Surprising new study finds that Earth's mantle is hotter than we thought
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 in the journal Science, could change how scientists think about many issues in Earth science including how ocean basins form.
temperature of Earth's interior affects everything from the movement of
tectonic plates to the formation|
of the planet [Credit: Jack Cook, Woods Hole Oceanographic Institution]
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."
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."
Based on her results, Sarafian concluded that mantle melting had to be starting at a shallower depth under the seafloor than previously expected.
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.
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.
Source: Woods Hole Oceanographic Institution [March 02, 2017]