Volcanologists: Magma is wetter than we thought

Researchers have determined magma is wetter than previously thought.

Volcano experts knew volatiles in magma play an important role in influencing the size and power of an eruption. But measuring volatile levels is difficult. In the wake of a violent eruption, all the volatiles have evaporated and escaped into the atmosphere.

The best technique scientists have for measuring volatile levels is locating and analyzing tiny bits of magma trapped in crystals ejected during the eruption. By measuring the amount of hydrogen in the magma inclusions, researchers can estimate the amount of water that was dissolved in the magma prior to the eruption.

Scientists have accepted the technique as a reasonable way to determine the lower limit of volatile levels in a volcanic magma. But researchers at Washington University in St. Louis wanted to establish an upper limit.

"What people have never looked at before and what we're trying to measure now is, how big is this bucket?" Michael J. Krawczynski, assistant professor of earth and planetary sciences, said in a news release. "You can imagine if it rained a lot, and your rain gauge was full, then you don't really know how much it rained. It could have rained more! We just can't say."

Similarly, if a melt inclusion is too small to hold all the water dissolved in the magma, then scientists aren't measuring the upper limit of volatile levels.

Researchers took to the lab to test how much water a melt inclusion can hold. To create a synthetic melt inclusion, scientists replicated the conditions of a magma chamber, complete with high temperatures and extreme pressure. They melted and rapidly cooled a magma sample, confirming that glass formed. Scientists repeated the process, adding more water each time until the rapid cooling failed to form glass.

"We found out that if you have a lot of water, then eventually you don't have a glass," said Maxim Gavrilenko, a former postdoctoral fellow in Krawczynski's lab, now a researcher at the University of Nevada.

Non-glassy melt inclusions are relatively common, but they are rarely studied by scientists. The authors of the latest study -- published this week in the journal American Mineralogists -- suggest this bias has prevented scientists from accurately modeling volatile levels inside volcanoes.

"If the deep magmas in these zones have more than 9 weight-percent of water, then they won't be correctly measured by the current gold-standard method," Gavrilenko said. "There is a need to find a new method for how to measure. We need a new, bigger bucket."

The findings could force researchers to reconsider the amounts of water being absorbed into the mantle at subduction zones.

"If more is going down into the mantle, more needs to be coming out back into Earth's crust," Krawczynski said. "That's what we're looking at here. We've understood that it's a cycle that needs to be balanced, but we haven't had a good grasp of the sizes of the different reservoirs."

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