As intensely as diamonds sparkle, their origin is often just as spectacular and mysterious. Originating at a depth of more than 120 kilometers in the earth’s mantle, the precious crystals rest there for millions of years in the glowing hot, tough plastic mass of rock. Violent volcanic eruptions suddenly catapult them through narrow vents at breakneck speed to the surface of the earth. Embedded in a black-blue debris rock called kimberlite, the diamonds arrive undamaged despite the heavy transport. The best-known mines where diamond-bearing kimberlite rock is mined today are in South Africa near the town of Kimberley, hence the name.
“What on earth makes these kimberlites spring up from the depths after spending millions or even billions of years down there – sometimes with diamonds in their luggage?” Thomas Gernon asked himself that, now an earth scientist at the University of Southampton, England, as a student 20 years ago. Now, along with other geoscientists, Gernon has come closer to solving the kimberlite mystery, like the team in the trade journal Nature reported.
What is unusual is that kimberlite volcanoes are located in the middle of the great continental plates and not, like the many volcanoes of the Pacific Ring of Fire, at the edge of the plates. Scientists have long suspected that kimberlite volcanoes are fed by hot magma streams called “mantle plumes”. These rise from the core-mantle boundary at a depth of 2900 kilometers and press against the earth’s crust, such as under Hawaii or Yellowstone, the US national park known for its hot springs. Yet the kimberlites “have no hint of plumes in their chemistry,” says Gernon.
As if a cutting torch would repeatedly pierce the continental plates from below
Another hypothesis: The kimberlite explosions could be related to the breaking up of supercontinents. In the course of the earth’s history, the continental plates, which drift across the globe at speeds of several centimeters per year on the molten interior of the earth, joined together several times to form a single, gigantic continent and later broke apart again into individual plates. Pangea was the last supercontinent. It began gradually breaking up into the continents that drift across Earth today 230 million years ago.
Gernon therefore initially processed vast amounts of data on plate tectonic movements and petrological databases on kimberlites. In addition to statistical methods, AI programs were used. He discovered that the kimberlite eruptions around the world always first occurred about 20 to 30 million years after the rupture of the supercontinents. Subsequent eruptions shifted further and further towards the centers of the continental plates over the course of millions of years. Geochemical investigations and age dating of kimberlite rocks in Africa, North and South America confirmed this result.
But what causes the delay and how can it be that the eruptions move – as if a cutting torch were used to repeatedly pierce the continental plates from below at intervals?
Now the members of the team specializing in simulations were called upon, including Sascha Brune, head of the section for geodynamic modeling at the Geoforschungszentrum (GFZ) in Potsdam. “When continents break up,” says the scientist, “this leads to destabilization in the Earth’s mantle.” The mantle rock under the continents, which is in a kind of viscous state due to the extreme pressure-temperature conditions and moves slowly, is disturbed in its flow – far below the continents.
The diamonds shoot up at the speed of sound
Brune illustrated this hard-to-imagine process in a video animation. According to this, turbulence occurs at the fracture point at the edge of the continents in the hot, viscous molten rock of the Earth’s mantle, which continues from the fracture point under the continental plates. The scientists speak of Rayleigh-Taylor instability. Like the revolving shovels of the huge excavators in brown coal open-cast mines, these vortices scrape off fragments several kilometers thick from the underside of the continental plates on their way towards the center of the plate. This even happens at the roots of the “cratons”, the thickest and most stable parts of the continents at a depth of 150 to 200 kilometers. The fragments sink and merge with the underlying magma.
According to Gernon, who led the study, kimberlite magma, which is rich in water vapor and carbon dioxide, can form during this process. These gases propel the magma up through the gaps left by the scraped-off fragments on the underside of the plates through the mantle with such force that it ruptures the crust at the speed of sound. In the process, areas studded with diamonds are also carried away. Due to the high speed, the gemstones in the kimberlite, which are made of pure carbon, reach the earth’s surface as complete crystals undamaged. If they rose slower, the heat would turn them into soft graphite.
The analyzes could also interest the operators of diamond mines, says Gernon, because they provide clues to possible future discovery sites. However, according to statistics, only every hundredth kimberlite pipe contains diamonds in a mineable concentration. The youngest kimberlite volcano discovered to date is in the Igwisi Hills in Tanzania. Its formation, only about 11,000 years ago, may be related to the “Great Rift Valley” that has opened up in East Africa over the past 20 million years and where the African continental plate could split in two in the distant future. The kimberlite there contains all the minerals typical of its rock type, including olivine and garnet, but no diamonds have been found there.