The outer core of the Earth, consisting of a 2000 km thick layer of liquid iron alloy, influences our surface habitability. An experimental observation of the formation of silicon-rich crystals in an iron-hydrogen alloy liquid at high pressure and high temperature — conditions similar to those of the Earth’s outer core — has just been completed by scientists at Arizona State University.
The silicon crystals are expected to be lighter than the liquid; therefore the crystals will rise in liquid iron metal.
Sang-Heon Dan Shim of Arizona State University said: “The crystallization of a silicon-rich alloy was found during our experiments on snowy winter days in Chicago during the pandemic. Interestingly, crystallization behavior can lead to rising silicon-rich snow in the outer core.”
For the experiment, scientists focused their lab observations on predicting that silicon-rich crystals in the outer core may snowball, but instead of sinking, they will rise.
At ASU, an iron-silicon alloy is put into a hydrogen-argon gas mixture for this experiment. The samples were then placed in a diamond anvil cell and compressed to the expected pressure for the core. To observe crystallization in a diamond anvil cell using extremely bright X-rays, the sample is heated by laser beams to temperatures expected for the core while kept under high pressure at the Advanced Photon Source, a U.S. Department of State user facility. Energy at DOE’s Argonne National Laboratory.
Shim said, “Creating high enough temperatures to melt iron alloys in high-pressure hydrogen was very difficult. The reason is that hydrogen can diffuse into diamond anvils and break them and the experiments fail.”
“Our team has developed a new method in which hydrogen is mixed with argon in diamond anvil cells. Argon does not react with the sample, but suppresses the diffusion of hydrogen in diamond anvils, allowing us to achieve the extreme conditions in the laboratory.”
In the transition between the metallic core and the rocky mantle, where dozens of puzzling small-scale structures have been discovered in seismic imaging studies over decades, this phenomenon can create silicon-rich snow mounds.
Suyu Fu of Arizona State University said: “If silicon and hydrogen are the two main light elements in the outer core with the right amounts, such a rising silicon-rich snow can form.”
In the transition between the metallic core and the rocky mantle, where dozens of puzzling small-scale structures have been discovered in seismic imaging studies over decades, this phenomenon can create silicon-rich snow mounds.
These rising silicon-rich crystal snows can appear as a fine-shell structure with extremely low seismic velocities at the lower mantle if the convection mantle current can trap some of them. If so, it could explain the ultra-low speed zones observed in seismic studies in the area for decades.
Fu said, “Our study also predicts that the silicon-rich snow starts in the outer core region and may progress to greater depths with further secular cooling of the Earth.”
Magazine reference:
- Fu, S., Chariton, S., Prakapenka, V.B. et al. Nuclear origins of seismic velocity anomalies at Earth’s core-mantle boundary. Nature (2023). DOI: 10.1038/s41586-023-05713-5
