The presence of salt trapped in many diamonds has puzzled scientists for years, but now, researchers believe it may hold an important clue as to how the sparkly minerals are formed from recycled ancient seabeds buried deep beneath the Earth鈥檚 crust.
, researchers led by geoscientists from Macquarie University in Sydney, Australia say they have discovered how most diamonds grow by recreating the extreme conditions that formed the salt they contain.
鈥淭here was a theory that the salts trapped inside diamonds came from marine seawater, but couldn't be tested,鈥� lead author Michael Forster said in a press release. 鈥淥ur research showed that they came from marine sediment.鈥�
Diamonds are crystals of carbon that typically form beneath the Earth鈥檚 crust in very old parts of the mantle, the researchers explained. They are brought to the surface in volcanic eruptions of a special kind of magma called kimberlite.
Because the Earth鈥檚 mantle is so thick (approximately 3,000 kilometres), scientists haven鈥檛 been able to drill down deep enough to understand exactly how the diamonds are formed within this layer before they surface in volcanic eruptions.
While most people think of diamonds as the crystal clear gems used in engagement rings and fine jewelry, there is another class of diamonds called 鈥渇ibrous diamonds鈥� that are far more common.
These fibrous diamonds have a clouded appearance because they contain traces of salt, potassium, and other minerals from their environment. This makes them less attractive to jewelers, but more valuable to scientists seeking to learn about their origin.
Because fibrous diamonds grow more quickly than clear, pure carbon diamonds, they trap 鈥渢iny samples of fluid鈥� from their environment during their formation.
鈥淲e knew that some sort of salty fluid must be around while the diamonds are growing,鈥� Forster said.
According to the study鈥檚 authors, fibrous diamonds can only be created when a large slab of sea floor slips down to a depth of more than 200 kilometres below the Earth鈥檚 surface in the process of one tectonic plate slides beneath another known as subduction.
鈥淭he rapid descent is required because the sediment must be compressed to more than four gigapascals (40,000 times atmospheric pressure) before it begins to melt in the temperatures of more than 800°C found in the ancient mantle,鈥� the study said.
In order to test this theory, the team conducted a series of experiments to see if they could recreate the salt for themselves using the same extreme pressure and heat that is found in that part of the Earth鈥檚 mantle.
To do this, they placed marine sediment samples in a vessel with a rock called peridotite that is commonly found in the part of the mantle where diamonds are formed. Next, they increased the pressure and heat in the vessel to allow the sediment samples and rock to react to each other.
They found that when the pressure was between four and six gigapascals and the temperatures were between 800 C and 1,100 C, corresponding to depths of between 120 and 180 kilometres below the Earth鈥檚 surface, the salt formed.
鈥淲e demonstrated that the processes that lead to diamond growth are driven by the recycling of oceanic sediments in subduction zones,鈥� Forster said.
What鈥檚 more, Forster said their experiments also formed minerals that are necessary ingredients for the creation of kimberlite magma, which is used to transport diamonds to the Earth鈥檚 surface in volcanic eruptions, further supporting their theory.
The study titled 鈥淢elting of sediments in the deep mantle produces saline fluid inclusions in diamonds鈥� is .
