Advanced Photon Source

An Office of Science National User Facility

Why Rocks Flow Slowly in the Earth's Middle Mantle

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The original Arizona State University press release by Robert Burnham can be read here.

For decades, researchers have studied the interior of the Earth using seismic waves from earthquakes. Now a recent study, carried out at the U.S. Department of Energy’s (DOE’s) Advanced Photon Source (APS), has re-created in the laboratory the conditions found deep in the Earth, and used this to discover an important property of the dominant mineral in Earth's mantle, a region lying far below our feet.

The researchers from Arizona State University (ASU), the Smithsonian Institution, the China University of Geosciences at Wuhan, Argonne National Laboratory, the University of Wisconsin-Madison, the Carnegie Institute of Washington, and The University of Chicago combined x-ray techniques utilized at three beamlines at the APS and atomic resolution electron microscopy at Arizona State University to determine what causes unusual flow patterns in rocks that lie 600 miles and more deep within the Earth. Their results were published in the Proceedings of the National Academy of Sciences.

Planet Earth is built of layers. These include the crust at the surface, the mantle and the core. Heat from the core drives a slow churning motion of the mantle's solid silicate rocks, like slow-boiling fudge on a stove burner. This conveyor-belt motion causes the crust's tectonic plates at the surface to jostle against each other, a process that has continued for at least half of Earth's 4.5 billion-year history.

The research team focused on a puzzling part of this cycle: Why does the churning pattern abruptly slow at depths of about 600 to 900 miles below the surface?

Recent geophysical studies have suggested that the pattern changes because the mantle rocks flow less easily at that depth," said team leader Dan Shim of ASU. "But why? Does the rock composition change there? Or do rocks suddenly become more viscous at that depth and pressure? No one knows."

To investigate the question in the lab, the team studied bridgmanite, an iron-containing mineral that previous work has shown is the dominant component in the mantle.

"We discovered that changes occur in bridgmanite at the pressures expected for 1,000 to 1,500 km depths," Shim said. "These changes can cause an increase in bridgmanite's viscosity — its resistance to flow."

The team synthesized samples of bridgmanite in the laboratory and subjected them to the high-pressure conditions found at different depths in the mantle. They obtained diffraction patterns from double-sided laser heating experiments carried out at the GSECARS and High Pressure Collaborative Access Team beamlines 13-ID-D and 16-ID-B, respectively, at the APS. In addition, nuclear forward scattering studies were conducted at the X-ray Science Division 3-ID-B,C,D beamline, also at the APS (The APS is an Office of Science user facility at Argonne).

The experiments showed the team that, above a depth of 1,000 kilometers and below a depth of 1,700 km, bridgmanite contains nearly equal amounts of oxidized and reduced forms of iron. But at pressures found between those two depths, bridgmanite undergoes chemical changes that end up significantly lowering the concentration of iron it contains.

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."

See: Sang-Heon Shim1*, Brent Grocholski2‡, Yu Ye1,2, E. Ercan Alp4, Shenzhen Xu5, Dane Morgan5, Yue Meng6, and Vitali B. Prakapenka7, “Stability of ferrous-iron-rich bridgmanite under reducing midmantle conditions,” Proc. Natl. Acad. USA, Early Edition (June 2017) . DOI: 10.1073/pnas.1614036114
Author affiliations: 1Arizona State University, 2Smithsonian Institution, 3China University of Geosciences at Wuhan, 4Argonne National Laboratory, 5University of Wisconsin, 6Carnegie Institute of Washington, 7The University of Chicago Present address: American Association for the Advancement of Science

Correspondence: *shdshim@gmail.com

This work was supported by National Science Foundation (NSF) Grants EAR1316022 and EAR1338810 (to S.-H.S.). GeoSoilEnviroCARS is supported by NSF Grant EAR-1634415 and DOE Grant DE-FG02-94ER14466. The High Pressure Collaborative Access Team facility is supported by DOE-National Nuclear Security Administration Grant DE-NA0001974. YM acknowledges the support of DOE-Basic Energy Sciences Grant DE-FG02-99ER45775. This research used resources of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

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Published Date: 
06.12.2017