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Researchers discover previously unknown mineralogy of deep Earth

earth mantle

Credit: Pixabay/CC0 Public Domain

What is the structure of the world? For starters, it consists of several layers: crust, upper and lower mantle, and core. The mantle makes up most of our planet’s volume – 84%. The lower mantle represents 55% of Earth’s volume; it is also warmer and denser than the upper mantle.

The lower mantle has played an important role in Earth’s evolution, including how the Earth cooled over billions of years, how materials circulated and how water was stored and transported deep inland on a geological time scale.

For more than seven decades, the mineralogy of the lower mantle has been extensively studied. Decades of work, including laboratory experiments, computational simulations, and the study of inclusions in deep diamonds, led to the conclusion that the lower mantle is composed of three major minerals: bridgmanite, ferropericlase, and davemaoid.

In a recently published study NatureByeongkwan Ko, former Ph.D. Sang-Heon Dan Shim, a student at ASU, currently a postdoctoral fellow at Michigan State University and Professor in the School of Earth and Space Studies, and Navrotsky Professor of Materials Studies at ASU, have completed a new high-pressure experiment that uses some different styles. Heating to reveal an additional mineral found in the lower mantle.

Among these three major minerals, bridgmanite and davemaoite both have perovskite type crystal structures. This structure is also widely known in physics, chemistry and materials engineering because some materials with perovskite type structure have shown superconductivity.

At shallow depths, minerals with similar crystal structures often coalesce and typically become single minerals in a high temperature environment. For example, the mineral diopside contains both calcium and magnesium and is stable in the crust. However, despite the structural similarity, current studies have shown that calcium-rich davemaoid and magnesium-rich bridgmanite remain separate throughout the lower mantle.

“Why don’t davemaoid and bridgmanite combine even though they have very similar structures at the atomic scale? This question has fascinated researchers for two decades,” Shim said. “Many attempts have been made to find the conditions under which these two minerals meet, but the answer from the experiments has consistently been two separate minerals. We felt we needed some new ideas in the experiments.”

The new experiment was an opportunity for the research group to try various heating techniques to compare methods. Instead of slowly increasing the temperature in traditional high-pressure experiments, they increased the temperature very quickly to the high temperature associated with the lower mantle, reaching 3000-3500 F in one second. This is because when two perovskite minerals are formed, it becomes very difficult for them to combine even if they enter the temperature conditions where a single perovskite mineral must be stable.
By rapidly heating the samples to target temperatures, Ko and Shim were able to prevent the formation of two perovskite structured minerals at low temperatures. Once they reach the lower mantle temperature, they watch which minerals form for 15-30 minutes using X-ray beams at the Advanced Photon Source. They found that only a single perovskite mineral formed, unexpected from previous experiments. They found that at sufficiently high temperatures, greater than 3500 F, davemaoid and bridgmanite became a single mineral in perovskite-type structure.

“It was believed that a large size difference between calcium and magnesium, which are the main cations of davemaoite and bridgmanite, respectively, should prevent these two minerals from combining,” Ko said. “However, our work shows that in warm environments it can overcome such difference.”

Experiments show that the deeper lower mantle with sufficiently high temperatures should have a different mineralogy than the shallower lower mantle. Because the mantle was much warmer on early Earth, the group’s new results show that most of the lower mantle then had a single perovskite-structured mineral, meaning the mineralogy was different from today’s lower mantle.

This new observation has a number of important implications for our understanding of the deep Earth. Many seismic observations have shown that the features of the deeper lower mantle are different from the shallow lower mantle. The changes are reportedly gradual. The combination of bridgmanite and davemaoid has been shown to be gradual in the research group’s experiments.

Also, the properties of a rock containing three major minerals, bridgmanite, ferropericlase, and davemaoid, do not quite match those of the deeper lower mantle. Ko and collaborators speculate that these unresolved problems can be explained by the combination of bridgmanite and davemaoite with a new single perovskite structured mineral.


Quantum mechanical simulations of Earth’s lower mantle minerals


More information:
Byeongkwan Ko et al, Calcium dissolution in bridgmanite in Earth’s deep mantle, Nature (2022). DOI: 10.1038/s41586-022-05237-4

Provided by Arizona State University

Quotation: Researchers discover previously unknown mineralogy of deep Earth (October 2022, 2022) retrieved from https://phys.org/news/2022-10-previously-unknown-mineralogy-deep-earth.html on Oct. 21, 2022

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