Big and rare diamonds like the Cullinan Diamond, Lesedi La Rona and the Kohinoor have a story to tell beyond their beauty – they reveal to scientists what goes on deep inside our Earth’s mantle at great depths.
Diamonds have been pegged as scientists’ best friends because they are quite handy when it comes to cutting down some of the hardest materials on Earth. Beyond this, diamonds have been revealing to us a great deal about Earth’s interiors at depths of up to 200 km where they are formed. While these depths are huge, consider the case wherein diamonds are formed at depths of up to 750 km in the convecting mantle. While scientists have no way of reaching such depths, diamonds formed that these depths do tell us a lot about what’s going on inside our planet that those depths.
Researchers at Gemological Institute of America and their colleagues have published a study wherein they have revealed details about the composition of diamonds formed at great depths. Authors of the study point out that they have been intrigued by origin of large diamonds like the Cullinan Diamond, Lesedi La Rona and the Kohinoor. They say that these bigger and rare diamonds have a different story to tell than their smaller counterparts.
According to the team they found metallic iron slivers in more than 30 exceptionally large stones and this is indicative of the fact that they would have formed at extreme depths – as deep as 750 km in the convecting mantle – compared to depths at which smaller gems are formed – at depths of up to 200 km.
Scientists pointed out that the metallic inclusions in these rare diamonds are a solidified mixture of iron, nickel, carbon and sulfur. Beyond these, the diamonds also contain traces of fluid methane and hydrogen in the thin tiny space between the metallic phases and the encasing diamond. Diamonds are formed at these depths after pure carbon in this metallic mix crystallizes and during this process metallic liquid gets occasionally trapped within the diamonds as they grew.
While this process was already theorized there haven’t been any conclusive evidence to point out the existence of small amounts of metallic iron and supply of oxygen in mantle below the depths of 250km. The latest study and its findings that reveal metallic inclusions and their surrounding methane and hydrogen jackets in these diamonds provide consistent, systematic physical evidence to support this prediction
Though the extent of metal distribution is uncertain, this key observation has broad implications for understanding the behavior of the deep Earth, including the recycling of surface rocks into the convecting mantle, and the deep storage and cycling of carbon and hydrogen in the mantle through geologic time. This result is important for understanding how volatile substances like carbon might cycle through Earth’s interior over time.