Diamonds Show Carbon Cycle May Reach Earth's Lower Mantle
September 16, 2011

Diamonds Show Carbon Cycle May Reach Earth’s Lower Mantle


By studying the chemistry of an unorthodox set of Brazilian diamonds, a team of researchers have discovered startling new information about the carbon cycle and the theory of plate tectonics.

Researchers from the University of Bristol, Universidade de Brasilia, and the Carnegie Institution for Science studied what are known as "superdeep" diamonds, some of which originated from depths of 435 miles (700km) or more before erupting to the surface as volcanic rocks.

Most diamonds originate from depths of 120 miles (200 km) or less, and by studying these unique samples obtained from the Juina region of Brazil, the researchers were able to determine that their mineralogical composition was consistent with that of oceanic crust.

According to a press release from the Carnegie Institution, "This finding is the first direct evidence that slabs of oceanic crust sank or subducted into the lower mantle and that material, including carbon, is cycled between Earth's surface and depths of hundreds of miles."

Furthermore, in a separate media advisory, the University of Bristol noted, "The authors also observe that four of the six diamonds studied have extremely low amounts of 13C, a feature never previously seen in diamonds from the lower mantle. Low 13C is consistent with an origin of the carbon in oceanic crust at the Earth's surface."

"These and future results from investigations of diamonds and their inclusions could transform our understanding of the oxidation state, volatile content, and geological history of the lower mantle. They certainly mean that recycling of crustal materials, including carbon, is not limited to the upper mantle but extends deeper into the lower mantle," the UK university added.

Their findings were published online Friday at the Science Express website, the online version of 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," Nick Wigginton, associate editor at Science, said in a statement. "Results of this kind offer a broader perspective of planet Earth as an integrated, dynamic system."

Essentially, the study shows that the carbon cycle--the movement of the element through Earth's atmosphere, oceans, and crust--goes much deeper into the planet than originally though. Furthermore, according to a University of Bristol press release, the discovery also demonstrates that oceanic crust can reach the lower mantle, and then find its way back to the surface.

"I find it astonishing that we can use the tiniest of mineral grains to show some of the largest scale motions of the Earth's mantle," said Dr. Steven Shirey of the Carnegie Institution.


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