Diamond formation may be triggered by oxidation reactions of sulphide minerals in the Earth’s mantle, according to a new study published in Nature Communications this week. These findings constitute direct evidence that diamonds may form by nucleating onto sulphides in the mantle.
Diamonds commonly form from carbon-bearing fluids and melts in the mantle in a variety of tectonic settings. Tiny mineral and fluid inclusions within diamonds can be used as a window into the processes occurring deep within the Earth. Sulphur is a very rare element in the Earth’s mantle, but, despite its rarity, is abundant in diamond inclusions. In particular, iron- and nickel-rich sulphide mineral inclusions (FeNi sulphides) - such as pyrrhotite - have been found within diamonds as inclusions, but no direct evidence could explain their presence until now.
Dorrit Jacob and colleagues used a technique known as Transmission Kikuchi Diffraction to image and map the microscopic structures of FeNi sulphide inclusions found in a diamond from Botswana. Using this technique, they were able to unravel the history of this diamond’s formation. They found that the natural oxidation reaction of pyrrhotite to magnetite, at a depth of 320 km to 330 km in the upper mantle, can trigger diamond precipitation. As the diamond nucleates and grows around magnetite inclusions, it traps them, thereby preserving them in the diamonds. The authors suggest that areas in the Earth’s mantle which are locally sulphide-enriched and where this reaction takes place could act as ‘diamond factories’.
Although diamonds may form in a variety of different ways according to the tectonic setting, this study suggests that sulphide minerals may play a key role in diamond formation.