LIQUIFIED MOLTEN ROCK DISCOVERED IN EARTH'S MANTLE

Lava fountain at Kilauea in Hawaii.  Credit:  Wikimedia Commons/USGS.

FROM: NATIONAL SCIENCE FOUNDATION
Scientists Discover Layer of Liquified Molten Rock in Earth's Mantle

Scientists have discovered a layer of liquified molten rock in Earth's mantle that may be responsible for the sliding motions of the planet's massive tectonic plates.

The finding may carry far-reaching implications, from understanding basic geologic functions of the planet to new insights into volcanism and earthquakes.

The research was funded by the National Science Foundation (NSF), and is reported in this week's issue of the journal Nature by Samer Naif, Kerry Key, and Steven Constable of the Scripps Institution of Oceanography (SIO), and Rob Evans of the Woods Hole Oceanographic Institution.

"This new image greatly enhances our understanding of the role that fluids, both seawater and deep subsurface melts, play in controlling tectonic and volcanic processes," said Bil Haq, program director in NSF's Division of Ocean Sciences, which funded the work through the NSF Directorate for Geosciences' MARGINS (now GeoPRISMS) Program.

The scientists discovered the magma layer at the Middle America trench off Nicaragua's shores.

Using advanced seafloor electromagnetic imaging technology pioneered at SIO, the scientists imaged a 25-kilometer- (15.5-mile-) thick layer of partially melted mantle rock below the edge of the Cocos plate where it moves beneath Central America.

The new images of magma were captured during a 2010 expedition aboard the research vessel Melville.

After deploying a vast array of seafloor instruments that recorded natural electromagnetic signals to map features of the crust and mantle, the scientists realized they had found magma in a surprising place.

"This was completely unexpected," said Key. "We went out looking to get an idea of how fluids are interacting with plate subduction, but we discovered a melt layer we weren't expecting to find."

For decades scientists have debated the forces that allow the planet's tectonic plates to slide across the Earth's mantle.

Studies have shown that dissolved water in mantle minerals results in a more ductile mantle that would facilitate tectonic plate motions, but for many years clear images and data required to confirm or deny this idea were lacking.

"Our data tell us that water can't accommodate the features we are seeing," said Naif. "The information from the new images confirms the idea that there needs to be some amount of melt in the upper mantle. That's what's creating this ductile behavior for plates to slide."

The marine electromagnetic technology employed in the study was originated by Charles "Chip" Cox, an emeritus oceanographer at SIO, and in recent years further advanced by Constable and Key.

They have been working with the energy industry to apply this technology to map offshore oil and gas reservoirs.

The researchers say their results will help geologists better understand the structure of the tectonic plate boundary and how that affects earthquakes and volcanism.

"One of the longer-term implications of our results is that we are going to understand more about the plate boundary, which could lead to a better understanding of earthquakes," said Key.

The researchers are now trying to find the source that supplies the magma in the newly discovered layer.

The Seafloor Electromagnetic Methods Consortium at SIO also supported the research.

-NSF-

IN DEEP MAGMA

Credit:  Wikimedia Commons.

FROM: NATIONAL SCIENCE FOUNDATION

Magma in Earth's Mantle Forms Deeper Than Once Thought Study simulating pressures in mantle beneath the ocean floor shows that rocks can melt at depths up to 250 kilometers


Magma forms far deeper than geologists previously thought, according to new research results.

A team led by geologist Rajdeep Dasgupta of Rice University put very small samples of peridotite, rock derived from Earth's mantle, under high pressures in a laboratory.

The scientists found that the rock can and does liquify, at least in small amounts, at pressures equivalent to those found as deep as 250 kilometers down in the mantle beneath the ocean floor.

Dasgupta said that this answers several questions about Earth's inner workings.

He is the lead author of a paper that appears today in the journal Nature. The research was funded by the National Science Foundation (NSF).

