Despite humanity’s intense fascination with glittering chunks of carbon, it seems there is still a lot to learn about how diamonds form deep within our planet.
New research has discovered that two different types of rare diamonds share a common origin story – the recycling of organisms once living more than 400 kilometers (250 miles) below the surface.
There are three main types of natural diamonds. The first are lithospheric diamonds, which form in the lithospheric layer about 150 to 250 kilometers (93 to 155 miles) below the Earth’s surface. These are by far the most common and probably the type of diamond you will find on an engagement ring.
Then there are two rarer types – super deep oceanic and continental diamonds.
Oceanic diamonds are found in oceanic rocks, while deep continental diamonds are those that form 300 to 1,000 kilometers (186 and 621 miles) below the Earth’s surface.
To put this in perspective, we categorize space as 100 kilometers (62 miles) above sea level, the ISS orbits around 400 km (250 miles) above Earth, and humans have never managed to dig deeper more than 12.2 km (7.6 miles) into the ground. Thus, ultra-deep continental diamonds are formed … Great deep in earthly mantle.
As you might expect, oceanic and ultra-deep continental diamonds look quite different. Because the variation of an isotopic signature of carbon called ??13C (delta carbon thirteen) can be used to determine whether carbon has an organic or inorganic origin, former researchers have suggested that oceanic diamonds were originally formed from organic carbon that was once found in living things.
Super deep continental diamonds, on the other hand, have an extremely variable amount of δ13C. It is difficult to say whether they are made of organic carbon or not.
But in this new paper, led by Curtin University geologist Luc Doucet, the team found that ultra-deep continental diamond cores have a similarity.13C composition. Surprisingly, this means that, like oceanic diamonds, these gemstones also contain the remains of once-living creatures.
“Bringing new meaning to the old adage of trash trash, this research found that the Earth’s engine actually turns organic carbon into diamonds several hundred kilometers below the surface.” said Doucet.
“The bloating of rocks in the Earth’s deeper mantle, called mantle plumes, then bring the diamonds back to the Earth’s surface via volcanic eruptions for humans to enjoy as sought-after gems.”
Back in the lithosphere, some of these deep diamonds become cores enveloped in crusts of inorganic diamonds, whose isotopes correspond to diamonds in the lithosphere. This explains why their δ13The composition in C is so variable.
In recent years, we have learned a surprising amount about scientists second preferred form of carbon.
Contemplating defective diamonds can help researchers discover their first moments; the structure of these crystals remains in place even under a pressure five times greater than that of the earth’s core; in 2019 we even discovered a diamond with an entirely different diamond inside.
But this new research is not the end of the story – not by far. Scientists don’t know why these rare, deep diamonds found deeper than the lithosphere use this recycled organic carbon.
“It could have something to do with the physico-chemical environment there”, Curtin University geologist Zheng-Xiang Li explained.
“It is not uncommon for a new scientific discovery to raise more questions that require further investigation.”
The research was published in Scientific reports.