The process starts with driving oxidized iron out of the bridgmanite. The oxidized iron then consumes the small amounts of metallic iron that are scattered through the mantle like poppy seeds in a cake. This reaction removes the metallic iron and results in making more reduced iron in the critical layer.

Where does the reduced iron go? The answer, said Shim's team, is that it goes into another mineral present in the mantle, ferropericlase, which is chemically prone to absorbing reduced iron.

"Thus the bridgmanite in the deep layer ends up with less iron," explained Shim, noting that this is the key to why this layer behaves the way it does.

"As it loses iron, bridgmanite becomes more viscous," Shim said. "This can explain the seismic observations of slowed mantle flow at that depth."

Other members of the Shim team include Brent Grocholski (Smithsonian Institution), Yu Ye (former ASU post-doctoral student now at the University of Geosciences in Wuhan, China), Ercan Alp (Argonne National Laboratory), Shenzhen Xu and Dane Morgan (University of Wisconsin), Yue Meng (Carnegie Institution of Washington) and Vitali Prakapenka (University of Chicago). About half of the high-pressure samples were analyzed using the electron microscopes at John M. Cowley Center for High Resolution Electron Microscopy at ASU.

Robert Burnham

Science writer, School of Earth and Space Exploration