To get from the ground to the jewelry store, diamonds undergo a journey that spans millions of years and hundreds of kilometers.
That journey depends on kimberlite, a type of magma originating in the mantle that transports diamonds to the surface in a rare type of volcanic vent. Most kimberlite eruptions occurred millions of years ago.
Because no one has ever observed a kimberlite eruption, much remains unknown about the phenomenon. A new study in Nature Geoscience, however, modeled how pillars of heat 2,900 kilometers below ground may drive these enigmatic events. Researchers used supercomputers to show how blasts of heat traveling from Earth’s core can be matched to known kimberlite eruption sites around the world. Use of the technique may lead to a better understanding of where diamonds and other mineral deposits are located.
“Kimberlites are exciting for two reasons. First, they are the main carriers of diamonds,” said geochemist Andrea Giuliani of the Institute of Geochemistry and Petrology at ETH Zürich, who was not involved in the study. “Second, because these are the deepest magmas…that have reached the Earth’s surface, kimberlite really gives you a unique perspective on the deep Earth.”
Pillars of Heat
Research in the 1970s and 1980s suggested that kimberlite eruptions might be caused by upwelling mantle plumes—blasts of material traveling from a depth of nearly 3,000 kilometers at the boundary between the mantle and the core. A study in 2010 further suggested that the plumes arose from the edges of two very hot blobs of such material underneath Africa and the Pacific Ocean. More recently, through computer modeling, researchers examined how these blobs may have moved over time and hinted that they could be where the mantle plumes driving kimberlite eruptions originate.
“We want to understand why these kimberlite eruptions have been happening at certain places and at certain times,” said geodynamicist Ömer Bodur of the University of Wollongong in Australia and one of the authors of the study. “That is our big question.”
Bodur and his colleagues used supercomputers to create global mantle convection models to simulate the flow of material as well as continent locations over the past billion years. These simulations were able to model plumes of heat coming from the core-mantle boundary. The plumes then could be correlated to known kimberlite eruption sites in Africa, the Americas, and Asia.
“These geodynamic models clearly show, in space and time, the link between kimberlite eruptions and the upwelling in Earth’s mantle,” said Oğuz Göğüş, a geophysicist at Istanbul Technical University who was not involved in the study. “The significance of this work is connecting the deep Earth to the surface. Their 3D models show how these deep structures are interlocked with kimberlite.”
The images that Bodur observed in the new models were not small, feathery plumes, he explained, but more like what he called pillars of heat. The form “was more like an ancient Greek column,” he said. These upwellings were able to provide the heat needed to push kimberlite from the mantle through the crust, picking up diamonds along the way.
Despite correlations between the modeling and known sites of kimberlite eruptions, there were some anomalies. In different places at different times, the models indicated an ancient eruption at a site with no known kimberlite deposits. “Is it because we haven’t explored there yet?” Bodur wondered. “Maybe our model is wrong or misinterpreted. It’s not a perfect model in the end; it’s just useful.”
Not Just for Diamonds
“Finding a diamond mine is finding a proverbial needle in the haystack,” Giuliani said. “Many are below the surface; they’re covered by other rocks or are located in harsh environments—the Arctic or the jungle. They’re not easy to discover.”
Diamond mining companies have an incentive to use models such as the one from Bodur and colleagues to pinpoint untapped reserves. These models also can be used by other types of mining industries to find different minerals formed by high heat flow from deep below the ground. Currently, researchers in Sydney are using these models to locate reserves of nickel and rare earth minerals that might be useful in the global transition to greener energy.
“When the mining industry was founded, they didn’t have the ability to make this connection between the deep Earth and the surface,” Göğüş said. “We know today that we need to understand the deep roots of these deposits.”
—Rebecca Owen (@beccapox), Science Writer