EDMONTON — A unique combination of minerals trapped inside a “superdeep” diamond that originated hundreds of kilometres beneath Earth’s surface sheds new light on plate tectonics, says a University of Alberta diamond expert.
One of the inclusions found in the diamond was the mineral olivine — a variety of which is more commonly known as the gemstone peridot — likely the most magnesium-rich ever found in nature. Most olivine found on Earth has some iron in it, so the purity of this olivine speaks to the unique conditions under which it was formed, says Nature study co-author Graham Pearson, professor in the Department of Earth and Atmospheric Sciences.
The olivine’s purity, as well as some of the other minerals that were inclusions in this diamond, indicate a far deeper origin than usual for a diamond, between what’s called the transition zone and the lower mantle zone — 420 kilometres to 660 kilometres beneath Earth’s surface.
“The complex mineral assemblage is best explained by buckling of an oceanic slab as it descends, via plate tectonics, into Earth’s deep mantle,” explains Pearson. “During slab buckling, a complex gradient of oxygen activity is generated, which can drive extreme mineral compositions.”
An understanding of these oxygen gradients helps explain how plate tectonics — the geological processes that give rise to mountains, oceans and continents — brings volatile elements back into the mantle, and can also offer clues to how superdeep diamonds are formed.
“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,” he says.
“These diamonds provide a unique trace of that detailed chemical evolution as the slab’s going down. It’s amazing to document the buckling of these huge oceanic plates as they descend into the bowels of the Earth, by probing minerals that are 10s of microns in size, trapped in diamonds.”
Superdeep diamonds, which originate from depths of more than 300 kilometres below Earth’s surface, act almost as time capsules 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.
“There are many things at the surface of the Earth that can only be explained by processes happening at great depths,” says Pearson.
“If you want to explain things you see at the surface, whether it’s economic mineralisation, surface uplift, or subsidence phenomena related to oil-bearing basins, you need an understanding of the structure, properties and mechanics of the deep Earth. Diamond is uniquely able to bolster that understanding.”
To speak with Graham Pearson about the study, please contact:
Debra Clark
U of A communications associate
debra.clark@ualberta.ca