When Old Rose dropped the diamond necklace into the ocean at the end of Titanic, it turns out she was probably just returning it to where it originally came from. Researchers in Australia and Germany have found that many diamonds begin life as sediment on the bottom of the ocean, before being swallowed up by the Earth's mantle and forged into our favorite shiny stones.
Generally held up as symbols of status, wealth and romance, diamonds may be starting to lose a little of their mystique in recent years, as scientists learn more about them. The gems are far more common than you might think, with a quadrillion tons of diamond newly discovered deep within the Earth, and there's plenty of them floating around out in space. They've been found to contain brand new types of ice and, of course, lab-made diamonds have become virtually indistinguishable from natural ones – to the disdain of the jewelry industry.
The new study sheds more light onto the life cycle of these gems. It's long been known that diamonds are formed deep within the Earth's mantle, as extreme heat and pressure compresses carbon into shiny, ultra-hard crystals. These are then brought up towards the surface through volcanic activity.
One particular mystery was where the material originally came from, before becoming a diamond. Clues are found inside "impure" fibrous diamonds, in the form of traces of sodium, potassium or other minerals. These diamonds tend to form much faster than pure diamonds, so they capture samples inside more readily.
Salt is a common impurity, and the researchers on the new study believe they've discovered why; many fibrous diamonds appear to be made up of material from the ocean floor.
"There was a theory that the salts trapped inside diamonds came from marine seawater, but couldn't be tested," says Michael Förster, lead author of the study. "We knew that some sort of salty fluid must be around while the diamonds are growing, and now we have confirmed that marine sediment fits the bill."
The team tested the idea by placing samples of marine sediment into a chamber that would simulate the high pressure and high temperature deep within the Earth. Samples of peridotite, the most common type of rock in the mantle, was also added, before the mix was cooked.
Sure enough, the team found that salts similar to those found in diamonds began to form at pressures between four and six gigapascals, and temperatures between 800 and 1,100° C (1,472F and 2,012° F). These are the conditions you'd expect at depths between 120 and 180 km (74.5 and 112 mi) below the surface of the Earth.
"We demonstrated that the processes that lead to diamond growth are driven by the recycling of oceanic sediments in subduction zones," says Förster. "The products of our experiments also resulted in the formation of minerals that are necessary ingredients for the formation of kimberlite magmas, which transport diamonds to the Earth's surface."
The team also noted that for this process to work, large sections of the seafloor would have to be swallowed up very quickly, and drop to a depth of more than 200 km (124 mi). That's because the pressure needs to work its magic first, before the heat melts the material down. That might sound unlikely, but it happens all the time – the Mariana Trench is an active subduction zone that's sending huge amounts of seawater and sediment down into the mantle.
The research was published in the journal Science Advances.
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