A million billion tons. It would be the number "1", followed by eighteen "0" if it was measured in kilograms... A colossal, disproportionate, unimaginable mass. Especially for a mineral famous for its rarity: diamonds. But it is in these large quantities that the gemstone would lie hidden in the depths of the Earth, according to the estimates just released by American geologists.
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Published in mid-June in the journal Geochemistry, Geophysics, Geosystems, a study conducted by scientists at MIT, the Massachusetts Institute of Technology, revealed the incredible diamond deposit that could lie deep in the depths of our planet. An underground El Dorado that is unfortunately inaccessible... These abundant sites are believed to be located a little over 160 kilometres below the surface. Well beyond the drilling capabilities of even the most efficient mining tools.
1. Precious stones lying in the depths of the Earth: This unsuspected deposit could be dispersed at the base of "cratons", large areas of continental land that are particularly stable. Their "roots", the thickening located directly above this original network of mountain ranges, are believed to contain about 1 to 2% of diamonds. A low grade a priori that in relation to the total volume of craton roots buried underground, leads researchers to raise their astronomical estimate by one million billion tons.
"This shows that diamonds may not be so much of an exotic mineral [as we had imagined], but on the [geological] scale of things, it is relatively common," says Ulrich Faul from the MIT Earth, Atmospheric and Planetary Sciences Department. "We can't look for them, but there are still many more diamonds out there than we previously imagined" says the researcher.
2. A method based on the study of seismic waves: To make this unexpected discovery, geologists didn't travel "to the centre of the Earth" like the heroes of Jules Verne. They have appointed this risky mission to slightly more daring explorers: seismic waves. Produced by tectonic events that regularly shake the Earth, they spread to depths inaccessible to humans and rise to the surface where they can be detected to determine their velocity.
The time it takes for them to spread varies according to the nature of the materials they pass through. At the craton root level, they pass quickly, much faster than the predicted for this density of the rocks.
"The measured speeds are greater than we think we can reproduce with sensible assumptions about what is there," explains Ulrich Faul, who continues: "We had to bring ourselves to say that there was a problem. That's how our project started.
3. A revealing 3D model: The first stage of their work consisted of making a 3D model of the propagation rates of seismic waves through terrestrial cratons. Then, scientists integrated different rocks into their model in order to reproduce the astonishing speeds measured at the cratons.
The result? A single rock made of 1 to 2% diamond was the only way to match the results of the numerical simulation to the realities on land. This proportion is at the same time large enough to explain the increase in measured speed, and small enough not to increase the density of cratons, structures that are naturally lighter than the environment around them.
"They're like pieces of wood floating on the water," explains Ulrich Faul. "We therefore have determined that it is only necessary to have 1 to 2% diamond for the cratons to be stable and not sink". This hypothesis is supported by the high pressure and temperature conditions found at these depths of more than 150 kilometres below the surface, which are essential for the formation of diamonds.
3. Precious stones rising from the depths: It is only in the chance of magmatic ascents that occur every ten million years that these diamonds are formed in the centre of the earth and rise to the surface. An ascent that they carry out via a diatreme: a volcanic chimney punctuated by holes. These spaces are sometimes filled with kimberlite, the only magmatic rock on earth that contains diamonds.
The 'few' gemstones that reach depths accessible to mining would therefore only come from the tip of an 'iceberg', made of nearly a million billion tons of diamonds... A theory that does not convince all specialists. If cratons contained 1 to 2% diamonds, this would indeed shake up some of the knowledge about the geology of our planet.
'My quick estimate indicates that this would almost double the amount of carbon that we generally think the Earth's mantle can contain, which would have profound implications for the evolution of the earth's mantle and crust,' said Gizmodo, Wendy Panero, professor at Ohio State University, not involved in the study she finds interesting.
If the presence of these diamonds is not challenged, there may be fewer than estimated in the new study. "I look forward to seeing more research in this area to see if this hypothesis holds up after further investigation," concluded Wendy Panero.
