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Diamond Flaws Reveal Secrets Of How Earth's Continents Formed

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Diamonds can reveal so much about geological processes. By analyzing inclusions in the diamonds, scientists learned more about how mantle keels are formed under continents.  ( Lars Plöger | Pixabay )

Flaws in diamonds may make it less valuable for jewelry collectors or newly engaged women, but for scientists, these tiny defects only make it more precious.

Since diamonds are formed deep in the Earth's mantle, tiny grains of minerals get trapped inside these beautiful rocks. These are considered imperfections in the jewelry trade, but scientists analyze these imperfections to learn about the conditions of the environment when the rock formed.

"We've found a way to use traces of sulfur from ancient volcanoes that made its way into the mantle and eventually into diamonds to provide evidence for one particular process of continent building," said Karen Smit, from the Gemological Institute of America and lead author of the study, in a report from Carnegie Science.

Mantle Keels

Under the surface of the Earth, about 93 to 124 miles (150 to 200 kilometers), there are formations known as mantle keels that stabilize the continental crust. For these buoyant keels to preserve the landmass even amid the destructive tectonic activities of the planet, the material that makes up these mantle keels should thicken, stabilize, and cool right under the continent.

The question is: how? Scientists debate on the answer, with some suggesting mantle keels are produced by subduction, which is when one tectonic plate slides under another and sinks into the ocean's depths. On the other hand, others theorize they form through hot magma rising from deep within the planet.

Figuring out how mantle keels form is an important part of piecing together the history of the continents and learning about their continuous survival, according to co-author Steve Shirey from Carnegie.

"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," Shirey added.

Diamonds Are A Geologist's Best Friend

Fortunately, diamonds can help answer this question since these sparkling rocks form within these mantle keels. By studying the mineral grains embedded in the diamonds — known as the imperfections or inclusions — scientists can reveal the nature and origins of the mantle keels where the diamond comes from.

For the study published in the journal Science, scientists analyzed the sulfur-rich minerals embedded within diamonds from Sierra Leone and discovered that there have been two subduction events in the region's history.

The researchers reached this conclusion because the chemistry of the mineral grains within the diamond was last seen in the Earth's surface over 2.5 billion years ago, before oxygen even became abundant in the atmosphere. Thus, the sulfur in the mineral inclusions must have been found on the surface in the past and then made its way to the mantle by the process of subduction.

Diamonds in Botswana displayed similar evidence, but diamonds from northern Canada did not. Researchers suggest that the mantle keel in the latter region may have been formed by a different process that didn't include surface material.

"Our work shows that sulfide inclusions in diamonds are a powerful tool to investigate continent construction processes," Smit said.

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