Explosive neue tektonische Entdeckung könnte helfen, die Lage von Silber- und Golderzvorkommen zu erklären

Mikroskopfoto eines Krustengesteins des unteren Bogens, das in der Studie verwendet wurde und die Mineralien Granat (rot) und Klinopyroxen (grün) zeigt. Kredit: Mit freundlicher Genehmigung der Forscher

Magma unter tektonischen Kollisionszonen ist feuchter als bisher angenommen

Neue Forschungsergebnisse könnten helfen zu erklären, wie sich die Erdkruste bildet, wo sich Erzvorkommen befinden und warum manche Vulkane explosiver sind als andere.

Eine neue Studie hat herausgefunden, dass kollidierende Kontinentalplatten mehr Wasser ziehen können als bisher angenommen. Die Ergebnisse könnten dabei helfen, die Brisanz mancher Vulkanausbrüche sowie die Verteilung von Erzvorkommen wie Kupfer, Silber und Gold zu erklären. Die Forschung wurde von Geologen der Woods Hole Oceanographic Institution (WHOI) durchgeführt.

MIT
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The findings are based on an analysis of ancient magmatic rocks recovered from the Himalayan mountains — a geologic formation that is the product of a subduction zone, where two massive tectonic plates have crushed against each other, one plate sliding beneath the other over millions of years.

Subduction zones can be found around the world. As one tectonic plate slides beneath another, it can take ocean water with it, drawing it deep into the mantle, where the liquid can merge with rising magma. The more water magma contains, the more explosive an eruption may be. Subduction zones therefore are the sites of some of the strongest and most destructive volcanic eruptions in the world.

Kohistan Paleo-Arc Rock

Rocks rich in the minerals garnet (red) and amphibole (black) from the Kohistan paleo-arc, similar to samples analyzed in the present study (hammer shown for scale). Credit: Courtesy of Othmar Müntener

Their analysis, published on May 26, 2022, in the journal <span class="glossaryLink" aria-describedby="tt" data-cmtooltip="

Nature Geoscience
<span class="st"> Nature Geoscience is a monthly peer-reviewed scientific journal published by the Nature Publishing Group that covers all aspects of the Earth sciences, including theoretical research, modeling, and fieldwork. Other related work is also published in fields that include atmospheric sciences, geology, geophysics, climatology, oceanography, paleontology, and space science. </span><span class="st">It was established in January 2008.
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” data-gt-translate-attributes=”[{” attribute=””>Nature Geoscience, finds that magma at subduction zones, or “arc magmas,” can contain up to 20 percent water content by weight — about double the maximum water content that has been widely assumed. The new estimate suggests that subduction zones draw down more water than previously thought, and that arc magmas are “super-hydrous,” and much wetter than scientists had estimated.

The study’s authors include lead author Ben Urann PhD ’21, who was a graduate student in the MIT-WHOI Joint Program at the time of the study (now at the University of Wyoming); Urann’s PhD advisor Véronique Le Roux of WHOI and the MIT-WHOI Joint Program; Oliver Jagoutz, professor of geology in MIT’s Department of Earth, Atmospheric and Planetary Sciences; Othmar Müntener of the University of Lausanne in Switzerland; Mark Behn of Boston College; and Emily Chin of Scripps Institution of Oceanography.

Deep bends

Previously, estimating the amount of water drawn down in subduction zones was done by analyzing volcanic rocks that have erupted to the surface. Scientists measured signatures of water in these rocks and then reconstructed the rocks’ original water content, when they first absorbed the liquid as magma, deep beneath the Earth’s crust. These estimates suggested that magma contains about 4 percent water by weight on average.

But Urann and Le Roux questioned these analyses: What if there are processes the rising magma undergoes that affect the original water content in a way that scientists did not anticipate?

“The question was, are these rocks that rose quickly and erupted representative of what’s really going on down deep, or is there some surface process that skews those numbers?” Urann says.

Benjamin Urann

Benjamin Urann, who graduated from the MIT-WHOI Joint Program in 2021 and is now a NSF postdoctoral fellow at U of Wyoming, analyzes water in minerals with a secondary ion mass spectrometer at the Woods Hole Oceanographic Institution. Credit: Benjamin Urann

Taking a different approach, the team looked to ancient magmatic rocks called plutons, that remained deep beneath the surface, never having erupted in the first place. These rocks, they reasoned, would be more pristine recorders of the water they originally absorbed.

Urann and Le Roux developed new analytical methods by secondary ion mass spectrometry at WHOI to analyze water in plutons collected previously by Jagoutz and Müntener in the Kohistan arc — a region of the western Himalayan mountains comprising a large geologic section of rock that crystallized long ago. This material was subsequently upheaved to the surface, exposing layers of preserved, unerupted plutons, or magmatic rock.

“These are incredibly fresh rocks,” Urann says. “There is no evidence of the rocks’ crystals being disturbed in any way, so that was the driver for using these samples.”

Urann and Le Roux selected the freshest samples and analyzed them for signs of water. They combined water measurements with the composition of minerals in each crystal and plugged these numbers into an equation to back-calculate the amount of water that must have been absorbed originally by magma, just before it crystallized into its rock form.

In the end, their calculations revealed that the arc magmas contained an original water content of more than 8 percent by weight.

The team’s new estimates may help to explain why volcanic eruptions in some parts of the world are stronger and more explosive than others.

“This water content is key to understanding why arc magmas are more explosive,” says Cin-Ty Lee, professor of geology at Rice University who was not involved in the research. “The water content of arc magmas is a bit of a mystery because it’s so hard to reconstruct original water content. Most of the community uses [erupted volcanic rock], aber sie sind weit entfernt von ihren tiefen Quellen. Wenn Sie also direkt zum Mantel gehen können, ist dies der richtige Weg. Das [rocks in the current study] sind so nah, wie man kommen kann.“

Die Ergebnisse könnten auch auf Orte in der Welt hinweisen, an denen Erzvorkommen – und hohe Konzentrationen von Kupfer, Silber und Gold – gefunden werden könnten.

„Es wird angenommen, dass sich diese Ablagerungen aus magmatischen Flüssigkeiten bilden – Flüssigkeiten, die sich vom ursprünglichen Magma getrennt haben und Kupfer und andere Metalle in Lösung tragen“, sagt Urann. „Das Problem war schon immer, dass diese Ablagerungen viel Wasser benötigen, um sich zu bilden – mehr als man von Magmen mit 4 Prozent Wassergehalt bekommt. Unsere Studie zeigt, dass superwasserhaltige Magmen die besten Kandidaten für die Bildung wirtschaftlicher Erzvorkommen sind.“

Referenz: „High water content of arc magmasrecorded in cumulates from subduction zone lower crust“ von BM Urann, V. Le Roux, O. Jagoutz, O. Müntener, MD Behn und EJ Chin, 26. Mai 2022, Natur Geowissenschaften.
DOI: 10.1038/s41561-022-00947-w

Diese Forschung wurde von der National Science Foundation und dem Ocean Venture Fund der Woods Hole Oceanographic Institution unterstützt.

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