It’s how Earth recycles ocean floor into diamonds

Most diamonds are formed from recycled ancient seabeds that became buried deep beneath the Earth’s crust, a study has found. Others are created by crystallization of melts deep in the Earth’s mantle.

How do they form?

200+ kilometres --- the depth a large slab of sea floor slips below the earth’s surface quite rapidly, in a process known as subduction in which one tectonic plate slides beneath another.

4 + gigapascals (40,000 times atmospheric pressure) level to which the sediment must be compressed

800+ degrees Celsius temperature, found in the ancient mantle, on which the sediment begins to melt. After that the crystallization of the melts deep in the Earth’s mantle forms the diamonds.

Kimberlite: Diamonds, which are crystals of carbon that form beneath the Earth’s crust in very old parts of the mantle, are brought to the surface in volcanic eruptions of a special kind of magma called kimberlite.

The experiment

Fibrous diamonds grow more quickly than gem diamonds. It means they trap tiny samples of fluids around them while they form.

To test the process, researchers from Macquarie University in Australia carried out a series of high-pressure, high-temperature experiments.

They placed marine sediment samples in a vessel with a rock called peridotite that is the most common kind of rock found in the part of the mantle (pic in middle) where diamonds form.

Then they turned up the pressure and the heat, giving the samples time to react with one another in conditions like those found at different places in the mantle.

At pressures between 4 and 6 gigapascals and temperatures between 800 and 1,100 degrees Celsius, corresponding to depths of between 120 and 180 kilometres below the surface, they found salts formed with a balance of sodium and potassium that closely matches the small traces found in diamonds.