isotope ratio of helium changes from time to time from a low value, similar to the helium found in the Earth’s crust, to a high value, like in the Earth’s mantle, when magma activity increases. This was based on observations of helium isotope ratios in the gas of a cold spring on the island of El Hierro in the Canary Islands (in the Atlantic Ocean off the northwest coast of Africa), where an eruption occurred in 2011,” explained Professor Hirochiko Sumino of the Advanced Research Center of science and technology. “But we didn’t know why we had more helium derived from the mantle during magmatic disturbances.”

Hirochiko Sumino (dark green jacket), Tomoya Obase (blue jacket) and Hiroshi Shinohara (orange jacket) collect gas samples from fumaroles at the Tateyama Jigakudani (“Hell Valley”) geothermal area in Toyama Prefecture, Japan. Gas sampling from fumaroles is dangerous due to toxic gas and hot steam, so a gas mask, goggles, helmet and gloves are required. But Sumino says the results of this study show that the insights gleaned from the samples are worth challenging. Author: Yuki Hibiya

To gain further insight, Sumino and the research team decided to monitor gases from six fumaroles around the active Kusatsu-Shirane volcano, located about 150 kilometers (90 miles) northwest of Tokyo in Gunma Prefecture. The team collected samples every few months for seven years between 2014 and 2021. After collecting the samples, they returned them to the laboratory and analyzed them using a modern piece of equipment called a noble gas mass spectrometer. This allowed them to precisely measure the isotopic composition, including supertrace (small but important) isotopes such as helium-3, which is usually more abundant in the mantle than in the crust or air.

“We were able to detect changes in magma-derived argon-40/helium-3 ratio associated with magmatic disturbances. Using computer models, we found that the ratio reflects the extent to which the magma underground foams, creating bubbles of volcanic gases that separate from the liquid magma,” Sumino explained. “How much magma foams up depends on how much magmatic gas enters the hydrothermal system beneath the volcano and how buoyant the magma is. The first is related to the risk of phreatic eruption, in which an increase in water pressure in a hydrothermal system causes an eruption. The latter would increase the rate of magma ascent, which would lead to a magmatic eruption.

“When you compare a volcano to a human body, the conventional geophysical methods represented by observations of earthquakes and crustal deformation are similar to listening to breasts and measuring body dimensions. In these cases, without a detailed medical examination, it is difficult to determine what health problem is causing the chest noise or sudden weight gain. On the other hand, the analysis of the chemical and isotopic composition of elements in fumarole gases is similar to the analysis of breath or blood. That means we’re looking at actual material directly derived from the magma to know exactly what’s going on with the magma.”

Currently, gas samples must be collected in the field and returned to the laboratory for analysis, a complex and time-consuming process. However, Sumino has experience improving noble gas mass spectrometers and hopes to develop a new instrument that will allow them to perform the same analysis, but in real time and in the field.

“We want to be able to detect changes in magma activity as quickly as possible,” Sumino said. “We are now developing a portable mass spectrometer for real-time in situ monitoring of noble gas isotope ratios in fumarole gases. Our next step is to create a noble gas analysis protocol with this new instrument to make it a reality that all active volcanoes—at least those that could cause disaster for local residents—are monitored 24 hours a day, seven days a day. week”.

Reference: Tomoya Obase, Hirochika Sumino, Kotaro Toyama, Kaori Kawana, Kohei Yamane, Muga Yaguchi, Akihiko Terada, and Takeshi Oba “Monitoring a Magmatic-Hydrothermal System by Isotope Composition of Noble Gases and Carbon in Fumarole Gases” Scientific reports.
DOI: 10.1038/s41598-022-22280-3

Funding: This work was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan under the Comprehensive Program for Next Generation Volcano Research and Human Resource Development (Program Grant Number JPJ005391) and the Second Earthquake and Volcano Hazard Observation and Research Program (Research on reducing the danger of earthquakes and volcanoes).