If you’ve ever held or beheld a diamond, there’s a good chance it came from a kimberlite. Over 70% of the world’s diamonds are mined from these unique volcanic structures. Yet despite decades of study, scientists are still working to understand how exactly kimberlites erupt from deep in Earth’s mantle to the surface.
Kimberlites — carrot-shaped volcanic pipes that erupt from mantle depths greater than 150 km — have long fascinated geologists as windows into the deep Earth. Their mantle-derived melt ascends rapidly through the mantle and crust, with some estimates suggesting ascent rates of up to 80 miles per hour before kimberlites erupt violently at the surface. Along the way, the magma captures xenoliths and xenocrysts, fragments of the rocks encountered on its path.
“They’re very interesting and still very enigmatic rocks,” despite being well-studied, says Ana Anzulović, a doctoral research fellow at the University of Oslo’s Centre for Planetary Habitability.
In a study published this month in the journal Geology, Anzulović and colleagues from the University of Oslo have taken a major step toward solving the puzzle. By modelling how volatile compounds like carbon dioxide and water influence the buoyancy of proto-kimberlite melt relative to surrounding materials, they quantified for the first time what it takes to erupt a kimberlite.
Diamonds make it to the surface in kimberlites because their rapid ascent prevents them from reverting to graphite, which is more stable at shallow pressures and temperatures. But the composition of the kimberlite’s original melt — and how it rises so fast — has remained mysterious.
“They start off as something that we cannot measure directly,” says Anzulović. “So we don’t know what a proto-kimberlite, or parental, melt would be like. We know approximately but everything we know basically comes from the very altered rocks that get emplaced.”
To constrain the composition of these parental melts, the team focused on the Jericho kimberlite, which erupted into the Slave craton of far northwest Canada. Using chemical modelling, they tested different original mixtures of carbon dioxide and water.
“Our idea was, well, let’s try to create a chemical model of a kimberlite, then vary CO2 and H2O,” says Anzulović. “Think of it as trying to sample a kimberlite as it ascends at different pressure and temperature points.”
The researchers used molecular dynamics software to simulate atomic forces and track how atoms in a kimberlite melt move under varying depths. From these calculations, they determined the density of the melt at different conditions and whether it remained buoyant enough to rise.
“The most important takeaway from this study is that we managed to constrain the amount of CO2 that you need in the Jericho kimberlite to successfully ascend through the Slave craton,” Anzulović says. “Our most volatile-rich composition can carry up to 44% of mantle peridotite, for example, to the surface, which is really an impressive number for such a low viscosity melt.”
The study also shows how volatiles play distinct roles. Water increases diffusivity, keeping the melt fluid and mobile. Carbon dioxide helps structure the melt at high pressures but, near the surface, it degasses and drives the eruption upward. For the first time, researchers demonstrated that the Jericho kimberlite needs at least 8.2% CO2 to erupt; without it, diamonds would remain locked in the mantle.
“I was actually pretty surprised that I can take such a small scale system and actually observe, ‘Okay, if I don’t put any carbon in, this melt will be denser than the craton, so this will not erupt,'” says Anzulović. “It’s great that modeling kimberlite chemistry can have implications for such a large-scale process.”