An analysis of the first confirmed large diamonds to come from the depths of Earth’s surface supports initial predictions that the Smithsonian’s famous Hope diamond may be “super deep,” originating more than three times as deep on Earth. than most diamonds. It also suggests, in a new finding, that the Cullinan “Crown Jewels” diamond may also be a super deep diamond.
Introducing the paper at the Goldschmidt geochemistry conference, Dr. Evan Smith of the Gemological Institute of America (GIA) confirmed: “We examined the first confirmed large gem diamonds that originate from Earth’s lower mantle, which is multiple times deeper than most other diamonds. The results support earlier predictions based on smaller gems, suggesting that diamonds with properties similar to those studied, including Cullinan and Hope diamonds, are super deep diamonds. “
Diamonds form under high pressure in the Earth’s mantle, the intermediate layer between the surface crust and the central core. While most diamonds form at the base of continental tectonic plates, at depths of 150-200 km, some rare diamonds form more deeply in the mantle. These “super deep” diamonds originate below rigid and stable continental plates, where the mantle slowly moves or convinces. Hope diamond is classified as a type IIb diamond, which contains the element boron, which can cause a blue tint. Until now there has been uncertainty as to whether “type IIb” diamonds formed in a shallow or deep environment. In particular, the uncertainty revolves around the origin of large Type IIb diamonds, larger than 3 carats (approximately the size of a pea). It is only in recent years that scientists have begun to understand where these dazzling blue crystals form on Earth.
Now researchers Drs Evan Smith and Wuyi Wang, who work at the GIA laboratory in New York, have detected the remains of the bridgmanite mineral in a large type IIb diamond. Smith said:
“Finding these remains of the elusive Bridgmanite mineral is significant. It is very common on deep Earth, in extreme pressure conditions of the lower mantle, below a depth of 660 km, even deeper than most super deep diamonds. Bridgmanite does not exist in the upper mantle or on the surface. What we really see in diamonds when they reach the surface is not bridgmanite, but the minerals that remain when it decomposes as pressure decreases. Finding these minerals trapped in a diamond means that the diamond itself must have crystallized to a depth where bridgmanite exists, deep within the Earth. “
Smith examined a large 20-carat Type IIb blue diamond from a mine in South Africa. By laser targeting the small inclusions trapped within this diamond, they discovered that the way light was scattered (using a Raman spectrometer) was characteristic of bridgmanite decomposition products.
He said: “We also examined a 124-carat large diamond from the Letseng mine in Lesotho. This diamond, which is about the size of a walnut, is very pure, does not contain nitrogen in its crystalline structure, and is known as a “CLIPPIR” diamond. This is from the same class of diamonds as the famous Cullinan Diamond, which is now the centerpiece of the British Crown Jewels. This large diamond showed the same characteristic bridgmanite decomposition products, meaning that it had also formed as a super deep diamond. What is special about this is that it is the first CLIPPIR diamond to which we can firmly assign a lower mantle origin, that is, below 660 km. Previously, we knew that CLIPPIR diamonds are super deep, and we speculated that their depth of origin could span a depth of 360 to 750 km, but we have not actually seen any that were definitely from the deeper end of this window. This gives us a better idea of where CLIPPIR diamonds come from, such as Crown Jewel diamonds. What we have learned here is that there is some overlap in the birthplace of CLIPPIR diamonds, such as Cullinan, and Type IIb diamonds, such as Hope. This is the first time this has been found. “
Type IIb boron-rich diamonds, such as the Hope diamond, are rare; less than 1 in a thousand diamonds classified as type IIb.
“Discovering the origin of the deep mantle means that the material in these diamonds undergoes a remarkable journey. We believe that boron, which gives Hope diamond its distinctive blue color, originates from the bottom of the oceans. From there, plate tectonics drags it hundreds of kilometers towards the mantle, where it can be incorporated into the diamond. It shows that there is a gigantic recycling route that carries elements from Earth’s surface to Earth, and then occasionally returns beautiful diamonds to the surface, like passengers in volcanic eruptions. “
Commenting, Dr. Jeff Post, Curator in Charge for Gems and Minerals at the Smithsonian’s National Museum of Natural History, said: “This fascinating work confirms that Hope Diamond is extraordinary and special, and truly one of the rarest objects in the Land”.
Dr. Christopher Beyer of Ruhr University, Bochum, Germany commented: “The discovery of traces of decomposition products of bridgmanite in large gem-quality diamonds shows that the inclusions in diamonds are capsules that reach us from deep Earth. that would otherwise be inaccessible. Furthermore, the unique signature of boron in Type IIb diamonds supports the theory of convection of the entire mantle with subducting slabs that descend to Earth’s lower mantle. Diamonds are crystallized from a fluid, so more studies are now needed to trace the composition of the fluid and the conditions that facilitate the growth of these large rare diamonds. “
NOTE: Neither Dr. Post nor Dr. Beyer participated in this work, these are independent comments.
The Goldschmidt Conference thanks the Smithsonian Museum of Natural History for their help in preparing this press release.
The Goldschmidt Conference is the world’s leading geochemistry conference, organized by the Geochemical Society and the European Geochemical Association. It is conducted annually and covers materials such as climate change, astrobiology, planetary and stellar development and conditions, the chemistry of Earth’s materials, pollution, the underwater environment, volcanoes, and many other topics. By 2020, the scheduled Hawaii congress moved online and will take place June 21-26, see https: /