Researchers have discovered that the world's largest, most famous diamonds were formed in a different part of the Earth's mantle and through a different process to the smaller, more common diamonds that make up the vast majority. The research has provided new insights into the Earth's mantle and the geological evolution of the planet.

Contrary to what many of us were taught, diamonds are not formed within coal, but deeper in the Earth's mantle – a layer of rock that makes up 84 percent of our planet's volume. From there, diamonds are usually deposited on or near the surface by deep-source volcanic eruptions.

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Quick primary school science refresher: the Earth is made up of three layers – the outermost shell or crust, which is around 25 miles (40 km) deep; the vast mantle, which is made up mostly of silicate rocks and minerals; and the core, which is composed mainly of iron and nickel.

We're still learning about the Earth's mantle, but it seems to be predominantly solid, with areas of viscous and semiliquid rock, because temperatures approaching the core are close to melting point.

Scientists have known for some time that the mantle is made up of around 44.8 percent oxygen, 21.5 percent silicon and 22.8 percent magnesium, along with iron, aluminum, calcium, sodium and potassium. These elements are all bound together in silicate rocks, all of which are bonded to oxygen in the form of oxides – the most common being silicon dioxide and magnesium oxide.

Large gem diamonds, such as the Cullinan and the Lesotho Promise, provide new insights into the mantle because they tend to be "superdeep" diamonds, forming at depths below 240 miles (386 km).

When the US research team gained the opportunity to examine some of these diamonds, they found the first physical evidence that confirms a theory that there are pockets of iron-nickel metal within the mantle. Preserved within these diamonds, the researchers found tiny metallic grains made up of a mixture of metallic iron and nickel – as well as carbon, sulfur, methane and hydrogen.

A cut and polished diamond with metallic inclusions – the most obvious group of inclusions looks like black spots on the left side, middle. (Credit: Jae Liao)

This tells scientists that the availability of oxygen is different in different parts of the mantle. Deep below the oxidized parts of the mantle, it now appears, some parts of the mantle are the opposite of oxidized. They are "reduced," meaning oxygen is being removed rather than added, which allows the iron and nickel metal to form there.

"The fact that reduced regions can be found in the Earth's mantle has been theoretically predicted, but never before confirmed with actual samples" said Steven Shirley, a researcher on the study from the Carnegie Institution for Science.

The metal iron-nickel phase revealed within these diamonds – some of which must have formed more than 1.2 billion years ago – would have to change the way we think about the "life cycle" and evolution of elements within the mantle. The metal phase would dissolve other elements such as carbon and sulfur and change their behaviors. It would regulate and reduce the availability of oxygen, producing phenomena such as the basaltic magma found at mid-ocean ridges, which is depleted in silicon but rich in iron and magnesium.

As well as access to some of the world's largest diamonds, researchers were given access to some the offcuts associated with them – these diamonds could be analyzed by destructive means such as polishing to expose inclusions) whereas polished gemstones could only be studied non-destructively. (Credit: Evan Smith)

We still have much to learn about the Earth's mantle – for instance, earlier this year, scientists discovered a previously unknown layer in the lower mantle estimated to contain 8-10 times more oxygen that the planet's surface.

Studying the planet's largest and most active layer yields insights into the evolution of our planet, as well as giving us a greater understanding of the planet's mineral resources, plate tectonics, volcanic and earthquake activity.

You can read more about the research in the journal Science.

Source: Carnegie Institution of Washington

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