Tuesday, September 20, 2011

Diamonds: Telling the Tale of an Epic Journey

Diamonds formed in the lower mantle carry the record of Earth's carbon cycle.


[Like an insect in amber, mineral inclusions trapped in diamonds can reveal much about the Earth’s deep interior. Image credit: Science/AAAS]

Hundreds of millions of years ago, six diamonds formed in our planet's lower mantle began a remarkable journey up to the surface of the world. They emerged in a diamond mine in Brazil, a travelogue of an epic voyage.

In the Sept. 16 issue of Science magazine, scientists from the United States, Brazil and the U.K. detailed their findings that the small diamonds show just how colossal are the forces that shape the Earth, and how the planet recycles itself, particularly the element carbon. It is the first physical proof of a theory born in geology labs.

Scientists have known for decades that our planet's crust -- the part we stand on -- is formed in mid-ocean ridges. The crustal plates move toward other plates and are eventually forced underneath, or subducted. The subduction explains earthquakes and volcanoes, and where the continents sit. The subducted material slides down toward the center of the earth where it is eventually recycled. No one is quite sure how deep it goes and what happens to it. The diamonds tell the story.

"Diamonds can exist for half the life of the earth," said Steven B. Shirey of the Carnegie Institution for Science in Washington, in D.C., one of the researchers. "That's part of their charm."

Diamonds are formed when carbon is subjected to intense heat and pressure. Temperatures have to be higher than 2,240 degrees Fahrenheit, usually found 75-90 miles down. Our planet's lower mantle is about 430 miles down.

These six diamonds were found in the Juina region of Brazil. They were deposited at the end of their journey after riding up plumes of magma called kimberlites in the Cretaceous Era, about the time the dinosaurs were dying off.

The diamond in a typical engagement ring could be a billion years old. Shirey said these are 100-300 million years old.

What was important about these super-deep diamonds are the tiny amounts of other materials embedded in four of them as they formed, called "inclusions." The inclusions, which incorporate basalt material, are typical of what would be expected of materials from the oceanic crust, the crust that formed in the mid-ocean ridges and rode toward the plates.

That means the carbon that became the diamonds were deposited in the ocean crust, sank to the lower mantle, became embedded as the diamonds formed and then rode back up again to the surface. It is the first physical proof of that carbon cycle.

The discovery, by scientists from the University of Bristol in the U.K., Universidade de Brasilia, and the Carnegie, also means that diamonds could serve as a useful tool in understanding what is happening deep in the earth.

"The important thing to me is that we can use diamonds as a tracer of mantle circulation in a way that's sensitive enough to pick up this subduction process and trace it all the way to the lower mantle," Shirey said. "As long as the diamond is being dissolved by the magma carrying it is a pretty good container for bringing phases up from deeper than we can get them any other way."

"For a long time we've known about the geochemistry of things that come up in the plumes that appear to be contaminated with remains of oceanic crust that had been subducted a long time ago and had been churning around," said David Walker of the Lamont-Doherty Earth Observatory, in Palisades, N.Y., who was not involved in the research. "This would be physical pieces of something coming back."

It also demonstrates the importance of what scientists call the carbon cycle.

"Climate scientists and the media have focused the carbon cycle as pertaining just to the atmosphere, organic life at the Earth surface, and human activity and the oceans and their contained organisms, but these crustal environments are only a thin skin of the Earth which is only part of Gaia [the Earth] as a whole," said Chris Smith, a Bristol researcher.

Joel N. Shurkin, ISNS Contributor
Inside Science News Service

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