Super-Deep Diamonds Reveal Secrets of Ancient Supercontinent

Diamonds that formed deep below the Earth's surface are revealing the secrets of an ancient supercontinent that existed hundreds of millions of years ago.

A study of these "super-deep" diamonds has found that they contain evidence of the rocks that helped buoy and grow the supercontinent known as Gondwana. It is thought to have formed between 800 million and 550 million years ago, incorporating the landmasses that now make up South America, Africa, the Middle East, India and Australia.

The latest findings, published in the journal Nature, demonstrate that super-deep diamonds can shed light on the growth and formation process of ancient supercontinents.

"These diamonds allow us to see how deep plate tectonic processes relate to the supercontinent cycle," Steven Shirey, an author of the study who is with the Carnegie Institution for Science, said in a press release.

"By revealing the geological processes that contributed to Gondwana's growth, scientists can better understand the forces that shaped Earth's history and the phenomenon of continental stability, which is—of course—fundamental to the eventual success of life on our planet," he said.

For billions of years, Earth's landmasses have been ripped apart and smashed back together again in a process known as plate tectonics. Tectonic plates are gigantic pieces of the Earth's crust (our planet's outer layer) and uppermost mantle (the layer that lies below the crust). These plates are made up of oceanic crust and continental crust.

At various points in Earth's history, the tectonic plates have aligned in such a way that supercontinents have formed. But the records of these formation events are poorly preserved.

In part, this is because the oceanic crust is young and continually sinks beneath the planet's surface in a process known as subduction. In addition, the continental crust provides only a limited view of what is going on deep beneath the Earth.

In the latest study, the researchers examined diamonds found in the Juína area of Brazil and the West Africa nation of Guinea that formed between 300 and 700 kilometers (186 and 435 miles) below Earth's surface. The diamonds were unearthed during mining activities in the last few decades.

"Super-deep diamonds are very rare and usually only represent less than one percent of the diamonds recovered at a diamond mine. Certain locations like the Juína area in Brazil have an unusual high abundance of superdeep diamonds though," Suzette Timmerman, an author of the study from the University of Bern in Switzerland, told Newsweek.

The latest investigations revealed how material was added to the base of Gondwana, according to the study.

"We had two key findings. First, we showed that super-deep diamonds from Brazil and Guinea were formed 650 to 450 million years ago, at depths of 300-700 kilometers. Second, at that time Earth's landmasses were amalgamated together in what we call a supercontinent located at the South Pole," Timmerman said.

"In order for deep diamonds to move together with the continents to their present-day locations, the diamonds first had to move vertically, to the base of the continent, and then horizontally together with the continents. The vertical movement upwards with buoyant material, and the long residence at the base of the continent are key findings."

Geologic formations known as "mantle keels"—found around 40 to 250 kilometers (25 to 155 miles) beneath the Earth's surface—serve as the foundation of continental crusts. These strong, buoyant structures are capable of resisting the relentless destructive forces of Earth's tectonic activity.

Remnants of the mantle rocks that helped to form the keels can be found hidden inside super-deep diamonds, which provide the deepest available samples of Earth's mantle. The remnants come in the form of what the researchers call "inclusions"—meaning any material that is trapped inside a gemstone. In this case, the inclusions consist of silicate and sulfide compounds.

"The age of these inclusions provides a record of when buoyant mantle was added to Gondwana from below, thereby scaffolding, underpinning, and growing the supercontinent," Shirey said.

Around 120 million years ago, Gondwana began to break up. Then, 30 million years later—or roughly 90 million years ago—the super-deep diamonds examined in the study were transported to the Earth's surface in violent volcanic eruptions.