Analysis of ancient, super-deep diamonds from mines in Brazil and West Africa has revealed new processes of how continents formed and moved during the early evolution of complex life on Earth.
These diamonds, formed between 650 and 450 million years ago at the base of the supercontinent Gondwana, were analysed by an international team of experts and have revealed how supercontinents like Gondwana formed, stabilised and moved around the planet.
“Superdeep diamonds are extremely rare and we now know that they can tell us a lot about the whole process of continent formation,” says Dr Karen Smit from the Wits School of Geosciences, who was involved in the study. “We wanted to date these diamonds to try and understand how the earliest continents formed.”
Formed millions to billions of years ago, diamonds can shed light on the darkest and oldest parts of the Earth’s mantle. Continents drift across the Earth’s surface, creating “supercontinents” and destroying them. Together these migrations are known as the ‘supercontinent cycle’ and diamonds are one of the few minerals strong enough to survive and record these ancient cycles of creation and destruction.
Supercontinents can concentrate deep oceanic plate subduction – the driver of plate tectonics – in very specific regions. Such deep geological processes have been very difficult to study directly, especially in the past, because the oceanic crust is young and the continental crust provides only a limited view of the Earth’s deep interior. Old diamonds provide a direct window into the deep plate tectonic engine and how it might relate to the supercontinent cycle.
By dating the tiny silicate and sulphide inclusions inside the diamonds, the team led by Dr Suzette Timmerman of the University of Bern, Switzerland, dated the diamonds that formed 300 to 700 km beneath the base of Gondwana. The aim was to trace how material was added to the keel of the supercontinent. The team discovered a previously unknown geological process. The research has been published in the journal Nature.
“The geochemical analyses and dating of inclusions in the diamonds, combined with existing plate tectonic models of continent migration, showed that diamonds were formed at great depths beneath Gondwana when the supercontinent covered the South Pole between 650 and 450 million years ago,” says Smit.
The diamonds’ host rocks became buoyant during diamond formation, transporting subducted mantle material along with the diamonds. This material was added to the base of Gondwana’s root, essentially ‘growing’ the supercontinent from below.
“About 120 million years ago, Gondwana began to break apart to form today’s oceans, such as the Atlantic Ocean. About 90 million years ago, the diamonds, carrying trapped tiny inclusions of the host rock, were brought to the Earth’s surface in violent volcanic eruptions”.
The current locations of these volcanic eruptions are on the continental fragments of Brazil and West Africa, two of the key components of Gondwana. So the diamonds must have migrated with different parts of the former supercontinent as it broke up, “glued” to its base.
“This complex history of the diamonds shows that they are remarkably well-travelled, both vertically and horizontally, within the Earth – tracing both the formation of the supercontinent and the later stages of its evolution. The accretion of relatively young material at the roots of the continents thickens and welds together these ancient continental fragments, suggesting a possible new mode of continental growth”.
Smit carried out the isotopic analyses of the sulphide inclusions at the Carnegie Institution for Science. Smit is now based at the University of the Witwatersrand, where she is part of a team developing a new isotope laboratory and methods so that diamond inclusion analyses can eventually be carried out at Wits.
“We have installed the necessary equipment in 2022 and are working to bring together the highly specialised skills and equipment so that we can do this type of diamond work in South Africa where previously it could only be done overseas,” says Smit.
“We need this kind of research to understand how continents evolve and move. Without continents, there’s no life. This research gives us an insight into how continents form, and it is linked to how life evolved and what makes our planet, the Earth, different from other planets”.
