Researchers discover previously unknown mineralogy in the deep Earth

What is the structure of the earth? 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 hotter and denser than the upper mantle.

The lower mantle has played an important role in Earth’s evolution, including how Earth has cooled over billions of years, how elements have circulated, and how water has been stored and transferred to/from the deep interior over geologic time scales.

For more than seven decades, the mineralogy of the lower mantle has been extensively studied. Decades of study, including laboratory experiments, computational simulations, and studies of deep diamond inclusions, have concluded that the lower mantle contains three main minerals: bridgmannite, ferropericlase, and davemoite.

In a recently published study, Dr the nature, a team of scientists—including Byeongkwan Ko, a former Ph.D. student at ASU, now a postdoctoral researcher at Michigan State University, and Sang-Heon Dan Shim, a professor in the School of Earth and Space Exploration and a Novrotsky Professor of Materials Science at ASU, completed a new high-pressure experiment employing a few different styles of a Excessive exposure to heating minerals Lives in the lower mantle.

Among these three main minerals, the two minerals bridgmannite and davmaoite both have so-called perovskite-type crystal structures. This structure is also widely known in physics, chemistry and materials engineering, as some materials with perovskite-type structures have shown superconductivity.

At shallow depths, minerals with similar crystal structures often coalesce and become single minerals, usually in high-temperature environments. For example, the mineral diopside contains both calcium and magnesium and is stable in the crust. Despite the structural similarities, however, existing studies have shown that calcium-rich davmaoite and magnesium-rich bridgmannite are distinct throughout the lower mantle.

“Why don’t davmaoite and bridgmanite merge into one even though their atomic-scale structures are very similar? This question has fascinated researchers for two decades,” says Shim. “Many efforts have been made to find the conditions under which these two minerals combine, yet the answer from experiments has consistently been two separate minerals. Here we felt we needed some new ideas in our experiments.”

The new experiment was an opportunity for the research group to try different heating techniques to compare the methods. Instead of increasing the temperature slowly in conventional high-pressure tests, they increased the temperature very rapidly to the higher temperature associated with the lower coating, reaching 3000-3500 F within a second. This was because once two perovskite-forming minerals were formed it was very difficult for them to coalesce even if they entered temperature conditions where a single perovskite mineral should be stable.
By rapidly heating the samples to the target temperature, Ko and Shim were able to avoid the formation of two perovskite-forming minerals at low temperatures. Once they reach the lower mantle temperature, they observe what minerals form for 15-30 minutes using X-ray beams in an advanced photon source. They found that only single perovskite mineral forms, unexpected from previous experiments. They found that at sufficiently high temperatures of over 3500 F, davemoite and bridgmannite become single minerals in a perovskite-type structure.

“It’s believed that a large size difference between calcium and magnesium, the major cations in davemaoite and bridgmanite, respectively, would prevent these two minerals from combining,” Ko said. “But our research shows that they can overcome such differences in hot environments.”

The experiments suggest that the deeper lower mantle with sufficiently high temperatures should have a different mineral composition than the shallower lower mantle. Because Earth’s early mantle was much warmer, the group’s new findings indicate that much of the lower mantle then contained a single perovskite-composed mineral, meaning the mineralogy was different from that of the lower mantle today.

These new observations have considerable implications for our understanding of the deep Earth. Many seismic observations have shown that the properties of the deeper lower mantle differ from those of the shallower lower mantle mantle. The changes are said to be gradual. The amalgamation of bridgmanite and davmaoite has been shown to be gradual in experiments by the research group.

Also, the characteristics of a rock with three major minerals, bridgmanite, ferropericlase, and davemoite, do not match well with depth characteristics. lower cover. Ko and collaborators predicted that these unresolved issues could be explained by the amalgamation of bridgmannite and davmaoite into a new single perovskite-composed mineral.