When geologists go out into the field, they search out just the right volcano, hydrothermal vent, or rock outcrop that will let them test out their hypotheses. However, geologists interested in the mantle processes occurring hundreds of kilometers beneath the surface can’t simply travel to their field site, so they have to let the mantle come to them. One way to access tiny bits of deep mantle minerals is when inclusions are trapped during diamond crystallization at ultra-deep depths.
To access these unusual inclusion-bearing diamonds, Steve Shirey (Carnegie Institution for Science, USA), Graham Pearson, and Thomas Stachel (both at University of Alberta, Canada), members of the DCO Reservoirs and Fluxes Community, went with four Brazilian geologists to the town of Juína in Mato Grasso State, Brazil. For Shirey and Pearson this was their second trip; both went in 2014.
Many of the diamonds from this region are brownish, full of specks, and will never end up in jewelry, but these stones are scientifically valuable nonetheless for they have the potential to hold minerals from as deep as 600 to 700 kilometers beneath the surface. Analysis of these inclusions can reveal information about the mineralogy of the mantle, the processes that led to the diamond’s formation, and the ultimate source of the carbon.
“Typically, they’re not great diamonds, said Shirey, “but for science they are pretty important because quite a few of them have inclusions of mantle minerals that were trapped at great depths.”
The diamonds, which are really widely distributed in the soil of the region, originated from many, now buried kimberlite eruptions. Kimberlite is a type of volcanic eruption that rises rapidly from deep in the mantle and is the main magma known to carry diamonds. Overlying soil mixed with the kimberlites, so the gems must be separated from surface dirt instead of mined from rocks deep underground, as occurs at more traditional diamond mines. During a peak in the 1980s, the Juína region produced 400,000 carats of diamonds per month. Large mining companies including de Beers and Rio Tinto had diamond exploration operations in the area in the 1980s and 1990s, respectively, but currently independent prospectors called garimpeiros mine the stones. “They pan them in basically the same way that you pan for gold,” said Shirey. For farmers, who can be more systematic about separating diamonds, the gemstones are another way to gain income from their land.
Juína is not a typical tourist destination. Shirey, Pearson, and Stachel flew to Cuiabá, Brazil, which is the closest city large enough to rent a car, and then drove through farmland for a day to reach Juína, a small farming community of about 40,000 people.
The researchers worked with a local farmer, entrepreneur, and diamond dealer named Paolo Traven to obtain the stones. They examined samples under a microscope in Traven’s Juina town house and instructed Traven on the types of diamonds they wanted to purchase. “It’s a bit of a voyage of discovery,” said Shirey. “We don’t really know what we’ve got until we actually start studying them in much more detail, so you just have to go on faith.”
At the very affordable rate of $20 per carat, Shirey selected hundreds of carats in the hope that at least a few of the diamonds will have rare inclusions from the deep mantle not yet studied. Once Traven delivers the gems, Shirey will work with colleagues to map out the minerals inside the inclusions in the most promising diamonds using high-intensity X-rays.
Shirey suspects that diamonds found in this region form from a different process and depth than other types of ultra-diamonds. A previously studied Juína diamond enclosed a tiny sample of the mineral ringwoodite , the first sample that had been found on Earth. Pearson used the inclusion to estimate the water content of the mantle transition zone in 2014.
The diamonds from this trip will become part of the Diamonds and Mantle Geodynamics of Carbon (DMGC) Collection. The DMGC is an initiative to understand the movement of carbon through Earth’s deep interior in deep time, through the study of diamonds and diamond-forming fluids. Currently the diamonds are available for study through the Carnegie Institution for Science, but ultimately will be housed at the Smithsonian Institution.