Volcanic mountains

Study explores water content of deep-earth volcanic magma

Newswise – May 26, 2022 – Magmatic volatile contents, such as water, are abundant in volcanic arc melts and influence magma evolution, volcanic eruption dynamics, and ore deposit formation.

Ben Urann, National Science Foundation (NSF) postdoctoral researcher in ocean science at the University of Wyoming Department of Geology and Geophysics, was part of a research team that found magma in volcanic rocks that never erupted – but instead solidified deep in the earth – recorded much wetter levels than previously thought. This deep underground magma, or molten rock stored in the earth’s crust, contained 8 to nearly 20% by weight water. Previous conventional studies analyzed magma on the Earth’s surface, which contained an average of 4% water by weight.

“The water contained in the magmas influences the explosiveness of a volcanic eruption; the higher the water content, the more explosive it is,” says Urann. “Water is important in the formation of economic ore deposits: for example, copper, gold, silver and molybdenum. The wetter magmas we found help explain how these large ore deposits form.

Urann is lead author on a paper titled “High Water Content of Arc Magmas Recorded in Lower Crustal Cumulates of the Subduction Zone,” which was published today (May 26) in nature geoscience. The journal covers all aspects of earth science, including theoretical research, modeling and fieldwork.

The study was carried out while Urann was a Ph.D. student at the Woods Hole Oceanographic Institution in Woods Hole, Mass., in collaboration with Véronique Le Roux, his Ph.D. advisor. Other contributors to the article 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 arrived at UW in October 2021 while the paper was undergoing peer review. Urann spent time rewriting and editing the paper at UW, with support from his advisers, Michael Cheadle and Barbara John, both professors of geology and geophysics.

The research team studied igneous rocks in the Himalayas, a mountain range in northwest Pakistan, where subduction tipped these rocks from an ancient subduction zone.

Subduction zones are where two tectonic plates collide – one plunges beneath the other. These collision zones, called arcs, tend to carry a lot of water with them.

“Some of this water forms magmas, which rise to the surface and form volcanoes – think Mount St. Helens – while some of the water is returned to the depths of the earth,” says Urann. “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 burst on the surface.

As the magma cools, some crystals begin to form in a process known as crystal splitting. Eventually, all of the liquid magma will crystallize, leaving behind solidified rock, such as granite. Magma from the lower crust typically crystallizes at depths of 20 to 50 kilometers, according to the article.

“Consider placing a bottle of water in the freezer. As it gets colder, more and more ice forms until the whole bottle is solid ice,” says Urann. Crystal splitting is important because it allows magmas to change composition to form the continents we live in. Without crystal subduction and splitting, the planet would be very different.”

The study was funded by NSF and Ocean Venture funds.