"The results show that in some parts of the Earth, melting, or magma formation, happens very deep beneath Earth's surface," said geologist Jennifer Wade, a program director in NSF's Division of Earth Sciences, which funded the research.

"It also means that some carbon dioxide and water could come from different sources--and deeper within the Earth--than we believed."

The mantle is the planet's middle layer, a buffer of rock between the crust--the top five miles or so--and the Earth's core.

If one could compress millions of years of observation of the mantle to mere minutes, the mantle would look like a rolling mass of rising and falling material.

This slow but constant churning convection brings materials from deep within the Earth to the surface, and higher, through volcanic eruptions.

The team focused on the mantle beneath the ocean because that's where crust is created and where, Dasgupta said, "the connection between the interior and surface world is established."

Magma rises with convective currents, then cools and spreads out to form ocean-floor crust.

The starting point for melting has long been thought to be at 70 kilometers beneath the seafloor.

That had confounded geologists who had suspected, but could not demonstrate, the existence of deeper magma, said Dasgupta.

For example, when scientists try to determine the mantle's density, they do so by measuring the speed of a seismic wave after an earthquake, from its origin to other points on the planet.

Because such waves travel faster through solids (e.g., crust) than through liquids (e.g., magma), geologists had been surprised to detect waves slowing down, as though passing through liquid, in a zone that should be the mantle's faster "express lane."

"Seismologists have observed anomalies in velocity data as deep as 200 kilometers beneath the ocean floor," Dasgupta said.

"It turns out that trace amounts of magma are generated at this depth, which would potentially explain that" slower velocity.

The research also offers clues to the electrical conductivity of the oceanic mantle.

"The magma at such depths has a high enough concentration of dissolved carbon dioxide that its conductivity is very high," Dasgupta said.

But, because scientists have not yet been able to sample the mantle directly, researchers have had to extrapolate from the properties of rocks carried up to the surface.

So, in a previous study, Dasgupta determined that melting in Earth's deep upper mantle is caused by the presence of carbon dioxide.

The present study shows that carbon helps to make silicate magma at significant depths. And, the researchers also found that carbonated rock melts at significantly lower temperatures than non-carbonated rock.

"This deep melting makes the silicate differentiation [changes in silicate distribution that range from the dense metallic core, to the less-dense silicate-rich mantle, to the thinner crust] of the planet much more efficient than previously thought," Dasgupta said.

"Deep magma is the main agent that brings all the key ingredients for life--water and carbon--to the surface of the Earth."

In Dasgupta's high-pressure lab, volcanic rocks are windows to the planet's interior. The researchers crush tiny rock samples that contain carbon dioxide to find out how deep magma forms.

"We have all the necessary tools to simulate very high pressures--to nearly 750,000 pounds per square inch--and temperatures," he said. "We can subject small amounts of rock to these conditions to see what happens."

The geologists use powerful hydraulic presses to partially melt rocks that contain tiny amounts of carbon, simulating what they believe is happening under equivalent pressures in the mantle.

"When rocks come from deep in the mantle to shallower depths, they cross . . . the solidus [boundary], where rocks begin to undergo partial melting and produce magmas," Dasgupta said.

"Scientists knew the effect of a trace amount of carbon dioxide or water would lower this boundary, but our new estimation made it 150-180 kilometers deeper from the known depth of 70 kilometers," he said.

"What we are now saying is that with just a trace of carbon dioxide in the mantle, melting can begin as deep as around 200 kilometers.

"When we incorporate the effect of trace water, the magma generation depth becomes at least 250 kilometers."

The extent of magma generation is larger than previously thought, he said, and, as a consequence, has the capacity to affect the geophysical and geochemical properties of the entire planet.

Co-authors of the paper are Ananya Mallik and Kyusei Tsuno at Rice University; Anthony Withers and Marc Hirschmann at the University of Minnesota; and Greg Hirth at Brown University.

The study was also supported by a Packard Fellowship to Dasgupta.