Superdeep diamond reveals new info on earth’s geological processes

Superdeep diamonds that originate hundreds of kilometers beneath earth’s surface are like time capsules revealing how they were formed. (Image courtesy of the University of Alberta).

A recent paper published in the journal Nature points to a “superdeep” diamond recovered in Kankan, Guinea, as the messenger of new information on plate tectonics, the geological processes that give rise to mountains, oceans and continents.

One of the inclusions found in the diamond was a very pure example of the mineral olivine, a variety of which is more commonly known as the gemstone peridot. Most olivine found on our planet has some iron in it, so the purity of this olivine speaks to the unique conditions under which it was formed.

The olivine’s purity, as well as some of the other minerals that were inclusions in the precious rock, indicate a far deeper origin than usual for a diamond, between what is called the transition zone and the lower mantle zone—420 kilometres to 660 kilometres beneath earth’s surface. It also shows that the environment between these zones has an extremely variable oxygen content.

“To make this extreme composition [of olivine] and the overall mineral assemblage that we’ve got, the only way of doing that is to have a very deeply subducted oceanic plate or slab that goes down into the mantle, so you’re essentially pushing material from the surface of the earth into the depths of the earth,” Graham Pearson, study co-author and director of the Diamond Exploration and Research Training School at the University of Alberta, said in a media statement.

“You get huge gradients in oxygen activity when you do that, and these big gradients are very conducive to driving extreme variations in the composition of minerals,” he noted.

An understanding of these oxygen gradients helps explain how plate tectonics bring volatile elements back up into the mantle, and can also offer clues to how superdeep diamonds are formed—knowledge that can’t be gained any other way.

“You can see oceanic slabs descending into the earth in seismic images, but you don’t have any idea of the detailed structures they develop, or the mechanisms and chemistry going on in those slabs,” Pearson said. “These diamonds provide a unique trace of that detailed chemical evolution as the slab’s going down.”

As researchers gain more insight into the movement of those slabs into the mantle, called subduction, they can better understand plate tectonics.

“Subduction drives the whole of plate tectonics. If you don’t understand the details of subduction, that limits your understanding of how plate tectonics work,” the scientist said

Superdeep diamonds, which originate from depths of more than 300 kilometres below earth’s surface, are a treasure trove of scientific information because diamonds are uniquely able to preserve information about where they’re formed, including many of the physical and chemical processes that occurred during their formation.

Most other minerals lose much of that information by the time they make their way to the surface but, as Pearson explained, diamonds act almost as time capsules.

“There are many things at the surface of the earth that can only be explained by processes happening at deep depths,” he pointed out. “If you want to explain things you see at the surface—whether it’s economic mineralization, surface uplift or subsidence phenomena related to oil-bearing basins—you need an understanding of the structure, mechanics and properties of the deep earth. Diamond is uniquely able to bolster that understanding.”