At 1,800-miles-thick, Earth's mantle makes up roughly 84 percent of the planet's volume. However, the layer of mostly solid rock is characterized by intense heat and crushing pressure, which makes it difficult for geologists to study. Instead, they study the minerals and rocks that come to the surface, typically via volcanic eruptions. Now, a team of scientists have discovered a new mineral trapped within a diamond, according to a new study published last week in the journal Science.
The researchers named the mineral davemaoite, after the well-known geophysicist Ho-kwang (Dave) Mao. The mineral—calcium silicate perovskite—originated more than 400 miles underground and provides geologists with a glimpse of the chemical makeup of the lower mantle, reports Harry Baker for Live Science.
Before this discovery, scientists had managed to synthesize the mineral in a lab using a high-pressure environment, but its chemical structure immediately rearranged itself once the mineral was removed from 20 gigapascals of pressure. Scientists knew that this mineral likely existed in the mantle, making it presumably impossible to find a natural sample without drilling miles deep beneath the ocean floor, reports Stephanie Pappas for Scientific American.
"The chances, we thought, of finding it were so low that we never really actively looked for it," lead author Oliver Tschauner, a mineralogist at the University of Nevada, Las Vegas, tells Scientific American.
Typically, deep-Earth minerals reform as they are pushed from the mantle and towards the crust, but this mineral hitched a ride in a diamond, reports Carolyn Gramling for Science News. Diamonds form in the mantle and are blasted to the Earth's crust with volcanic eruptions. Under high temperatures and pressure, carbon atoms form super strong bonds with each other, which ultimately allows diamonds to withstand changes in pressure—such as from the mantle to the surface—without rearranging their structure, Smithsonian mineralogist Jeffrey Post said in a Q&A.
The davemaoite appeared as small black flakes embedded in the diamond, which was mined more than 30 years ago in Botswana. When Tschauner and his collaborators got their hands on the diamond a few years ago, they used X-rays and other techniques to reveal the chemical makeup of the specks, which they concluded was a new-to-science mineral that originated several hundred miles deep at the upper boundary of the lower mantle, reports Alexandra Witze for Nature.
Around 5 to 7 percent of the lower mantle could be made up of davemaoite. Although the new mineral is mostly made up of calcium silicate, there could be traces of radioactive elements such as thorium and uranium. When these elements decay, they release heat; estimates suggest they produce about a third of the heat in the lower mantle. By identifying the radioactive traces that make up davemaoite, scientists have a better understanding of where in the mantle those elements lurk, reports Science News.
"The work by Tschauner [and collaborators] inspires hope in the discovery of other difficult high-pressure phases in nature," Yingwei Fe, a geophysicist at the Carnegie Institution for Science writes in a commentary on the study. "Such direct sampling of the inaccessible lower mantle would fill our knowledge gap in chemical composition of the entire mantle of our planet."