Diamonds might be a girl’s best friend‚ but they are a geologist’s soulmate.
New research into gems of exceptional size and quality‚ many of them unearthed in South Africa‚ has revealed how they were formed.
More significantly‚ findings by geologists including Steve Richardson of the University of Cape Town reveal some of the secrets of Earth’s deep mantle‚ between 360km and 750km beneath the surface.
The research‚ published in Science magazine‚ looked into diamonds similar to the 3‚106.75-carat Cullinan‚ which was found 30km east of Pretoria in 1905.
“Large gem diamonds like the Cullinan have a set of physical characteristics that distinguish them from other kinds of diamonds‚” said the lead researcher‚ Evan Smith of the Gemological Institute of America.
The new research shows these Cullinan-like gems sometimes have small metallic inclusions — or internal characteristics — trapped within them alongside traces of fluid methane and hydrogen.
“In addition to the metallic inclusions‚ some of these exceptional diamonds contain mineral inclusions that show the diamonds formed at extreme depths‚ likely within 360-750km in the convecting mantle‚” said Smith.
“This is much deeper than most other gem diamonds‚ which form in the lower part of continental tectonic plates at depths of 150-200km.
“This new understanding ... resolves one of the major enigmas in the study of diamond formation — how the world’s largest and most valuable diamonds formed. The composition of the inclusions‚ however‚ provides the story.”
The metallic inclusions were a solidified mixture of iron‚ nickel‚ carbon and sulphur‚ also containing traces of fluid methane and hydrogen in the thin tiny space between the metallic phases and the encasing diamond.
Pure carbon crystallised in this mix of molten metallic liquid in the Earth’s deep mantle to form diamonds. Small droplets of this metallic liquid were occasionally trapped within the diamonds as they grew‚ said Smith.
During cutting and polishing‚ parts of the diamond that contained inclusions were often cut off or polished away. These cut diamond pieces were not normally available for research‚ but Smith’s team were given access to them.
“Previous experiments and theory predicted for many years that parts of the deep mantle below about 250km depth contain small amounts of metallic iron and have limited available oxygen‚” said Smith.
“Now‚ the 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 was uncertain‚ this key observation had broad implications for understanding the behaviour 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.
Tiso Black Star Group Digital/Sunday Times
Massive South African diamonds uncover some of Earth's deepest secrets
New research into gems of exceptional size and quality‚ many of them unearthed in South Africa‚ has revealed how they were formed.
More significantly‚ findings by geologists including Steve Richardson of the University of Cape Town reveal some of the secrets of Earth’s deep mantle‚ between 360km and 750km beneath the surface.
The research‚ published in Science magazine‚ looked into diamonds similar to the 3‚106.75-carat Cullinan‚ which was found 30km east of Pretoria in 1905.
“Large gem diamonds like the Cullinan have a set of physical characteristics that distinguish them from other kinds of diamonds‚” said the lead researcher‚ Evan Smith of the Gemological Institute of America.
The new research shows these Cullinan-like gems sometimes have small metallic inclusions — or internal characteristics — trapped within them alongside traces of fluid methane and hydrogen.
“In addition to the metallic inclusions‚ some of these exceptional diamonds contain mineral inclusions that show the diamonds formed at extreme depths‚ likely within 360-750km in the convecting mantle‚” said Smith.
“This is much deeper than most other gem diamonds‚ which form in the lower part of continental tectonic plates at depths of 150-200km.
“This new understanding ... resolves one of the major enigmas in the study of diamond formation — how the world’s largest and most valuable diamonds formed. The composition of the inclusions‚ however‚ provides the story.”
The metallic inclusions were a solidified mixture of iron‚ nickel‚ carbon and sulphur‚ also containing traces of fluid methane and hydrogen in the thin tiny space between the metallic phases and the encasing diamond.
Pure carbon crystallised in this mix of molten metallic liquid in the Earth’s deep mantle to form diamonds. Small droplets of this metallic liquid were occasionally trapped within the diamonds as they grew‚ said Smith.
During cutting and polishing‚ parts of the diamond that contained inclusions were often cut off or polished away. These cut diamond pieces were not normally available for research‚ but Smith’s team were given access to them.
“Previous experiments and theory predicted for many years that parts of the deep mantle below about 250km depth contain small amounts of metallic iron and have limited available oxygen‚” said Smith.
“Now‚ the 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 was uncertain‚ this key observation had broad implications for understanding the behaviour 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.
Tiso Black Star Group Digital/Sunday Times
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