Launched by a diamond? Unveiling Davemaoite's deep Earth mysteries

Still, while perovskites are proving crucial for life on Earth's surface, they also play another engineering (not to mention existential) role right beneath our feet. That is, for decades, Earth scientists have long held that 'silicate' perovskites exist in Earth's deep mantle- the area between the planet's core and crust, some 670 to 2700 kilometers below us. But there was one catch: not one had been observed in nature. 

In a world-first, tiny black specks of the never-before-seen mineral, now officially named davemaoite, made the unlikely trip to Earth's surface. 

To learn more,  Interesting Engineering (IE) spoke with Oliver Tschauner, the mineralogist from the University of Nevada, Las Vegas, who oversaw the discovery. Better yet, we explored what other mysteries may await us in the lower mantle.

What is davemaoite, and why had it never been seen before?

"Davemaoite is a natural calcium silicate (CaSiO₃) perovskite, which is a structure only stable at the high pressures of the deep Earth," Tschauner told IE.

Earth scientists have long struggled to obtain one due to the perovskite's tendency to fall apart at lower pressures near the planet's surface.

Essentially, the mineral's crystal structure must be cubic to truly be davemaoite. This means that the unit cell - the smallest repeated arrangement of atoms- must be a cube. 

Mother Nature (being Mother Nature) typically causes davemaoite's unit cell to expand in one direction as it ascends, forming a warped version of davemaoite instead. This begs the question: how did davemaoite retain its cubic structure in this case?

Like a message in a bottle, davemaoite minerals were delivered to the surface as entrapments in a deep-mantle diamond. According to Tschauner, the diamond was actually found back in the 1980s in an African mine. It was launched toward the surface by a volcanic eruption between around 92 million and 93 million years ago. 

It was only recently, when Tschauner and his colleagues applied cutting-edge tools to the diamond, that the novel crystalline compound within was exposed. 

These tools included the study of the diamond in the Advanced Photon Source (APS), a vast complex at the Argonne National Laboratory of the U.S. Department of Energy in Lemont, Illinois. The APS pretty much functions as a giant, high-energy X-ray microscope that can see through thick materials. Significantly, it can illuminate the structure and chemistry of matter at the molecular and atomic levels.

Tschauner said to IE that diamonds are elastically very stiff, hard materials. Therefore, "Inclusions trapped inside a diamond can retain pressures in the range of several 10,000 times the atmospheric pressure," he explained. In other words, they can withstand pressures hundreds of thousands of times greater than that of the air around us on the surface. 

"These high remnant pressures prevent the conversion of minerals like davemaoite to lower pressure minerals," he said.

He also admitted that the sample's survival over the arduous voyage to the surface is extraordinary in and of itself. He believes that if the specks were any larger, the diamond would have broken, causing Davemaoite's structure to disintegrate, as per an article in Las Vegas Review-Journal. 

What does the discovery of davemaoite tell us about Earth's lower mantle?

"It's the conservation of the high-pressure crystal structure that tells us where a mineral has formed," explained Tschauner. "It allows us to trace back the conditions where the growing diamond has entrapped the mineral."

The remnant pressure pinpoints davemaoite's origin to the lower mantle. He also explained that the mineral makes up around 5-7 percent of the material there. 

And that's not all. The specks of davemaoite mineral were found to host radioactive isotopes of the elements uranium, thorium, and potassium. It could therefore have some role in how our planet manages its internal heat since these elements are well known to contribute to 20-30 percent of the heat in the Earth's interior. 

Tschauner suspects that regional differences in the amount of davemaoite in the deep mantle ultimately affect how buoyant and mobile the rocks in this region are. 

Surprisingly, "the actual specimen of davemaoite contains an appreciable amount of [potassium], he added. "This indicates a more efficient mobilization of [potassium] in the deep mantle than we previously had assumed."

We also now know that carbon, potentially in vast quantities, is present at that depth. After all, the diamond that entrapped the davemaoite minerals must also have formed in that high-pressure region. But how did carbon get to that depth initially? This remains to be known. 

"It may be from former atmospheric CO₂ that was chemically bound and transported by plate tectonic processes ('subduction') that deep," he explained. 

IE is also told that the same diamond contains ice-VII, a high-pressure form of ice. Could these inclusions confirm the existence of water-rich regions in the Earth's mantle, as scientists have long hypothesized?

"Hence, mobility of potassium, carbonaceous and aqueous fluid in the deep mantle appear to be linked together- if one sample makes a case!" Tschauner concluded.

So then, what other mysteries await us in the lower mantle?

"We expect to find more of these specimens," he said. "In particular, we hope to find a specimen of bridgmanite, the main mineral of the deep Earth."

He also explained that over 80 percent of the bridgmanite already found had come from shock-compressed meteorites.

"We already found two more high-pressure minerals (neither of which we had expected to find!)," revealed Tschauner. He hopes that with these samples at hand - and more to come - the team will be able to better understand the effect of pressure on the chemistry of elements on Earth. 

Additionally, he hopes to address the following deep mantle questions: Does the potassium in davemaoite come from deep subducted former Earth crust? How is it mobilized at a depth of mantle where no melting occurs? Does the carbon of the diamond come from CO₂ of that crust or was it bound as iron-carbide in the mantle for a long time? 

As Tschauner and his team progress, they hope to establish a fuller chemistry and geochemistry of high pressures. This would be akin to finding about inorganic chemistry at ambient pressure made in the nineteenth and twentieth centuries, but with high pressures. 

Ultimately, Tschauner highlights the importance of finding samples in nature. "Natural samples represent the complexity of reality, the unexpected and unpredicted," he said. "Experiments only address the particular aspects that we set out to study in our laboratories."  

He concluded that Earth is a complex system. Hence, we need both: samples from Earth and experiments, which help us interpret these samples fully.