Super-deep diamonds, which form more than 380 km deep in Earth’s mantle, are invaluable tools for deep carbon scientists. Not only do they harbor clues about how they formed and therefore the reactions taking place inside Earth, they also trap small samples of mantle minerals, so-called inclusions, within their carbon crystal structure as they grow. These tiny samples of Earth’s deep interior from the region where the diamond forms are preserved under high pressure within a super-strong, unreactive diamond shell.
Many super-deep diamonds are small, have poor clarity, and are not generally used as gemstones. However, in a paper published in the journal Science, a team of researchers led by Evan Smith of the Gemological Institute of America (GIA) and including Deep Carbon Observatory DMGC (Diamonds and Mantle Geodynamics of Carbon) collaborators Steven Shirey (Carnegie Institution for Science, USA) and Fabrizio Nestola (Università degli Studi di Padova, Italy), suggests giant gemstone diamonds, like the 3106 carat Cullinan, are super-deep diamonds formed under special mantle conditions .
Co-author Professor Fabrizio Nestola explains the method of X-ray diffraction at the University of Padova, Italy. This method was used to first identify the presence of cohenite (an iron-nickel carbide) within the metallic inclusions. (credit Chiara Anzolini and Fabrizio Nestola)
When gem diamonds are polished and cut, expert diamond cutters often remove sections of the stones with inclusions. These offcut diamond pieces are not normally made available to scientists, and are usually considered waste, but the team made special efforts to get their hands on some.
“The project started with our collaborators at the GIA who have the opportunity to observe a number of large gem diamonds and have access to some of their offcut pieces,” said Shirey. “Evan Smith, a GIA postdoctoral researcher had a hypothesis that large diamonds could form deep in the mantle from metallic liquid, but we needed to the samples to figure it out.”
When they analyzed the offcuts, the team discovered multi-mineral metallic inclusions containing iron-nickel metal, an iron-carbide mineral known as cohenite, and the iron-sulfide mineral pyrrhotite. There were also traces of fluid methane and hydrogen in the thin space between the mineral phases and the encasing diamond. At the original pressure and temperature deep in Earth’s mantle, the composition of these multi-mineral inclusions suggested to the research team that a much larger mass of molten metallic liquid existed from which pure carbon crystallized to form diamonds. As each diamond grew, small droplets of the metallic liquid got trapped. As the diamonds were brought to Earth’s surface by volcanic eruption, the liquid droplets crystallized to the individual minerals.
“My motivation in this work was to solve this long-standing mystery about how these especially large and alluring diamonds form," said Smith. "Everything about them suggests they form in a special way and that means they might tell us something new about the behavior of mantle carbon. In this research I was chasing an idea that I published a couple years ago, that the low nitrogen content and large size of these (CLIPPIR) diamonds might be linked to metallic iron in the mantle. I was thrilled when I started finding the first few inclusions. With the expertise of everyone involved we saw the observations unfold into an amazing story from the deep Earth.”
As well as diamonds with only the metal inclusions, the team found additional similar diamonds with silicate mineral inclusions –that coexisted with smaller amounts of metal. This assemblage suggests that all the metal-containing diamonds formed between 360 and 750km deep inside Earth. This is much deeper than most other gem diamonds, which form in the lower part of continental tectonic plates at depths of 150–200 km.
These two observations together show not only that Earth’s largest gemstone diamonds form extremely deep in the mantle, but also in regions of the mantle with metallic iron, the first time these aspects of the largest gem diamonds have been recognized.
“The idea of metallic iron in the silicate mantle at far shallower levels than Earth’s iron core , is something Earth scientists have expected for a while,” said Shirey. “A number of experiments and simulations predicted it, but now we have physical evidence that this is the case.”
Previous experiments and theory suggested for many years that small amounts of metallic iron existed in parts of the deep mantle below about 250 km depth. Though it’s still unclear how much metallic iron is present in the lower mantle, this is a key observation for our understanding of Earth and the conditions under which it formed and evolved. Because the metallic liquid at these pressures and temperatures contains carbon and hydrogen it plays a hitherto undetected role in the geochemical cycles of these elements in the deep mantle.
"This result provides a direct link between diamond formation and deep mantle conditions, addressing a key goal of the Deep Carbon Observatory," said DCO Executive Director Robert Hazen (Carnegie Institution for Science, USA). "The fact that it was made possible by a hugely successful collaboration between our Diamonds and Mantle Geodynamics of Carbon group and the Gemological Institute of America is also very exciting, highlighting the importance of academic connections with industry and their important role in providing postdoctoral funding and the key specimens for this research."
Images: Top Left: Science cover from SCIENCE volume 354, Issue 6318, 16 December 2016. Reprinted with permission from AAAS. Middle Left: Assortment of CLIPPIR diamond offcuts used in the study. The largest is 9.6 carats. These diamonds could be analyzed by destructive means (polishing to expose inclusions) whereas many other diamonds studied were polished gemstones that were only borrowed and studied non-destructively. Credit: Evan Smith. Bottom: A cross section though a metallic inclusion, exposed by polishing down the surface of a diamond until it intersects the inclusion. This is a false-colored X-ray element map from a scanning electron microscope, showing the three main phases within a metallic inclusion. These solid phases were originally a molten metallic liquid mixture of iron-nickel-carbon-sulfur. The host diamond surrounding the inclusion appears as black. Credit: Evan Smith.