Focus on

THE SOURCE OF DIAMONDS

CONTINENTAL DRIFT REVEALS THE SECRETS
OF THE ORIGIN OF BLUE DIAMONDS

 

The incidence of rough diamond deposits has long been associated with the science of tectonics, relating to a theory that the earth’s outer crust, or lithosphere, is made up of several plates that slowly glide over the mantle, thereby creating the continents and oceans.

Where two tectonic plates meet are called subduction zones. There, one layer slides beneath the other back into the earth’s mantle, releasing energy in the form of molten rock or lava that flows from deep underground to the surface, giving rise to volcanic activity.

Geologists believe that natural diamonds were formed from carbon deposits that were subject to extreme pressures and temperatures as they were dragged through tectonic movement into the earth’s mantle. Consequently, they were delivered to the surface by deep-source volcanic eruptions, producing the kimberlite and lamproite ore pipes that are mined today.

It now appears that not only are diamonds the result of a tectonic action, but so is the color of fancy-color diamonds, or at the very least blue fancy-color diamonds, according to an article just published by a group of researchers in the Nature periodical, entitled “Deep Blue,” whose lead author was Evan Smith, a scientist at the Gemological Institute of America (GIA).

BLUE DIAMONDS FORMED AT THREE TIMES THE DEPTHS OF OTHERS

The source of the color in blue diamonds are boron atoms, embedded in the crystal structure of rare Type IIb diamonds. The article in Nature reveals they are “superdeep” diamonds, which means that they crystallized considerably deeper below the earth than most other gem diamonds.

To reach their findings, the scientists studied mineral inclusions trapped in the crystal structures of 46 Type IIb diamonds,  concluding that they are only are found together when the diamond had been subject to especially high pressures, such as those that existing in subduction transition zones, starting at 410 kilometers beneath the earth’s surface, reaching into the lower mantle, at a depth of 660 kilometers beneath the earth’s surface.

In comparison most other kinds of diamonds are believed to have been formed the base of old continents, at depths of about 150 kilometers to 200 kilometers.

A subduction zone, which starting at about 410 kilometers below the earth’s surface is the depth at which it believed blue diamonds began forming.

A PRODUCT OF EARTH AS IT WAS MILLIONS OF YEARS BACK

What puzzled the scientists is that, ordinarily, the concentration of boron in the earth’s mantle is extremely low, whereas higher concentrations, such as those existing at the earth’s surface, would be required to result in the boron atoms presence in Type IIb diamonds.

What the researchers now believe is that the boron may have originally been derived from ancient waterbodies, separating the continents as they once existed, which over time was deposited in the ocean crust that eventually was carried down to the lower mantle during the process of subduction. At that stage the increased temperature and pressure broke down the deposited minerals, which released a boron-enriched fluid which triggered diamond growth.

“Most previous studies of super-deep diamonds had been carried out on diamonds of low quality,” explained Steven Shirey of the Carnegie Institute, one of the scientists involved in the study. “But between our 2016 finding that the world’s biggest and most-valuable colorless diamonds formed from metallic liquid deep inside Earth’s mantle and this new discovery that blue diamonds also have super-deep origins, we now know that the finest gem-quality diamonds come from the farthest down in our planet.”

The August 2018 Nature magazine, where the cover story tackles the subject of origin in blue diamonds.