Although the formation of diamonds is now fairly well understood, the processes that allow these crystals formed at great depths in the Earth’s mantle to rise are still a matter of debate. However, a new study shows that they came to the surface through violent eruptions fueled by huge plumes of heat that lodged on the surface of Earth’s core.
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If diamonds are so valuable, it is not only because of their brilliance, but above all because of their rarity. Diamonds are actually formed under very specific conditions, inside the earth. Finding them requires tracking down very specific volcanic rocks: kimberlites. These ultra-basic (low-silica) rocks are exceptionally rich in volatile elements (H20 and CO2). No kimberlite eruption has been observed by humans, but this chemistry suggests that its formation was violent and associated with highly explosive volcanic activity and an excessively rapid rise in magma.
Stones that you don’t find everywhere
Currently, kimberlites are found in the form of large dikes or “tubes,” a type of vertical chimney that supplied magma to ancient volcanic eruptions. The diamonds can be found there as inclusions. Hundreds of such sites have been identified, mostly in Canada, South Africa, Angola and Brazil, regions where the continental crust is very old. This geographic distribution has long puzzled scientists and suggests a connection to the formation of deep magmas. However, the processes leading to the formation of kimberlites are still poorly understood.
For several decades it has been assumed that kimberlites are associated with a rapid ascent of hot material through the Earth’s mantle and that these plumes would arise in the interior of the Earth, at the core-mantle boundary, i.e. about 2,900 kilometers from depth. But not anywhere. In 2010, researchers suggested that these large plumes rooted around the perimeter of two superstructures lurking at the bottom of the mantle, one beneath Africa and the other beneath the Pacific, and named LLSVP (Large Low-Shear-Velocity Province) have received. While the origin and nature of these “blobs” are still a matter of debate, they appear to be the source of the large hotspot volcanoes found in the Indian Ocean in particular. It is therefore hypothesized that this deep activity was also the source of kimberlite volcanism. It remained to understand how the heat was transported almost 2,900 kilometers from these great depths to the surface.
Huge columns of heat rise from the base of the mantle
A new study published in the journal Nature Geoscience provides some answers to this question, thanks to the use of supercomputers that have made it possible to create complex, three-dimensional geodynamic models of the Earth’s mantle. These models take into account the dynamics of the mantle, but also the movement of the continents over the past billion years.
The results show that the kimberlites are formed by the rise of giant thermal columns that take root at the base of the Earth’s mantle and allow for the formation of magma at depths of 450 to 150 kilometers. In the central part of these heat columns, the cladding material would rise fastest, transporting the densest elements to the surface. This mechanism would thus explain the unique chemical composition of kimberlites.
It should be noted that the diamonds found in the kimberlite tubes were not formed concurrently with the magma at the origin of these eruptions. These crystals are much older. Their formation is associated with a metasomatic process (reactions involving liquids) at a depth of typically 150 to 250 kilometers, although some specimens come from much deeper areas of the Earth’s mantle. However, their arrival at the surface is directly linked to volcanic episodes, the rapid rise of magmas that carry the diamonds and trap them in the kimberlites during the cooling and crystallization of the rock.
The model can explain almost all eruptions of this type that have occurred in the last 200 million years and have been observed worldwide. It also suggests the presence of as yet undiscovered deposits in Antarctica and Western Australia.