New study shakes up understanding of deep Earth
To help understand the seismic structure of the deep Earth, researchers have studied the elastic behavior of an iron-containing mineral under extremely high pressures.
Scientists, led by Jonathan Crowhurst of the Laboratory’s Chemical Sciences division within the Chemistry, Materials, Earth and Life Sciences Directorate specifically looked at the mineral ferropericlase (an iron-magnesium oxide).
The Earth’s mantle is a 2,900-kilometer thick rocky shell that makes up about 70 percent of the Earth’s volume. It’s mostly solid and overlies the Earth’s iron-rich core. The lower mantle, which makes up more than half of the Earth by volume, is subject to high pressure-temperature conditions with a mineral collection made mostly of ferropericlase and silicate perovskite. The Earth’s lower mantle varies in pressure from 22 GPa (220,000 atmospheres) to 140 GPa (1,400,000 atmospheres) and in temperatures from approximately 1,800 K to 4,000 K. (One atmosphere equals the pressure at the Earth’s surface).
In the new research, the team for the first time was able to determine the complete elasticity of ferropericlase through the pressure-induced high-spin to low-spin electronic transition. For the composition, with approximately 6 percent iron at room temperature, the researchers discovered that the transition manifests as a softening of the material over an extended range of pressures from 40 GPa (400,000 atmospheres) to 60 GPa (600,000 atmospheres).
“The lower mantle constitutes a large fraction of the entire planet, and profoundly influences many terrestrial phenomena including those that are of direct relevance to us as inhabitants,” Crowhurst said. “A great effort is being made by both experimentalists and theoreticians to precisely determine the physical properties of the lower mantle to provide data that can be used to test and refine geophysical models. Since knowledge of this deep and inaccessible region is derived largely from seismic data, it is particularly desirable to measure under relevant conditions the acoustic characteristics of candidate materials or mixtures.”
The lower mantle contains a significant fraction of ferropericlase (10 percent-20 percent by volume). Ferropericlase is a mineral obtained by substituting a small fraction of iron atoms for magnesium iron atoms in the compound MgO and is written as (Mg,Fe)O.
This limited substitution can profoundly affect the behavior of the material under certain conditions. At low pressure the iron exists in what as known as the "high-spin" state. This refers to a specific electronic configuration which changes dramatically to the "low-spin" state when plenty of external pressure is applied.
“The electronic spin transition in ferropericlase has been observed in various experiments over the last several years, but until the present research its detailed effect on the elasticity and acoustic characteristics of the mineral were not known,” Crowhurst said.
The research appears in the Jan. 25 edition of the journal, Science.
Other collaborators include the University of Washington, the Carnegie Institution of Washington and Northwestern University.