Researchers locate mantle's spin transition zone, leading to clues about Earth's structure

By looking at the electronic spin state of iron in a lower-mantle mineral at high temperatures and pressures relevant to the conditions of the Earth’s lower mantle, Lawrence Livermore National Laboratory researchers and colleagues have for the first time tracked down exactly where this occurs.

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 (an iron-magnesium oxide) and silicate perovskite (an iron-magnesium silicate). 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 atmospheres equals the pressure at the Earth’s surface).

The scientists identified the ratios of the high-spin and low-spin states of iron that define the spin transition zone. By observing the spin state, scientists can better understand the Earth’s structure, composition, and dynamics, which in turn affect geological activities on the surface.

“Locating this pressure-temperature zone of the spin transition in the lower mantle will help us understand its properties, in particular, how seismic waves travel through the Earth, how the mantle moves dynamically and how geomagnetic fields generated in the core penetrate to the Earth’s surface,” said Jung-Fu Lin, a Lawrence fellow in LLNL’s Physics and Advanced Technologies Directorate.

“The spin transition zone (STZ) concept differs from previously known structural transitions in the Earth’s interior (e.g., transition zone (TZ) between the upper mantle and the lower mantle), because the spin transition zone is defined by the electronic spin transition of iron in mantle minerals from the high-spin to the low-spin states.”

The research appears in the Sept. 21 issue of the journal, Science.

Lin and colleagues determined that the simultaneous pressure-temperature effect on the spin transition of the lower mantle phase is essential to locating the exact place where this occurs.

The scientists studied the electronic spin states of iron in ferropericlase and its crystal structure under applicable lower-mantle conditions (95 GPa [950,000 atmospheres] and 2,000 K) using X-ray emission spectroscopy and X-ray diffraction with a laser-heated diamond anvil cell. The diamond cell is a small palm-sized device that consists of two gem-quality diamonds with small tips pushing against each other. Because diamonds are the hardest known materials, millions of atmospheres in pressure can be generated in the small device. The sample between the tips was then heated by two infrared laser beams, and the spin states of iron in ferropericlase were probed in situ using synchrotron X-ray spectroscopes at the nation’s Advanced Photon Source at Argonne National Laboratory.

Ferropericlase (which is made up of magnesium, iron and oxygen) is the second most abundant mineral in the lower mantle and its physical properties are important for understanding the Earth’s structure and composition. A high- to low-spin transition of iron in ferropericlase could change its density, elasticity, electrical conductivity and other transport properties. Pressure, temperature and characteristics of the spin transition of ferropericlase are therefore of great importance for the Earth sciences, Lin explained.

“The spin transition zone of iron needs to be considered in future models of the lower mantle,” said Choong-Shik Yoo, a former staff member at LLNL and now a professor at Washington State University. “In the past, geophysicists had neglected the effects of the spin transition when studying the Earth’s interior.

Since we identified this zone, the next step is to study the properties of lower mantle oxides and silicates across the zone. This research also calls for future seismic and geodynamic tests in order to understand the properties of the spin transition zone.”

“The benchmark techniques developed here have profound implications for understanding the electronic transitions in lanthanoid and actinoid compounds under extreme conditions because their properties would be affected by the electronic transitions,” said Valentin Iota, a staff member in LLNL’s Physics and Advanced Technologies Directorate.