New research in the journal Science explains how diamonds of exceptional size and quality formed — from metallic liquid deep inside Earth’s mantle.
World’s biggest and most-valuable diamonds like the Cullinan, Constellation, and Koh-i-Noor formed in deep mantle metallic liquid. Image credit: Colin Behrens.
To date, researchers have puzzled over the origin of large diamonds like the famous Cullinan Diamond, discovered in South Africa in 1905; the Koh-i-Noor, found in India in the 13th century; or the widely publicized Lesedi La Rona stone, uncovered in Botswana in 2015.
Historically, research into such diamonds has been nearly impossible due to the high value of the jewels and the fact that they rarely contain inclusions that might shed light on their geological origin.
“Some of the world’s largest and most valuable diamonds exhibit a distinct set of physical characteristics that have led many to regard them as separate from other, more common diamonds,” said study senior author Dr. Wuyi Wang, director of research and development at the Gemological Institute of America.
“However, exactly how these diamonds form and what they tell us about the Earth has remained a mystery until now.”
Dr. Wang and co-authors studied large gem diamonds by examining their so-called ‘offcuts,’ which are the pieces left over after the gem’s facets are cut for maximum sparkle.
The researchers determined that these diamonds sometimes have tiny metallic grains trapped inside them.
In addition to the metallic inclusions, some of these exceptional diamonds contain mineral inclusions that show the diamonds formed at extreme depths, likely within 224-466 miles (360-750 km) in the convecting mantle.
This is much deeper than most other gem diamonds, which form in the lower part of continental tectonic plates at depths of 93-124 miles (150-200 km).
“This new understanding of these large diamonds resolves one of the major enigmas in the study of diamond formation — how the world’s largest and most valuable diamonds formed,” said study first author Dr. Evan Smith, also from the Gemological Institute of America.
“The composition of the inclusions, however, provides the story.”
The metallic inclusions are a solidified mixture of iron, nickel, carbon and sulfur, also containing traces of fluid methane and hydrogen in the thin tiny space between the metallic phases and the encasing diamond.
Pure carbon crystallized in this mix of molten metallic liquid in Earth’s deep mantle to form diamonds.
Small droplets of this metallic liquid were occasionally trapped within the diamonds as they grew. During cutting and polishing, parts of the diamond that contain inclusions are often cut off or polished away to craft exquisite polished gems with minimal flaws.
“Previous experiments and theory predicted for many years that parts of the deep mantle below about 155 miles (250 km) contain small amounts of metallic iron and have limited available oxygen,” Dr. Smith said.
“Now, the metallic inclusions and their surrounding methane and hydrogen jackets in these diamonds provide consistent, systematic physical evidence to support this prediction.”
The results are also important for understanding how volatile substances like carbon might cycle through Earth’s interior over time.
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Evan M. Smith et al. 2016. Large gem diamonds from metallic liquid in Earth’s deep mantle. Science 354 (6318): 1403-1405; doi: 10.1126/science.aal1303
Source link: https://www.sci.news/geology/diamonds-metallic-liquid-earths-mantle-04459.html
