A schematic cross section of the Earth’s interior below oceanic ridges shows the conditions of magma generation. Through experimentation on mantle rocks, Rice University researchers found evidence that magma forms as deep as 250 kilometers inside the Earth’s, much deeper than previously thought. Such melting would help explain apparent geophysical and geochemical contradictions that have puzzled geologists for years. Image: Dasgupta Group/Rice UniversityMagma forms far deeper than geologists previously thought, according to new research at Rice University.


Dasgupta is lead author of the paper in Nature.

The mantle is the planet's middle layer, a buffer of rock between the crust—the top 5 miles or so—and the core. If one could compress millions of years of observation down to minutes, the mantle would look like a rolling mass of rising and falling material. This slow but constant convection brings materials from deep within the planet to the surface—and occasionally higher through volcanoes.

The Rice team focused on mantle beneath the ocean because that's where the crust is created and, Dasgupta says, "the connection between the interior and surface world is established." Silicate melts—also known as magma—rise with the convective currents, cool and spread out to form the ocean crust. The starting point for melting has long been thought to be at 70 kilometers beneath the seafloor.

That has confounded geologists who suspected but could not demonstrate the existence of deeper silicate magma, says Dasgupta, an assistant professor of Earth science at Rice.


The research also offers clues to the bulk electrical conductivity of the oceanic mantle, he says. "The magma at such depths has a high enough amount of dissolved carbon dioxide that its conductivity is very high," Dasgupta says. "As a consequence, we can explain the conductivity of the mantle, which we knew was very high but always struggled to explain."

Rice University geologist Rajdeep Dasgupta uses powerful hydraulic presses to crush rock samples in pressures that simulate those found deep inside the Earth. New research by Dasgupta and his colleagues revealed that magma can form much deeper in the planet’s mantle than previously thought. Image: Jeff Fitlow/Rice UniversityBecause humans have not yet dug deep enough to sample the mantle directly—though some are trying—researchers have to extrapolate data on rocks carried up to the surface. A previous study by Dasgupta determined that melting in the Earth's deep upper mantle is caused by the presence of carbon dioxide. The present study shows that carbon not only leads the charge to make carbonate fluid but also helps to make silicate magma at significant depths.

The researchers also found carbonated rock melts at significantly lower temperatures than non-carbonated rock. "This deep melting makes the silicate differentiation of the planet much more efficient than previously thought," Dasgupta says. "Not only that, this deep magma is the main agent to bring all the key ingredients for life—water and carbon—to the surface of the Earth."

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

"Our field of research is called experimental petrology," he says. "We have all the necessary tools to simulate very high pressures (up to nearly 750,000 pounds per square inch for these experiments) and temperatures. We can subject small amounts of rock samples to these conditions and see what happens."


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


Source: Rice University