Rising 'snow' deep in the Earth

By recreating the high pressures and temperatures 1,800 miles below the Earth's surface, researchers have shed light on the origin of mysterious deep-Earth structures and how they might be related to volcanoes on the Earth's surface.

By recreating the extremely high pressures and temperatures 1,800 miles below the Earth's surface, researchers from Arizona State University and University of Chicago have discovered an intriguing phenomenon in this region: the formation of snow like, silicon-rich crystals that float upwards. The finding provides strong evidence that the rocks in some volcanoes may contain materials from deep in the Earth's core.

The research team combined laser heating and X-ray characterization techniques at the GeoSoilEnviroCARS (GSECARS) beamline at the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science user facility at DOE's Argonne National Laboratory. GSECARS is a University of Chicago-managed experiment station with unique capabilities.

Mysterious structures at the core-mantle boundary

The Earth is comprised of a thin outer layer of rock (called the crust), a much thicker layer of rock underneath (the mantle), a liquid metal outer core and a solid metal inner core. The layers grow progressively hotter toward the core, which is mostly metal alloys of iron and nickel.

By understanding the structure and dynamics at the boundary between the mantle and the outer core, researchers can gain new insights on how the Earth has cooled and redistributed materials over geologic time scale. For example, volcanic activity along the ocean floor can form new crust from melted mantle materials.

The researchers wanted to better understand the origin of two mysterious zones discovered at the core-mantle boundary. First, there are regions at the lowermost mantle in which seismic waves slow down dramatically, suggesting that the regions are denser than surrounding areas and contain molten materials. Second, there are small solid areas in the liquid outer core that may affect the Earth's magnetic field.

The team was investigating a hypothesis to explain the origin of these anomalies. According to the hypothesis, the presence of hydrogen in the outer core promotes the formation of silicon-rich iron crystals. The high silicon content makes the solid crystals less dense than the surrounding metallic liquids. As a result, the crystals rise to the core-mantle boundary, rather than sink.

"This process, which might look like upward snowing, creates 'piles' of snow that could be the solid areas in the liquid outer core," said Sang-Heon Dan Shim, one of the study's authors and a professor in Arizona State University's School of Earth and Space Exploration. "If the silicon-rich iron snow enters the lowermost mantle, it would slow down seismic waves. That's because iron-silicon alloy is much denser than mantle materials. This could explain the mysterious, dense zones in the lowermost mantle."

Demonstrating these processes could support the idea that materials from deep in the Earth's core end up being included in the molten rock inside hotspot volcanoes on the Earth's surface. Hotspot volcanoes are formed by heat rising from the mantle.

Here's how these materials might make the long journey: the snow like solids in the unusual outer core areas rise to form the dense zones in the lowermost mantle. Some materials in the dense zones are incorporated into the mantle's upwelling flows and rise to the surface. In fact, imaging of the Earth's interior has shown that some volcanic rocks may come from material rising from these mantle zones.

Lasers, X-Rays and Diamond Anvils

The researchers faced a significant technical barrier to testing their hypothesis. Recreating the high pressure conditions in the upper core is usually achieved by compressing a material sample in a diamond anvil. But when iron alloy samples are melted at the outer core's high temperatures (about 6,200 degrees Fahrenheit), hydrogen diffuses into the diamond, causing it to break.

The team developed a clever solution that involves mixing hydrogen with argon, which stops hydrogen infiltration into the diamond. Argon does not react with the iron alloy and diamond anvil.

At the APS, the team loaded an iron-silicon alloy along with the hydrogen-argon mixture into a diamond anvil. They used laser beams to heat the pressurized sample and then probed the sample with extremely high-energy X-ray beams. The X-ray beams were scattered by the electrons of the atoms in the material, and a detector measured the scattering. This technique, known as synchrotron X-ray diffraction, tracks the structural changes in the material. With these techniques, the researchers observed the formation of iron crystals with a high silicon concentration—demonstrating the researchers' hypothesis.

"GSECARS at the APS is the only facility in the world with the set of capabilities needed to successfully execute this experiment," said Vitali Prakapenka, beamline scientist at University of Chicago and one of the study's authors. "We combined high-energy X-rays, super-fast pulsed laser heating, high-resolution imaging and other tools to record the patterns of X-rays scattered by the sample at high pressures and temperatures. All of this was done in microsecond time. This allowed us to suppress hydrogen infiltration into the diamond anvil."

An in-progress upgrade to the APS is expected to make this line of research even more powerful.

"With the APS upgrade, our X-ray beams will be about 100 times brighter and 10 times smaller," said Prakapenka. "Coupled with faster X-ray detectors and more sophisticated lasers, the upgrade will enable us not only to track material structural changes. It will also allow us to closely monitor the paths of chemical reactions as well as the partitioning of different elements into materials—and at much higher pressures and temperatures. These advanced capabilities will open up a new era of research on even more extreme, deep-interior conditions found in planets larger than Earth."

The study was published in Nature in February. Besides Shim and Prakapenka, the research team included Suyu Fu, Arizona State University, and Stella Chariton, University of Chicago.

More information:
Suyu Fu et al, Core origin of seismic velocity anomalies at Earth's core–mantle boundary, Nature (2023). DOI: 10.1038/s41586-023-05713-5

Provided by Argonne National Laboratory