Science

Earth’s deepest diamonds are made up of former organisms, study finds

Earth’s deepest and most sought-after diamonds are made up of former living organisms, a new study finds. 

Ultra-rare ‘super-deep continental diamonds’ have levels of a carbon isotope that suggest they’re formed from organic matter, researchers in Australia reveal. 

These super-deep diamonds, which adorn the Crown Jewels, are formed more than 250 miles (400 km) below Earth’s surface before being released during violent eruptions.  

Most natural diamonds form in the Earth’s mantle at depths of at around 100 miles (150 km), under enormously high pressures and temperatures exceeding 2,700°F. 

Diamonds, which are made entirely of carbon atoms arranged in a dense lattice, are the hardest materials on Earth. 

Diamonds are forever: Experts say super-deep continental diamonds’ have levels of a carbon isotope that suggest they’re formed from organic matter

DIAMOND TYPES 

– Lithospheric

Depths between 80 and 125 miles

– Oceanic

Found on ocean floor 

– Super-deep continental 

More than 186 miles (300 km) below the continental crust 

‘This research not only helps to understand Earth’s carbon cycle, but also has the potential to unlock more secrets of the Earth’s dynamic history through tracking the past locations of mantle plumes and superplumes,’ said study author Professor Zheng-Xiang Li at Curtin University.

‘This can be achieved by mapping out the distribution of both continental and oceanic diamonds.’  

There are three main types of natural diamonds – ‘lithospheric’, ‘oceanic’ and ultra-rare ‘super-deep continental’ diamonds. 

Lithospheric, formed at depths between about 80 and 125 miles (130 and 200 km) are the most common, representing 99 per cent of all mined diamonds. 

Oceanic, meanwhile, are found on the ocean floor, while super-deep continental diamonds are formed at more than 186 miles (300 km) below the continental crust.

The continental crust is the outermost layer of what’s called the lithosphere, Earth’s rocky, outermost shell. 

Cross section of the Earth shows the continental crust - the outermost layer of the lithosphere (Earth's rocky, outermost shell)

Cross section of the Earth shows the continental crust – the outermost layer of the lithosphere (Earth’s rocky, outermost shell)

All three diamond types are formed at different levels of the mantle with a varying mixture of organic and inorganic carbon, which can be determined by variations in a carbon isotope signature called δ13C (delta carbon thirteen).

Diamonds formed from organic carbon would suggest they originated from a living organism, because organic carbon compounds are produced in living things. 

Previous research has already suggested δ13C levels in oceanic diamonds are suggestive of an organic origin. 

According to the researchers, super-deep continental diamonds contain a ‘surprising’ amount of δ13C, similar to oceanic diamonds – and therefore suggest an organic origin too.

One of the main differences between oceanic and super-deep continental diamonds is that the latter have highly variable levels of δ13C.

The diamonds in the Crown Jewels 

The famous Hope Diamond, may also be 'super deep'

The famous Hope Diamond, may also be ‘super deep’

The giant diamond cut into gems that now adorn the Crown Jewels was formed 400 miles below Earth’s surface — three times deeper than other precious stones. 

Analysis of similar diamonds by the Gemological Institute of America revealed that the Cullinan was a ‘super-deep’ diamond, and one of the rarest objects on Earth.

The largest gem-quality rough diamond ever found, weighing 3,106.75 carats, the Cullinan was unearthed from a mine in South Africa in the January of 1905.

In 1907, the diamond was bought by the Transvaal Colony government, who presented it to King Edward VII as a gift.

The King had the rough stone cut by Joseph Asscher & Company of Amsterdam — forming nine major stones (Cullinan I–IX) as well as 96 minor brilliant stones.

The largest two stones — Cullinan I and II — now form centrepieces in the Crown Jewels, and sit in the Sovereign’s Sceptre with Cross and the Imperial State Crown.

Despite originally remaining in Amsterdam, the other seven major stones have over time also been acquired or gifted into the possession of the British royal family.

The recent study also concluded that the famous Hope Diamond — presently held in the collections of the Smithsonian Museum in the US — may also be ‘super deep’.

The study authors think this is because super-deep cores become wrapped in inorganic crusts in the lithosphere, prior to being ejected out during eruptions. 

Both oceanic and super-deep continental diamonds form in the mantle transition zone – 250 to 372 miles (400 to 600 km) deep – using subducted organic carbon, and are then brought to the lithosphere by mantle plumes.  

‘Bringing new meaning to the old trash to treasure adage, this research discovered that the Earth’s engine actually turns organic carbon into diamonds many hundreds of kilometres below the surface,’ said study author Dr Luc Doucet at Curtin University.

Diagram from the paper explains the origin of diamond types. (A) Oceanic and super-deep continental diamonds (cores only) form in the mantle transition zone using subducted organic carbon, and are then brought to the lithospheric levels by mantle plumes

Diagram from the paper explains the origin of diamond types. (A) Oceanic and super-deep continental diamonds (cores only) form in the mantle transition zone using subducted organic carbon, and are then brought to the lithospheric levels by mantle plumes

‘Ballooning rocks from the Earth’s deeper mantle, called mantle plumes, then carry the diamonds back up to the Earth’s surface via volcanic eruptions for humans to enjoy as sought-after gemstones.

‘While recycling is becoming a modern-day necessity for our sustainable survival, we were particularly surprised to learn, through this research, that Mother Nature has been showing us how to recycle with style for billions of years.’        

The research provides a model that explains the formation and locations of all three major types of diamonds, according to the team. 

Ballooning rocks from the Earth's deeper mantle, called mantle plumes, carry diamonds to Earth's surface via volcanic eruptions (stock image)

Ballooning rocks from the Earth’s deeper mantle, called mantle plumes, carry diamonds to Earth’s surface via volcanic eruptions (stock image) 

‘This is the first time that all three major types of diamonds have been linked to mantle plumes, ballooning hot rocks driven by plate tectonics and the supercontinent cycle from deeper Earth,’ said study author Professor Li.

It remains a mystery as to why diamonds formed in the mantle transition zone are formed of recycled organic carbon only.

‘This might have something to do with the physical-chemical environment there,’ Professor Li added.

‘It is not uncommon for a new scientific discovery to raise more questions that require further investigation.’          

The study has been published in Scientific Reports.  

HOW ARE DIAMONDS FORMED?  

Naturally occurring diamonds were formed over 3 billion years ago deep within the Earth’s crust under conditions of intense heat and pressure. 

These conditions cause carbon atoms to crytallise, forming diamonds. 

Diamonds are found at a depth of approximately 150 to 200 kilometres (93 – 124 miles). 

Here, temperatures average 900 to 1,300 degrees Celsius, with pressures of 45 to 60 kilobars (which is around 50,000 times that of atmospheric pressure at the Earth’s surface).

Under these conditions, molten lamproite and kimberlite (known as magma) are also formed within Earth’s upper mantle, and they expand at rapid rates. 

This expansion causes the magma to erupt, forcing it to Earth’s surface and taking along with it diamond bearing rocks.

The magma erupts by forming a ‘pipe’ to the surface, and as it cools the magma hardens to form Kimberlite, settling in vertical structures called kimberlite pipes. 

These pipes are the most significant sources of diamonds, yet only about 1 in every 200 kimberlite pipes contain gem-quality diamonds.

The name ‘Kimberlite’ comes from the South African town of Kimberley, where the first diamonds were found in this type of rock. 

Source: Cape Town Diamond Museum

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