By dating the tiny silicate and sulphide inclusions inside superdeep diamonds dug up from mines in Brazil and Western Africa, a team led by Suzette Timmerman at the University of Bern, Switzerland, realized that the gems formed between 650 and 450 million years ago and 300 to 700 kilometres deep under the base of the ancient supercontinent Gondwana.
The goal of their study, which was published in the journal Nature, was to trace how material was added to the keel of the supercontinent. While doing this, the team recognized a previously unknown geologic process.
“The geochemical analyses and dating of inclusions in the diamonds, combined with existing plate tectonic models of continent migration, showed that diamonds formed at great depths beneath Gondwana when the supercontinent covered the South Pole, between 650–450 million years ago,” Karen Smit, co-author of the study and a researcher at South Africa’s University of Witwatersrand, said in a media statement.
Smit explained that the host rocks to the diamonds became buoyant during diamond formation, transporting subducted mantle material plus the diamonds. This material was added to the base of the root of Gondwana, in essence, ‘growing’ the supercontinent from below.
“Around 120 million years ago, Gondwana started to break apart to form the present oceans such as the Atlantic. At 90 million years ago, the diamonds, carrying trapped tiny inclusions of the host rock, were brought to earth’s surface in violent volcanic eruptions,” she said.
The current locations of these volcanic eruptions are on the continental fragments of Brazil and Western Africa, two of the key components of Gondwana. Thus, the diamonds must have migrated together with different parts of the former supercontinent as it dispersed, “glued” to their base.
“This complex history of the diamonds shows that they are remarkably well-traveled, both vertically and horizontally, within the earth—tracing both the formation of the supercontinent and the latter stages of its evolution,” Smit said. “The accretion of relatively young material to the roots of the continents thickens and welds together these ancient continental fragments indicating a potential new mode of continent growth.”
Continents drift across earth’s surface creating “supercontinents” and destroying them. Collectively, 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 focus deep oceanic plate subduction—the driver of plate tectonics—in very specific regions. Such deep geologic processes, especially in the past, have been very difficult to study directly because the oceanic crust is young, and the continental crust only provides a limited view of the earth’s deep workings.
Old diamonds offer a direct window into the deep plate tectonic engine and how it might relate to the supercontinent cycle.