A beautiful blue flaw in a gem-quality diamond from Botswana is actually a tiny fragment of the Earth’s deep interior – and it suggests that our planet’s mantle contains water from the oceans.
The defect, technically called an inclusion, looks like a fish’s eye: a deep blue center surrounded by a white haze. But it’s really a pocket of the mineral ringwoodite 660 kilometers deep, at the boundary between the upper and lower mantle. This is only the second time scientists have discovered 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 incredibly rare to even have a super-deep diamond, and then to have inclusions is even rarer,” says Suzette Timmerman, a mantle geochemist and postdoctoral fellow at the University of Alberta, who didn’t not participated in the new discovery. Finding a ringwoodite inclusion is even more mind-boggling, she says.
The discovery indicates that this very deep area of the Earth is soggy, with large amounts of water trapped in the minerals there. Although this water is chemically linked to the structure of minerals and does not flow like a real ocean, it probably plays an important role in how the mantle melts. This in turn affects global geology, such as plate tectonics and volcanic activity. For example, water could contribute to the development of mantle upwelling areas called plumes, which are hotspots for volcanoes.
The stunning piece of diamond-covered mantle was discovered by Tingting Gu, a mineral physicist now at Purdue University who was doing research at the Gemological Institute of America at the time. His job was to study the rare inclusions found in diamonds. Inclusions are undesirable for jewelry because they obscure the brilliance of a diamond. But they are often of interest to scientists because they trap pieces of the environment where the diamond formed millennia earlier.
The vast majority of diamonds form between about 150 and 200 km below the Earth’s surface. But a handful comes from much deeper. It’s often difficult to pinpoint exactly how deep, but the new sample was remarkably accommodating on that front, Gu and colleagues reported Monday in a study published in nature geoscience. Ringwoodite can only form at incredibly high pressures. It is not found in the earth’s crust, but is sometimes seen trapped in meteorites that have suffered major cosmic trauma. In the Earth’s mantle, ringwoodite exists at present pressures up to 660 km. The only other sample of terrestrial ringwoodite found, which was discovered in a diamond in 2014, could simply be considered to have formed less than 135 km from this depth. The other two minerals found in the new inclusion, ferropericlase and enstatite, can only be found together 660 km and deeper, indicating where the diamond formed.
This is an important depth because it is the boundary between layers of the mantle, where seismic waves traveling through the Earth’s interior mysteriously change speed. Ringwoodite holds water better than ferropericlase and enstatite, so the mineral probably releases a lot of water when it undergoes changes at this boundary. The modification of the minerals and the possible release of water could explain why the seismic waves propagate differently in this region.
The ringwoodite inclusion contains a tiny amount of water bound to the molecules that make up the mineral, much like the 2014 sample. amounts of water, there has been little direct evidence that this is the case. The discovery of ringwoodite in 2014 was the first clue, but this second sample makes up 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 exceed the water on the planet’s surface. “If you only have one sample, it could just be a local waterbody,” she says, “whereas now that we have the second sample, we can already tell it’s not is not a single event. It is likely to be generalized.
The next step is to figure out where that water is coming from, says Oliver Tschauner, 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 rocks. 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 travel. It is also possible that the water has been there since the formation of the Earth. Understanding water cycles between Earth’s depths and surface could help explain how it developed into such a hydrated planet over its 4.5 billion year history.
To learn more, researchers will need to analyze the trace elements in the new inclusion, says Tschauner. They may also hope to find more deep mantle ringwoodite in diamonds in the future. It would be lucky, but again, so was this discovery, Gu says. “If someone asks you for a diamond and you find an inclusion,” she adds, “don’t say no.”