When it comes to knowing what actually lies deep inside the Earth, diamonds are a geologist’s best friend.
A team of researchers affiliated with a host of institutions in the U.S. has found a sample of a mineral previously believed to be unable to exist in nature. In their paper published in the journal Science, the group describes their study of a diamond found in Orapa, Botswana and the mineral specks they found trapped inside. Yingwei Fei with the Carnegie Institution for Science has published a Perspectives piece in the same journal issue outlining the work by the team and explaining why the mineral find is important to geology.
In 1975, a team of researchers created what Fei describes as a "high-pressure phase of aSiO3 "—a mineral that had been theorized to exist, but only under certain conditions. To synthesize the mineral, the researchers had to place its building materials under high heat and pressure conditions.
They noted that as soon as the pressure was relieved, the mineral immediately changed to a form of glass. This finding suggested that it was not likely that the mineral could exist in nature. That assumption has now been proven wrong, as the diamond found in Botswana contained three tiny samples of it.
Because the mineral, now called calcium silicate perovskite, can only form under extreme heat and pressure conditions, the researchers suspect the specks they found in the diamond had to form deep below the Earth's surface—perhaps as far down as 660 to 900 km.
They note that calcium silicate is found in multiple forms, one of which is wollastonite—a mineral found in the Earth's crust. They note also that material in the crust and mantle moves between the two because rocks are pushed around by plate tectonics. Prior research has shown that as they move, minerals in the rocks often undergo transformations, but some become trapped in diamond.
Diamond is a sort of base material; once conditions arise for its formation, the resulting diamond remains intact regardless of what goes on around it. The researchers suspect a form of calcium silicate (such as wollastonite) found itself in just the right conditions to form calcium silicate perovskite far below the surface and shortly thereafter found itself surrounded by carbon that was in the process of being pressed into a diamond.
After that, the material surrounding the diamond carried it upward until it reached the surface, where it was found. The researchers have named it "davemaoite," after noted geologist Ho-Kwang "Dave" Mao.
Diamonds act like time capsules, locking in the original mineral forms on their journey to the surface. The discovery of davemaoite is not only a confirmation of its existence, but it also reveals the location of some sources of heat deep inside Earth. Although it’s a calcium silicate mineral, davemaoite is also host to a rogue’s gallery of different elements that sneak into its crystal structure. That includes radioactive elements such as uranium, thorium and potassium, as well as rare-earth elements. Such radioactive elements have long been thought to produce about a third of the heat circulating in the lower mantle (the other two-thirds is left over from the planet’s original formation 4.55 billion years ago). By identifying the chemical makeup of davemaoite, researchers can now confirm where those elements reside.
That’s because the Botswana diamond also contained a high-pressure form of ice as well as another high-pressure mineral known as wüstite. The presence of those inclusions helped narrow down the rough pressures at which the davemaoite might have formed: somewhere between 24 billion pascals and 35 billion pascals, Tschauner says. It’s hard to say exactly what depth that corresponds to, he adds. But the discovery directly links heat generation (the radioactive materials), the water cycle (the ice) and the carbon cycle (represented by the formation of the diamond itself), all in the deep mantle, Tschauner says.
Another intriguing aspect of this new mineral is that it’s surprisingly rich in potassium compared with laboratory predictions, says Sang-Heon Shim, a geophysicist at Arizona State University in Tempe. Most experimental efforts to create the mineral came up with “nearly pure calcium silicate perovskite,” Shim says. Scientists can only speculate right now what the source was for the extra potassium, but this unexpected composition hints that the lower mantle may be a more motley mix than thought, with complexity difficult to predict from lab studies alone.
The study was published in the journal Science X Network.