I’m squinting at a diamond in the palm of my hand. As gems go, it’s nothing special: smaller than a grain of rice and full of impurities, it would fetch a poor price. But for researchers like , those impurities are invaluable for the information they reveal about geological processes under way deep within Earth – all the more so given that some of them look unlike anything we have encountered before. “I hope that we will discover a new mineral,” he says.
This particular diamond formed around 600 kilometres below the surface, “close to the border between the upper mantle and lower mantle”, says Korolev, a geologist at the American Museum of Natural History in New York City. That makes it among the deepest-formed objects that have found their way back to the surface of the planet and into the hands of geologists.
Korolev and his colleagues at the museum are using such rare super-deep diamonds to directly study the material that makes up Earth’s interior – an underground realm we still know surprisingly little about. The work could reveal important new information, such as how much water is carried to the lower mantle when slabs of Earth’s rocky crust are dragged into the planet’s interior at subduction zones. We could even learn how, or if, this deep water influences the behaviour of Earth’s tectonic plates.
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I’m in the museum’s all-purpose imaging lab, a few floors above the public galleries echoing with the sounds of school children gawking at Tyrannosaurus rex and the model blue whale. In the relative quiet of the lab, Korolev plucks the diamond from my hand with a pair of tweezers to situate it underneath a microscope. After a few seconds, a magnified picture of the crystal appears on a nearby monitor, revealing golden flecks of deeply formed minerals lodged among the diamond’s transparent facets.
Diamonds aren’t the only view we have into Earth’s deep interior. We can image it to some extent by studying the behaviour of seismic waves passing through the planet, while high-pressure experiments in the labĚý˛ą˛Ô»ĺ computer models can simulate conditions far underground. We also have xenoliths, which are rocks spewed out by volcanoes that sample the uppermost 250 kilometres of the mantle. But only diamonds are strong enough to maintain their crystal structure – as well as the structure of minerals trapped inside – through the extreme changes in pressure and temperature encountered en route from deeper regions to the surface.
Exactly how these deep diamonds make that journey remains enigmatic. “It’s a question a lot of researchers are breaking their brains about,” says , a petrologist at the museum and Korolev’s adviser. Her favoured hypothesis is that the diamonds are carried upwards with convecting material in the lower mantle, taking hundreds of millions of years to reach the rocky roots of the continents. From there, they can be rocketed to the surface via deeply rooted kimberlite volcanoes. An alternative theory is that the diamonds are carried to the base of kimberlites in columns of hot, buoyant rock known as mantle plumes.
While the shape and composition of such diamonds offer some information about their origin, the researchers are most interested in the pockets of minerals and fluids – inclusions – captured within the diamond’s carbon matrix. The diamond under the microscope was secured from a mine in Brazil and provided to the researchers by a collaborator. We know it formed in the lower mantle because it contains inclusions of a silicate perovskite mineral called davemaoite that only occurs at such depths. The mystery mineral might reveal more about conditions so far below ground.
To discover whether it can, the geologists have already sent a fragment of the tiny diamond to France, where it is now being bombarded with powerful synchrotron X-rays to determine the mystery mineral’s precise crystal structure.
The results aren’t in yet, but it wouldn’t be the first new mineral discovered in a diamond. In a previous study, Korolev and his colleagues analysed a diamond from South Africa and that had never been encountered in nature, even though experimental evidence indicates that it is one of the most abundant minerals inside Earth. The presence of this calcium silicate mineral also served as direct evidence that slabs of oceanic crust subducted at the surface really do descend all the way to the lower mantle.
With the new inclusions, the researchers now aim to get better numbers on how much water these slabs of oceanic crust transport to the lower mantle. Water molecules can become incorporated into the crystal structure of the new minerals that form as oceanic crust reaches the transition zone between the upper and lower mantle – a depth at which it is transformed by pressure and heat. More water molecules in the crystals indicates more water in the mantle. “We do not know very well and cannot estimate the fate of water in the lower mantle because currently we do not have pristine inclusions of all minerals from that region,” says Korolev.
A clearer picture of this deep water cycle, in turn, connects to a “hundred questions” about the deep Earth, from how much water was present after the formation of the planet to the contentious question of how plate tectonics began, says Kiseeva. “We have very little direct evidence from the depths,” she says. They just need to get their hands on a few more diamonds.
