BYLINE: Jess Hunt-Ralston

News – Ten years ago, Samer Naif made a surprising discovery in the Earth’s mantle: a narrow pocket, proposed to be filled with magma, hidden about 60 kilometers below the sea floor of the Cocos Plate.

Mantle melt is liquid and usually floats towards the surface – think of underwater volcanoes erupting to form islands. But Naif’s imaging instead showed a clear slice of semi-molten rock: low grade partial meltingstill clamped to the base of the plate about 37 miles below the ocean floor.

Then the observation provided an explanation for how tectonic plates can gradually slide, lubricated by partial melting. The study “also raises some questions about why magma is stored in a thin channel—and where the magma originates from,” says Naif, an assistant professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology.

Colleagues continued to share competing interpretations of the channel’s cause—including studies that argued that magma was needed to explain the observation.

So Naif went straight to the source.

“I actually went on a multi-year hunt, akin to a Sherlock Holmes detective story, looking for evidence of a mantle magma that we first saw in 2013 The nature study,” he says. “This involved bringing together evidence from several independent sources, including geophysical, geochemical and geological data (direct sampling of the seabed).

Now the results of that search are detailed in the new one Scientific progress article, “Episodic intraplate magmatism fed by a long-live melt channel of distal plume origin”, written by Naif and scientists from the US Geological Survey at Woods Hole Coastal and Marine Science Center, Northern Arizona University, Lamont-Doherty Earth Observatory of Columbia. University, Department of Geology and Geophysics Woods Hole Oceanographic Institution and GNS Science of Lower Hutt, New Zealand.

Reset

A relatively young oceanic plate – about 23 million years old – the Cocos Plate drifts down the west coast of Central America, curves west to the Pacific Plate, then north to meet the North American Plate off the Pacific coast of Mexico.

Sliding between the two plates caused the devastating 1985 Mexico City earthquake and the 2017 Chiapas earthquake, while similar subduction between the Cocos and Caribbean plates led to the 1992 Nicaragua tsunami and earthquake and the 2001 El Salvador earthquakes.

Scientists study the edges of these oceanic plates to understand the history and formation of volcanic chains—and to help scientists and institutions better prepare for future earthquakes and volcanic activity.

It is in this active zone that Naif and colleagues recently studied to document a series of magma intrusions just below the ocean floor, the same area where the team first discovered a magma channel in 2013.

Plumbing in depth

For the new study, the team combined geophysical, geochemical and seafloor drilling results with seismic reflection data, a technique used to image layers of sediment and rock below the surface. “It helps us see the geology where we can’t see it with our own eyes,” Naif explains.

First, the researchers observed an abundance of widespread magma within the plate. “Volcanism where it’s not expected,” says Naif, “basically away from plate boundaries: subduction zones and mid-ocean ridges.”

Think Hawaii, where “a mantle plume of hot rising material melts as it rises and then forms the Hawaiian volcanic chain in the middle of the Pacific Ocean,” just like with the Cocos plate, where the team imaged the volcanism that fed magma. at the lithosphere-asthenosphere boundary – the base of the flowing tectonic plates.

“Down there is the formation mantle,” adds Naif. “The tectonic plates move around on the Earth’s surface because they slide on the asthenosphere beneath them.”

The researchers also found that this channel below the lithosphere is regionally extensive — over 100,000 square kilometers — and is a “long-lived feature originating from the Galápagos plume,” a mantle plume that formed the volcanic Galápagos Islands and fed melt in a series of volcanic events over the past 20 million years , and persist today.

Importantly, the new study also suggests that these cylinder-lined melt channels may be widespread and long-lived sources of magma in the slab itself—as well as for mantle metasomatismwhich occurs when the Earth’s mantle reacts with fluids to form minerals from original rock.

Connecting (hot spot) the dots

“This confirms that magma was present in the past – and some of it leaked through the mantle and erupted near the ocean floor,” says Naif, “in the form of ledge intrusions and seamounts: basically volcanoes located on the ocean floor.”

The work also provides compelling evidence that magma may still be stored in the channel. “What’s more surprising is that the magma that erupted has a chemical fingerprint linking its source to the Galápagos mantle plume.

“We found that the magma channel has been around for at least 20 million years, and occasionally some of that magma seeps down to the ocean floor where it erupts,” adds Naif.

The source of the magma the team has found, the Galápagos plume, “is more than 1,000 kilometers away from where we found this volcanism.” It’s not clear how magma can stay in the mantle for so long, only to seep out all of a sudden.

Jump hunters wanted

The evidence the team compiled is “really quite subtle and requires a detailed and detailed study of the suite of seafloor observations to connect the dots,” says Naif. “Fundamentally, the signature of such volcanism, while quite clear here, also requires high-resolution data and several different types of data to be able to detect such subtle seafloor features.”

So, “if we can see such subtle evidence of volcanism here,” Naif explains, “it means that similar, detailed analysis of high-resolution data in other parts of the ocean floor could lead to similar discoveries of volcanism elsewhere, caused by other mantle plumes.” .”

“There are numerous mantle plumes on the planet. There are also numerous seamounts – at least 100,000 of them! — that covers the ocean floor, and it is possible to guess how many of them were formed in the middle of the tectonic plates by magma from distant mantle plumes that leaked to the surface.

Naif looks forward to continuing that quest, from the ocean floor to the hydrosphere.

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Funding: National Science Foundation: OCE-0625178, US Science Support Program

Quote: DOI: 10.1126/sciadv.add3761

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