Magazine article Science News

Microscopic Diamonds Crack Geologic Mold

Magazine article Science News

Microscopic Diamonds Crack Geologic Mold

Article excerpt

Tiny diamond grains, discovered in rocks from southwestern Norway, are forcing geologists to rethink cherished ideas about Earth's continents.

"This is a spectacular discovery. This is a wake-up call," comments geologist Stephen E. Haggerty, a diamond expert at the University of Massachusetts in Amherst.

The diamond fragments were found by Larissa F. Dobrzhinetskaya of the Russian Academy of Science in Moscow, who collaborated with Norwegian, British, and U.S. colleagues. The team discusses its work in the July Geology.

At only 20 to 80 micrometers in size, the diamonds are too small to see without a microscope. Yet they have dazzled scientists because they formed within the continental crust, an unlikely birthplace for the world's hardest natural mineral.

According to geologic textbooks, diamonds can grow only in Earth's mantle, at depths of 120 kilometers or more. It takes the exceedingly high pressures and temperatures of the mantle--40,000 atmospheres and 900oC--to squeeze carbon into the ultracompact crystal structure of a diamond. The gems reach the surface when explosive volcanic eruptions force them up narrow conduits through the mantle and crust.

The Norwegian diamonds break the standard mold because they do not come from volcanic mantle rocks. Instead, they appear in metamorphic rocks that originally formed as ancient sedimentary deposits at Earth's surface. The layers of sediments were compacted and cooked hundreds of millions of years ago, when another continent rammed into what is now Scandinavia.

Although such continental collisions can metamorphose crustal rocks, they are considered far too docile for making diamonds. At their worst, conditions in the deep crust reach only 15,000 atmospheres and temperatures of 650oC--not nearly enough to mold carbon into diamond, says Haggerty. …

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