If you are lucky enough to own a diamond, perhaps perched on your finger or hanging around your neck, it is most likely made from recycled marine material. It is the conclusion of an investigation published in the journal Science Advances that, through the analysis of traces of salt trapped in the crystals of diamonds, showed that these gemstones were formed from ancient sea beds that were buried deep under the crust. land.
Most of the diamonds found on the Earth’s surface are formed in this way; instead, others were formed by the crystallization of molten material deep in the mantle.
In experiments that recreate the extreme pressures and temperatures found 200 kilometers underground, Dr. Michael Förster, Professor Stephen Foley, Dr. Olivier Alard, and their colleagues have shown that seawater in sediments at the bottom of the Ocean reacts in a very specific way, which results in the balance of the salts found in the diamond.
The mystery behind the origin of diamonds
Diamonds are carbon crystals that form under the earth’s crust in very old parts of the mantle. They rise to the surface in volcanic eruptions of a special type of magma called kimberlite.
The origin of diamonds is a long debated question. This study is not the first to suggest that the origin of diamonds resides in marine sediments; but it is the first that has managed to carry out the empirical verification of this hypothesis.
While gem diamonds are generally made of pure carbon, so-called fibrous diamonds, which are cloudy and less attractive to jewelers, often include tiny traces of sodium, potassium, and other minerals that reveal information about the environment where they were formed. . Why? Fibrous diamonds form more quickly than gem diamonds, which means they trap small samples of fluids around them as they form. These fibrous diamonds are generally ground and used in technical applications.
Scientists knew that there must be some kind of salty liquid in the diamond formation process; Now, the confirmation of the marine sediment fits perfectly with the suspicions of the scientists.
How are diamonds formed?
For this process to occur, a large slab of seafloor would have to slide to a depth of more than 200 kilometers below the surface fairly quickly, in a process known as subduction, in which one tectonic plate slides under another. .
The rapid descent is necessary 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 in the mantle.
To test the idea, team members from the Johannes Gutenberg Universität Mainz and the Goethe Universität Frankfurt in Germany conducted a series of high-pressure, high-temperature experiments.
They placed samples of marine sediments in a container with a rock called peridotite, which is the most common type of rock found in the part of the mantle where diamonds are formed. Then they increased the pressure and heat, giving the samples time to react with each other under conditions like those found in different places in the mantle.
At pressures between four and six gigapascals and temperatures between 800 ° C and 1100 ° C, corresponding to depths between 120 and 180 kilometers below the surface, they found salts formed with a balance of sodium and potassium that closely resembles small footprints found in diamonds.
Therefore, it can be deduced that the processes that lead to the growth of diamonds are driven by the recycling of oceanic sediments in subduction zones.
The researchers described it this way: “The products of our experiments also resulted in the formation of minerals, which are necessary ingredients for the formation of kimberlite magmas , and which transport diamonds to the surface of the Earth.”
The research has been led by geoscientists from Macquarie University in Sydney, Australia.
Study reference:
Michael W. Förster, Stephen F. Foley, Horst R. Marschall, Olivier Alard, Stephan Buhre. Melting of sediments in the deep mantle produces saline fluid inclusions in diamonds. Science Advances, 2019; 5 (5): eaau2620 DOI: 10.1126 / sciadv.aau2620
