Imperfect diamonds may be a geoscientist’s best friend, acting as “mineralogical emissaries from the Earth’s depths,” according to Steve Shirey of the Carnegie Institution for Science.
Shirey and a group of geoscientists from Carnegie, the Gemological Institute of America, and the University of Alberta, published a paper in Science showing how they were able to study inclusions or imperfections in diamonds to reveal clues about how the Earth’s continents were formed. The diamonds contain traces of sulfur from ancient volcanoes that existed in Earth’s early atmosphere.
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By studying the diamonds, researchers can begin to solve the mystery of how the continents formed and stabilized, a critical factor in making Earth inhabitable for life at the surface.
“Solving this mystery is key to understanding how the continents came to exist in their current incarnations and how they survive on an active planet,” Shirey explained. “Since this is the only tectonically active, rocky planet that we know, understanding the geology of how our continents formed is a crucial part of discerning what makes Earth habitable.”
Sulfur-rich minerals called sulfides are found in samples from more than 2.5 billion years ago. Then, the atmosphere changed as oxygen became abundant. So locating sulfides in diamonds found buried deep in the Earth suggests that the diamonds’ beginnings were at the Earth’s surface.
The sulfides and carbon were pushed down deep into the ground, where the extreme pressures of shifting tectonic plates and hellish temperatures produced a diamond. The geological process is called subduction.
Subduction is ideal for producing lots of diamonds.
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Carnegie Science noted that these findings are helping to resolve a debate among geologists. At up to 93 to 124 miles down, formations called mantle keels stabilize the continents above them. The scientists disagree about how the critically important mantle keels form.
“Some scientists think mantle keels form by a process called subduction, by which oceanic plates sink from the Earth’s surface into its depths when one tectonic plate slides beneath another. Others think keels are created by a vertical process in which plumes of hot magma rise from much deeper in the Earth.”
The lead author on the group’s paper, Karen Smit of the Gemological Institute of America, says diamonds from the Zimmi region of Sierra Leone formed by subduction.
According to Smit, the Zimmi locality is known for producing yellow diamonds with abundant sulfide inclusions. To extract the inclusions, the team laser cut and polished the diamonds, then used hammers and steel crackers to break out the sulfides for study.
“Our technique shows that the geologic activity that formed the West African continent was due to plate tectonic movement of ocean crust sinking into the mantle,” said Smit.
"We now know that the finest gem-quality diamonds come from the farthest down in our planet," explains @CarnegiePlanets' Steve Shirey of his team's @nature cover work on the origin of blue #diamonds. 💎💎💎https://t.co/GTFgEMJTbP pic.twitter.com/zNoLbmTxAN
— Carnegie Science (@carnegiescience) August 1, 2018
The group also studied diamonds from northern Canada that didn’t have the sulfide inclusions. Thus, the scientists believe the Earth’s crust formed differently in North America. It remains a mystery how the mantle under Canada was formed.
“The sulfide minerals inside Canadian diamonds do not tell the researchers how this keel formed, only how it didn’t.”
The inclusions in diamonds tell many stories to geologists, also revealing the diamond’s age. In this way, a perfect diamond really is timeless.
“Inclusions in diamonds have given geoscientists information about water in the earth’s interior, mineralogy of the deep earth and metallic phases in the deep earth. Also, because there is no direct way to determine a diamond’s age from the diamond itself, mineral inclusions trapped within diamonds provide the only way to date diamonds. So, although inclusions in diamonds are often considered to be undesirable in the gem trade, they are extremely valuable scientific samples,” wrote Smit for GIA.
A prior study by the geologists found that rare blue diamonds are created through the subduction process, some 254 to 410 miles below the planet surface. The beautiful blue color comes from the element boron which is found high up in the Earth’s crust. The researchers believe that boron on the seafloor was pushed down when tectonic plates collided.
Even though they often have inclusions, the team determined that the “finest gem-quality diamonds come from the farthest down in our planet.”
Last month, a 20.46 carat blue diamond from Botswana was found called the “Okavango Blue.” The diamond now thought to be formed by subduction is further evidence of how the continent of Africa was formed. It may also be worth more than $350 million dollars.
— Melissa Knowles (@Knowlesitall) April 20, 2019
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