An international team of scientists led by the University of Southampton has discovered that the breakup of tectonic plates is the main driving force behind the generation and eruption of diamond-rich magmas from deep inside the Earth.
It is posited that the findings could shape the future of the diamond exploration industry, informing where diamonds are most likely to be found.
Diamonds, which form under great pressures at depth, are hundreds of millions, or even billions, of years old. They are typically found in a type of volcanic rock known as kimberlite.
Kimberlites are found in the oldest, thickest, strongest parts of continents – most notably in South Africa, home to the diamond rush of the late nineteenth century. However, how and why they got to Earth’s surface has, until now, been unknown.
The new research examined the effects of global tectonic forces on these volcanic eruptions spanning the last billion years. The findings were published on July 26 in the journal Nature.
Southampton researchers collaborated with colleagues from the University of Birmingham, the University of Potsdam, the GFZ German Research Centre for Geosciences, Portland State University, Macquarie University, the University of Leeds, the University of Florence and Queen’s University, Ontario.
“The pattern of diamond eruptions is cyclical, mimicking the rhythm of the supercontinents, which assemble and break up in a repeated pattern over time.
“But previously we didn’t know what process causes diamonds to suddenly erupt, having spent millions – or billions – of years stashed away 150 km beneath the Earth’s surface,” explains University of Southampton Earth Science associate professor and principal research fellow Dr Tom Gernon, who was the lead author of the study.
To address this question, the team used statistical analysis, including machine learning, to forensically examine the link between continental breakup and kimberlite volcanism. The results showed the eruptions of most kimberlite volcanoes occurred 20-million to 30-million years after the tectonic breakup of Earth’s continents.
“Using geospatial analysis, we found that kimberlite eruptions tend to gradually migrate from the continental edges to the interiors over time at rates that are consistent across the continents,” explains University of Southampton senior research fellow Dr Thea Hincks.
GEOLOGICAL PROCESSES
This discovery prompted the scientists to explore what geological process could drive this pattern. They found that the Earth’s mantle – the convecting layer between the crust and core – is disrupted by rifting (or stretching) of the crust, even thousands of kilometres away.
“We found that a domino effect can explain how continental breakup leads to formation of kimberlite magma. During rifting, a small patch of the continental root is disrupted and sinks into the mantle below, triggering a chain of similar flow patterns beneath the nearby continent,” explains University of Birmingham Earth Systems associate professor Dr Stephen Jones, who co-authored the study.
GFZ Potsdam Geodynamic Modelling Section head Dr Sascha Brune, also a co-author on the study, ran simulations to investigate how this process unfolds.
“While sweeping along the continental root, these disruptive flows remove a substantial amount of rock, tens of kilometres thick, from the base of the continental plate,” he explains.
The typical migration rates estimated in models matched what the scientists observed from kimberlite records.
“Remarkably, this process brings together the necessary ingredients in the right amounts to trigger just enough melting to generate kimberlites,” Gernon says.
The team’s research could be used to identify the possible locations and timings of past volcanic eruptions tied to this process, offering insights that could enable the discovery of diamond deposits in the future.
Gernon, who was recently awarded a major philanthropic grant from the WoodNext Foundation to study the factors contributing to global cooling over time, said the study also sheds light on how processes deep within the Earth control those at the surface
“Breakup not only reorganises the mantle, but may also profoundly impact Earth's surface environment and climate, so diamonds might be just a part of the story.”