(Image courtesy of Surachit via Wikimedia Commons)

Improved imaging of the Earth’s interior has unlocked new subsurface mysteries, including an area 600 miles (1,000 kilometers) down where the mantle’s usual flow pattern changes.

Now, at a lab bench on the planet’s surface, a team of researchers might have found the reason why: a swap of iron from one mineral type to another within mantle rocks at that depth.

Arizona State University, the Smithsonian Institution, China University of Geosciences at Wuhan, Argonne National Laboratory, University of Wisconsin–Madison, Carnegie Institute of Washington, DC, and the University of Chicago contributed to the research, which was published early online by the journal PNAS.

For something so essential to our existence, the Earth’s interior remains largely mysterious.

“We’re now living in the era where we send cameras and robots to other planets that are really, really far away. But, because the Earth’s interior is made up of dense rocks, it’s really difficult to send anything to scientifically understand what’s going on in the interior,” said Dan Shim of ASU’s School of Earth and Space Exploration.

That includes the rocky mantle that wraps the Earth’s iron-nickel outer core, transferring heat to the surface crust via a sluggish circulation that occurs roughly at the rate that fingernails grow. To understand this rocky nougat, we rely on indirect measurements, such as how quickly earthquake energy travels through the planet’s interior.

Based on this information, scientists divide the mantle roughly into three sections: the upper mantle (20-250 miles or 30-400 km), the mantle transition zone (250-430 miles or 400-700 km) and the lower mantle (430-1860 miles or 700-3,000 km).

But, as Shim explained, the discontinuity breaks that pattern.

“What is interesting about this recently observed flow pattern is that it isn’t happening in between these known boundaries — at, for example, 400 kilometers or 700 kilometers. Instead, it’s happening at 1,000 kilometers, where seismologists have never seen any global-scale sudden changes.”

Medical personnel can compare images of the human body with what they find during surgery. Geophysicists like Shim don’t have that option, so his team did the next best thing: They subjected the lower mantle’s dominant mineral, bridgmanite, to pressures found at that depth using a high-pressure diamond anvil cell — essentially a vise that squeezes minerals between two gem-quality diamonds.

“We create different pressures that simulate the conditions at different depths, from about 600 kilometers [270 miles] all the way down to 2,500 kilometers [1,550 miles]. We generate different pressures, and we put the rocks and minerals that we expect to exist in the mantle, and then we look at how they respond to the pressure.”

Bridgmanite, aka silicate-perovskite, is named for Nobel-winning physicist Percy Williams Bridgman, a pioneer in exposing materials to extremely high pressures. The most abundant mineral on Earth, it consists mainly of magnesium, silicon and oxygen, along with a little bit of iron and aluminum.

The team focused specifically on the behavior of bridgmanite’s iron content because iron responds readily to pressure changes. It paid off: At the pressures studied, the sample’s iron content changed form and moved into another mineral, ferropericlase.

“When this iron behavior changes in bridgmanite at 1,000 kilometers, the second mineral sitting there starts taking this iron away from bridgmanite,” said Shim.

Many kinds of iron occur in nature, in part because the metal freely gives up and accepts electrons. The differing crystalline structures of some minerals — including bridgmanite and ferropericlase — cause them to prefer one type of iron over another.

Moreover, previous research has shown that pressure influences the number of electrons orbiting iron atoms.

“The pressure is so high that it actually forces atoms to find a different arrangement,” said Shim.

As Shim explained, as the iron in bridgmanite gains an electron from surrounding oxygen, it changes to a form that bridgmanite dislikes and ferropericlase prefers.

“The key point is that it doesn’t require the change in chemical composition; chemical composition just stays the same.”