Volcanic arcs may be wetter than previously assumed, which has scientific and economic ramifications

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Arc volcanoes, which form over subduction zones, may contain a far higher percentage of water than previously thought.

In a recent report published in Nature Geoscience, researchers explain how the Earth’s lower crust is formed, how magma erupts through the crust, and how economically significant mineral ore deposits form.

According to the publication, the water concentrations in primitive arc magmas measured in this study are more varied and much greater than the average water content of roughly 4 weight percent discovered in prior studies.. After substantial crystal fractionation in the lower arc crust, H2O saturation can rise to a maximum of 20wt percent, according to the paper’s findings.

“In the grand scheme of things, plate tectonics relies on water as a lubricant. Tectonic plates’ movement will be affected by a variety of factors, including the amount of water in the environment “MIT-WHOI Joint Program in Oceanography/Applied Ocean Science and Engineering doctoral student at the time of the study, Benjamin Urann, is the paper’s principal author.

As a result of this study, we can better estimate how much water is being subducted deep into the mantle globally; quantify different water reservoirs, including surface and deep reservoirs; and better understand the transport between these different water reservoirs,” says Urann, who is currently a National Science Foundation Ocean Sciences Postdoctoral Research F fellow. As Urann pointed out, water content is also discussed in the report, which emphasizes the importance of porphyry copper deposits. According to the US Geological Survey, these deposits account for around 60 percent of the world’s copper supply.

Melt inclusions, which are tiny drops of magma confined by crystals growing around them, and lava and other volcanic deposits erupted to the Earth’s surface have been used in many earlier research. There are intrinsic limits to these approaches that obscure H2O in arc magmas across their whole range, the report concludes.

Véronique Le Roux, Urann’s doctoral supervisor and co-author of the paper, helped develop methods to measure water content in minerals using the Secondary Ion Mass Spectrometry instrument at WHOI. Their work builds on previous efforts that suggested arc magmas should contain significantly more H2O than inferred from melt-inclusion measurements.

For their study of deep crustal magmas, the researchers decided to look at samples that had not yet risen to the surface of the Earth.

“Magma that has solidified deep in the crust can still be sampled at these depths, but you can’t get the liquid form of it. With plate tectonics, some of those really deep crusts get exhumed at the surface, which is fortunate for us “In the Geology and Geophysics department, Le Roux is an associate scientist, and a member of the MIT-WHOI Joint Program. For this study, researchers relied on summaries from the Kohistan paleoarc terrane in Pakistan’s Himalaya Mountains that had been collected by the paper’s co-authors.

Magma—lower crustal cumulates—that had crystallized deep down in the crust at a high enough pressure to retain its original water content signature was investigated instead of surface rocks that travel far up through the crust and lose much of their water content in the process.

“A new promising way to access the deep depths of the crust in subduction zones is studying water in cumulate minerals,” according to Le Roux.

Magma studied by researchers included between 10% and 20% water, depending on the magma’s composition, according to their findings. Although this weight percent of water had been anticipated experimentally, it had never been observed in wild samples, Le Roux added.

In the end, Urann concluded that arc magmas can be wetter than previously anticipated.

Further information: Benjamin Urann, High water content of arc magmas recorded in cumulates from subduction zone lower crust, Nature Geoscience (2022). DOI: 10.1038/s41561-022-00947-w. www.nature.com/articles/s41561-022-00947-w

Journal information: Nature Geoscience

Source: Woods Hole Oceanographic Institution