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Despite their incredible reputation of being extraordinarily rare and precious, diamonds are more common than you think. Other planets, like Uranus, are probably full of them. They float on space rocks, even now. It turns out that we, the Terrans, are sitting on top of a diamond chain – a quadrillion tons, actually.
More than 100 miles below the surface of the Earth is a section of geology known as cratonic roots. They sit under what geologists call a craton, the oldest and most stable rock section of a continent. Some penetrate deep into the Earth's interior – able to extend up to 200 miles deep, through the outer crust and into the mantle of the planet – hence the name "Roots"
"Cratons are intimately associated with the most significant events in the Earth's history, such as the initiation of plate tectonics and the formation of continents," says Joshua Garber, postdoctoral fellow at Penn State University and lead author of the new study. "They've been stable since the Archean era, at least 2.5 billion years ago, so they've had a lot of time for a lot of things to happen to them … they have been simmering chemically in this coat for a long time. "In a new article published in Geochemistry, Geophysics, Geosystems this week, an international team of estimates from researchers that about 1 to 2% d Earth's cratonic roots are diamond. This may seem like a small amount, but this tiny fraction makes up enough of these massive geological formations to represent a quadrillion tonnes of diamonds. It is the number 1 followed by 15 zeros !
The research team was not looking for bright gems when it made this discovery. For years, scientists have collected a huge amount of data relating to the Earth's seismic activity. This data can be used to create a kind of 3D map that describes what the interiors of the Earth look like.
But not all seismic data is equal. Much is made using sound wave measurements emanating from the Earth after they were triggered by earthquakes, explosions, and other events. These sound waves (also known as shear velocities or friction velocities) move at different speeds depending on the type and temperature of the material being traversed, and they move faster through the roots of the old craters because they are colder and less dense the surrounding mantle rock. But shear rates accelerate through craton roots at higher speeds than expected.
"High shear rates are expected if temperatures are colder than average at this depth," says Barbara Romanowicz, co-author of the new study. Seismology researcher at the University of California, Berkeley. "But you would need unrealistic low temperatures compared to what we know from these coat regions, for standard rock compositions."
The team wanted to know what would explain the unusually fast velocity readings. They began their study using the data they had to create a 3D model of seismic waves zipping through the craton roots, followed by laboratory tests of sound waves traveling through various terrestrial mineral combinations, heated to temperature profiles that we would expect to find in the craton roots.
Cratons are mainly peridotite, but can not have high shear rates. There were only two other minerals in the Earth's mantle that could have explained the velocities: eclogite (subducted oceanic crust descending into the mantle zone) and diamond
You would need an unrealistic amount of d & rsquo; Eclipses to facilitate the observed shear rates. Meanwhile, "the diamond has extremely fast shear rates, so you would only need a small part of it, especially if it is combined with eclogite, to match the observations, "explains Romanowicz
. the speeds observed in nature had only a pinch of diamonds, from 1 to 2%. This was sufficient to reconcile the differences in the sound wave data without affecting the overall measured densities of the craton roots.
Although the results are a bit unexpected, they make a lot of sense (though maybe not quadrillion tons of meaning). "Diamond is a really appealing answer because it's much stiffer than most other minerals," says Garber. "One result of this study is that, if the answer is not the diamond, there is a little more than another rigid constituent than the diamond of which we have even less evidence."
In addition, Romanowicz says that it is possible craton diamonds could come from carbonaceous fluids from subducted slabs (diamonds are essentially stable forms of carbon, created by extremely high temperatures and pressures).
But do not be too excited now: you'll never see those pretty rocks in the flesh. Again, they are located more than 100 miles underground – "it's about 10 times deeper than the deepest hole ever drilled in a continent," Romanowicz explains. "The drills go very fast and soften when you reach high temperatures in excess of several hundred degrees Celsius, and even water cooling is not enough to compensate. Frankly, I do not think it will happen anytime soon. "
" On the other hand, "she continues," the diamonds rise to the surface, carried by an ancient volcanism. We may need to be patient, but when it comes to time at the geological scale, this kind of patience should last beyond our species. Plus, would you really like? If a quadrillion tons of diamonds circulated on the surface of the Earth, these precious stones would not suddenly appear as precious as they would be. Diamonds would probably become as valuable as a copper penny (although Lincoln's face engraved on the face of a diamond would certainly be a brilliant change.)
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