Large diamonds reveal what Earth looks like 750 kilometers below the surface

Metal trapped in diamonds up to 10 carats provides new insight into Earth’s geologic evolution.

Diamonds usually form in depths of up to 200 kilometers below the surface. But stones like the Cullinan, the largest known gem ever recovered that adorned the scepter of King Edward VII, come from far greater depths. Evan Smith and his team at the Gemological Institute of America report in their latest study what metal inclusions in these large, rare stones tell us about the Earth’s mantle and its evolution.

ResearchGate: Which large diamonds did you study, and who owned them?

Evan Smith: Most of the diamonds we studied, especially the largest ones, over 10 carats, belong to many different owners and were submitted to the Gemological Institute of America for grading services. When a particular diamond of scientific interest came into the laboratory, a request was made to borrow the diamond for a few hours in order to make a detailed examination of the inclusions. Some additional large, high-purity diamond fragments were either purchased or donated for the purposes of this study.

The nine major stones cut from the rough diamond. Top: Cullinans II, I, and III. Bottom: Cullinans VIII, VI, IV, V, VII and IX. Source: Wikipedia Commons.
The nine major stones cut from the rough diamond. Top: Cullinans II, I, and III. Bottom: Cullinans VIII, VI, IV, V, VII and IX. Source: Wikipedia Commons.

ResearchGate: What did you find?

Smith: We analyzed large gem diamonds with physical characteristics like the renowned Cullinan diamond, the largest known gem diamond ever recovered. The study revealed three main things. First, the physical characteristics and inclusions trapped in large, relatively pure gem diamonds are different from other varieties of diamonds. They are not simply larger versions of the more “common” diamonds. This tells us they must have formed in a unique way. Secondly, these diamonds contain high-pressure mineral inclusions that indicate they came from extreme depths in the Earth, likely within 360-750 km deep, in the Earth’s convecting mantle. For comparison, most common diamonds come from a depth of about 150-200 km, from the lower parts of the rigid continental tectonic plates. Finally, the diamonds contain metallic inclusions, and these are the most abundant included material. In fact, in 38 of the 53 diamonds, the metallic inclusions were the only kind of inclusion present. This is the most exciting finding. These diamonds give us confirmation that the deep Earth mantle rocks are “speckled” with small amounts of iron-nickel metal. This is something that has been predicted from theory and experiments, but now we have physical evidence that small amounts of metal really do exist deep in the Earth. This is important because the metal has a strong control over the availability of oxygen, something that has broad implications for the geologic evolution of the Earth.

This is a close-up view of a metallic inclusion. Credit: Evan Smith; © GIA.This is a close-up view of a metallic inclusion. Credit: Evan Smith; © GIA.

ResearchGate: What implications for instance?

Smith: The biggest implication of the metallic inclusions is that it shows there really are regions of the deep Earth that have iron-nickel metal. This deep metal in turn affects the behavior of the mantle. One of the most important things it does is regulate and limit the availability of oxygen. In more scientific terms the metal should control the redox state. This affects the mechanisms of rock melting that produce basaltic magma at mid ocean ridges. A paper a few years back by Rohrbach and Schmidt talks about redox melting and freezing and how metal and carbon plays a role.

Another important thing this metallic iron-nickel phase does is dissolve other elements (carbon and sulfur, and others too) and change their behavior in terms of storage and cycling in the mantle. Any element that partitions into the metal phase could be affected. So this metal changes the way we think about the secret lives of some elements in the mantle, including deep carbon and hydrogen.

ResearchGate: Did you learn anything about the stones’ age?

Smith: While we don’t know very much about the age of this variety of diamond, we know for sure that some of them must have formed more than 1.2 billion years ago, based on the age of the rocks some of these diamonds are found in. We really don’t know how far back in time we can stretch this process.

ResearchGate: How is the formation of large diamonds different from normal-sized diamonds?

Smith: In general, we can say that these rare, exceptional gem diamonds like the Cullinan formed in a different part of the mantle by totally different processes compared to more common varieties of diamond.

ResearchGate: How do these large diamonds ever make it to the surface?

Smith: The final part of the journey from about 200 km to surface, we know has to do with special deep-seated volcanic eruptions called kimberlites. This is the same mechanism that brings most other kinds of gem diamonds to surface. The initial, deeper part of the journey, carrying diamonds up to the ~200 km mark is less clear. It may involve movement of buoyant mantle materials that carry the diamonds toward the surface.

ResearchGate: What did it feel like to work with large diamonds like these?

Smith: It is very exciting, not just because large diamonds are beautiful or expensive, but mainly because they are such astounding samples of the deep Earth. In a way, it is like examining a meteorite and realizing it is not at all like the other rocks around you.

ResearchGate: What fascinated you most about your findings?

Smith: It is astonishing that diamonds, the most valuable and prized of all gems, are coincidentally some of the most scientifically valuable samples of the Earth. You couldn’t ask for a better vessel than diamond to trap and preserve materials from the Earth’s interior. Each diamond has the potential to host real physical samples from a place we cannot go, from a time long since passed.

Challenges that have prohibited this kind of investigation in the past have been tackled in the present study by using the strengths of the Gemological institute of America, where I am a postdoctoral research fellow. The non-profit diamond grading operations at GIA have empowered us to pre-screen many thousands of diamonds in order to look for certain diamond samples for detailed examination. I think this is a great example of the benefits of collaboration between academia and industry.

Feature image: TVZ design