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According to a new study from MIT and other universities, there could be more than a quadrillion tons of diamonds hidden in the Earth's interior. But the new results are unlikely to trigger a diamond rush. Scientists believe that precious minerals are buried more than 100 miles below the surface, well beyond any drilling expedition.
The ultradeep cache can be scattered within cratonic roots – the oldest and most immobile rock sections under the center of most continental tectonic plates. In the form of inverted mountains, the cratons can extend up to 200 miles across the Earth's crust and into its mantle; geologists refer to their deepest sections as "roots".
In the new study, scientists estimate that cratonic roots can contain 1 to 2 percent of diamonds. Considering the total volume of cratonic roots in the Earth, the team estimates that about a quadrillion (1016) tons of diamonds are scattered in these ancient rocks, 90 to 150 miles below the surface.
"This shows that the diamond may not be this exotic mineral, but on the scale of things [geological] it is relatively common," says Ulrich Faul, researcher at the Department terrestrial, atmospheric and planetary sciences of MIT. "We can not reach them, but there are many more diamonds than we have ever imagined before."
Faul's co-authors include scientists from the University of California at Santa Barbara, the Globe Physics Institute. from Paris, the University of California at Berkeley, the Polytechnic School, the Washington Carnegie Institution, Harvard University, the University of Science and Technology of China, the University of Bayreuth, University of Melbourne and University College of London
A small sound problem
Faul and his colleagues came to their conclusion after being asked about an anomaly in the seismic data. In recent decades, organizations such as the United States Geological Survey have kept records of global seismic activity – essentially sound waves that cross the Earth and are triggered by earthquakes, tsunamis, explosions and other sources of shaking. Seismic receivers all over the world pick up sound waves from such sources, at different speeds and intensities, that seismologists can use to determine the origin of an earthquake for example.
Scientists can also use this seismic data to construct an image of what the Earth's interior might look like. Sound waves move at different speeds across the Earth, depending on the temperature, density, and composition of the rocks they pbad through. Scientists have used this relationship between seismic velocity and rock composition to estimate the types of rocks that make up the earth's crust and parts of the upper mantle, also known as the lithosphere.
However, using seismic data to map the interior of the Earth. Scientists have been unable to explain a curious anomaly: Sound waves tend to accelerate considerably by pbading through the roots of ancient cratons. The cratons are known to be cooler and less dense than the surrounding mantle, which in turn would produce slightly faster sound waves, but not quite as fast as what was measured.
"Measured speeds are faster than we think we can reproduce with reasonable badumptions about what exists," says Faul. "Then we have to say," There is a problem. "That's how this project began."
Diamonds in the deep
The team sought to identify the composition of cratonic roots that could explain the peaks in the seismic velocities. To do this, the team's seismologists first used USGS seismic data and other sources to generate a three-dimensional model of the seismic wave velocities traveling through the main cratons of the Earth.
Next, Faul and other past measured sound velocities across many types of minerals in the laboratory, used this knowledge to bademble virtual rocks, made from various combinations of minerals. Then, the team calculated the speed at which the sound waves cross each virtual rock and found only one type of rock producing the same velocities as those measured by the seismologists: 1 to 2 % of diamond, in addition to the typical peridotite of the upper mantle of the Earth) and minor amounts of eclogite (representing the subducted oceanic crust). This scenario represents at least 1000 times more diamonds than people had predicted before.
"Diamond in many ways is special," says Faul. "One of its special properties is, the speed of sound in the diamond is more than twice as fast as in the dominant mineral in the rocks of the upper mantle, olivine."
The researchers found that a composition of 1 to 2% of just enough to produce the higher sound velocities measured by seismologists. This small diamond fraction would not change either the overall density of a craton, which is naturally less dense than the surrounding mantle.
"They are like pieces of wood floating on the water," says Faul. "The cratons are a little less dense than their environment, so they are not brought back into the Earth but remain floating on the surface, so they keep the oldest rocks. the cratons are stable and do not flow. "
In a way, Faul says that cratonic roots made partly of diamonds make sense. Diamonds are forged in the environment at high pressure and high temperature of the deep Earth and only bring it closer to the surface by volcanic eruptions that occur every few tens of millions of years. These eruptions excavate geological "pipes" made up of a type of rock called kimberlite (named after the city of Kimberley, South Africa, where the first diamonds of this type of rock were found). Diamond, as well as magma from the depths of the Earth, can spread through kimberlite pipes on the surface of the Earth.
Kimberlite pipes were found on the edges of cratonic roots, as in some areas. from Canada, Siberia, Australia and South Africa. It would be logical, then, that cratonic roots should contain diamond in their makeup.
"It's circumstantial evidence, but we've pieced it all together," says Faul. "We have gone through all the different possibilities, from all angles, and it's the only one that remains a reasonable explanation."
Learn more:
Scientists discover that seismic imaging is blind to water
More information:
Joshua M. Garber et al. Multidisciplinary constraints on the abundance of diamond and eclogite in the cratonic lithosphere, Geochemistry, Geophysics, Geosystems (2018). DOI: 10.1029 / 2018GC007534
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