A diamond from a South African mine, seen wrapped in aluminum foil for lab analysis, contains a grain of perovskite, a mineral long suspected of making up a large portion of Earth’s deep interior but never before detected at the surface.M. Kopylova
New findings about Earth's deep interior have come to light, with two teams of scientists this week announcing separate discoveries of rare minerals trapped in diamonds.
In one case, Maya Kopylova, a geologist at the University of British Columbia, and post-doctoral researcher Nester Korolev identified an unusual inclusion in a diamond from South Africa's Cullinan mine. The mine is world-renowned as the source of the world's largest gem-quality diamond – now among Britain's crown jewels – but the specimen the UBC researchers examined was only 2.5 millimetres across.
At first, the team did not know what they had found; their initial analysis suggested it was a material that had not been seen before in samples.
"You know, when you conduct any scientific work and find something extremely unusual, your first reaction is that it is most likely a mistake," Mr. Korolev said.
Working with collaborators around the world, the team used X-rays to determine that a small grain trapped inside the diamond was made of calcium silicate in a mineral form known as perovskite. Geologists have long suspected that a large share of Earth's mantle is made of perovskite but have been unable to prove it until now because it can only exist under high pressure.
"Many people thought we'd never see a natural sample because it's so unstable at the Earth's surface," said Graham Pearson, a team member and professor of Earth science at the University of Alberta in Edmonton.
The structure and composition of the grain suggests it formed about 700 kilometres down, in Earth's deep mantle. Encapsulated in its diamond shell, it was carried toward the surface by molten rock moving up through a natural conduit called a kimberlite pipe.
Perovskite is particularly important to scientists' understanding of Earth's interior, Dr. Pearson said, because it can host trace amounts of radioactive metals that are collectively responsible for much of the planet's internal heat. The release of that heat is what drives the gradual movement of the continents and the recycling of the oceanic crust, the top layer of the oceanic portion of tectonic plates. The crust moves downward like a conveyor belt in places where it collides with the continents, such as along North America's West Coast.
Chemical clues suggest the diamond formed out of the remains of oceanic crust that disappeared from the surface a billion years ago. It shows that the recycling process brings crust material all the way down to the deep mantle – another hypothesis that scientists have been unable to verify until now.
"This is really key to working out the mechanics of what goes on inside the Earth," Dr. Pearson said.
The findings are described in this week's edition of the journal Nature.
Another find, published in the journal Science this week, suggests that water is indeed a big part of Earth's interior. After looking at diamonds from mines in Africa and Asia, a second team identified two diamonds containing water in a rare crystalline form known as ice VII ("ice seven"). This discovery marks another first; previously, ice VII has only been seen in the laboratory, though it is thought to exist in the interiors of some watery moons and planets.
Kim Tait, the curator of mineralogy at the Royal Ontario Museum in Toronto, was involved in the study of the ice VII-bearing diamonds, one of which will become part of the museum's permanent research collection. She says the discovery provides direct evidence not only of the water content of Earth's interior but of the pressures and conditions at different depths and locations.
"A lot of people don't understand that we've only been able to go down about 12 kilometres," she said. "These opportunities with diamonds are really special and tell us a lot about Earth."
The findings could also help refine theories about the interior of Mars, which recent studies suggest absorbed a large portion of its original water into its mantle.
"That's where my brain goes immediately," Dr. Tait said. "If we can understand more about our planet, what does that tell us about other planets?"
Ivan Semeniuk