Diamonds Journey to the Center of the Earth


Diamonds are a symbol of deep love. How deep? From the bottom of the ocean to the center of the Earth.

Rare “superdeep” diamonds may be formed as far down as 435 miles (700 kilometers) beneath the surface in the Earth's lower mantle.

But a recent discovery shows they didn't start out there. Some Brazilian superdeep diamonds may have begun as undersea lava flows. That means Earth's carbon cycle is involved in the process of recyling the diamonds and reaches far deeper than anyone previously knew, said researchers from the University of Bristol and the Carnegie Institution in the journal Science.

"This study shows the extent of Earth's carbon cycle on the scale of the entire planet, connecting the chemical and biological processes that occur on the surface and in the oceans to the far depths of Earth's interior," said Nick Wigginton, associate editor at Science in a news release.

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The diamonds, from a mine near Junia, Brazil, showed the chemical fingerprint of basalt rock that forms on the ocean's floor from lava eruptions.

Jules Verne might say the rocks formed 20,000 leagues under the sea then journeyed to the center of the Earth.

Plate tectonics, movements of the Earth's crust caused by magma flowing underneath in the mantle, propelled the basalt downward into the lower mantle.

The basalt was then compressed into diamonds and returned to near the Earth's surface by magma plumes. The plumes hardened into diamond rich kimberlite deposits. The long-distance diamonds were then recovered by miners and studied by scientists.

"We looked at the variations in the isotopes of the carbon atoms in the diamonds,” said study co-author Steven Shirey of the Carnegie Institution in a press release.

“Carbon originating in a rock called basalt, which forms from lava at the surface, is often different from that which originates in the mantle, in containing relatively less carbon-13. These super-deep diamonds contained much less carbon-13, which is most consistent with an origin in the organic component found in altered oceanic crust," said Shirey.

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The carbon signature told scientists the diamonds were made from surface lava flows, but now they had to prove they were formed deep beneath the surface. Most diamonds form at depth of less that 120 miles (200 km). Some form at deeper depths from surface rock, but until now, no lower mantle superdeep diamonds had been shown to form from surface rock.

To figure out just how deep the diamonds formed the researchers looked at inclusions, or tiny mineral deposits in the diamonds. They found inclusions with the chemical composition expected from melted and crystallized basalt. Basalt only forms those types of inclusions when it is produced under the extreme heat and pressure of the earth's lower mantle.

"I find it astonishing that we can use the tiniest of mineral grains to show some of the motions of the Earth's mantle at the largest scales," said Shirey.

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"Inclusions in diamonds are fantastically useful for studying the inaccessible part of the deep Earth. It's a bit like studying extinct insects in amber. Although we can't extract DNA and grow dinosaurs, we can extract their chemical compositions and tell where they formed by growing minerals in the lab at extreme conditions!" said Micheal Walter of the University of Bristol in a press release.


Raw diamonds are screened for those hosting inclusions. The key to discovering the minute inclusions is meticulous polishing of the diamonds on a jeweler's polishing wheel. (Science/AAAS)

A cut-away view of the Earth's interior. (Wikimedia Commons)

This panel of images shows a raw diamond from Juina, Brazil, with a small window polished into it to see if any inclusions are inside. Once an inclusion is spotted, the diamond is further polished very slowly and carefully to expose the mineral, examples of which are shown in the middle panel. The chemical information obtained from the mineral inclusions together with the carbon isotopic composition obtained from the diamonds leads to a the conceptual model for deep recycling shown at the right. The model shows that the diamonds and inclusions form in the lower mantle in subducted oceanic crust, are then transported by mantle flow to the upper mantle, and finally to the surface in a kimberlite magma. (Science/AAAS)

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