Lower mantle chemistry breakthrough

Breaking research news from a team of scientists led by Carnegie’s Ho-kwang “Dave” Mao reveals that the composition of the Earth’s lower mantle may be significantly different than previously thought. These results are to be published by Science.

The lower mantle comprises 55 percent of the planet by volume and extends from 670 and 2900 kilometers in depth, as defined by the so-called transition zone (top) and the core-mantle boundary (below). Pressures in the lower mantle start at 237,000 times atmospheric pressure (24 gigapascals) and reach 1.3 million times atmospheric pressure (136 gigapascals) at the core-mantle boundary.

The prevailing theory has been that the majority of the lower mantle is made up of a single ferromagnesian silicate mineral, commonly called perovskite (Mg,Fe)SiO3) defined through its chemistry and structure. It was thought that perovskite didn’t change structure over the enormous range of pressures and temperatures spanning the lower mantle.

Recent experiments that simulate the conditions of the lower mantle using laser-heated diamond anvil cells, at pressures between 938,000 and 997,000 times atmospheric pressure (95 and 101 gigapascals) and temperatures between 3,500 and 3,860 degrees Fahrenheit (2,200 and 2,400 Kelvin), now reveal that iron bearing perovskite is, in fact, unstable in the lower mantle.

The team finds that the mineral disassociates into two phases one a magnesium silicate perovskite missing iron, which is represented by the Fe portion of the chemical formula, and a new mineral, that is iron-rich and hexagonal in structure, called the H-phase. Experiments confirm that this iron-rich H-phase is more stable than iron bearing perovskite, much to everyone’s surprise. This means it is likely a prevalent and previously unknown species in the lower mantle. This may change our understanding of the deep Earth.

“We still don’t fully understand the chemistry of the H-phase,” said lead author Li Zhang, also of Carnegie. “But this finding indicates that all geodynamic models need to be reconsidered to take the H-phase into account. And there could be even more unidentified phases down there in the lower mantle as well, waiting to be identified.”

High pressure experiments reproduce mineral structures 1,800 miles deep

X-ray diffraction image of the post-perovskite phase of the mineral magnesium silicate glass (MgSiO3), produced in a diamond-anvil cell under 1.85 million times atmospheric pressure and heated to 3500 Kelvin. -  Advanced Light Source/LBNL
X-ray diffraction image of the post-perovskite phase of the mineral magnesium silicate glass (MgSiO3), produced in a diamond-anvil cell under 1.85 million times atmospheric pressure and heated to 3500 Kelvin. – Advanced Light Source/LBNL

University of California, Berkeley, and Yale University scientists have recreated the tremendous pressures and high temperatures deep in the Earth to resolve a long-standing puzzle: why some seismic waves travel faster than others through the boundary between the solid mantle and fluid outer core.

Below the earth’s crust stretches an approximately 1,800-mile-thick mantle composed mostly of a mineral called magnesium silicate perovskite (MgSiO3). Below this depth, the pressures are so high that perovskite is compressed into a phase known as post-perovskite, which comprises a layer 125 miles thick at the core-mantle boundary. Below that lies the earth’s iron-nickel core.

Understanding the physics of post-perovskite, and therefore the physics of the core-mantle boundary, has proven tough because of the difficulty of recreating the extreme pressure and temperature at such depths.

The researchers, led by Yale post-doctoral fellow Lowell Miyagi, a former UC Berkeley graduate student, used a diamond-anvil cell to compress an MgSiO3 glass to nearly 1.4 million times atmospheric pressure and heated it to 3,500 Kelvin (more than 3,000 degrees Celsius, or nearly 6,000 degrees Fahrenheit) to create a tiny rock of post-perovskite. They then further compressed this to 2 million times atmospheric pressure and zapped the substance with an intense X-ray beam from the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory to obtain a diffraction picture that reveals the deformation behavior of post-perovskite.

They found that the orientation of post-perovskite’s crystals in the deformed rock allowed some seismic waves – those polarized parallel to the core-mantle boundary – to travel faster than those polarized perpendicular to it. This anisotropic structure may explain the observations of seismologists using seismic waves to probe the earth’s interior.

“For the first time, we can use mineral physics with diamond-anvil cells at the ALS to get information about how this mineral, post-perovskite, performs under intense pressure,” said co-author Hans-Rudolf Wenk, a Professor of the Graduate School in UC Berkeley’s Department of Earth and Planetary Science and Miyagi’s Ph.D. thesis advisor. “People had suggested this as an explanation for the anisotropy, but now we have experimental evidence.”

“Understanding how post-perovskite behaves is a good start to understanding what’s happening near the mantle’s lower reaches,” Miyagi said. “We can now begin to interpret flow patterns in this deep layer in the earth.”

The study, which appears in the Sept. 24 issue of the journal Science, has important implications for understanding how the earth’s internal heating and cooling processes work.

“This will give seismologists confidence in their models by matching what these observations predict with the seismic data they get,” said coauthor Waruntorn “Jane” Kanitpanyacharoen, a UC Berkeley graduate student.

Post-perovskite was first recognized as a high-pressure phase in the mantle in 2004, and subsequent experiments in diamond-anvil cells have produced the mineral. Wenk and his colleagues in 2007 conducted experiments that they thought had determined the deformation behavior of post-perovskite, but which now appear to have been related to the phase transformation to post-perovskite. This transition takes place at about 1,300,000 times atmospheric pressure (127 gigaPascals) and 2,500 Kelvin (4,000 degrees Fahrenheit).

The current experiment showed that post-perovskite’s crystal structure is deformed by pressure into a more elongated shape. Because seismic waves travel faster in the stretched direction, this matches the observed difference in velocity between seismic waves polarized horizontally and vertically traveling through the post-perovskite zone above the earth’s core.

If scientists can gain a better understanding of the core-mantle boundary’s behavior, it will give them clues as to how Earth’s internal convection works there, where cool tectonic plates descend from the ocean floor through the mantle eventually nearing the dense, liquid-iron outer core, heat up, and begin moving upward again in a repeated cycle that mixes material and heat through the mantle.