The Earth’s mantle is 1,800 miles deep and accounts for around 84 percent of the planet’s volume. The stratum of largely solid rock, on the other hand, is characterized by high heat and crushing pressure, making it a challenge to be analyzed by geologists. Instead, they investigate the minerals and rocks that are brought to the surface by volcanic eruptions. According to a research study published last week in the journal Science, a team of scientists has identified a new mineral trapped within a diamond.
The mineral was given the name davemaoite by the researchers after the well-known geophysicist Ho-Kwang (Dave) Mao. The mineral, calcium silicate perovskite, formed more than 400 miles down and gives geologists a view into the lower mantle’s chemical makeup, according to Live Science’s Harry Baker. These tiny black specks found in an African mine diamond are being hailed as a vital component of the deep earth, uncovered in nature for the first time after decades of seeking.
Davemaoite: a rare marvel
According to Oliver Tschauner, a geochemist at the University of Nevada, Las Vegas, it is a rare view of a mineral that ordinarily cannot exist on the earth’s surface but plays a significant role in heat transport deep within the globe. He headed the team that published the discovery in Science on November 11th.
Davemaoite is primarily calcium silicate (CaSiO3), although it may scavenge radioactive uranium, thorium, and potassium isotopes. These isotopes create great heat in the earth’s mantle, the layer between the planet’s crust and core. As a result, davemaoite plays a crucial role in controlling how heat travels through the deep earth and how heat cycles between the mantle and crust drive processes, including plate tectonics.
The mineral could exist only under extreme pressures and temperatures, such as those found in the lower mantle, 660–2,700 kilometers below the surface. However, these specific specks were lodged within a diamond, transporting the material intact from deep within the earth. It is the diamond’s strength that keeps the inclusions under strain, explained Tschauner.
The diamond, greenish and octahedral in form, was discovered decades ago in Botswana at the Orapa mine, the world’s biggest opencast diamond mine. A mineral trader sold it in 1987 to George Rossman, a mineralogist at the California Institute of Technology in Pasadena. Tschauner, Rossman, and their colleagues began investigating it a few years ago as part of a study on minerals trapped in deep-earth diamonds. The study of these minerals and how inclusions are formed in them could also shed new light on growing lab diamonds in a high-pressure high heat environment, thus making the process of making them more efficient.
Research around the mineral
The scientists used X-rays to examine the diamond at the Advanced Photon Source in Lemont, Illinois and discovered that the inclusions were rich in a calcium mineral. Further research revealed that it was calcium silicate, which may take on multiple crystalline forms depending on the pressure and temperature at which it was formed.
As explained by Tschauner, the version in the diamond has a perovskite crystal structure that only occurs at temperatures and pressures found between 660 and 900 kilometers underground. Scientists had previously created high-pressure calcium silicate perovskite in the laboratory. They thought it should exist in the lower mantle, but they had yet to observe it in geological samples decisively.
Another study team discovered calcium silicate perovskite in a deep-earth diamond from South Africa in 20183. However, those scientists did not claim to have made an official discovery of a new mineral. Tschauner’s team did, which is why he contacted Mao and requested whether the mineral might be named after him. “Of course, I gladly agreed,” said Mao, the head of Shanghai’s Center for High-Pressure Science and Technology Advanced Research.
Last year, the International Mineralogical Association adopted the name davemaoite. It is the second high-pressure mineral named after Mao; in 2018, Chinese researchers discovered a separate one, maohokite, in rocks from an impact crater.
Tschauner claims that davemaoite is one of three significant minerals in the earth’s lower mantle, accounting for 5–7 percent of the material there. It is, however, only one of the three that contains uranium and thorium. Given the mineral’s crystalline structure, calcium can be easily exchanged for these elements, which create heat through radioactive decay.
Other diamonds may also include traces of davemaoite. According to Yingwei Fei, a geochemist at the Carnegie Institution for Science in Washington DC, one strategy to locate additional davemaoite would be to hunt for deep diamonds in potassium-rich locations.
Admittedly, though much can be learned from these minerals, the problem is getting those rare natural samples. The search for super-deep diamonds is laborious, and geochemists believe there are no straightforward techniques to predict where gems from the deep earth could appear.
But hope cannot be eliminated. While commenting on the study, Yingwei Fe, a geophysicist at the Carnegie Institution for Science, said that the work of Tschauner and his partners inspires optimism in the finding of additional challenging high-pressure phases in nature. Such direct sampling of the inaccessible lower mantle would close the gap in our understanding of the chemical composition of our planet’s whole mantle.