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During their formation, diamonds have been shown to capture evidence that slabs of the ocean floors, descending deep beneath the earth’s surface, recycled carbon between earth’s mantle and the ocean.
According to a New York Times article by Nicholas Wade, understanding those slabs’ fate will help scientists understand the earth’s carbon cycle and all the processes that depend on it.
Some of the processes that depend on the carbon cycle: the carbon dioxide in the atmosphere, all the carbon compounds that live in organisms, and the formation of hydrocarbons in oil and gas.
Since objects that resemble ocean slabs lie far too deep for any drill to sample, diamonds provide a good alternative for study.
Researchers find that the impurities found in them contain chemical signatures of the extinct ocean floor. Those impurities provide evidence that the slabs have been cycled deep into the earth’s mantle.
These microscopic impurities, derived from rock and from organic material in creatures that once lived on an ancient ocean floor, have undergone an amazing journey. The ocean floor rock, 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.
After the slab of ocean floor plunged 435 miles beneath the surface, minerals from the basalt were encapsulated inside the diamonds formed at those depths.
Diamonds then continued to descend with the slab until they experienced two elevator rides back to the surface.
By means of a rising mass of solid rock known as a “mantle plume,” they were carried slowly back toward the upper mantle. The heat of the plume then propelled an explosive jet of molten “kimberlite” (a volcanic rock that preserves diamonds) to the surface.
The research team was let by Michael J. Walter of the University of Bristol in England. The diamonds tested were those mined eons later by the Rio Tinto Group from Juina, in Brazil.
The company allowed researchers to sift through stones not deemed to be of “gem quality.” After examining thousands of diamonds, the researchers found only six that seemed to be of superdeep origin.
Those impurities that make superdeep diamonds useless to jewelers make them invaluable to the scientist.
Dr. Walters’s team was able to infer the existence of two minerals from these inclusions — two minerals that form only in conditions that exist 435 miles or deeper below the earth’s surface.
The composition of the two minerals match the basalt of the ocean floor. This shows that slabs of the ocean floor had reached this depth.
The report was published in the September 15th issue of the journal Science.
Researchers also showed that the carbon in the impurities contained less than usual of the isotope known as carbon-13, which is a signature of organic carbon at the surface of the earth which was processed by living organisms.
Researchers were also delighted that so much information about great geological processes can be gleaned from the microscopic impurities.
According to a member of Dr. Walters’s team, Steven B. Shirey of the Carnegie Institution, “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.”
And Thomas Stachel, diamond geochemistry expert at the University of Alberta, says
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.”
This discovery raises the question of how much of the ocean’s floor and sediments are carried to the deep mantle. Since carbon is so important to life, scientists want 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. 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.
sole source: Nicholas Wade’s NY Times article on September 16, 2011. Visit http://www.nytimes.com/2011/09/16/science/16diamonds.html?_r=1&ref=nicholaswade
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