Magmatic volatile contents, such as water, are abundant in volcanic arc melts and influence magma evolution, dynamics of volcanic eruptions and the formation of ore deposits.

Ben Urann, a National Science Foundation (NSF) Ocean Sciences Postdoctoral Research Fellow in the University of Wyoming Department of Geology and Geophysics, was part of a research team that discovered magma in volcanic rocks that never erupted - but rather solidified deep in the earth - recorded far wetter levels than previously thought. This deep underground magma, or molten rock stored in Earth's crust, contained 8 to nearly 20 weight percent water. Previous conventional studies analyzed magma on Earth's surface, which had an average of 4 weight percent water.

"Water in magmas influences how explosive a volcanic eruption can be; the higher the water content, the more explosive," Urann says. "Water is important in forming economic ore deposits: for example, copper, gold, silver and molybdenum. The wetter magmas we found help to explain how these large ore deposits form."

Urann is lead author of a paper titled "High water content of arc magmas recorded in cumulates from subduction zone lower crust," which was published (May 26) in Nature Geoscience. The journal covers all aspects of the earth sciences, including theoretical research, modeling and fieldwork.

The study was performed while Urann was a Ph.D. student at the Woods Hole Oceanographic Institution in Woods Hole, Mass., working with Veronique Le Roux, his Ph.D. adviser. Other contributors to the paper were from the Massachusetts Institute of Technology; University of Lausanne in Lausanne, Switzerland; Boston College; and the Scripps Institution of Oceanography at the University of California-San Diego.

Urann came to UW in October 2021 while the paper was in peer-review status. Urann spent time rewriting and editing the paper while at UW, with the support of his advisers, Michael Cheadle and Barbara John, both professors of geology and geophysics.

The research team studied magmatic rocks from the Himalayas, a mountain range in northwestern Pakistan, where subduction tilted these rocks from an ancient subduction zone.

Subduction zones are where two tectonic plates collide - one dives under the other. These collision zones, called arcs, tend to carry lots of water down with them.

"Some of this water forms magmas, which rise to the surface and form volcanoes - think Mount St. Helens - whereas some water is returned to the deep earth," Urann explains. "Water is what makes volcanoes explosive and facilitates the formation of economic ore deposits. So, geologists really want to know how much water is in these subduction zone magmas. Most geologists analyze magmas that have erupted on the surface."

As magma cools, some crystals begin to form in a process known as crystal fractionation. Eventually, all of the liquid magma will crystallize, leaving behind a solidified rock, for example, a granite. Lower-crustal magma typically crystallizes at depths of 20-50 kilometers, according to the paper.

"Think of placing a bottle of water into the freezer. As it cools, more and more ice forms until the whole bottle is solid ice," Urann says. "Crystal fractionation is important because it allows magmas to change in composition to form the continents on which we live. Without subduction and crystal fractionation, the planet would look vastly different."

Research Report:High water content of arc magmas recorded in cumulates from subduction zone lower crust

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