Cach of carbon

Geology & Climate

Diamonds that once lay more than 435 miles beneath the earth’s surface have provided researchers with an unexpected window into the planet’s history. The diamonds, during their formation, captured evidence that slabs of the ocean floors descend deep beneath the surface, recycling carbon between the oceans and the earth’s mantle, the shell of rock, about 1,800 miles thick, that lies directly beneath the surface.

Understanding the fate of the slabs will help scientists better understand the earth’s carbon cycle and all the processes that depend on it, from the carbon dioxide in the atmosphere to the carbon compounds in living organisms and the formation of hydrocarbons in oil and gas.

Objects that resemble ocean slabs can be seen in seismic recordings, but they lie far too deep for any drill to sample. Impurities in the diamonds contain chemical signatures of the extinct ocean floor, evidence that the slabs have been cycled deep into the earth’s mantle, says a research team led by Michael J Walter of the University of Bristol in England.

These microscopic impurities, derived from rock and organic material in creatures that once lived on an ancient ocean floor, have undergone an amazing journey. The ocean floor rock, known as basalt, along with the sediment that built up on top of it, was drawn down at the edge of an ocean as part of the conveyor-belt mechanism that moves the continents.

When the slab of ocean floor had plunged 435 miles beneath the surface, minerals from the basalt were encapsulated inside the diamonds that formed at these depths. The diamonds continued to descend with the slab of ocean floor until they experienced two elevator rides back to the surface. A rising mass of solid rock known as a mantle plume carried them slowly back toward the upper mantle, and the heat of the plume then propelled to the surface an explosive jet of molten kimberlite, a volcanic rock that preserves diamonds.

Eons later, the diamonds were mined by the Rio Tinto Group from Juina in Brazil. The company allowed members of the research team to sift through stones not deemed to be of gem quality. After examining thousands of diamonds, the researchers found just six that seemed to be of superdeep origin. Despite their deep origin, the Juina diamonds are comparatively young as diamonds go. They were formed only 100 million years ago.

Most gem-quality diamonds are 1 billion to 3.5 billion years old, and originate at shallower depths, in the keels beneath the cratons, the ancient blocks of rock that form the hearts of the earth’s continental masses.

Invaluable to scientists
The impurities that make the superdeep diamonds useless to the jeweller are invaluable to the scientist. From the inclusions in the six Juina diamonds, Walter’s team was able to infer the existence of two minerals that form only in conditions that exist 435 miles or deeper below the surface. The composition of the two minerals matched the basalt of which the ocean floor is made, showing that slabs of ocean floor had reached this depth, the researchers reported in the journal Science.

In another test, they showed that the carbon in the impurities contained less than usual of the isotope known as carbon-13, a signature of organic carbon at the surface of the earth that has been processed by living organisms. Researchers are delighted that so much information about major geological processes can be gleaned from the microscopic impurities in the superdeep diamonds.

“The superdeeps will probably emerge in the next 10 years as some of the strongest evidence for deep movements and pathways in the earth’s mantle,” said Steven B Shirey of the Carnegie Institution of Washington, a member of Walter’s team. Thomas Stachel, an expert on diamond geochemistry at the University of Alberta in Canada, said, “Here you have a beautiful demonstration that the oceanic plate cycle is not relatively shallow, as many people assume, but that the subducted plate makes it down to the deep mantle and is brought back to the surface by a mantle plume.”

In Walter’s laboratory, the superdeep diamonds are polished with a jeweller’s polishing wheel until the precious impurities within them are exposed. With a variety of spectroscopic tests, the researchers then measure the composition of the minerals within the impurities.

The discovery that carbon from the ocean floor can be mixed so deep within the mantle raises the larger question of how much of the ocean floor and sediments are carried to the deep mantle. Given the importance of carbon to life, scientists seek to understand the major reservoirs of carbon in the earth and the exchanges between them, both in space and in time.

“The mantle is the biggest reservoir of carbon, and we know very little about it,” Walter said. “This won’t affect climate tomorrow, but what our results tell you is that carbon from the surface can go all the way into the lower mantle, which may be a long-term sink for carbon.”