Super-rare diamonds form at extreme depths

Blue diamonds, which account for less than 0.02% of all diamonds ever mined, form up to four times deeper in the Earth’s crust than other types, geologists have established.

Mineral impurities found in common diamonds indicate that the gems form in the upper part of the planet’s mantle, a region known as the cratonic lithosphere, less than 200 kilometres beneath the surface.

Writing in the journal Nature, a team led by Evan Smith of the Gemological Institute of America, New York, reveals that Raman spectroscopy conducted on 46 type IIb blue diamonds collected over two years showed that the stones formed as deep as 750 kilometres below ground, in the transition zone between the upper and lower mantles.

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That depth, and the source of their blue colour, could reveal valuable information about how the Earth’s crust recycles material from the surface into its depths and back again – a process, write the researchers, that is “both driven and obscured by plate tectonics”.

“These so-called type IIb diamonds are tremendously valuable, making them hard to get access to for scientific research purposes,” comments Smith. 

The impurities that give the diamonds their trademark blue hue are microscopic flecks of an element called boron. This came as a surprise. Boron is relatively rare on Earth (and in the solar system), and tends to be found near the planet’s surface. How, then, did it end up in diamonds formed at record-breaking depths?

The findings revealed that its most likely source was from the ocean floor, where it crystallised during geochemical reactions between seawater and the rocks of the oceanic plate.

These ocean floor areas were then pushed down into the Earth’s mantle when one tectonic plate slide over top of another, a process called subduction.

It was then, at these extreme depths and under intense pressure and heat of the deep mantle, that the boron was incorporated into the diamonds, turning them blue.

Over the next millions of years, the diamonds slowly made their way back towards the surface following currents in the mantle, incorporated back into the crust – from where they were eventually mined.

Even more intriguingly though, is that the boron source in oceanic plates that is most likely to remain stable during subduction is a mineral called serpentinite, which can also hold water. And this, say the researchers, means the findings also reveal a possible major pathway for the recycling of water deep into the Earth.

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