Studying material properties under the extreme pressures and temperatures that exist in planetary interiors, including Earth’s interior, presents extraordinary experimental challenges. By pushing diamond anvil cell techniques literally to the breaking point, Jung-Fu “Afu” Lin, a mineral physicist and new assistant professor in the Department of Geological Sciences, has probed material properties and discovered new insights into their behavior under the pressure-temperature conditions that exist deep within Earth and other planetary bodies.
Researchers have long been able to achieve very high pressures using diamond anvil cells, which can reach millions of atmospheres of pressure at small scales. More recent work has also used laser heating to simulate the high temperatures that exist in Earth’s iron-rich core and lower mantle, which is made up primarily of silicate perovskite, an iron-containing magnesium silicate, and ferropericlase, an iron-containing magnesium oxide.
Getting to the Core
“Mineral physicists’ quest to the Earth’s core falls largely around stably creating pressure-temperature conditions and simultaneously measuring properties of the subject iron alloys, and that is also true for the Earth’s lower mantle,” Lin said. “In the past we really didn’t have such capabilities. But with the advances in diamond anvil cell techniques, especially laser-heated diamond anvil cells, combined with synchrotron X-ray spectroscopies, we are now in a position to probe the properties of planetary materials under relevant pressure-temperature conditions.”
These improved techniques are yielding new results in mineral physics, such as the discovery of the phase transition from perovskite to post-perovskite at the bottom of the lower mantle.
“What is interesting about the perovskite to post-perovskite transition is that for a long time people kind of gave up on looking at another transformation in the lower mantle,” Lin said, because it was believed that perovskite was a stable mineral form in the lower mantle. “But people had only looked at pressure-temperature conditions up to about 120 GPa [gigapascals], instead of 136 GPa where the transition occurred.”
In his recent work, Lin has pioneered experimental capabilities to examine the electronic spin transition of iron in ferropericlase, the second most abundant mineral in the lower mantle. He discovered a transition from an electronic spin-unpaired (high-spin) to a spin-paired (low-spin) state occurs gradually in iron over a pressure-temperature range in the middle part of the lower mantle, extending from about 1,000 kilometers in depth and 1,900 degrees Kelvin to 2,200 kilometers and 2,300 degrees Kelvin in the lower mantle.
This finding, together with its associated effects on the properties of the mineral, will help give scientists a better understanding of how seismic waves travel through the mantle, how the mantle moves dynamically and how geomagnetic fields generated within the Earth reach the surface.
“That is what I’m most excited about in my research in recent years,” Lin said. “It’s really a new research direction. When people talk about the Earth’s interior, they usually talk about temperature, pressure, or compositional variables. This spin transition research is a new ‘spin’ on our understanding of the properties of the Earth’s interior. What is really interesting is that just tiny electrons of iron in the Earth’s lower mantle oxide can significantly affect our understanding of the Earth’s interior.”
After demonstrating the high-spin to low-spin transition and its effects in ferropericlase, Lin has recently turned his attention to the spin transition in perovskite and post-perovskite.
“We now see not just a high-spin to low-spin transition, but there is actually another spin state, called the intermediate-spin state in which electrons of ferrous iron are partially paired, that occurs in both perovskite and post-perovskite under lowermost mantle conditions,” he said.
Lin performs many diamond anvil experiments at the Jackson School, but he also performs research at synchrotron radiation centers, such as the Advanced Photon Source, the nation’s brightest synchrotron X-ray source, at Argonne National Lab in Chicago. His goal is a better understanding of the full array of properties that describe candidate materials under deep-earth pressure-temperature conditions, such as crystal structures, phase relations, sound velocities, electronic spin transitions and chemical reactions. He is also particularly interested in the role of volatiles, such as H20 and CO2, in planetary interiors.
“These volatiles consist of very light elements, so it’s really difficult to detect some of their properties under high pressure-temperature conditions,” Lin said. “On the other hand, these volatiles are also very intriguing in the sense that, since they could be so chemically active, they can play very important roles in many properties of minerals under high pressures and temperatures.”
Lin continues to work with his colleagues to push for even higher pressures and temperatures within diamond anvil cells, and to refine techniques to probe mineral physics properties of deep-earth materials. Pressures at the center of the Earth’s core are estimated at 360 gigapascals (GPa), and Lin has reached 250 GPa and high temperatures in recent experiments on iron-silicon alloys – though he broke four pairs of diamonds in the process. Reaching relevant temperatures of the core, somewhere from 4,000-7,000 degrees Kelvin, is much more difficult.
“High-quality diamonds can handle the strain of the pressure – that’s not a big problem,” he said. “Normally when we shine the laser through the diamond to the sample, the sample is the only medium that we want to supply with heat from the laser. With the use of an insulating material between the sample and the diamonds, the heat is mostly kept in the sample. But once under very high pressures and high temperatures there’s just enough space to insert sufficient insulating materials to keep the heat from dissipating into the diamonds. So when you provide more heat to the sample in order to reach to higher temperatures, the diamond anvil gives up – a popping sound that signals the end of the experiment.”
Lin is constantly thinking of how to solve this heating problem. “There must be ways to statically reach the pressure-temperature conditions of the Earth’s core,” he said. “One way to really improve the situation is to use super hard materials as the gasket to sustain a very thick sample chamber under such extreme conditions.”
Lin recently received funding from the National Science Foundation to continue his research into the electronic spin transitions of iron in the lower mantle to determine how the spin transitions affect density distribution, speeds of seismic waves and heat transport, among other properties in the deep mantle. He plans to incorporate this research into the training of the next generation of mineral physicists at the Jackson School.
For more information about the Jackson School contact J.B. Bird at email@example.com, 512-232-9623.