Super-Deep Diamond Holds First Glimpse of Long-Expected Mantle Mineral

In theory, calcium silicate perovskite is one of the most common minerals on Earth, but until recently, no one had ever held it in their hands. Lab experiments, modeling, and seismology data all suggest that calcium silicate perovskite makes up a large portion of the deep mantle. But when researchers synthesized it under high pressure and temperature conditions in the lab, its structure would break down at surface pressure and room temperature.

The recent discovery of an unusual inclusion in a rare, super-deep diamond means that calcium silicate perovskite (CaSiO3, abbreviated as Ca-Pv) is no longer just theory. DCO Reservoirs and Fluxes Community members Fabrizio Nestola (University of Padua, Italy) and Graham Pearson (University of Alberta, Canada) worked with Maya Kopylova (University of British Columbia, Canada) and colleagues to confirm the identity of a pocket of Ca-Pv in a diamond that formed 780 kilometers deep inside Earth. Their analysis also revealed that the carbon in the surrounding diamond originally came from ocean crust, suggesting that surface carbon travels incredibly deep into the mantle to be recycled. Their study appears in a new paper in the journal Nature [1].

Thanks to a collaboration with Petra Diamonds, Kopylova initially picked out the unusual diamond from a pile of rejects mined from the Cullinan mine in South Africa. Most diamonds form at depths of 130 to 250 kilometers, but 6% are “super-deep” and grow at depths of more than 380 kilometers, including the diamond in this study. These diamonds look more like brownish icebergs than the sparkling gems in engagement rings, but they can hold incredible souvenirs from the deep mantle.

Kopylova discovered what looked like Ca-Pv in an inclusion within the diamond but reached out to colleagues to confirm the find. The researchers used three complementary techniques, X-ray diffraction, Raman spectroscopy, and electron backscatter diffraction, to determine the compound’s identity as calcium silicate and to establish definitively that it had the high-pressure crystal structure of perovskite.

“Perovskite is probably the fourth most abundant mineral in the entire planet and yet no one has seen it at Earth’s surface, because of this difficulty in stabilizing it,” said Pearson. “There are probably zettatons of this mineral down inside Earth.” (A zettaton is 1 sextillion tons.)

The researchers also detected calcium titanite (CaTiO3) interspersed in the inclusion. Its titanium-rich nature suggests that the inclusion likely crystalized from material that was once in oceanic crust. Researchers also analyzed the carbon isotope profile of the region of the diamond that surrounded the Ca-Pv, to determine the ratio of “heavy” carbon atoms with an extra neutron compared to the more common “light” atoms with just six neutrons. They saw that the diamond has a higher percentage of heavy carbon atoms compared to typical mantle carbon, and is closer in composition to carbonates in Earth’s crust. “This is probably the strongest evidence ever seen of oceanic plates descending into the lower mantle,” said Pearson. “It’s direct evidence of the great depths at which carbon is cycled.”

In the future, Nestola plans to measure the oxygen isotopes in the inclusion. The necessary technique can destroy the sample, so it may be tricky, he says, but the results would reveal useful information about oxygen isotope profiles of the deep mantle. “This would be the first time that we can measure the oxygen isotopes so deep in Earth,” said Nestola. “Then we can retrieve information about what happens to oxygen isotopes when they move from the surface to the deep.”

The study also highlights the importance of studying diamonds and their inclusions for a better understanding of the deep carbon cycle. They are “the perfect trap” for preserving deep minerals and bringing them to the surface intact.

 

This 232 carat diamond came from the Cullinan mine in South Africa. The same mine also produced the Cullinan Diamond, which was the largest gem-quality diamond ever found, in 1905. Credit: Image courtesy of Petra Diamonds
 
 
Petra Diamond's Cullinan mine is a source of super-deep diamonds, including the one in the current study. Credit: Image courtesy of Petra Diamonds

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