The composition of magma foam could reveal when 'things are heating up'
Scientists have found a way to use the ratio of atoms in specific gases created by volcanic fumaroles (gaps in the Earth’s surface) to detect what’s happening to magma deep below.
Much like obtaining a blood test to examine your health, the new method, published in Nature by a team at the University of Tokyo, could indicate when things are “heating up.” Significantly, it could aid in the prediction of future volcanic eruptions.
Scientists now show that they can determine how frothy a magma is by observing changes in the ratio of argon-40 to helium-3. The degree of “foaminess” can then be used to predict the likelihood of a specific type of eruption.
Professor Hirochika Sumino from the Research Center for Advanced Science and Technology and coauthor of the study explained in a press release, “We knew that the helium isotope ratio occasionally changes from a low value, similar to the helium found in the Earth’s crust, to a high value, like that in the Earth’s mantle, when the activity of magma increases.”
“But we didn’t know why we had more mantle-derived helium during magmatic unrest,” he added.
Sumino and his team chose to monitor the gases coming from six fumaroles near the active Kusatsu-Shirane volcano in Gunma Prefecture, which is located about 150 kilometers northwest of Tokyo. Here they took samples from 2014 to 2021 at intervals of a few months.
After collecting the samples, they returned to the lab and used cutting-edge machinery called a noble gas mass spectrometer to evaluate them. Thanks to this, they could accurately determine isotopic compositions, including ultra-trace isotopes like helium-3, usually found in greater abundance in the mantle than in the crust or air.
“We succeeded in detecting changes in the magma-derived argon-40/helium-3 ratio, related to magmatic unrest. Using computer models, we revealed that the ratio reflects how much the magma underground is foaming, making bubbles of volcanic gases which separate from the liquid magma,” said Sumino.
Poignantly, “how much magma foams controls how much magmatic gas is provided to the hydrothermal system beneath a volcano and how buoyant the magma is,” he added.
The researcher claims that the risk of a “phreatic eruption” correlates with how much gas is supplied to the hydrothermal system beneath a volcano. A phreatic eruption is when an increase in water pressure in the hydrothermal system causes this eruption.
Additionally, the magma’s buoyance influences the rate of magma ascent.
“When you compare a volcano with a human body, the conventional geophysical methods represented by observations of earthquakes and crustal deformation are similar to listening to the chest and taking body size measurements,” said Sumino.
He further explained that these examinations make it challenging to determine what health issue is causing a rumbling in your chest or a sudden rise in weight in these situations. They are simply not detailed enough.
“On the other hand, analyzing the chemical and isotope composition of elements in fumarolic gases is like a breath or blood test. This means we are looking at actual material directly derived from magma to know precisely what is going on with the magma,” he said.
For the time being, the team has to collect gas samples in the field and transport them back to the lab for analysis. They reveal that this operation is not only difficult, but also time-consuming.
With this in mind, Sumino intends to create a new tool that would allow them to conduct the same analysis in real-time and out in the field.
“We want to be able to detect changes in magma activity as soon as possible,” stated Sumino. “Now we are developing a portable mass spectrometer for on-site, real-time monitoring of noble gas isotope ratios from fumarolic gases.”
“Our next step is to establish a noble gas analysis protocol with this new instrument, to make it a reality that all active volcanoes — at least those which have the potential to cause disaster to local residents — are monitored 24 hours a day, seven days a week.”
Abstract:
We repeatedly measured isotopic compositions of noble gases and CO2 in volcanic gases sampled at six fumaroles around the Kusatsu-Shirane volcano (Japan) between 2014 and 2021 to detect variations reflecting recent volcanic activity. The synchronous increases in 3He/4He at some fumaroles suggest an increase in magmatic gas supply since 2018. The increase in magmatic gas supply is also supported by the temporal variations in 3He/CO2ratios and carbon isotopic ratios of CO2. The 3He/40Ar* ratios (40Ar*: magmatic 40Ar) show significant increases in the period of high 3He/4He ratios. The temporal variation in 3He/40Ar* ratios may reflect changes in magma vesicularity. Therefore, the 3He/40Ar* ratio of fumarolic gases is a useful parameter to monitor the current state of degassing magma, which is essential for understanding the deep process of volcanic unrest and may contribute to identifying precursors of a future eruption. These results provide additional validation for the use of noble gas and carbon isotopic compositions of fumarolic gases for monitoring magmatic–hydrothermal systems.
Sade Agard Sade is a geology graduate and Fellow of The Geological Society of London who she has experience publishing for. She enjoys writing and editing features of a range of scientific topics, with special interest in Earth science. Based in Windsor, Sade enjoys travelling to various sites of outstanding nature and has also lived in Thailand for a year.
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