A gem-quality diamond from Botswana has been found to have a flaw that is actually a tiny fragment of the Earth’s deep interior. Thanks to this, scientists were able to determine that the mantle of our planet contains oceanic reserves of water, reports Scientific American.
The flaw, technically called an inclusion, looks like a fish eye: a deep 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 just the second time scientists have found this mineral in a chunk of crystal from this zone, 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 chemical composition.
The discovery indicates that this very deep area of the Earth is wet, with vast amounts of water tightly locked in minerals. Although this water is chemically bound to the structure of minerals and does not flow around like a real ocean, it probably plays an important role in melting the mantle. It, therefore, affects general geology such as plate tectonics and volcanic activity. For example, water can contribute to the development of regions of mantle upwelling known as plumes, which are hotspots for volcanoes.
A piece of mantle in a diamond shell was discovered by Tingting Gu, a mineral physicist who now works at Purdue University, and then conducted research at the Gemological Institute of America. Her job was to study the rare inclusions found in diamonds. They are undesirable for jewelry because they dull the brilliance of the diamond. However, inclusions are often of interest to scientists, because they contain particles of the environment in which the diamond was formed over thousands of years.
The vast majority of diamonds form 150 to 200 km below the Earth’s surface, but a handful come from much deeper depths. It’s often hard to tell exactly how deep. The new sample turned out to be surprisingly well-suited for this, Gu and her colleagues reported. Ringwoodite can only form under incredibly high pressure. It is not found in the Earth’s crust, but is sometimes seen in meteorites. Ringwoodite exists in the Earth’s mantle at pressures up to 660 km. The only known sample of terrestrial ringwoodite, which was discovered in diamond in 2014, formed within 135 km of this depth. Two other minerals found in the new inclusion, ferropericlase and enstatite, can only occur together at depths of 660 km and deeper, pinpointing where the diamond formed.
A ringwoodite inclusion contains a tiny amount of water bound to the molecules that make up the mineral, just like the 2014 sample. This is important, because while previous laboratory experiments suggested that the Earth’s mantle could store vast amounts of water, there was little direct evidence that this was actually the case. The discovery of ringwoodite in 2014 was the first hint, but the second sample makes the speculation much more compelling. If the mineral is really significantly overmoistened in the transition zone of the mantle, then the amount of water stored in the depths of the Earth can easily exceed that on the surface of the planet.
The next step is to find out where this water comes from, says Oliver ZTschauner, a mineralogist at the University of Nevada, Las Vegas. He was part of the team that discovered a form of high-pressure water ice in ultradeep diamonds in 2018, but was not involved in the new study. Researchers know that oceanic plates carry water with them, because they are pushed into the mantle by tectonics. However, researchers debate how deep this water can travel. It is also possible that water has been there since the formation of the Earth. Understanding how water circulates between the depths and the surface could help explain how Earth became such a wet planet over its 4.5 billion year history.