Bigger and rarer diamonds have a unique story to tell – not only about their origins but also about Earth’s complex geology and what’s going on deep inside our planet’s mantle.
This is revelation of the study by researchers at Gemological Institute of America and colleagues who have been intrigued by origin of large diamonds like the Cullinan Diamond, Lesedi La Rona and the Kohinoor. The researchers found metallic iron slivers in more than 30 exceptionally large stones. While research on diamonds is a pretty expensive affair, GIA’s position enabled its researchers to go through thousands of diamonds submitted daily to the grading operations and found out that bigger and rares diamonds have something unique to tell that’s quite a story.
Researchers found that bigger stones are much different from other smaller diamonds and one of the primary reasons is the conditions under which they are formed. Scientists found metallic inclusions in some of the rarest diamonds 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.