Puzzling pockets of rock deep in Earth's mantle
Michigan State University and Arizona State University geoscientists have used computer modeling to find an explanation for how pockets of mushy rock have accumulated at the boundary between Earth's core and mantle.
The relatively small rock bodies reside roughly 1800 miles below the surface. Known as “ultra-low velocity zones” because seismic waves greatly slow down as they pass through them, they have been long thought to be partially molten. However, they have puzzled geoscientists for decades because many of these assumed hot rock zones are often observed in cooler regions of the deep mantle.
"These small regions have been assumed to be a partially molten version of rock that surrounds them," said Mingming Li, assistant professor in ASU's School of Earth and Space Exploration and lead author of the paper published in the current issue of Nature Communications. "But their global distribution and large variation of density, shape and size suggest that they have a different composition."
Li was a graduate student of Allen McNamara, coauthor and MSU earth and environmental scientist. The additional coauthors are Edward Garnero and Shule Yu of ASU.
“We don’t know what ultra-low velocity zones are,” McNamara said. “They are either hot, partially-molten portions of otherwise normal mantle or they are something else entirely, some other composition. In fact, the seismic evidence allows for both possibilities. We decided to perform computational modeling of mantle convection to investigate whether their shapes and positions can provide the answer to that question.”
A year ago, Garnero and McNamara reported that two gigantic structures of rock deep in the Earth are likely made of something different from the rest of the mantle. They called the structures thermochemical piles, or more simply "blobs." The origin and composition of the blobs were unknown, but the scientists suspected they held important clues as to how the Earth was formed and how it works.
The team’s computer modeling explained how these small, isolated pockets of rock, the ultra-low velocity zones, are associated with the location of the much bigger blobs. They also showed that most of these ultra-low velocity zones are different in composition than their surrounding mantle.
So the problem the team faced was finding a way to make small pockets of rock that explained these seismic observations, namely, that they existed near the margins of the larger thermochemical piles, and in some cases, far away from the hot massive blobs.
"How could partially melted mantle rock exist in the cold areas in tiny spots?” Garnero asked. “If it's hot enough to melt there, shouldn't we expect massive melt zones in the hottest regions? But we do not see that."
The team found that the hottest regions above the core-mantle boundary are well inside the blobs. This suggests that some pockets located well inside the large blobs may be caused by partial melting alone, Li added.
Li's computer modeling showed that pockets of distinctly different rock composition will migrate from anywhere on the core-mantle boundary towards the margins of the large blobs.
“The margins of the thermochemical piles,” McNamara said, “are where mantle flow patterns are converging, and therefore, these areas provide a collection depot for denser types of rock.”
The secret driving this movement is heat which powers mantle convection.
Earth's mantle is made of hot rock, but it behaves more like fudge simmering slowly on a stove. For the mantle, the heat is provided by both radioactivity of mantle rock and from the core which lies just below, the center of which is about as hot as Sun's surface. Mantle rock responds to this heat with a slow churning – convective – motion.
"The details are not completely clear," Li said. But the modeling shows that rocks of different composition respond to the convection in a way that gathers compositionally similar materials together. This moves the small pockets of chemically distinct rocks to the edges of the hotter blobs above the core-mantle boundary.
“We ran 3D high resolution computer modeling and we developed a method to track the movement of both the small pockets of ultra-low velocity zones and the much larger thermochemical piles,” Li said. “This allows us to study how the small pockets are moved around and how their locations can be related to their origin.”
What was different about their models?
“What was new and computationally challenging about our approach,” McNamara said, “is that our modeling simultaneously took into account vastly different scales of motion, from global mantle-scale convection patterns, to the large thermochemical piles in the lower mantle, to the very small-scale pockets of ultra-low velocity zone at the bottom.”
“What we ultimately found,” continues McNamara, “is that if ultra-low velocity zones are caused by melting of otherwise normal mantle, they should be located well inside of the thermochemical piles, where mantle temperatures are the hottest. If they are instead caused by some other composition, then although they could originate outside of the piles, they would continually be carried toward the edges of piles by mantle convection, where they collect. This is consistent with what we see in the seismic observations.”
Where do the different materials come from in the first place?
“There are several possibilities,” Garnero said. "Some of the material might be associated with former basaltic oceanic crust that got subducted deeply. Or it might be associated with chemical reactions between the outer core's iron-rich fluid and the crystalline silicate mantle."
The origin of ultra-low velocity zones is currently unsolved, but the process of collecting the material into small pockets of rock is clear.
"You can have various mechanisms, such as plate tectonics, that push rock of differing chemistries into the deepest mantle anywhere on Earth,” Garnero said. "But once these different rocks have gone down deep, convection wins and sweeps them to the hot regions, namely, where the continental-sized thermochemical piles reside. "