Washington, DC-Diamonds contain evidence of the mantle rocks that helped buoy and grow the ancient supercontinent Gondwana from below, according to new research by a team of scientists led by Suzette Timmerman – formerly of the University of Alberta and now at the University of Bern – and including Carnegie’s Steven Shirey, Michael Walter and Andrew Steele. Their findings, published in Nature, show that superdeep diamonds can provide a window through space and time into the process of supercontinent growth and formation.
For billions of years, the Earth’s landmasses have been ripped apart and reassembled by plate tectonics, periodically forming giant supercontinents. This process is driven by large-scale convection in the Earth’s mantle. But the record of these events is poorly preserved because the oceanic crust is young and constantly sinking beneath the surface in a process called subduction, while the continental crust provides only a limited view of Earth’s deep workings.
Surprisingly, the research team was able to show that superdeep diamonds, formed between 300 and 700 kilometres below the Earth’s surface, can reveal how material was added to the base of a once mighty supercontinent.
“These diamonds allow us to see how deep plate tectonic processes relate to the supercontinent cycle,” said Shirey.
The supercontinent of Gondwana is thought to have formed between 800 and 550 million years ago in the Neoproterozoic Era. Starting from the present-day location of the South Pole, it encompassed the landmasses that make up present-day South America, Africa, the Middle East, India and Australia.
“By revealing the geological processes that contributed to the growth of Gondwana, scientists can better understand the forces that shaped Earth’s history and the phenomenon of continental stability, which is of course fundamental to the ultimate success of life on our planet,” added Walter.
About 40 to 250 kilometres below the surface, geological formations called mantle keels act as the foundation of the continental crust. The material that forms these keels thickened, stabilised and cooled beneath the continental blocks to form strong, buoyant structures that can withstand the relentless destructive forces of Earth’s tectonic activity.
Remnants of the mantle rocks that helped form the keel can be found in tiny silicate and sulphide inclusions hidden within these super-deep diamonds. Typically flaws in ordinary gem diamonds, these inclusions are a geoscientist’s best friend. They were identified, isolated, crystallographically studied and then radiometrically dated to determine their geological age.
The work was carried out by researchers from the University of Alberta and the Carnegie Institution for Science, as well as other teams of diamond specialists from the Vrije Universiteit Amsterdam, the University of Bristol and the University of Padua. It involved many steps, including shipping the diamonds around the world several times and using some of the most precise mass spectrometers and X-ray diffractometers available.
“Studying such rare samples with a variety of measurement techniques required a great deal of teamwork. But what is even more remarkable is how careful analyses of such tiny amounts of material can shed light on the evolution of the Earth’s largest continental landmasses,” said Timmerman.
“The age of these inclusions provides a record of when the buoyant mantle was added to Gondwana from below, thus framing, supporting and growing the supercontinent,” Shirey added.
Then, about 120 million years ago, the supercontinent once buoyed by the rocks containing these diamonds began to break up, and finally, 30 million years later – about 90 million years ago – the diamonds – and the inclusions trapped inside them – were brought to the Earth’s surface in violent volcanic eruptions of diamond-bearing kimberlite magma.
By combining their laboratory analysis with existing models of tectonic movement and continental migration, the researchers can now use these remarkably well-travelled diamonds to understand how material welds continental fragments together from below, stabilising such a super-sized continental landmass.
