When it comes to knowing what really lies deep inside the Earth, diamonds are a geologist’s best friend.
A tiny piece of rock embedded in a diamond now opens a whole new window into what the planet’s lower mantle looks like. Inside the diamond is a recently identified silicate mineral called davemoite which could have formed only in the Earth’s lower mantle, researchers reported on November 12 Science . This is the first time that scientists have been able to conclusively prove that this type of lower mantle mineral, previously only predicted as a result of laboratory experiments, actually exists in nature. The team named the mineral after the famous high-pressure experimental geophysicist Ho-Kwan (Dave) Mao.
The diamond with prominent mineral inclusions came from a mine in Botswana and formed at a depth of more than 660 kilometers, at the upper limit of the Earth’s lower mantle. Using analytical techniques including X-ray diffraction, X-ray fluorescence imaging, and infrared spectroscopy, mineralogist Oliver Schauner of the University of Nevada, Las Vegas, and his colleagues determined the chemical composition and structure of the new mineral, tying it to a type of calcium. silicate perovskite.
Scientists have previously estimated that about 5 to 7 percent of the lower mantle must be made up of this mineral, Chauner says. But it is devilishly difficult to directly observe such deep minerals of the Earth. That’s because minerals that are stable under the high pressure of the lower mantle — which extends as far as 2,700 kilometers below Earth’s surface — begin to rearrange their crystal structures once the pressure drops.
Even the planet’s most abundant mineral, a lower-mantle magnesium-iron silicate known as bridgmanite, was largely theoretical until 2014, when it was discovered to have been caught in a meteorite that crashed into Australia with a force that caused devastating, deep mantle damage. pressure in the rock. To date, bridgmanite is the only silicate mineral under high pressure, the existence of which has been confirmed in nature.
Diamonds act as time capsules, locking in the original mineral forms during their journey to the surface. The discovery of davemoite is not only a confirmation of its existence, but also reveals the location of some heat sources deep inside the Earth. Although it is a calcium silicate mineral, davemoite also contains a gallery of different elements that permeate its crystal structure. This 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 being left over from the planet’s initial formation 4.55 billion years ago). By determining the chemical composition of davemaoite, researchers can now determine where these elements are found.
That’s because the Botswana diamond also contained a form of high-pressure ice, as well as another high-pressure mineral known as wustite. The presence of these inclusions helped narrow down the rough pressure at which davemoite could have formed: somewhere between 24 billion pascals and 35 billion pascals, Chauner says. It’s hard to say exactly what depth this corresponds to, he adds. But the discovery directly links heat (radioactive materials), the water cycle (ice) and the carbon cycle (represented by the formation of the diamond itself), all of which occur in the deep mantle, Chauner says.
Another intriguing aspect of this new mineral is that it is surprisingly rich in potassium compared to laboratory predictions, says Sang-Heung Shim, a geophysicist at Arizona State University in Tempe. Most experimental efforts to create the mineral have resulted in “almost pure calcium silicate perovskite,” Shim says. For now, scientists can only speculate on what the source of the extra potassium was, but this unexpected composition hints that the lower mantle may be a more colorful mix than previously thought, the complexity of which is difficult to predict based on laboratory studies alone.
