A University of Alberta study of "superdeep" diamonds reveals new knowledge about the formation of Earth's ancient supercontinent Gondwana — offering scientists a glimpse into the evolution of deep plate tectonics.
The research, led by former Banting postdoctoral scholar Suzette Timmerman, maps the rise of rocks containing superdeep diamonds within the Earth's mantle — the largest solid layer between the core and crust — to the base of Gondwana by examining tiny radioactive particles found in the diamonds to date the formation of the precious stones.
The age of the studied superdeep diamonds at the base of Gondwana between 650 million and 450 million years ago provides researchers with previously unknown information about when such diamonds form and their subsequent history of rising and sticking to the continental root.
"Diamonds are one of the most fascinating research materials because diamonds are chemically inert," says Timmerman. "There is basically no exchange between the surroundings and what is inside the diamonds."
"They are perfect time capsules."
Superdeep diamonds come from depths of up to 700 kilometres or more in the Earth and are the best resource to understand the deep plate tectonic cycle. Deep plate tectonic cycles control several processes on the modern Earth, such as the carbon cycle and the rate of carbon sequestering into the mantle.
"Superdeep diamonds give you unique information about the Earth at depths that are simply unobtainable any other way," says study co-author Graham Pearson, professor in the Department of Earth & Atmospheric Sciences.
"We knew that there was a subduction system surrounding Gondwana, because that brought the continents together," says Pearson.
"But we didn't understand what was going on underneath, to what extent it was growing the continent and what processes were at play in keeping it together or growing it apart."
Host rocks carrying the superdeep diamonds became buoyant during diamond formation, transporting subducted mantle material and the diamonds to a structure resembling a ship's keel at the base of Gondwana, growing the supercontinent from below.
Ninety million years ago, the diamonds were brought to Earth's surface in violent volcanic eruptions. The location of these eruptions are on continental fragments of Brazil and Western Africa, two key components of Gondwana and superdeep diamond sources.
Scientists can infer that the diamonds must have migrated together with different parts of the ancient supercontinent as it dispersed to form our current continental landmasses.
Timmerman says there are still a lot of unknowns when it comes to deep plate tectonic cycles because it is so difficult to get samples from Earth's mantle.
The inclusions, or impurities, observed in the superdeep diamonds are quite rare and difficult to find, Timmerman explains, with teams spending weeks looking at the diamonds under microscopes to find interesting material.
"That's why we worked with such a big team to ask other researchers if they had any of such inclusions in their collections to put them all together to be able to do this study," Timmerman explains.
Chemical inclusions in superdeep diamonds are also microscopic, requiring analytical techniques and tools available at the U of A's Arctic Resources Geochemistry Laboratory.
"Even minute amounts of material can give us really important information," says Timmerman.
The study, "Sublithospheric diamond ages and the supercontinent cycle," was published in Nature. Funding for the research was provided through the Banting Postdoctoral Fellowship administered by the Natural Sciences and Engineering Research Council of Canada.
