Taking earth's inner temperature: Surprising new study finds that the mantle is hotter than we thought

March 2, 2017
Each of the tiny rocks in this circular mount is about half of a synthetic mantle sample -- after it has been heated and crushed in the piston-cylinder apparatus, then cut open and polished. Sarafian puts her samples in this mount in order to analyze them for their water content using secondary ion mass spectrometry (SIMS). Credit: Photo by Jayne Doucette, Woods Hole Oceanographic Institution

The temperature of Earth's interior affects everything from the movement of tectonic plates to the formation of the planet.

A new study led by Woods Hole Oceanographic Institution (WHOI) suggests the mantle—the mostly solid, rocky part of Earth's interior that lies between its super-heated core and its outer crustal layer—may be hotter than previously believed. The new finding, published March 3 in the journal Science, could change how scientists think about many issues in Earth science including how ocean basins form.

"At mid-ocean ridges, the tectonic plates that form the seafloor gradually spread apart," said the study's lead author Emily Sarafian, a graduate student in the MIT-WHOI Joint Program. "Rock from the upper mantle slowly rises to fill the void between the plates, melting as the pressure decreases, then cooling and re-solidifying to form new crust along the ocean bottom. We wanted to be able to model this process, so we needed to know the temperature at which rising mantle rock starts to melt."

But determining that temperature isn't easy. Since it's not possible to measure the mantle's temperature directly, geologists have to estimate it through laboratory experiments that simulate the high pressures and temperatures inside the Earth.

Water is a critical component of the equation: the more water (or hydrogen) in rock, the lower the temperature at which it will melt. The peridotite rock that makes up the upper mantle is known to contain a small amount of water. "But we don't know specifically how the addition of water changes this melting point," said Sarafian's advisor, WHOI geochemist Glenn Gaetani. "So there's still a lot of uncertainty."

Image of one of the team's lab mimicry experiments, which was conducted in a capsule made of gold-palladium alloy. The black boxes highlight the locations of olivine grains, and the dark pits in the olivines are actual measurements for the water content of the olivine. The peridotite is the super fine-grained matrix. Credit: Emily Sarafian.

To figure out how the water content of mantle rock affects its melting point, Sarafian conducted a series of lab experiments using a piston-cylinder apparatus , a machine that uses electrical current, heavy metal plates, and stacks of pistons in order to magnify force to recreate the high temperatures and pressures found deep inside the Earth. Following standard experimental methodology, Sarafian created a synthetic mantle sample. She used a known, standardized mineral composition and dried it out in an oven to remove as much water as possible.

Until now, in experiments like these, scientists studying the composition of rocks have had to assume their starting material was completely dry, because the mineral grains they're working with are too small to analyze for water. After running their experiments, they correct their experimentally determined melting point to account for the amount of water known to be in the mantle rock.

"The problem is, the starting materials are powders, and they adsorb atmospheric water," Sarafian said. "So, whether you added water or not, there's water in your experiment."

Sarafian took a different approach. She modified her starting sample by adding spheres of a mineral called olivine, which occurs naturally in the mantle. The spheres were still tiny—about 300 micrometers in diameter, or the size of fine sand grains—but they were large enough for Sarafian to analyze their water content using secondary ion mass spectrometry (SIMS). From there, she was able to calculate the water content of her entire starting sample. To her surprise, she found it contained approximately the same amount of water known to be in the mantle.

Based on her results, Sarafian concluded that mantle melting had to be starting at a shallower depth under the seafloor than previously expected.

In her laboratory experiments, Sarafian used a piston-cylinder apparatus--the red machine behind her--to simulate the high pressures and temperature of the Earth's mantle. The heavy stainless steel plates visible on the table are stacked on the apparatus, with the tiny synthetic mantle sample inside a 'pressure vessel' underneath them. Once the machine is turned on, pistons apply massive pressure from above and below the sample, which is simultaneously heated with electrical current. Credit: Photo by Veronique LaCapra, Woods Hole Oceanographic Institution

To verify her results, Sarafian turned magnetotellurics—a technique that analyzes the electrical conductivity of the crust and mantle under the seafloor. Molten rock conducts electricity much more than solid rock, and using magnetotelluric data, geophysicists can produce an image showing where melting is occurring in the mantle.

But a magnetotelluric analysis published in Nature in 2013 by researchers at the Scripps Institution of Oceanography in San Diego showed that mantle rock was melting at a deeper depth under the sea floor than Sarafian's experimental data had suggested.

At first, Sarafian's experimental results and the magnetotelluric observations seemed to conflict, but she knew both had to be correct. Reconciling the temperatures and pressures Sarafian measured in her experiments with the melting depth from the Scripps study led her to a startling conclusion: The oceanic upper mantle must be 60°C (~110°F) hotter than current estimates," Sarafian said.

A 60-degree increase may not sound like a lot compared to a molten mantle temperature of more than 1,400°C. But Sarafian and Gaetani say the result is significant. For example, a hotter mantle would be more fluid, helping to explain the movement of rigid tectonic plates.

Explore further: Deep mantle chemistry surprise: Carbon content not uniform

More information: "Experimental constraints on the damp peridotite solidus and oceanic mantle potential temperature," Science, science.sciencemag.org/cgi/doi … 1126/science.aaj2165

Related Stories

Deep mantle chemistry surprise: Carbon content not uniform

January 13, 2017

Even though carbon is one of the most-abundant elements on Earth, it is actually very difficult to determine how much of it exists below the surface in Earth's interior. Analysis by Carnegie's Marion Le Voyer and Erik Hauri ...

New insight into the temperature of deep Earth

May 22, 2014

Scientists from the Magma and Volcanoes Laboratory (CNRS) and the European Synchrotron, the ESRF, have recreated the extreme conditions 600 to 2900 km below the Earth's surface to investigate the melting of basalt in the ...

Is there an ocean beneath our feet?

January 27, 2014

(Phys.org) —Scientists at the University of Liverpool have shown that deep sea fault zones could transport much larger amounts of water from the Earth's oceans to the upper mantle than previously thought.

Oceanic crust breakthrough: Solving a magma mystery

November 29, 2012

Oceanic crust covers two-thirds of the Earth's solid surface, but scientists still don't entirely understand the process by which it is made. Analysis of more than 600 samples of oceanic crust by a team including Carnegie's ...

Recommended for you

Mountain glaciers shrinking across the West

October 22, 2017

Until recently, glaciers in the United States have been measured in two ways: placing stakes in the snow, as federal scientists have done each year since 1957 at South Cascade Glacier in Washington state; or tracking glacier ...

Carbon coating gives biochar its garden-greening power

October 20, 2017

For more than 100 years, biochar, a carbon-rich, charcoal-like substance made from oxygen-deprived plant or other organic matter, has both delighted and puzzled scientists. As a soil additive, biochar can store carbon and ...

Cool roofs have water saving benefits too

October 20, 2017

The energy and climate benefits of cool roofs have been well established: By reflecting rather than absorbing the sun's energy, light-colored roofs keep buildings, cities, and even the entire planet cooler. Now a new study ...

2 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

Whydening Gyre
5 / 5 (1) Mar 03, 2017
Smart girl. Good, solid science...
Osiris1
5 / 5 (1) Mar 03, 2017
All in the name of modeling.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.