A beautiful blue flaw in a gem-quality diamond from Botswana is actually a tiny fragment of Earth’s deep interior — and suggests that our planet’s mantle contains as much water as the oceans.
The defect, technically called an inclusion, looks like a fisheye: a dark blue center surrounded by a white haze. But it’s really a pocket of the mineral ringwoodite from 660 kilometers down, at the boundary between the upper and lower mantle. This is only the second time scientists have found this mineral in a piece of crystal from this area, and the sample is the only one of its kind currently known to science. The last example was destroyed during an attempt to analyze its chemistry.
“It’s extremely rare to even have a super-deep diamond, and then to have inclusions is even rarer,” said Suzette Timmerman, a mantle geochemist and postdoctoral fellow at the University of Alberta who was not involved in the new discovery. Finding the inclusion of ringudite is even more mind-blowing, she says.
The discovery shows that this very deep area of the Earth is wet, with huge amounts of water locked tightly in the minerals there. Although this water is chemically bound to the structure of the minerals and does not flow like a real ocean, it probably plays an important role in how the mantle melts. This in turn affects big picture geology such as plate tectonics and volcanic activity. For example, water can contribute to the development of zones of mantle upwelling known as plumeswhich are hotspots for volcanoes.
The stunning piece of diamond-encrusted mantle was discovered by Tingting Gu, a mineral physicist at Purdue University who was doing research at the Gemological Institute of America at the time. Her job was to study rare inclusions found in diamonds. Inclusions are undesirable for jewelry because they dull the diamond’s brilliance. But they are often interesting to scientists because they capture particles from the environment where the diamond formed millennia earlier.
The majority of diamonds form between about 150 to 200 km below the Earth’s surface. But a handful come from much deeper. It’s often difficult to determine exactly how deep, but the new sample was remarkably comfortable on that front, Gu and her colleagues reported Monday in a study published in Nature Geoscience. Ringwoodite can only form under incredibly high pressure. It is not found in the Earth’s crust, but is sometimes seen trapped in meteorites that have suffered major space trauma. In the Earth’s mantle, ringudite exists at pressures up to 660 km. The only other terrestrial sample of ringwoodite found, which was found in diamond in 2014, can simply be said to have formed within 135 km of this depth. The two other minerals found in the new inclusion, ferropericlase and enstatite, can occur together only 660 km and deeper, determining where the diamond formed.
This is an important depth because it turns out to be the boundary between mantle layers – where seismic waves traveling through the Earth’s interior mysteriously change speeds. Ringwoodite holds water better than ferropericlase and enstatite, so the mineral probably releases a lot of water as it undergoes changes at this boundary. The change in minerals and the possible release of water may explain why seismic waves travel differently through this region.
The the inclusion of ringwoodite retains a small amount of water bound to the molecules that make up the mineral, as the 2014 sample did. This is important because – although previous laboratory experiments have suggested that the mantle can store vast amounts of water – there is little direct evidence that this is actually the case. The discovery of ringudite in 2014 was the first hint, but this second sample makes a much more compelling story, Timmerman says. If the mineral is indeed largely waterlogged in the mantle transition zone, the water stored deep within the Earth could easily outstrip the water on the planet’s surface. “If you only have one sample, it might just be a local water region,” she says, “whereas now that we have the second sample, we can now say it’s not just a single event. It is likely to be widespread.
The next step is to figure out where that water comes from, says Oliver Schauner, a mineralogist at the University of Nevada, Las Vegas, who was part of a team that discovered a form of high-pressure water ice in ultra-deep diamonds in 2018 but was not involved in the new study. Researchers know that oceanic plates carry water with them as they are pushed into the mantle by plate tectonics, but they debate how deep that water can go. It is also possible that water has been there since the Earth was formed. Understanding how water cycles between Earth’s depths and the surface may help explain how it evolved into such a hydrated planet over its 4.5 billion-year history.
To learn more, researchers will need to analyze trace elements in the new inclusion, Tschauner says. They can also hope to find more ringudite in the deep mantle in diamonds in the future. That would be lucky — but then again, so was this discovery, says Gu. “If someone proposes to you with a diamond and you find a turn on,” she adds, “don’t say no.